Prof. Dr.-Ing. Wolfgang Tillmann

Lehrstuhl für Werkstofftechnologie
TU Dortmund University

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  • A numerical method to improve the representativeness of real microstructure cut-outs applied in finite element simulations
    Schneider, Y. and Wasserbäch, W. and Schmauder, S. and Zhou, Z. and Zielke, R. and Tillmann, W.
    Crystals 11 (2021)
    To improve the representativeness of a real microstructural cut-out for modeling purposes, a numerical method named as “boundary pixel color alteration (BPCA)” is presented to modify measured 2D microstructure cut-outs. Its physical background is related to the phase growth. For the application, the precondition is that the representativeness of the microstructure is already satisfied to a certain extent. This method resolves the problem that the phase composition of a small cut-out can have a large discrepancy to the real one. The main idea is to change the pixel color among neighboring pixels belonging to different phases. Our process simultaneously maintains most of the characteristics of the original morphology and is applicable for nearly all kinds of multi-phase or polycrystalline metallic alloys, as well. From our axisymmetric finite element (FE) simulations (ABAQUS ) applied with 2D real microstructures, it shows that the volume ratios of microstructural phases, as a function of the structure position to the symmetric axis, converge to phase area ratios in the 2D cut-out, even though the axisymmetric element volume is position dependent. A mathematical proof provides the reason for the aforementioned convergence. As examples to achieve real compositions and to numerically prove the aforementioned convergence, four different materials including multiphase polycrystals are implemented. An improvement of the predicted FE result is presented for the application of a modified microstructure (with a higher representativeness) compared to the original one. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/cryst11040382
  • Bias-voltage effect on the TiN nanoparticle injection into magnetron sputtered CrN thin films towards nc-TiN/nc-CrN composites
    Tillmann, W. and Kokalj, D. and Stangier, D. and Fu, Q. and Kruis, F.E.
    Applied Surface Science Advances 6 (2021)
    The formation of a nanocomposite structure is thermodynamically driven by spinodal decomposition in at least two phases. With respect to the suppression of solid solution formation, artificial nc-TiN/nc-CrN composites were deposited using a novel hybrid-process, in which TiN nanoparticles and CrN thin film were separately synthesized and simultaneously deposited during composite growth. The bias-voltage is known as a crucial deposition parameter concerning the structural and mechanical properties in thin film technology. However, it is still unclear whether an externally injected nanoparticle jet is influenced by the bias-voltage applied to the substrate. In this work, composite thin films were DC sputtered applying bias-voltages of 0 V, -100 V and -200 V in DC mode, as well as -100 V in MF and HiPIMS mode. TEM-investigations reveal the successful embedment of the nanoparticles in the film. Growth defects in the interface between nanoparticle and thin film can be reduced using a pulsed bias-voltage. Based on 2D GI-XRD experiments using synchrotron radiation, a bias-voltage of -200 V DC and -100 V MF enables the reinforcement of a higher nanoparticle content in the thin films. Similar to an increased bias-voltage, the injection of nanoparticles results in a decrease of the crystallite size. In principle, the residual stresses are increased by the nanoparticle embedding, as is the case for an increasing bias-voltage. In the event of a pulsed-bias voltage, however, the residual stresses can be reduced by the embedding of the nanoparticles. The mechanical properties of the CrN thin films can be maintained when nanoparticles are injected. © 2021
    view abstract10.1016/j.apsadv.2021.100149
  • Coatability of diamond-like carbon on 316L stainless steel printed by binder jetting
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Schaak, C. and Höges, S.
    Additive Manufacturing 44 (2021)
    Diamond-like carbon (DLC) coatings are efficient in improving surface properties of functional components and parts. For conventionally manufactured materials and products, the DLC coatings are on a high level of development and industrially well established. However, with the advent of additive manufacturing technologies and the unique microstructure of the produced parts, so far the properties of the DLC coatings on these surfaces have not been extensively investigated. Additively manufactured materials possess process-related structural characteristics, such as residual porosity or an anisotropic material behavior, thereby leading to distinguished properties of the substrate/coating system compared to conventionally fabricated substrate materials. Therefore, 316L substrates are produced with an intended residual porosity and different building directions by binder jetting and subsequently coated with DLC in a magnetron sputtering process. Conventionally manufactured 316L substrates were also coated to evaluate the manufacturing effects on the DLC properties. Based on the analysis of DLC coated open pores, a model is developed to describe the growth mechanisms of thin PVD coatings on open pores of different size. The thin DLC coating entirely covers residual porosity when the pore size (opening diameter) is smaller than or equal to the coating thickness of ~3 µm. Independently of the residual porosity or building orientation, the DLC coating provides a high hardness of 24 GPa and reveals a high adhesion strength to all binder jetted 316 L substrates. Compared to conventional manufacturing routes, the combination of additive manufacturing and DLC deposition is a competitive approach to fabricate complex-shaped components and parts with enhanced surface properties. © 2021 Elsevier B.V.
    view abstract10.1016/j.addma.2021.102064
  • Correction to: Microstructural and Tribological Characteristics of Sn-Sb-Cu-Based Composite Coatings Deposited by Cold Spraying (Journal of Thermal Spray Technology, (2020), 29, 5, (1027-1039), 10.1007/s11666-020-01054-1)
    Tillmann, W. and Hagen, L. and Kensy, M.D. and Abdulgader, M. and Paulus, M.
    Journal of Thermal Spray Technology (2021)
    view abstract10.1007/s11666-021-01226-7
  • Correlation between pre- and post-treatments of additively manufactured 316L parts and the resulting low cycle fatigue behavior
    Garthe, K.-U. and Hoyer, K.-P. and Hagen, L. and Tillmann, W. and Schaper, M.
    Rapid Prototyping Journal (2021)
    Purpose: The currently existing restrictions regarding the deployment of additively manufactured components because of poor surface roughness, porosity and residual stresses as well as their influence on the low-cycle fatigue (LCF) strength are addressed in this paper. Design/methodology/approach: This study aims to evaluating the effect of different pre- and post-treatments on the LCF strength of additively manufactured 316L parts. Therefore, 316L specimens manufactured by laser powder bed fusion were examined in their as-built state as well as after grinding, or coating with regard to the surface roughness, residual stresses and LCF strength. To differentiate between topographical effects and residual stress-related phenomena, stress-relieved 316L specimens served as a reference throughout the investigations. To enable an alumina coating of the 316L components, atmospheric plasma spraying was used, and the near-surface residual stresses and the surface roughness are measured and investigated. Findings: The results have shown that the applied pre- and post-treatments such as stress-relief heat treatment, grinding and alumina coating have each led to an increase in LCF strength of the 316L specimens. In contrast, the non-heat-treated specimens predominantly exhibited coating delamination. Originality/value: To the best of the authors’ knowledge, this is the first study of the correlation between the LCF behavior of additively manufactured uncoated 316L specimens in comparison with additively manufactured 316L specimens with an alumina coating. © 2021, Emerald Publishing Limited.
    view abstract10.1108/RPJ-01-2021-0017
  • Design of Cu- and Ag-containing amorphous carbon multilayers with improved tribo-mechanical properties
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Matveev, S. and Arne Thomann, C. and Debus, J.
    Materials Letters 284 (2021)
    The modification of amorphous carbon (a–C) films by adding either Cu or Ag is a common approach to tailor the film properties. These films become less hard, while they demonstrate higher friction and lower wear resistance than a–C. To enhance tribologically relevant features, multilayers of alternating a–C and a–C:Cu or a–C:Ag layers are synthetized by magnetron sputtering. The a–C/a–C:Cu and a–C/a–C:Ag multilayers possess a bilayer period of ~200 nm, a layer ratio of 1, and a bilayer number of 5. These structures are characterized by higher hardness and lower friction and wear against 100Cr6 counterparts as compared to monolayered a–C:Cu and a–C:Ag. © 2020 Elsevier B.V.
    view abstract10.1016/j.matlet.2020.128905
  • Experimental and statistical analysis of the wear of diamond impregnated tools
    Malevich, N. and Müller, C.H. and Dreier, J. and Kansteiner, M. and Biermann, D. and De Pinho Ferreira, M. and Tillmann, W.
    Wear 468-469 (2021)
    Diamond impregnated tools are considered which are used to machine concrete. During their application, the bonding as well as the diamonds need to wear down in a certain way to gain a sharp tool. This required wear is called self-sharpening and means a continuous exposure of new diamonds. Within the development phase of diamond tools, time and cost intensive testing is necessary for the assessment of the tool performance. Hence, an extrapolation based on a minimal amount of testing is desirable to forecast the tool lifetime. A further reduction of the development and testing cost can be achieved by reducing the data needed to forecast the tool performance. Within this paper, the development of a statistical model is shown which was used to forecast the lifetime of the single diamonds on the tool. The statistical analysis is based on single segment tests which were carried out with different segment specification. During the tests, the exposed and broken out diamonds were counted to serve as the necessary input data for the statistical analysis. The counting of the diamonds on the segment was done in two different ways: based on the 2-dimensional microscopic pictures made after every minute of drilling and based on the 3-dimensional surface measurements made after every 5 min of drilling. It turns out that these two approaches of the wear analysis provide similar results. © 2020 Elsevier B.V.
    view abstract10.1016/j.wear.2020.203574
  • Experimental investigations of micro-meso damage evolution for a co/wc-type tool material with application of digital image correlation and machine learning
    Schneider, Y. and Zielke, R. and Xu, C. and Tayyab, M. and Weber, U. and Schmauder, S. and Tillmann, W.
    Materials 14 (2021)
    Commercial Co/WC/diamond composites are hard metals and very useful as a kind of tool material, for which both ductile and quasi-brittle behaviors are possible. This work experimentally investigates their damage evolution dependence on microstructural features. The current study investigates a different type of Co/WC-type tool material which contains 90vol.% Co instead of the usual < 50vol.%. The studied composites showed quasi-brittle behavior. An in-house-designed testing machine realizes the in-situ micro-computed tomography (µCT) under loading. This advanced equipment can record local damage in 3D during the loading. The digital image correlation technique delivers local displacement/strain maps in 2D and 3D based on tomographic images. As shown by nanoindentation tests, matrix regions near diamond particles do not possess higher hardness values than other regions. Since local positions with high stress are often coincident with those with high strain, diamonds, which aim to achieve composites with high hardnesses, contribute to the strength less than the WC phase. Samples that illustrated quasi-brittle behavior possess about 100–130 MPa higher tensile strengths than those with ductile behavior. Voids and their connections (forming mini/small cracks) dominant the detected damages, which means void initiation, growth, and coalescence should be the damage mechanisms. The void appears in the form of debonding. Still, it is uncovered that debonding between Co-diamonds plays a major role in provoking fatal fractures for composites with quasi-brittle behavior. An optimized microstructure should avoid diamond clusters and their local volume concentrations. To improve the time efficiency and the object-identification accuracy in µCT image segmentation, machine learning (ML), U-Net in the convolutional neural network (deep learning), is applied. This method takes only about 40 min to segment more than 700 images, i.e., a great improvement of the time efficiency compared to the manual work and the accuracy maintained. The results mentioned above demonstrate knowledge about the strengthening and damage mechanisms for Co/WC/diamond composites with > 50vol.% Co. The material properties for such tool materials (> 50vol.% Co) is rarely published until now. Efforts made in the ML part contribute to the realization of autonomous processing procedures in big-data-driven science applied in materials science. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14133562
  • Force reduction by electrical assistance in incremental sheet-bulk metal forming of gears
    Wernicke, S. and Hahn, M. and Detzel, A. and Tillmann, W. and Stangier, D. and Lopes Dias, N.F. and Tekkaya, A.E.
    Journal of Materials Processing Technology 296 (2021)
    Producing load-adapted and functionally integrated components by flexible and resource efficient processes has gained importance within industries like the automotive sector in recent years. A promising new class of processes that enables a local adaption of the sheet thickness is Sheet-Bulk Metal Forming (SBMF). While the incremental procedure (iSBMF) only requires a moderate forming force, forming of high strength steels leads to a tool load resulting in a significantly reduced tool life. One approach to reduce tool loads is the utilization of the so called electroplastic effect (EPE). This study for the first time identifies the potential of the EPE on a temporary reduction of the forming force during the iSBMF of gears targeting an improvement of the tool life. The steel grades DC04 and HSM700 HD are characterized considering the EPE under uniaxial tension. Based on the characterization, the current density and temperature increase are modelled numerically and analytically for the incremental gear forming process. Moreover, the impact of EPE on strain hardening, grain texture and forming force is determined. By a local insulation of the forming tool based on a PVD coating and the application of an electrical current, a temporary force reduction of up to 55 % is observed whereas the strain hardening effect remains almost unaffected. © 2021
    view abstract10.1016/j.jmatprotec.2021.117194
  • Impact of structure on mechanical properties and oxidation behavior of magnetron sputtered cubic and hexagonal MoNx thin films
    Tillmann, W. and Kokalj, D. and Stangier, D.
    Applied Surface Science Advances 5 (2021)
    Molybdenum nitride films exhibit superior tribological properties, due to the possibility to form Magnéli phases when exposed to elevated temperatures. MoNx thin films were deposited by means of DC magnetron sputtering, while varying the bias-voltage and substrate temperature in regard to control the crystalline structure of the thin films. XRD experiments reveal a two-phase structure consisting of over stoichiometric cubic B1-NaCl and hexagonal MoNx phases for a high bias-voltage, whereas for lower bias-voltages the metastable cubic MoN phase was observed. Enhanced mechanical properties, obtained by means of nanoindentation, were analyzed for the thin films exhibiting the two-phase composite structure. The influence of the phase composition on the oxidation behavior, like oxidation state and formed oxide phases, was studied by means of in-situ XRD and XAS experiments using synchrotron radiation up to 700 °C. The oxidation processes start at 400 °C, forming oxides in amorphous state. Thin films composed of hexagonal MoN reveal a higher oxidation state up to 400 °C compared to cubic structured thin films, which changes above 400 °C. MoO2 and MoO3 were formed as primary oxides, independent from the crystalline structure of the deposited film. With an increase of temperature to 550 °C and 700 °C, the Magnéli-phases Mo9O26/Mo8O23 and Mo4O11 were formed. © 2021 The Author(s)
    view abstract10.1016/j.apsadv.2021.100119
  • Impact of tungsten incorporation on the tribomechanical behavior of alcrwxsin films at room and elevated temperature
    Tillmann, W. and Fehr, A. and Stangier, D.
    Coatings 11 (2021)
    AlCrWxSiN thin films (0 ≤ x ≤ 17.1 at.%) were synthesized by means of a hybrid magnetron sputtering process, merging direct current (DC) as well as tungsten high power impulse magnetron sputtering (HiPIMS) supplies. The influences of increasing the tungsten contents on the structural as well as the friction and wear behavior at room and high temperatures (500◦C) were elaborated. As a reference, a W61.4N38.6 system served to analyze synergetic effects on the oxidation behavior. Increased tungsten contents in AlCrWxSiN resulted in more distinctive (200)-, (202)-, and (311)-crystal orientations. A W/Cr ratio of ~1 could be correlated with a denser film growth, the highest hardness (24.3 ± 0.7 GPa), and a significantly decreased wear coefficient (&lt;0.3 × 10−5 mm3/Nm). Tribological tests performed at room temperature revealed that the coefficient of friction decreased with higher tungsten contents to µ~0.35. In contrast, at elevated temperatures, the coefficient of friction increased with higher W concentrations due to spotty oxidations in the wear track, which resulted in a locally increased surface roughness. Finally, a phase transformation of the WN film to m-WO3 did not contribute to a friction reduction at 500◦C. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/coatings11091033
  • Influence of Direct Splat-Affecting Parameters on the Splat-Type Distribution, Porosity, and Density of Segmentation Cracks in Plasma-Sprayed YSZ Coatings
    Tillmann, W. and Khalil, O. and Baumann, I.
    Journal of Thermal Spray Technology 30 (2021)
    The integrity and properties of ceramic coatings produced by atmospheric plasma spraying are highly controlled by the splat morphology and splat interconnection. In this study, the influence of selected parameters (spray angle, surface velocity of the spray gun, and substrate temperature) on splat morphology and coating microstructure was investigated. A favorite set of spray gun parameters, of which their effects on splat morphology and coating microstructure have been verified by previous experiments, were used to conduct the experiments for the present work. It was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats by about 60% at the expense of the fraction of irregular splats. Preheating the substrate also increases the pore count and level of coating porosity, while it does not influence the density of segmentation cracks. In contrast, the surface velocity of the spray gun does not affect the splat morphology while a slow speed decreases the coating porosity and plays a significant role in generating segmentation cracks. Shifting the spray angle by 15° distorts up to 20% of disk-like splats and slightly decreases the porosity level. However, changing the spray angle does not affect the generation of segmentation cracks. © 2021, The Author(s).
    view abstract10.1007/s11666-021-01180-4
  • Influence of spray gun parameters on inflight particle's characteristics, the splat-type distribution, and microstructure of plasma-sprayed YSZ coatings
    Tillmann, W. and Khalil, O. and Baumann, I.
    Surface and Coatings Technology 406 (2021)
    Ceramic coatings, fabricated with specific properties using the atmospheric plasma spray (APS) process, are widely used for many applications in which the porosity and splat interfaces are the main factors affecting the performance. Since the coating microstructure is composed of large numbers of molten and semi-molten particles impinged successfully at the substrate (known as splats), the produced coatings are characterized by the melting degree of these particles and their relative splat-type fractions. In the present work, the effect of process parameters settings has been studied systematically, relating the characteristics of impinging particles to splat formation and eventually to microstructure development and properties of the coating. Therefore, individual splats were collected on mirror-polished substrates and observed using image analysis (IA). These were evaluated and categorized into different splat types, based on their melting degree and morphology, under each combination of spray conditions. It was found that gun current and standoff distance have a profound effect on the characteristics of impinging particles. These, in turn, determine the relative fractions of splat types, layered structure, and final properties of the deposit. The effect of splat-type distribution on the bonding strength between layers, lamellar structure, and coating porosity was investigated. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.126705
  • Influence of the Brazing Paste Composition on the Wetting Behavior of Reactive Air Brazed Metal–Ceramic Joints
    Waetzig, K. and Schilm, J. and Mosch, S. and Tillmann, W. and Eilers, A. and Wojarski, L.
    Advanced Engineering Materials 23 (2021)
    Reactive air brazing (RAB) is a cost-effective way to produce ceramic–ceramic or ceramic–metal brazed joints in air, without applying a protective gas atmosphere or a vacuum. In addition to conventional furnace technology, the brazing with induction heating can also be used effectively. Within the scope of this study the shrinkage and wetting behavior of self-developed brazing pastes with different CuO contents and two qualities of silver powders with coarse and fine particle size are investigated by optical dilatometry on alumina (Al2O3, 99.7% purity). Thereby, the fine silver powder quality reveals a significant swelling effect at high temperatures, leading to an expansion of densified powder compacts caused by evolving gases. Joining tests are performed on ceramic–steel brazed joints using a muffle furnace and induction heating for short brazing cycles. The brazing seams and interfaces of the joints are investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). As a result, correlations between the brazing filler metal composition, the steel, and the brazing conditions are obtained. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/adem.202000711
  • Influence of the PVD process conditions on the incorporation of TiN nanoparticles into magnetron sputtered CrN thin films
    Tillmann, W. and Kokalj, D. and Stangier, D. and Fu, Q. and Kruis, F.E.
    Surface and Coatings Technology 409 (2021)
    CrTiN thin films are known to form a solid solution independent from the Ti content. Using a novel spatially separated synthesis approach, consisting of magnetron sputtering and atmospheric-pressure arc evaporation, artificial CrTiN nanocomposites were deposited. For the nanocomposite formation, TiN nanoparticles were synthesized using a transferred arc reactor and directly injected into growing CrN thin films using an aerodynamic lens system. The CrN and CrTiN thin films were deposited using various deposition conditions, such as heating power, substrate rotation velocity, nanoparticle injection distance, and cathode setup. The deposited thin films were analyzed regarding their physical structure, microstructure and mechanical properties. Based on the investigations, between 0.02 and 0.11 at.-% of TiN nanoparticles are embedded in the CrN matrix dependent on the deposition parameters. 2D GI-XRD experiments using synchrotron radiation confirm the nanocomposite structure for the two thin films with the highest TiN nanoparticle content. The crystallite size of the CrN thin film decreases from 9.4 ± 2.3 nm to 5.3 ± 1.2 nm due to the embedding of the nanoparticles. Concerning the physical structure, the nanoparticle injection leads to a change of the texture, as shown by the Debbye-Scherrer rings. Based on TEM-investigations, TiN nanoparticle agglomerates lead to a coarser microstructure of the CrN matrix. The hardness of the thin films is not significantly affected by the nanoparticle embedment. The nanoparticle injection distance and cathode setup reveal the highest impact on the film properties. © 2021 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2021.126935
  • Internal Diameter Coating by Warm Spraying of Fine WC-12Co Powders (− 10 + 2 µm) with Very Short Spray Distances up to 10 mm
    Baumann, I. and Tillmann, W. and Schaak, C. and Schmidt, K. and Hagen, L. and Zajaczkowski, J. and Schmidtmann, G. and Matthäus, G. and Luo, W.
    Journal of Thermal Spray Technology (2021)
    The internal diameter (ID) coating by means of thermal spraying is currently experiencing growing interest in science and industry. In contrast to the well-established plasma- and arc-based spray techniques, there is a lack of knowledge concerning kinetic processes such as HVOF, HVAF and warm spray (WS). A major challenge represents the necessity of short spray distances and the compact design of novel ID spray guns with reduced combustion power. Conventional WC-Co powders (− 45 + 15 µm) are not able to achieve a sufficient heat and momentum transfer. The use of fine powders &lt; 15 µm offers an approach to overcome this drawback as they feature a larger surface-to-volume ratio and a lower mass. However, the processing of fine powders requires suitable spray equipment and a sensitive parameter adjustment. In this study, warm spraying of fine WC-12Co powders (− 10 + 2 µm) with a novel ID spray gun (HVOF + N2) “ID RED” (Thermico Engineering GmbH, Germany) was investigated. First, the flame profile as well as the in-flight behavior of the particles along the spray jet (spray distances SD = 10-80 mm) was analyzed at different nitrogen flows NF = 15-115 L/min to find suitable spray parameter intervals. Subsequently, planar steel samples were coated with SD = 10-50 mm and constant NF = 90 L/min. Analyses regarding the microstructure, the mechanical properties and the phase evolution of the coatings were performed. The aim was to study spraying with the novel ID gun and to scrutinize shortest feasible spray distances. Finally, steel tubes (internal diameter of 81.6 mm and a wall thickness of 10.0 mm) were coated with SD = 20 mm and NF = 90 L/min to investigate in how far the results can be transferred to ID parts. Correlations between the particle behavior, the microstructure and the coating properties were made. © 2021, The Author(s).
    view abstract10.1007/s11666-021-01195-x
  • Investigation into deep hole drilling of austenitic steel with advanced tool solutions
    Felinks, N. and Rinschede, T. and Biermann, D. and Stangier, D. and Tillmann, W. and Fuß, M. and Abrahams, H.
    International Journal of Advanced Manufacturing Technology (2021)
    Deep hole drilling processes for high-alloyed materials are characterised by worn guide pads and chatter vibrations. In order to increase feed rates, process stability and bore quality in STS deep hole drilling, various investigations were carried out with adjustments to the tool. First, a new process chain for the production of tribologically optimised guide pads and their effects on the guide pad shape is described in detail. The results of these studies show that the shape change in the area of the axial run-in chamfer through a micro finishing process leads to a better bore hole quality. Furthermore, the influence of guide pad coating and cooling lubricant on the deep hole drilling process was investigated. In addition, the machining of the austenitic steel AISI 304 is analysed by using a conventional steel boring bar and an innovative carbon fibre reinforced plastic (CFRP)-boring bar. While the conventional drill tube oscillates with different eigenfrequencies, the CFRP-boring bar damps chatter vibrations of the drill head and stabilises the process. Even at higher feed rates up to f = 0.3 mm, it is possible to machine austenitic, difficult-to-cut-materials with significantly reduced vibrations. © 2021, The Author(s).
    view abstract10.1007/s00170-021-07989-1
  • Investigation of joints from laser powder fusion processed and conventional material grades of 18MAR300 nickel maraging steel
    Tillmann, W. and Wojarski, L. and Henning, T.
    Welding in the World 65 (2021)
    Even though the buildup rate of laser powder bed fusion processes (LPBF) has steadily increased in recent years by using more and more powerful laser systems, the production of large-volume parts is still extremely cost-intensive. Joining of an additively manufactured complex part to a high-volume part made of conventional material is a promising technology to enhance economics. Today, constructors have to select the most economical joining process with respect to the individual field of application. The aim of this research was to investigate the hybrid joint properties of LBPF and conventionally casted 18MAR300 nickel maraging steel depending on the manufacturing process and the heat treatment condition. Therefore, the microstructure and the strength of the hybrid joints manufactured by LPBF or vacuum brazing were examined and compared to solid material and joints of similar material. It was found that the vacuum-brazed hybrid joints using a 50.8-μm-thick AuNi18 foil provide a high tensile strength of 904 MPa which is sufficient for a broad field of application. Furthermore, the additively manufactured hybrid samples offered with 1998 MPa a tensile strength more than twice as high but showed a considerable impact of buildup failures to the strength in general. © 2021, The Author(s).
    view abstract10.1007/s40194-021-01096-1
  • Investigation of the stress corrosion cracking behavior on T24 material under the operational conditions in the water wall
    Ullrich, C. and Tillmann, W. and Rademacher, H.-G. and Zielke, R. and Körner, P.
    International Journal of Pressure Vessels and Piping 190 (2021)
    The requirements for materials and their strength significantly increased with the new generation of coal fired power plants operating at steam temperatures of up to 620 °C. Therefore, new materials were introduced to fulfill the defined needs. During the commissioning process of the first plant many cracks occurred in welds of T24 material. The cracks showed clear characteristics of stress corrosion cracking (SCC). Not knowing the exact parameters that lead to cracking, experiments in high temperature water were carried out. Slow tensile tests in a controlled environment are extremely well suited to generate information about material's SCC sensitivity. In the present paper, the influence of the temperature, the oxygen concentration of water, the pre-treatment of the specimen and the heat treatment to the SCC are investigated. Furthermore critical limits for the cracking are defined where possible. © 2021
    view abstract10.1016/j.ijpvp.2021.104317
  • Lapping and polishing of additively manufactured 316L substrates and their effects on the microstructural evolution and adhesion of PVD CrAlN coatings
    Tillmann, W. and Hagen, L. and Stangier, D. and Lopes Dias, N.F. and Görtz, J. and Kensy, M.D.
    Surface and Coatings Technology 428 (2021)
    Laser powder bed fusion (LPBF) enables the manufacturing of three-dimensional parts from metallic powder with a high degree of design-freedom. Due to the low hardness in order to protect tribologically stressed surfaces against severe wear, the surface of LPBF parts made of 316L stainless steel needs to be modified by coating processes such as physical vapor deposition (PVD) using hard coatings. Although 316L is among the most investigated material for LPBF, the deposition of PVD hard coatings such as ternary nitride coatings on additively manufactured 316L substrates has not been studied yet. This study aims at evaluating the effects of lapping and polishing of 316L stainless steel substrates processed by LPBF on the resulting surface integrity and its influence on the microstructural evolution and adhesion of PVD CrAlN coatings. A 316L substrate bulk material served as reference throughout the investigations. The 316L substrate CrAlN coating composites were examined with respect to the microstructural characteristics and residual stresses using nanoindentation and two-dimensional X-ray diffraction. To assess the adhesion of the CrAlN coating on the differently pre-treated 316L substrates, scratch and Rockwell C adhesion tests were conducted. The resulting scratch tracks and Rockwell C indents were metallographically examined in order to determine the failure modes. The results demonstrate that the deposited PVD CrAlN coatings differ in their texture and mechanical properties, such as hardness and residual stresses, depending on the 316L substrate surface conditions. The CrAlN coatings generally exhibit good adhesion to both the 316L LPBF substrate and 316L bulk material. However, the findings indicate improved adhesion of CrAlN coatings on lapped 316L substrates. The residual stress gradient at coating-substrate interface and imperfections at the surface of the 316L LPBF substrate are found to primarily influence the performance and failure mode of the CrAlN coating. © 2021 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2021.127905
  • Mechanical milling to foster the solid solution formation and densification in Cr-W-Si for hot-pressing of PVD target materials
    Tillmann, W. and Fehr, A. and Heringhaus, M.
    Advanced Powder Technology 32 (2021)
    Based on the significantly different melting points and high oxygen affinities, the fabrication of chromium-based tungsten silicides is restricted to powder metallurgical production routes. To foster particle contacts and diffusion processes between chromium and tungsten, which are known to necessitate long sintering times, mechanical alloying or milling processes prior to sintering are established. Nonetheless, due to spinodal decomposition of Cr and W, the solid solution formation is complex and yet little understood. For this reason, the influence of the mechanical milling time (0–24 h) on the crystal structure and the microstructural properties of hot-pressed 60Cr30W10Si (wt.–%) is examined. In this context, two different powders containing a different tungsten particle size (0.8 and 3 µm) were mechanically alloyed to analyze the impact on the phase formation and the particle distribution in the microstructure. It was shown that mechanical milling supported the mechanical clamping between the particles. However, the increased milling times significantly decreased the crystallite sizes of the particles and fostered the tungsten solubility in the Cr-rich (Cr, W) solid solution formed during sintering, thus supporting the densification. © 2021 The Society of Powder Technology Japan
    view abstract10.1016/j.apt.2021.04.001
  • Mechanical properties and adhesion behavior of amorphous carbon films with bias voltage controlled TixCy interlayers on Ti6Al4V
    Tillmann, W. and Lopes Dias, N.F. and Franke, C. and Kokalj, D. and Stangier, D. and Thomann, C.A. and Debus, J.
    Diamond and Related Materials 115 (2021)
    Amorphous carbon is a promising functional film material to enhance the surface properties of Ti-based alloys for orthopaedic applications. However, high adhesion of the amorphous carbon film on the orthopaedic implants is essential to fully exploit its potential under high load bearings. Interlayer films are generally employed to improve the adhesion. The applied bias voltage is a decisive deposition parameter and significantly influences the structure and mechanical properties during the interlayer growth, which in turn affect the properties of the amorphous carbon film. Therefore, chemically graded titanium carbide (TixCy) interlayers were deposited using bias voltages of −50, −100, and −150 V with a subsequent hydrogen-free amorphous carbon (a-C) top layer on Ti6Al4V by magnetron sputtering. The mechanical properties and adhesion behavior of single TixCy interlayers were evaluated to analyze the interaction effect of TixCy on bilayered TixCy/a-C structures. A high bias voltage generates dense TixCy of a more disordered and defected structure with high stresses, high hardness of ~16 GPa, and high elastic modulus of ~170 GPa. However, high compressive stresses provoke a low adhesion strength, while low compressive stresses ensure a good adhesion behavior of TixCy. Highly stressed TixCy interlayers lead to overall higher stresses for the entire TixCy/a-C film. Independently of TixCy, the a-C top layer exhibits hardness and elastic modulus values of ~16 and ~160 GPa, respectively. The TixCy/a-C films with TixCy interlayers deposited at high bias voltages possess a low adhesion strength, while a lower bias voltage favors a good adhesion of TixCy/a-C on Ti6Al4V. Therefore, a moderate bias voltage is crucial to deposit lowly stressed TixCy interlayers, which ensure a high adhesion of TixCy/a-C on Ti6Al4V. Consequently, the bias voltage allows controlling the mechanical properties and adhesion behavior of the interlayer and, hence, the adhesion strength of the entire amorphous carbon film structure on Ti-based alloys for orthopaedic applications. © 2021 Elsevier B.V.
    view abstract10.1016/j.diamond.2021.108361
  • MoS2-Dünnschichten für unsynchronisierte, trockenlaufende Schraubenmaschinen: Umweltfreundliche und nachhaltige Schmierungskonzepte als Herausforderung und Chance
    Aurich, D. and Wittig, A. and Stangier, D. and Debus, J. and Thomann, C.-A. and Tillmann, W. and Brümmer, A.
    Vakuum in Forschung und Praxis 33 (2021)
    MoS2 thin films for unsynchronized, dry-running screw machines. Unsynchronized, dry-running screw machines have great potential to provide a resource-saving alternative to conventional screw machine designs. By saving external synchronization and the absence of a liquid lubricant, there are economic and ecological advantages. The use of materials and energy are reduced and the purity of the process gas is increased, the impact on the environment is minimized. Vacuum coating processes are one of the key technologies for realizing an unsynchronized, dry-running screw machine. The synthesized thin films can be optimally conditioned thanks to the near-net-shape coating and the possibility of changing the structural properties of the coating in a targeted manner. MoS2 thin films as solid lubricant exhibit improved friction properties. As a result there is also a great potential to reduce friction and thus the use of energy. © 2021, John Wiley and Sons Inc. All rights reserved.
    view abstract10.1002/vipr.202100763
  • On the synthesis and structural evolution of artificial CrN/TiN nanocomposites
    Tillmann, W. and Kokalj, D. and Stangier, D. and Fu, Q. and Kruis, F.E. and Kesper, L. and Berges, U. and Westphal, C.
    Applied Surface Science 535 (2021)
    The synthesis of nanocomposites is limited to thermodynamically immiscible phases or to phase separation by exceeding the limits of solution. Hence, the formation of nanocomposites based on transition metals, revealing a nanocrystalline Metal-Nitride/nanocrystalline Metal-Nitride structure, is restricted. These restrictions can be overruled by a spatially separated synthesis of the two phases and a recombination during the deposition. With this approach, the limits of current systems can be expanded, enabling the synthesis of artificial nanocomposites based on a variety of materials. We demonstrate the synthesis of a composite of two nanocrystalline phases of the miscible transition metal-nitrides CrN and TiN. TiN nanoparticles were synthesized using an atmospheric-pressure arc reactor and in-situ injected into a growing CrN thin film. The thin films are analyzed regarding their physical- and microstructure using two-dimensional GIXRD, XPS based on synchrotron radiation and TEM. The CrTiN thin film reveals a two-phase structure consisting of nanocrystalline CrN and TiN phases with crystallite sizes of 9 nm and 4 nm according to GIXRD. XPS indicates bonding of Cr-N, Cr-Cr, and Ti-N. No hint for Cr-Ti bonding was found, excluding (Cr,Ti)N solid solution formation. Based on the TEM-investigations, TiN nanoparticles are embedded as agglomerates in the CrN matrix. © 2020 Elsevier B.V.
    view abstract10.1016/j.apsusc.2020.147736
  • Process characteristics, particle behavior and coating properties during HVOF spraying of conventional, fine and nanostructured WC-12Co powders
    Baumann, I. and Hagen, L. and Tillmann, W. and Hollingsworth, P. and Stangier, D. and Schmidtmann, G. and Tolan, M. and Paulus, M. and Sternemann, C.
    Surface and Coatings Technology 405 (2021)
    In recent years, great effort has been taken in science and industry to find novel material-related solutions, which provide improved properties for future technological applications. One of these approaches is the use of fine structured and nanostructured materials. Within the field of wear protection, the use of fine or nanostructured WC-Co powder feedstock in the thermal spray process enables the application of highly wear resistant, thin near net-shape coatings on parts with complex geometries. In this study, the processing of WC-12Co powders by means of High Velocity Oxy-Fuel (HVOF) flame spraying is fundamentally investigated and the results are compared to those obtained with conventional powders. The influence of process parameter and scaling effects on the spray process and the thermo-kinetic particle behavior in the flame, the heating of the substrate as well as on the coating properties, the microstructure, the behavior of elements and phases and the residual stress is discussed comprehensively. The investigations of this work have shown that HVOF spraying of fine and nanostructured WC-12Co powders instead of conventional ones leads to a significant alteration of the thermo-kinetic spray conditions. Under optimized spray conditions, achieved by the use of special spray equipment and statistical design of experiments (DoE), improvements in terms of the economy of the spray process (higher deposition efficiencies) and the mechanical properties (higher microhardness and fracture toughness, lower porosity and roughness) can be achieved. © 2020
    view abstract10.1016/j.surfcoat.2020.126716
  • Progress on effects of alloying elements on bainite formation and strength and toughness of high strength steel weld metal
    Zhang, T. and Yu, H. and Li, Z. and Kou, S. and Kim, H.J. and Tillmann, W.
    Materials Research Express 8 (2021)
    High-strength steel has excellent mechanical properties and develops rapidly, but the toughness of weld metal cannot be well solved, which hinders the large-scale application of high-strength steel to a certain extent. Thus it is urgent to improve the strengthening and toughening mechanism of high-strength steel weld metal and develop the corresponding welding consumables. This paper summarizes current main research methods of the influence of alloying elements on the microstructure transformation and mechanical properties, and the effects of alloying elements on the strength and toughness of high-strength steel weld metals. It briefly analyzes the influence mechanism of alloying elements on microstructure transformation and the relationship between alloy composition, microstructure transformation and mechanical properties. It is found that the multiple phase microstructure composed of bainite and acicular ferrite can make the weld metal obtain good toughness. In addition, the paper also discusses future development trend of high-strength steel welding, providing the guidance on the research and application of high-strength steel welding consumables. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstract10.1088/2053-1591/abea58
  • Residual stresses and tribomechanical behaviour of TiAlN and TiAlCN monolayer and multilayer coatings by DCMS and HiPIMS
    Tillmann, W. and Grisales, D. and Stangier, D. and Thomann, C.-A. and Debus, J. and Nienhaus, A. and Apel, D.
    Surface and Coatings Technology 406 (2021)
    The deposition of ternary nitrides with the incorporation of carbon atoms into its structure has demonstrated to be a promising approach in the pursuit of wear-resistant and self-lubricating coatings. Firstly, both TiAlN and TiAlCN monolayers were deposited using direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) onto quenched and tempered AISI H11 tool steel to be used as references. Acetylene was used as a carbon precursor, producing DCMS and HiPIMS TiAlCN coatings with 9.0 and 21.7 at.% C, respectively. Subsequently, TiAlN/TiAlCN multilayers of various designs were also developed as follows: 5×[10/500], 5×[50/500] and 5×[100/500] nm. Residual stresses of the coating systems were determined by X-ray radiation utilising an ETA-diffractometer with a Cu-Kα radiation source applying the sin2ψ method. Additionally, residual stresses depth gradients of the substrate before and after the deposition of the coatings were determined in a LEDDI 8-circle diffractometer equipped with a W-X-ray tube and operated in the energy-dispersive mode of diffraction. Great reduction of the compressive residual stresses in the coatings was observed after the introduction of carbon into the TiAlN coating structure, shifting from −1047 ± 149 to −307 ± 211 MPa for the DCMS and from −7035 ± 1361 to +989 ± 187 MPa for the HiPIMS coatings. In the multilayer coatings, compressive residual stresses increase along with the increment of the TiAlN interlayer. Additionally, residual stresses of the substrate in the near-surface are dragged from low compressive stresses (−218 ± 61) to tensile stresses in the range of 1000 to 2000 MPa for all the DCMS/substrate systems, a behaviour only presented in HiPIMS by the TiAlN monolayer. Wear coefficients of all the evaluated HiPIMS systems are notoriously lower than their DCMS counterparts. Compared to TiAlN, TiAlCN HiPIMS presented a lower coefficient of friction but a higher wear coefficient, which in turn was not reduced by the introduction of the multilayer systems. Finally, Scratch test and Rockwell C adhesion tests have shown higher adhesion of DCMS coatings than HiPIMS coatings, and a detriment of the monolayers adhesion by the implementation of TiAlN/TiAlCN multilayer systems. The understanding of the residual stresses, both in the coating and in the substrate, and the way they affect the tribomechanical performance of the system coating/substrate continues to be of great importance, especially for coatings deposited by new technologies such as HiPIMS and self-lubricating coatings. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.126664
  • Structure and tribo-mechanical properties of Si- and W-containing amorphous carbon based multilayers
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Matveev, S. and Thomann, C.-A. and Debus, J.
    Applied Surface Science Advances 5 (2021)
    Designing the film architecture of amorphous carbon based systems is effective in tailoring the tribo-mechanical properties. For this purpose, alternating a-C and a-C:X layers, with X = Si or W, were grown with a layer ratio of 1, a bilayer period of ~200 nm, and a bilayers number of 5 in a magnetron sputtering process. By comparing with a-C(:X) monolayers, the structure and tribo-mechanical properties of alternating a-C/a-C:X films were evaluated. Although the bonding state of the a-C network of a-C:X in a-C/a-C:X is comparable to monolayered a-C:X, the multilayer design significantly affects the tribo-mechanical properties. The a-C/a-C:X multilayers exhibit a higher hardness compared to a-C:X. With a coefficient of friction of 0.12 ± 0.01, a-C/a-C:Si shows a low friction as a-C:Si with 0.09 ± 0.01, but the wear rate is significantly lower for a-C/a-C:Si with (3.4 ± 0.7) × 10-7 mm³/Nm than a-C:Si with (8.3 ± 1.0) × 10-7 mm³/Nm. Contrarily, a-C/a-C:W and a-C:W provide similar wear rates of 1.2 to 1.4 × 10-7 mm³/Nm, but the coefficient of friction is lower for a-C/a-C:W with 0.29 ± 0.02 than a-C:W with 0.36 ± 0.01. Hence, the multilayer design is efficient in improving the tribo-mechanical properties of a-C:X based films. © 2021 The Author(s)
    view abstract10.1016/j.apsadv.2021.100105
  • Temperature-dependent tribological behavior of MoSx thin films synthesized by HiPIMS
    Tillmann, W. and Wittig, A. and Stangier, D. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
    Tribology International 153 (2021)
    Understanding the interaction between the structure and the tribological properties of sputtered molybdenum disulfide films at elevated temperatures is essential for their use in industrial applications. Therefore, the friction and wear behavior up to of 400°C of one stoichiometric MoS2 and a sub-stoichiometric MoS1.6 film are investigated against 100Cr6 counterparts. With an increasing temperature up to 200°C, the friction decreases, which is attributed to a thermally activated water desorption and an increasing intensity of the (002) basal plane. Due to a passivation mechanism caused by the sulfur defect sites, the friction is lower for the sub-stoichiometric film. Above this temperature the friction increases for both films and failure occurs at 400°C. Therefore, the friction at elevated temperatures result from a complex interaction of re-orientation mechanisms, desorption and oxidation processes. © 2020 Elsevier Ltd
    view abstract10.1016/j.triboint.2020.106655
  • Thermally assisted machine hammer peening of arc-sprayed znal-based corrosion protective coatings
    Wirtz, A. and Abdulgader, M. and Milz, M.P. and Tillmann, W. and Walther, F. and Biermann, D.
    Journal of Manufacturing and Materials Processing 5 (2021)
    Structural elements of offshore facilities, e.g., offshore wind turbines, are subject to static and dynamic mechanical and environmental loads, for example, from wind, waves, and corrosive media. Protective coatings such as thermal sprayed ZnAl coatings are often used for protection, mainly against corrosive stresses. The Machine Hammer Peening (MHP) process is an innovative and promising technique for the post-treatment of ZnAl coating systems that helps reducing roughness and porosity and inducing compressive residual stresses. This should lead to an enhancement of the corrosion fatigue behavior. In this paper, the effect of a thermally assisted MHP process was investigated. The softening of the coating materials will have a direct effect on the densification, residual porosity and the distribution of cracks. The investigation results showed the influence of thermally assisted MHP on the surface properties, porosity, residual stresses, and hardness of the post-treated coatings. The best densification of the coating, i.e., the lowest porosity and roughness and the highest compressive residual stresses, were achieved at a process temperature of 300◦ C. A further increase in temperature on the other hand caused a higher porosity and, in some cases, locally restricted melting of the coating and consequently poorer coating properties. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/jmmp5040109
  • Tribo-mechanical properties and biocompatibility of Ag-containing amorphous carbon films deposited onto Ti6Al4V
    Tillmann, W. and Lopes Dias, N.F. and Franke, C. and Kokalj, D. and Stangier, D. and Filor, V. and Mateus-Vargas, R.H. and Oltmanns, H. and Kietzmann, M. and Meißner, J. and Hein, M. and Pramanik, S. and Hoyer, K.-P. and Schaper, M. and Nienhaus, A. and Thomann, C.A. and Debus, J.
    Surface and Coatings Technology 421 (2021)
    The addition of Ag to amorphous carbon (a-C) films is highly effective in tailoring the tribo-mechanical properties and biocompatibility. For biomedical applications, Ag-containing a-C (a-C:Ag) represents a promising film material for improving the biofunctional surface properties of Ti-based alloys. In a sputtering process, a-C:Ag films, with Ag contents up to 7.5 at.%, were deposited with a chemically graded TixCy interlayer onto Ti6Al4V. The tribo-mechanical and biocompatible properties of a-C:Ag were evaluated. The influence of the Ag content on these properties was analyzed and compared to those of uncoated Ti6Al4V. Raman spectroscopy reveals that the amount of incorporated Ag does not induce significant structural changes in the disordered network, only a reduced number of vacancies and sp3-coordinated C bonds within the sp2-dominant a-C network is assigned to the films with high Ag concentration. With increasing Ag content, stresses, hardness, and elastic modulus decrease from (2.02 ± 0.07) to (1.15 ± 0.03) GPa, from (17.4 ± 1.5) to (13.4 ± 0.9) GPa, and from (171.8 ± 8.1) to (138.5 ± 5.8) GPa, respectively. In tribometer tests, the friction behavior against Al2O3 in lubricated condition with a simulated-body-fluid-based lubricant is not affected by the Ag concentration, but the Al2O3 counterpart wear is reduced for all a-C:Ag films compared to a-C. The friction against ultra-high-molecular-weight polyethylene (UHMWPE) decreases continuously with increasing Ag concentration and the counterpart wear is lower at higher Ag contents. Compared to a-C:Ag, Ti6Al4V demonstrates lower friction against UHMWPE and higher friction against Al2O3. The a-C:Ag films are not exposed to abrasion by Al2O3 or pronounced material transfer of UHMWPE. The hardness difference and chemical affinity between the friction partners are decisive for the tribological behavior of a-C:Ag. Compared to Ti6Al4V, the a-C:Ag films show antibacterial activity against Staphylococcus aureus, while the proliferation of osteoblast-like cells is reduced by Ag. © 2021 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2021.127384
  • A numerical method for the generation of hierarchical Poisson Voronoi microstructures applied in micromechanical finite element simulations—part I: method
    Schneider, Y. and Weber, U. and Wasserbäch, W. and Zielke, R. and Schmauder, S. and Tillmann, W.
    Computational Mechanics 66 (2020)
    Poisson Voronoi (PV) tessellations as artificial microstructures are widely used in investigations of material deformation behaviors. However, a PV structure usually describes a relative homogeneous field. This work presents a simple numerical method for generating 2D/3D artificial microstructures based on hierarchical PV tessellations. If grains/particles of a phase cover a large size span, the concept of “artificial phases” can be used to create a more realistic size distribution. From case to case, detailed microstructural features cannot be directly achieved by commercial or free softwares, but they are necessary for a deep or thorough study of the material deformation behavior. PV tessellations created in our process can fulfill individual requirements from material designs. Another reason to use PV tessellations is due to the limited experimental data. Concerning the application of PV microstructures, four examples are given. The FE models and results will be presented in consecutive works, i.e. “part II: applications”. © 2020, The Author(s).
    view abstract10.1007/s00466-020-01869-3
  • Adhesion of HVOF-Sprayed WC-Co Coatings on 316L Substrates Processed by SLM
    Tillmann, W. and Hagen, L. and Schaak, C. and Liß, J. and Schaper, M. and Hoyer, K.-P. and Aydinöz, M.E. and Garthe, K.-U.
    Journal of Thermal Spray Technology 29 (2020)
    Different studies have been demonstrated that the surface integrity of substrate bulk materials to be coated has a significant impact on the adhesion of thermally sprayed coatings. It is known that the surface integrity of parts processed by selective laser melting (SLM) differs from those obtained from bulk materials. Although 316L stainless steel is among the most investigated material for SLM, the adhesion of thermally sprayed coatings on 316L stainless steel substrates processed by SLM has not been studied yet. This study aims at evaluating the effect of various mechanical pre-treatments onto 316L stainless steel substrates processed by SLM and their effect on the adhesion of high velocity oxy-fuel (HVOF)-sprayed WC-Co coatings. To differentiate between topographical effects and residual stress-related phenomena, a stress-relief heat treatment of the SLM substrates served as a reference throughout the investigations. The differently pre-treated SLM substrates were investigated with regard to the surface roughness and residual stresses. For the HVOF-sprayed SLM composites, Vickers interfacial indentation tests were conducted to assess the resulting coating adhesion. The findings demonstrated that the HVOF-sprayed WC-Co coatings predominantly exhibit good adhesion to the SLM 316L substrates. However, it was found that the stress state in the SLM 316L substrate surface is more likely to affect the adhesion of the WC-Co coating, while the substrate surface roughness showed a marginal effect. © 2020, The Author(s).
    view abstract10.1007/s11666-020-01081-y
  • Aerosol synthesis of titanium nitride nanoparticles by direct current arc discharge method
    Fu, Q. and Kokalj, D. and Stangier, D. and Kruis, F.E. and Tillmann, W.
    Advanced Powder Technology 31 (2020)
    Arc discharge synthesis has industrial relevance due to its low cost and scale-up potential. The production of titanium nitride nanoparticles was achieved by direct current arc discharge in an atmospheric-pressured ambient composed of N2 and Ar. We systematically investigated the effect of the synthesis parameters, including quench gas velocity, quench gas composition, and applied arc current, on the particle quality, yield, and size. It is found that increasing quench gas velocity enables to produce particles with a primary size of 10–15 nm, while titanium nitride particles of 20–50 nm are produced at low quench gas velocity based on scanning electron microscope (SEM) analysis. X-ray diffraction (XRD) results indicated that titanium nitride particles produced at various nitrogen compositions are almost stoichiometric, while the crystallite size increases almost 20 nm when increasing nitrogen contents in the quench gas. Quench gas composition also has a significant impact on the arc voltage as well as particle production rate. When increasing the nitrogen concentration from 20% to 100%, the production rate can be enhanced by a factor of three. Besides, raising the applied arc current from 12 A to 50 A leads to a yield enhancement of factor 10. According to the Brunauer-Emmett-Teller (BET) measurement, the increase of applied arc current has a limited impact on primary particle size. The enhancement in particle production rate is mainly reflected by the larger agglomerate sizes and agglomerate number concentration. Additionally, based on experimental observations and previous studies, a mechanism is presented to explain the growth of deposits on the cathode tip. © 2020 The Society of Powder Technology Japan
    view abstract10.1016/j.apt.2020.08.012
  • Application of the eutectic high entropy alloy Nb0.73CoCrFeNi2.1 for high temperature joints
    Tillmann, W. and Wojarski, L. and Stangier, D. and Manka, M. and Timmer, C.
    Welding in the World 64 (2020)
    The eutectic high entropy alloy Nb0.73CoCrFeNi2.1 was manufactured by means of arc smelting and the obtained ingots were cut into 300-μm-thick foils, which were used as filler alloys to braze Crofer 22 APU to Hf-metallized yttria-stabilized zirconia (3YSZ). The brazing process was conducted in a vacuum furnace at 1200 °C for 5 min at a vacuum of 4.3·10–4 mbar. In order to minimize the intense diffusion and erosion of the steel substrate, a heating and cooling rate of 50 K/min was applied. Sound joints without any pores or flaws were obtained. The microstructure of the joints consisted of an HfO2 reaction layer at the ceramic interface and the same eutectic structure consisting of a Laves phase and a solid solution that was already detected in the smelted foil. The average hardness of the microstructure in the joint seam amounted to 352 ± 17 HV0.01 and the joints reached strength values up to 97 ± 7 MPa while the fracture area was always located at the ceramic interface in the HfO2 layer. Comparable joints, with AgCuTi3 as filler metal, brazed at 920 °C, only reached a shear strength of ~ 52 ± 2 MPa. © 2020, The Author(s).
    view abstract10.1007/s40194-020-00944-w
  • Application Progress of Annealing Treatment Process in the Study of Nano-multilayer Films [退火处理工艺在纳米多层膜材料研究中的应用进展]
    Li, H. and Xing, Z. and Hodúlová, E. and Hu, A. and Tillmann, W.
    Cailiao Daobao/Materials Reports 34 (2020)
    Compared with traditional bulk materials, nano-multilayer films exhibit unique optical, magnetic, electrical, mechanical and thermal properties due to their small-size effects, surface effects, quantum size effects, and quantum tunneling effects. Therefore, nano-multilayer films have been widely used in the areas of optical devices, semiconductors, electromagnetic protection, processing and manufacturing, surface protection and electronic packaging as optical absorbing materials, electromagnetic absorbing materials, magnetic recording materials, photovoltaic materials and low-temperature joining materials. There exists intrinsic size dependence in the physical and mechanical properties with the microstructure of nano-multilayer films. Due to the limitation of the preparation process, defects such as vacancies and dislocations can cause difficulty in fully meeting the requirements of heat resistance, wear resistance and corrosion resistance in the complex service environment, which limits the further development of nano-multilayer films. In the field of concentrating circuits and chip fabrication, nano-multilayer films devices are often working in a severe environment deviating from the normal temperature. However, metastable nano-multilayer films with high surface free energy tend to reach a state of low-energy and form a stable structure by interdiffusion of immiscible dual phases, interlayer detachment and interface evolution under heat. It might result in the extinction of melting point depression property, superhardness property and so on due to the destructions of the nano-multilayer structure. Therefore, studying on the microstructure evolution, thermal stability and failure mechanism of nano-multilayer films is particularly important for increasing the service life and reliability of nano-multilayer systems. As a common heat treatment method, the annealing process is widely used to eliminate defects in metals, so as to achieve to modify the properties. For nano-multilayer films operating at high temperatures, the annealing process is also an effective means of extending its service life. At present, the main directions of annealing process in nano-multilayer films research and application are: (i) improving nano-multilayer film performance by adopting different annealing temperature, holding time and cooling rate; (ii) investigating the effect of annealing temperature on the thermal stability of nano-multilayer films by increasing the annealing upper limit temperature and obtain a critical temperature that maintains stability of the interface of nano-multilayer. It is found that the appropriate annealing process can refine the nano-multilayer films grain structure, increase the density, decrease the defect density, induce the formation of special structures, reinforcing the interaction of atoms and dislocations. Therefore, the light transmittance of the film is increased with improvement of optical properties, as well as the magnetic, electrical and mechanical properties are significantly improved; (iii) in addition, the nano-multilayer film is annealed in a certain temperature range to observe the bilayer interface evolution, atomic diffusion and new phase formation using TEM, XRD and other means. Thus the structural stability, chemical stability and mechanical stability of nano-multilayer film can be studied. In this paper, the current progress and challenges of annealing process in nano-multilayer films modification and thermal stability research are reviewed. The influence of annealing parameters on the enhancement of nano-multilayer properties including optical properties, magnetic properties, electrical properties, mechanical properties is elaborated. Furthermore, it mainly focuses on the influencing mechanism of elevated temperature annealing on the thermal stability and microstructure evolution of immiscible nano-multilayer system. At last, the further development of annealing process for designing and preparing of high-strength and thermally stable nano-multilayer films are prospected, which has important theoretical significance and application value in materials welding/joining, integrated circuits, cutting tools, absorbing coatings, etc. © 2020, Materials Review Magazine. All right reserved.
    view abstract10.11896/cldb.19010159
  • Controlling the structural, mechanical and frictional properties of MoSx coatings by high-power impulse magnetron sputtering
    Kokalj, D. and Debus, J. and Stangier, D. and Moldenhauer, H. and Nikolov, A. and Wittig, A. and Brümmer, A. and Tillmann, W.
    Coatings 10 (2020)
    Tribology, as the science and technology of interacting surfaces, typically relies on liquid lubricants which reduce friction and wear. For environmentally friendly tribological purposes and applications requiring a liquid-free performance, solid lubricants, such as MoS2 coatings, play an essential role. It is crucial to understand the interplay between the parameters of the coating synthesis and the characteristics of the coating. The impact of the deposition parameters on the structural, mechanical and frictional properties of MoSx thin films, which are synthesized by high-power impulse magnetron sputtering, are studied. The morphology, topography and stoichiometry (2.02 &lt; x &lt; 2.22) of the films are controlled by, in particular, the bias-voltage and heating power applied during the sputtering process. In combination with a low pulse frequency the hardness and elastic stiffness of the MoSx films are enhanced up to 2 and 90 GPa, respectively. This enhancement is assigned to a shortening of the Mo-S bonding lengths and a strengthening in the interatomic coupling as well as to a formation of small-sized crystallites at the surface. The friction coefficient reduces to μ = 0.10 for films with an initial (100) orientation and the mean roughness of the MoSx films decreases below 15 nm by shortening the cathode pulses. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. All right reserved.
    view abstract10.3390/COATINGS10080755
  • Development of high entropy alloys for brazing applications
    Tillmann, W. and Ulitzka, T. and Wojarski, L. and Manka, M. and Ulitzka, H. and Wagstyl, D.
    Welding in the World 64 (2020)
    High entropy alloys are novel and innovative metallic materials, which have intensively moved into the focus of research over the last decade. The high entropy effect in those multi-component alloys promotes the formation of a characteristic crystal structure, the random solid solution, which features unique material properties, and reduces the number of possible brittle phases. In this publication, the influence of gallium as a melting point depressant on the melting range and the microstructure of the two-phased equimolar CoCrCoFeNi were determined. In order to integrate the vacuum brazing process into the solutioning heat treatment of the nickel-based super alloy Mar-M 247 between 1180 and 1270 °C, the liquidus temperature of CoCrCoFeNi was aimed to be below 1270 °C. The changes in the melting ranges due to the modified compositions were predicted by CALPHAD simulations and verified by differential thermal analysis measurements. The promising multi-component filler alloy CoCrCuFeNiGa was determined for further investigations. For this purpose, the microstructures of the filler metal itself and the brazement were conducted. A shear strength of 388 ± 73 MPa was achieved for a brazing gap of 200 μm. The crack, which led to joint failure propagated through high-entropic, fcc-structured phases in the brazing seam. © 2019, International Institute of Welding.
    view abstract10.1007/s40194-019-00824-y
  • Diamond-impregnated 316L metal matrix composites fabricated by powder bed fusion with laser beam - Influences of the energy input on the microstructural properties
    Wegner, J. and Fehr, A. and Platt, S. and Kleszczynski, S. and Witt, G. and Tillmann, W.
    Diamond and Related Materials 109 (2020)
    Fabricating diamond metal matrix composites (DMMCs) by means of powder bed fusion of metals using a laser beam (PBF-LB/M) is a new approach to extensively expand the spectrum of geometrical freedom for diamond tools. However, it must be borne in mind that the temperature input has a significant influence on the diamond condition since graphitizations are likely to occur. Therefore, it was analyzed how varying volume energy densities and substrate heating affect the microstructure and the densification of a 316 L stainless steel matrix, which was impregnated with 5 vol.-% Ni-coated diamonds. With regard to the densification, it was shown that an elevated substrate temperature (473 K) allowed to apply reduced volume energy densities and reduce stress-induced cracking. Thus, a relative density of 99.5% could be achieved. Furthermore, decreasing the volume energy density avoided graphitizations of the diamonds. Cr and Fe contents of the matrix material dissolved at the Ni-coated diamond surface revealing a diamond-metal interaction. A longer heat flux generally supported these diffusion processes at the interfaces. Finally, it became obvious that increased laser powers resulted in a higher densification, while low scan speeds and laser powers are desirable to foster diffusion and to avoid graphitizations. © 2020 Elsevier B.V.
    view abstract10.1016/j.diamond.2020.108040
  • Effect of substrate pre-treatment on the low cycle fatigue performance of tungsten carbide-cobalt coated additive manufactured 316 L substrates [Einfluss der Substratvorbehandlung auf das Kurzzeitermüdungsverhalten von Wolframkarbid-Kobalt beschichteten additiv gefertigten 316 L Substraten]
    Tillmann, W. and Hagen, L. and Garthe, K.-U. and Hoyer, K.-P. and Schaper, M.
    Materialwissenschaft und Werkstofftechnik 51 (2020)
    Numerous studies already identified that the fatigue strength of 316 L parts processed by laser beam melting (LBM) is distinctly affected by the surface integrity. Among others, surface defects as well as residual stresses are of crucial importance. Despite new findings in the field of surface engineering of laser beam melting (LBM) parts, the low cycle fatigue strength of thermally sprayed additively manufactured substrates has not been in the focus of research to date. This study aims at evaluating the effect of different pre-treatments onto 316 L substrates processed by laser beam melting (LBM) prior to the deposition of a high velocity oxy-fuel (HVOF) sprayed tungsten carbide-cobalt coating and their effect on the low cycle fatigue strength. Therefore, 316 L substrates were examined in their as-built state as well as after grit blasting with regards to the surface roughness, strain hardening effects, and residual stresses. To differentiate between topographical effects and residual stress related phenomena, stress-relieved 316 L substrates served as reference throughout the investigations. The tungsten carbide-cobalt coated and differently pre-treated 316 L substrates were mechanically tested under quasi-static and dynamic load conditions. Besides the low cycle fatigue strength, the fracture toughness as well as the fracture mechanism were identified based on fracture surface analysis. © 2020 The Authors. Materialwissenschaft und Werkstofftechnik published by Wiley-VCH GmbH
    view abstract10.1002/mawe.202000109
  • Effect of the bias voltage on the structural and tribo-mechanical properties of Ag-containing amorphous carbon films
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Nienhaus, A. and Thomann, C.A. and Wittrock, A. and Moldenhauer, H. and Debus, J.
    Diamond and Related Materials 105 (2020)
    The modification of amorphous carbon films by a mixture of Ag atoms is a promising approach to reduce the residual stresses in the coating and to improve its adhesion to the substrate. Besides the Ag concentration, the bias voltage has a crucial impact on the properties of carbon-based films. Therefore, the effect of the bias voltage on the structural and tribo-mechanical properties of hydrogen free a-C:Ag is investigated. The a-C:Ag films are sputtered from graphite targets with varying number of Ag pellets by setting the bias voltage to −100, −150, and −200 V. A non-modified a-C and two a-C:Ag film systems with different Ag content are synthetized to obtain a comprehensive understanding about the influence of the bias voltage on the properties of the a-C:Ag films. A high bias voltage leads to a reduction in the amount of Ag within the a-C:Ag films, since impinging ions remove Ag atoms during the film growth. Additionally, XRD analyses show the formation of large Ag nanocrystallites with rising bias voltage. In Raman scattering studies, an Ag-induced graphitization of the a-C films is identified. The graphitization is less pronounced at low Ag concentrations and high bias voltages. The residual stresses increase with rising bias voltage and decreasing Ag content, which also favor greater values of hardness and elastic modulus. While a high bias voltage results in a poor adhesion strength for the a-C films, a good adhesion behavior is observed for the a-C:Ag films. It is ascribed to lower stresses in the a-C:Ag films as compared to that in a-C. The friction behavior of the a-C:Ag films is not influenced by the bias voltage, since the coefficients of friction vary from 0.26 to 0.32 against a steel counterpart in tribometer tests. An agglomeration of Ag particles in the tribological contact is observed for all a-C:Ag films which contributes to the slightly higher friction when compared to non-modified a-C films. On the whole, it is demonstrated that the tribo-mechanical properties of a-C:Ag are not only affected by the Ag content, but also by the applied bias voltage. © 2020 Elsevier B.V.
    view abstract10.1016/j.diamond.2020.107803
  • Effects of acetylene flow rate and bias voltage on the structural and tribo-mechanical properties of sputtered a-C:H films
    Tillmann, W. and Ulitzka, H. and Lopes Dias, N.F. and Stangier, D. and Thomann, C.A. and Moldenhauer, H. and Debus, J.
    Thin Solid Films 693 (2020)
    The properties of sputtered a-C:H films are significantly influenced by the C2H2 flow rate and bias voltage. A suitable Design of Experiments allows to consider their effects on the mechanical and tribological properties. The a-C:H films are deposited by varying the C2H2 flow rate from 5.9 to 34.1 sccm and the bias voltage from −83 to −197 V, following the Central Composite Design. In Raman scattering studies, the presence of C[sbnd]H bands with increasing C2H2 flow rate is identified. Additionally, a decrease of the I(D)/I(G) ratio is observed with increasing C2H2 flow rate. Both observations indicate the formation of sp³-hybridized C[sbnd]H bonds. In contrast, a low C2H2 flow rate and a high bias voltage result in a higher I(D)/I(G) ratio and a lower intensity of the C[sbnd]H stretching bands, indicating a lower amount of C[sbnd]H bonds. The mechanical properties are also considerably influenced by these parameters. A higher C2H2 proportion results in a lower hardness and elastic modulus, which are related to a higher H content. However, a higher bias voltage increases the hardness and elastic modulus due to densification mechanisms, which increase the degree of distortion of the a-C:H films. Consequently, a low C2H2 flow rate and a high bias voltage ensure a high hardness of up to ~24 GPa due to a lower amount of C[sbnd]H bonds and a higher degree of distortion. In tribometer tests, most a-C:H films exhibit a low coefficient of friction against steel, ranging from 0.23 to 0.25. All a-C:H films are marked by a deformative wear, indicating a high resistance against abrasive wear when sliding against steel. © 2019 Elsevier B.V.
    view abstract10.1016/j.tsf.2019.137691
  • Effects of heat treatments on the microstructure and the adhesion of Cr, Ti, Al, Zr HiPIMS films deposited on APS Al2O3 and ZrO2-8Y2O3 coatings
    Tillmann, W. and Fehr, A. and Stangier, D.
    Surface and Coatings Technology 393 (2020)
    While several studies deal with thin film depositions on bulk ceramics, little is known about film applications on thermally sprayed ceramics. Hence, atmospheric plasma sprayed Al2O3 and ZrO2-8Y2O3 coatings served as substrates for the deposition of metallic films (Cr, Ti, Al, and Zr) by means of high power impulse magnetron sputtering (HiPIMS). Subsequently, these duplex coatings were heat treated in a vacuum furnace at 300 °C and 600 °C to investigate influences on the phase formation, residual stress state, mechanical properties, and film adhesion. The Cr and Al films crystallized in a cubic lattice and the Zr films exhibited a hexagonal lattice. The crystallization of the Ti film was substrate dependent, i.e. on alumina substrate the Ti film was cubic, while it was hexagonal on zirconia. Furthermore, heat treating the Ti and Zr films at 600 °C in vacuum led to an oxygen exchange generating c-TiO and m-ZrO. Heat treating the Cr film on Al2O3 substrate provoked a significant stress relaxation leading to an increased adhesion. However, depositing Al films on alumina and zirconia substrates resulted in the highest adhesion in the as-deposited state due to the low Young's modulus. In general, it was shown that phase transformations, increased lattice mismatches as well as differences in the thermal expansion behavior of the substrate and the film had a negative influence on the film adhesion. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.125766
  • Embedding behavior of ceramic particles in babbitt coatings and its effect on the tribological properties of low-pressure cold sprayed coatings
    Tillmann, W. and Abdulgader, M. and Hagen, L. and Hüning, S.
    Coatings 10 (2020)
    The low melting point of Sn-based Babbitt alloys often causes nozzle clogging in the low-pressure cold gas-dynamic spraying (LPCGDS) process, which impacts the process steadiness and the coating quality. Adding hard particles to the feedstock material eliminates this kind of interruption. A certain amount of these particles finds their way in the obtained coatings. These particles also trigger a kind of "hammering effect" due to their impulse forces. These forces are directly dependent on the mass and velocity of the impacting hard particles. However, these forces may lead to a decrease in the porosity and improve the adhesion of the obtained coating. In this study, the effect of the density and size of the hard particle was examined by three different hard materials, Cr3C2, Al2O3, and B4C, which have a material density of 6.68, 3.95, and 2.52 g/cm3, respectively. The used feedstock in this study is a powder mixture that contains 75 vol.% Babbitt and 25 vol.% of either B4C, Cr3C2, or Al2O3. The effect of the size distributions "particles with lower mass" was tested using two different Al2O3. The various hard particles show different embedding behaviors, as well as different effects on the coating build-up. It was found that the blended hard particles were enclosed with the Babbitt matrix, and their interface with Babbitt shows no clear evidence of pronounced diffusion. The size distribution of the blended hard particles has a direct effect on the splat formation and the obtained coating microstructure. It was found that the type of hard particles played a decisive role in the friction behavior. Nevertheless, the hard particle reinforced Sn-Sb-Cu-based composite coatings demonstrated a nearly constant coefficient of friction throughout the load-interval. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. All right reserved.
    view abstract10.3390/COATINGS10080769
  • Fatigue of brazed joints made of X5CrNi18-10 and Cu110 and derivation of reliable assessment approaches
    Baumgartner, J. and Tillmann, W. and Bobzin, K. and Öte, M. and Wiesner, S. and Sievers, N.
    Welding in the World 64 (2020)
    Brazing in a continuous belt furnace is a quite cost-effective way of joining components consisting of many individual parts. It is extensively used in many industrial sectors like the automotive or in the energy industry. Even though many components are cyclically loaded during service up to now, no approaches to assess the lifetime under in-service loads are available. In order to assess the fatigue strength of brazed joints, three different specimen types have been investigated: peel and shear as well as a component-like specimens. The specimens were characterized and tested under load control using constant and variable amplitudes. Subsequently, the fatigue life of these joints was assessed by linear-elastic notch stresses. A reference S-N curve and characteristic damage sums were derived that enable to perform a reliable fatigue assessment. Further parameters which seem to have a strong influence on the fatigue life, the surface topology, and the overall quality of the braze were identified. Their influence is discussed. © 2020, The Author(s).
    view abstract10.1007/s40194-020-00850-1
  • In situ sol-gel surface modification of alumina for a better wettability by brazing filler metals [In situ Sol-Gel Oberflächenmodifikation von Aluminiumoxid zur besseren Benetzbarkeit durch Hartlotwerkstoffe]
    Tillmann, W. and Pinho Ferreira, M.
    Materialwissenschaft und Werkstofftechnik 51 (2020)
    Alumina is an important technical high-performance material in the field of electronics and mechanical engineering. Therefore, it is often necessary to use these ceramics in combination with other materials such as metals. Brazing is a wide-spread and economical method to realize bonded joints between alumina and metallic components. Unlike the joining of pure metallic materials, ceramics cannot be easily brazed due to the poor wettability by molten metals. Hence, ceramic surfaces are either metallized prior to the brazing process, or a corresponding basic filler metal with active elements is used. Because of partially disadvantageous aspects concerning the manageability and profitability, this research paper describes a novel approach in which the ceramic surface is rendered wettable by the application of a sol-gel coating of semiconducting oxides. In order to make the surface modification economical and to avoid much additional expenditure, the drying of this sol-gel material is integrated into the sintering process of the aluminium oxide ceramic. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201700067
  • Influence of tailored surfaces and superimposed-oscillation on sheet-bulk metal forming operations
    Behrens, B.-A. and Tillmann, W. and Biermann, D. and Hübner, S. and Stangier, D. and Freiburg, D. and Meijer, A. and Koch, S. and Rosenbusch, D. and Müller, P.
    Journal of Manufacturing and Materials Processing 4 (2020)
    Producing complex sheet metal components in fewer process steps motivated the development of the innovative forming process called sheet-bulk metal forming (SBMF). In this process, sheet metal forming and bulk-metal forming are combined to create a unique forming process in which a component with external and internal gearing is produced in three production steps. However, the high degrees of deformation that occur using high-strength steels and the number of different process steps result in high process forces, strongly limiting the service life of tools. To reduce the forming force during SBMF processes, tool and process modifications were investigated. Therefore, plane-strain compression tests were conducted to examine the influence of a CrAlN PVD coating and tailored surfaces produced by high-feed milling (HF) of tool-active elements on the material flow of the specimens. In addition to the tool-sided modifications, the influence of an oscillation overlay during the forming process was investigated. Based on the results of the compression tests, the surfaces of the active tool elements of the SBMF process were modified in order to transfer the basic experimental results to the production of a functional component. The friction is thus adapted locally in the SBMF process. © 2020 by the authors.
    view abstract10.3390/jmmp4020041
  • Influence of the bias-voltage, the argon pressure and the heating power on the structure and the tribological properties of HiPIMS sputtered MoSx films
    Tillmann, W. and Wittig, A. and Stangier, D. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
    Surface and Coatings Technology 385 (2020)
    Understanding the growth process and its correlation to the structure of MoSx thin films is essential to control the friction behavior. Nevertheless, structural changes related to kinetic and thermal processes occurring during the deposition are not yet fully understood within the context of MoSx sputtered thin films. Therefore, MoSx films were synthesized by HiPIMS (High Power Impulse Magnetron Sputtering) technique using the one factor at a time method. By systematically changing the bias-voltage (0 to −200 V), the argon pressure (200 mPa to 600 mPa) or the heating power (0 to 3000 W) the interaction between the deposition parameters and their impact on the structure and the tribological properties was analyzed.The results show significant differences regarding the influence of kinetic and thermal effects. The investigation of the crystallographic orientation by XRD measurements reveals that a high kinetic energy induced by a high bias-voltage favors the growth of the (100) edge plane. A deposition process with a low deposition temperature and thus a low deposition rate leads to a more pronounced (002) basal plane due to the lower surface energy of the (002) surface. A high kinetic energy is also related to a densification of the morphology and a decrease in the sulfur content, which results in a thicker tribofilm and thus a lower wear and friction. Films deposited with a high heating power on the other show a low friction, but at the same time a columnar microstructure and high wear. Thus, the structure affects the amount of generated wear particles during the sliding, but more important is the ability of keeping them in the contact area during the tribo-tests. © 2020
    view abstract10.1016/j.surfcoat.2020.125358
  • Infuence of residual stresses in heat-treated high-speed steels on the adhesion of CrAlN coatings [Einfluss der eigenspannungen auf die adhäsion von CrAlN-beschichtungen bei wärmebehandelten schnellarbeitsstählen]
    Denkena, B. and Breidenstein, B. and Lucas, H. and Keitel, M. and Tillmann, W. and Stangier, D.
    HTM - Journal of Heat Treatment and Materials 75 (2020)
    The influence of different mechanical and chemical pre-treatments for heat-treated AISI M3:2 tool steel (∼ DIN S 6-5-3) on the coating adhesion of a CrAlN coating is investigated. Surface topography and residual stresses before and after the PVD coating process are examined. Nitriding and grinding with toric grinding pins results in compressive residual stresses in the subsurface, whereas polishing leads to tensile stresses. After the deposition process a decrease of the residual stresses in the substrate material is observed, whereas the stresses in the nitrided surface are not affected. The resulting residual stresses and roughness profiles are correlated with the adhesion of the PVD coating showing a clear dependency on the substrate pre-treatments. Additionally, interactions between the residual stresses in the subsurface and the resulting stress gradient in the PVD coating could be determined by depth profiles. Based on these findings an optimized grinding process can therefore make the additional process step of nitriding obsolete and can provide an improved coating adhesion for an enhanced wear resistance. © 2020 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110413
  • Investigation of compaction by ring rolling on thermal sprayed coatings
    Kuhlenkötter, B. and Glaser, T. and Fahle, S. and Husmann, S. and Abdulgader, M. and Tillmann, W.
    Procedia Manufacturing 50 (2020)
    Often mechanical parts have specific requirements in regard to the underlying alloy. Especially for axial symmetric parts like rings an often requested characteristic is a different requirement on the inner surface than on the outer. Therefore, different production processes exist. One strategy is to roll a basic ring e.g., via radial-axial ring rolling (RARR), and create a functional layer afterwards via thermal spraying. The temperature of the ring and the spraying particles influence the bonding behavior and compaction of the thermal sprayed layer benefits its mechanical properties. The hypothesis presented in this paper is a better performing thermal sprayed layer regarding the adhesion of the layer, reduced porosity and improved residual compressive stress through the combination of the thermal spraying process and the radial-axial ring rolling. After an introduction to the industrial processes of radial-axial ring rolling and thermal spraying, the experimental setup and the results are presented. In the first step, the approach of combining these processes includes an experimental design, in which the rolling of the basic ring, consisting of a 1.0570-steel, gets interrupted right before the target diameter is reached. The ring is then transported to the thermal spraying process where the ring gets covered with layers of 316L-Si, sprayed with a pressure of 8 bar, arc voltage of 34.0 V and a wire feedrate of 2.1 m/min. The coated rings were rolled to two different target diameters with a planned degree of deformation of 0.025 and 0.098 only in the radial direction. This deformation leads to an elongation of the outer circumference of 2.56 % and 10.26 %. Due to the rolling process, the applied coatings revealed a compacting effect and, thus, a significant reduction in the total porosity. The main goal of this study is to investigate the material pairings and parameters regarding thermal spraying and RARR to produce an intact coating with adequate adhesion to the substrate. The long term vision is to adapt thermally sprayed coating into the radial-axial ring rolling process. © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 18th International Conference Metal Forming 2020
    view abstract10.1016/j.promfg.2020.08.036
  • Investigation of wetting behaviour of hardenable copper based-alloys for brazing applications
    Tillmann, W. and Lehmert, B. and Manka, M. and Wojarski, L. and Holewa, M.
    Science and Technology of Welding and Joining 25 (2020)
    Tools for machining are exposed to high loads, wear, and elevated temperatures. Commonly, such tools consist of cemented carbides and tool steel. To combine the advantages of both materials, high-quality-joints with high strengths are desired. When brazing these materials, the main challenge is the mismatch of the Coefficient of Thermal Expansion (CTE) and the poor wettability of cemented carbides by molten filler alloys. In this regard, the feasibility of two custom-made alloys (CuNi12Si5, CuNi12Si5B0.4) was analysed. Besides being a cost-efficient alternative, these alloys offer the possibility to modify their mechanical properties by precipitation hardening to reduce stresses within the final joints. Thus, this paper shows that a temperature of 1060°C is suitable for wetting and brazing tests on different substrates. © 2019, © 2019 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute.
    view abstract10.1080/13621718.2019.1688458
  • LPBF-M manufactured Zr-based bulk metallic glasses coated with magnetron sputtered ZrN films
    Tillmann, W. and Fehr, A. and Wegner, J. and Stangier, D. and Kleszczynski, S. and Witt, G.
    Surface and Coatings Technology 386 (2020)
    The fabrication of Zr-based bulk metallic glasses (BMGs) by means of laser powder bed fusion of metals (LPBF-M) is recently emerging. This production route allows to widely overcome current geometrical restrictions of casting routes while maintaining the amorphous character, which is decisive for the unique mechanical properties, for instance. However, the roughness of the LPBF-M fabricated BMGs is still a challenging property, impeding the application of near-net shaped thin films that modify BMG surfaces, e.g. with respect to wear resistance. Zr59.3Cu28.8Al10.4Nb1.5 (at.%) substrates were manufactured by means of LPBFM, applying various exposure strategies, including laser remelting of the last solidified layer to influence the surface topography. Furthermore, BMG substrates were post-treated by grinding and polishing. Thus, varying degrees of crystallinity as well as surface roughness states were generated to analyze the effect of these characteristics on the microstructural properties of additionally applied magnetron sputtered ZrN films. Substrates that were fabricated with higher energy densities during LPBF-M exhibited (101)-Zr as well as (013)- and (110)-CuZr2 phases, which were accompanied by a decreased surface roughness. It was shown that all films had a crystalline structure on amorphous and partly crystalline BMG surfaces. A decreased surface roughness of the BMG substrates could be directly correlated with a higher hardness and a better adhesion of the ZrN film. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.125463
  • Mechanical behavior of reactive air brazed (RAB) Crofer 22 APU-Al2O3 joints at ambient temperature
    Tillmann, W. and Anar, N.B. and Wojarski, L.
    SN Applied Sciences 2 (2020)
    The evolution of innovative high-temperature electrochemical devices, such as high temperature solid oxide fuel cells (SOFCs), gas separators and gas reformers, consisting of metal–ceramic-joints is challenging. The seals have to be stable and gastight in isothermal high-temperature as well as in thermo-cyclic operation. Here, the reduction of porosity is the primary aim, to obtain air brazed joints with a long lifetime. In the last years, reactive air brazing (RAB) has gained rising interest for the joining of ceramic–ceramic and ceramic–metal compounds. In this paper an alternative brazing filler metal manufacturing process employing (physical vapor deposition (PVD)) is applied and its feasibility for the production of metal–ceramic composites has been investigated for Ag–4 wt%CuO. For RAB aluminum oxide with ferritic high chromium steel Crofer22APU have been joined. The pore formation in subordination of the braze and base materials can be monitored after brazing. By modifying the brazing process, the pore formation in the joints can be avoided. The microstructure of brazed joints with the developed braze foils is studied. Discussion of the results focuses on the influence of microstructural evolution on mechanical properties, the pore formation in the brazing seam and failure behavior of the brazed joints. A correlation between the process parameters brazing temperature and holding time and the achieved compound properties could be derived. Further, excellent wetting of the ceramic was obtained. The highest shear strength with 123 MPa was measured for a temperature of 1000 °C and 5 min, using the Ag4CuO alloy. © 2020, Springer Nature Switzerland AG.
    view abstract10.1007/s42452-020-2622-6
  • Microstructural and Tribological Characteristics of Sn-Sb-Cu-Based Composite Coatings Deposited by Cold Spraying
    Tillmann, W. and Hagen, L. and Kensy, M.D. and Abdulgader, M. and Paulus, M.
    Journal of Thermal Spray Technology 29 (2020)
    Sn-based Babbitt coatings are widely used for sliding in hydrodynamic bearings. The Babbitting of bearing surfaces is among others accomplished by casting; however, this implies some disadvantages such as segregations, or susceptibility to shrinkage defects. Thermal spraying represents a promising method to overcome these challenges. To date, no studies on Babbitt coatings deposited by means of low-pressure cold spraying (LPCS) are available in the literature. In this study, a first attempt is made to produce a Sn-Sb-Cu-based composite coating reinforced with alumina particles by means of LPCS which enables the coating of internal diameters (IDs) of cylindrical components. A tailor-made feedstock was utilized which consists of a powder mixture of Sn, Sb, Cu and alumina. The composite coating is investigated with regard to its microstructural and tribological characteristics using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), as well as dry sliding experiments. Metallographic investigations demonstrate the feasibility of depositing an alumina-reinforced Sn-Sb-Cu-based composite coating with a dense microstructure and low porosity. The composite coating mainly consists of Sn, SbSn, Cu and hexagonal CuSn. Despite a small fraction of alumina particles, the microhardness of the composite coating is primarily determined by the formation of SbSn intermetallic phases dispersed in the soft Sn-Sb-rich matrix. The composite coating possesses a coefficient of friction of 0.43 ± 0.01 and wear coefficient k of 17.27 ± 7.77 × 10−5 mm3 N m−1 sliding against a 100Cr6 counterbody. © 2020, ASM International.
    view abstract10.1007/s11666-020-01054-1
  • Microstructural characteristics in babbitt coatings deposited by LPCS
    Tillmann, W. and Hagen, L. and Abdulgader, M. and Kensy, M.D. and Paulus, M.
    Coatings 10 (2020)
    Studies have already established that themechanical properties of Babbitt coatings significantly depend on the microstructural characteristics, such as the amount and distribution of intermetallic compoundsdispersedin a soft solidsolutionmatrix. For Sn-Sb-Cu-based Babbitt coatings, the formation of SbSn-and CuSn-based precipitates has a substantial influence on the resulting microhardness and thus determines the maximum load carrying capacity. Thermal spraying of Sn-based Babbitt coatings results in a relatively more refined structure of these precipitates than in common manufacturing processes, such as casting, due to the thermal processing conditions. This study aims to evaluate the effect of the temperature of the propellant gas and substrate temperature on the microstructural characteristics of Sn-Sb-Cu-based Babbitt coatings deposited by low pressure cold spraying (LPCS). The deposits were examined for their phase composition, microhardness and mesoscopic structure. It was found that the coatings were mainly composed of Sb2Sn23, Sb0.49Sn0.51 and Sorosite (CuSn or CuSb0.115Sn0.835), regardless of the substrate temperature or temperature of the propellant gas to be investigated. For a gas temperature above 300 °C, an increased microhardness was observed, which correlates with the appearance of a more homogenous distribution of Sb0.49Sn0.51 dispersed in a soft Sn-rich solid solution matrix. © 2020 by the authors.
    view abstract10.3390/coatings10070689
  • Microstructure and Mechanical Properties of Reactive-Air-Brazed 3YSZ/Crofer 22 APU Joints at Ambient Temperature
    Tillmann, W. and Anar, N.B. and Wojarski, L. and Lassner, J.J.
    Metallography, Microstructure, and Analysis 9 (2020)
    The growth of inventive high-temperature electrochemical devices such as solid oxide fuel cells constitutes a major task in brazing technology of ceramic–metal joints. In this work, reactive air brazing was used and the joining characteristics of 3YSZ with Crofer 22 APU have been systematically analyzed for three different brazing temperatures (1000, 1050 and 1100 °C) and two dwell times (5 and 30 min). The joints have been brazed successfully using the Ag–4CuO filler alloy. This braze filler metal was manufactured by an arc PVD (physical vapor deposition) process. Further, sufficient wetting of the zirconium oxide was achieved. The morphology of the oxide reaction layer at the steel side had a major influence on the shear strength of the brazed joints. A maximum average shear strength of 101 ± 4 MPa was obtained for a temperature of 1050 °C and a dwell time of 5 min. © 2020, ASM International.
    view abstract10.1007/s13632-020-00663-0
  • Modeling the separation performance of depth filter considering tomographic data
    Hoppe, K. and Maricanov, M. and Schaldach, G. and Zielke, R. and Renschen, D. and Tillmann, W. and Thommes, M. and Pieloth, D.
    Environmental Progress and Sustainable Energy 39 (2020)
    Fibrous depth filters are frequently used for the purification of gas streams with low dust loadings, as well as processes where a high initial filtration efficiency is required (e.g., clean rooms for aseptic production). One tool suitable for supporting the development of optimized filter media is the use of numerical simulations. The drawback of this technique is the high computational resources required. In this work, a new and fast approach based on a one-dimensional model was applied. Structural characteristics (e.g., porosity distribution and fiber diameter) of two different filter media were successfully determined using a novel X-ray microscope. These characteristics were incorporated in the filtration model, and their influence on the calculations was evaluated. It was found that the porosity distribution does have an impact on local (microscopic) deposition rates, but only a minor influence on the macroscopic filtration efficiency (around 3%). Benefits of the model are the application of measured structural data and the low computational expense. Compared to experimental data (VDI 3926 / ISO 11057), the prediction of the filtration efficiency can be improved by incorporating the structural data in the model. © 2020 The Authors. Environmental Progress & Sustainable Energy published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers.
    view abstract10.1002/ep.13423
  • Nitrogen doping of MoSx thin films sputtered by reactive High Power Impulse Magnetron Sputtering
    Tillmann, W. and Wittig, A. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
    Thin Solid Films 713 (2020)
    Incorporating nitrogen into non-stoichiometric molybdenum disulfide (MoSx) thin films is a promising approach in order to improve the mechanical properties. Nevertheless, the adhesion between the film and the substrate is still challenging and the interaction between the mechanical and the tribological properties is not fully understood yet. Subsequently, reactive High Power Impulse Magnetron Sputtering (HiPIMS) is used to deposit nitrogen doped MoSx thin films with different nitrogen amounts on 16MnCr5 steel. The interaction between the structural changes, the mechanical properties and the tribological behavior depending on the nitrogen amount is investigated. The results prove that an increasing amount of nitrogen significantly affects the structure and the tribo-mechanical properties of the thin films. X-ray diffraction analysis reveals a transformation from crystalline to amorphous with an increasing amount of nitrogen from (7.1 ± 0.3) at.-% to (19.5 ± 0.5) at.-%. This transformation is related to a suppression of the columnar microstructure as well as an increasing hardness and Young‘s modulus from (0.14 ± 0.02) GPa, and (5.28 ± 0.32) GPa for the undoped film, to (5.12 ± 0.32) GPa and (92.5 ± 6.2) GPa, for the film with the highest nitrogen amount. The results of the Rockwell indentation tests show that the films with a small amount of nitrogen exhibit an improved adhesion behavior. The wear coefficient can be reduced to a quarter of the value of the undoped MoSx film, whereas coefficients of friction are at similar level of 0.2 in ambient air. Reactive HiPIMS has proven to be promising to deposit nitrogen doped MoSx thin films on steel substrates, which reveal improved mechanical properties and an excellent transfer film built-up during the tribo-tests without failures. © 2020 Elsevier B.V.
    view abstract10.1016/j.tsf.2020.138267
  • Static and oscillation superimposed ring compression tests with structured and coated tools for Sheet-Bulk Metal Forming
    Behrens, B.-A. and Meijer, A. and Stangier, D. and Hübner, S. and Biermann, D. and Tillmann, W. and Rosenbusch, D. and Müller, P.
    Journal of Manufacturing Processes 55 (2020)
    Forming tools with tailored surfaces or functional surface modifications offer great potential for the adaption and optimization of forming processes. However, the interaction of the resulting tribological conditions with additional process oscillations and lubricants has not yet been sufficiently investigated. In the field of sheet-bulk metal forming the superimposition of oscillations is a new, highly promising approach for optimizing the forming of metallic materials. The aim of this study is therefore to investigate the forming behavior of metallic materials with an oscillation superimposition in combination with structured tool surfaces. In order to examine to what extend the friction factor and the forming force can be influenced by structured surfaces and PVD-coatings ring compression tests were conducted to re-create the real process conditions. The experiments were carried out statically and with an oscillation superimposition in the main force flow of the machine under lubrication and dry conditions. Occurring interactions between surface applications, lubrication and oscillation superimposition are identified and analyzed within the context of this work. Thereby, three different deterministic surface structures of the ring compression tools were considered. A radial and tangential arrangement of a wave-like structure as well as an isotropic honeycomb structure were applied on the forming tools by means of micromilling. For a lubricated and vibration superimposed process, especially the isotropic honeycomb structure caused a significant decrease in friction. This was attributed to the formation of lubrication pockets. Despite the reduced friction, in some cases no reduction of the forming force occurred by using an oscillation superimposed forming process. This behavior was attributed to damping effects caused by the lubricating pockets. © 2020 The Society of Manufacturing Engineers
    view abstract10.1016/j.jmapro.2020.04.007
  • TiAlN-Beschichtungen mittels hoch-energetischer Kathodenzerstäubung: Eine Studie zur Auswirkung von Eigenspannungen auf die Leistungsfähigkeit von TiAlN-Dünnschichtsystemen
    Tillmann, W. and Stangier, D. and Grisales, D.
    Vakuum in Forschung und Praxis 32 (2020)
    TiAlN-coatings by means of high-energy sputtering. According to current knowledge, high-energy plasmas represent a promising tribological property profile for nitride hard material coatings and their applications. However, the increased ionization rate results in a high level of residual compressive stresses in TiAlN thin films (σTiAlN ∼ –6000 MPa), which can negatively influence the adhesion between substrate and coating and thus can lead to delamination and damage. In this context, the presented investigations prove that a hybrid process consisting of the synchronous use of direct current (DC) and high power pulse sputtering (HiPIMS) can be used to reduce the residual stress state in the coating. This allows to combine the advantages of both technologies and thus to qualify TiAlN thin films for an extended range of applications as tribological coatings for heat treated tool steels. © 2020, Wiley-VCH Verlag. All rights reserved.
    view abstract10.1002/vipr.202000737
  • Tribo-mechanical properties and adhesion behavior of DLC coatings sputtered onto 36NiCrMo16 produced by selective laser melting
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Hagen, L. and Schaper, M. and Hengsbach, F. and Hoyer, K.-P.
    Surface and Coatings Technology 394 (2020)
    The combination of selective laser melted nickel-chromium-molybdenum alloyed steel and diamond-like carbon (DLC) coatings is an aspiring approach to produce light-weight components with improved tribological properties. In order to assure the tribo-mechanical properties of the DLC coatings, good adhesion on the additively manufactured substrates under high loads is essential. 36NiCrMo16, produced by Selective Laser Melting (SLM) and wrought material techniques, served as substrate material for hydrogen-free a-C and hydrogenated a-C:H coatings with a chromium carbide interlayer. The structural and mechanical properties of 36NiCrMo16 were examined in order to consider their effects on the tribo-mechanical properties and adhesion of the DLC coatings deposited by magnetron sputtering. In x-ray diffraction analyses, retained austenite was identified for the 36NiCrMo16 substrate processed by SLM. This increased austenite content in the SLM built steel is attributed to the pre-heating temperature of 200 °C during fabrication. Additionally, a relative density of 99.98% was detected for the SLM manufactured tempering steel by using an x-ray microscope. In contrast to the conventional 36NiCrMo16, the SLM substrates exhibited a higher surface roughness, which is ascribed to the different phase composition and microstructure. In nanoindentation tests, the a-C and a-C:H coatings deposited onto 36NiCrMo16 exhibited a hardness of ~22 and ~19–20 GPa, respectively. In general, the DLC coatings revealed a good adhesion to the conventional and SLM 36NiCrMo16 in Rockwell C indentation tests. Local, limited spalling failures were identified for a-C and a-C:H on the SLM substrate, while no delamination was observed for the coatings on conventional steel. The differences in microstructure, such as the retained austenite and higher surface roughness, do appear to affect the adhesion of the DLC coatings. In tribometer tests against 100Cr6 counterparts, the DLC coatings significantly reduced the friction and increased the wear resistance of 36NiCrMo16. Therefore, the DLC coatings are promising to enhance the tribo-mechanical properties of SLM 36NiCrMo16 substrates, but it is crucial to consider the surface integrity of SLM steel on the adhesion behavior of DLC coatings. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.125748
  • Tribological behaviour of low carbon-containing TiAlCN coatings deposited by hybrid (DCMS/HiPIMS) technique
    Tillmann, W. and Grisales, D. and Marin Tovar, C. and Contreras, E. and Apel, D. and Nienhaus, A. and Stangier, D. and Lopes Dias, N.F.
    Tribology International 151 (2020)
    TiAlN monolayers and TiAlN/TiAlCN bilayers were successfully deposited by hybrid (DCMS/HiPIMS) technology. The increase in the acetylene flow has linearly augmented the percentage of carbon incorporated into the coating systems. The TiAlCN with the highest carbon content significantly increased roughness and reduced hardness and Young's modulus. Additionally, a great reduction of the compressive residual stress has been noticed. Response surface methodology has been used from a full factorial design of experiments in order to create friction and wear maps varying sliding velocity and normal load for each of the coating systems. Tribological results showed that the coefficient of friction is not as dependent on the carbon content as it is on both the sliding velocity and normal load. © 2020 Elsevier Ltd
    view abstract10.1016/j.triboint.2020.106528
  • Tribological investigation of surface structures processed by high-feed milling on HVOF sprayed WC-12Co coatings
    Tillmann, W. and Stangier, D. and Hagen, L. and Biermann, D. and Freiburg, D. and Meijer, A.
    Surface and Coatings Technology 395 (2020)
    Since properties of high-velocity oxy-fuel (HVOF) sprayed WC-12Co coatings are limited due to the inherent restrictions of the material, new approaches that involve tailored surfaces and designing friction properties of functional materials have attracted enormous interest. Within this scope, two different deterministic patterns are machined on as-sprayed and polished WC-12Co coatings by means of a high-feed milling (HFM) process, whereby different process parameters with respect to the feed per tooth as well as the lead angle are taken into account. The produced high-feed milled surfaces are investigated with respect to the resulting topography and tribological properties under dry sliding conditions. The tribological examination of the high-feed milled surfaces reveals a decreased coefficient of friction (COF) compared to the polished WC-12Co coating sliding against a 100Cr6 counter-body. The lower friction is attributed to the reduction of the contact area as well as the capability of the cavities to store the transferred material resulting from wear debris. With respect to the different surface patterns, higher values for the kurtosis Sku lead to a reduction of friction. In contrast to the results presented for 100Cr6, the WC-12Co surfaces sliding against Al2O3 counter-bodies exhibit an almost constant COF for all surfaces. Although the reduced contact area leads to increased tribological stresses, the processed surfaces show a high resistance to sliding wear. Finally, it is clarified that time- and cost-consuming mechanical surface treatments such as polishing or grinding prior to a subsequent HFM process do not have to be applied. Therefore, HFM enables a new manufacturing route to structure WC-12Co coatings as well as to modify the tribological properties. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.125945
  • Tribological studies on multi-coated forming tools
    Weikert, T. and Tremmel, S. and Stangier, D. and Tillmann, W. and Krebs, E. and Biermann, D.
    Journal of Manufacturing Processes 49 (2020)
    Processes of Sheet-Bulk Metal Forming combine operations of both sheet and bulk metal forming, enabling the production of closely-toleranced functional components. Locally differing friction conditions on a single tool surface are crucial to control the material flow and to reduce process forces and tool wear. In order to understand how different friction conditions on a multi-coated tool surface affect the material flow during forming operations, variants of amorphous carbon coatings (a-C:H:W) and Cr-based hard coatings (CrAlN, CrAlCN) were deposited in combinations side by side on punches of ring compression tests. Evaluating the rings’ shapes revealed the coatings’ effects on material flow in terms of ring inner contours with sections of varying curvatures. Characteristic friction mechanisms were observed, which allowed a local delimitation of friction conditions by selectively applied coatings. © 2019 The Society of Manufacturing Engineers
    view abstract10.1016/j.jmapro.2019.11.021
  • Al 2 O 3 /ZrO 2 -8Y 2 O 3 and (Cr,Ti)AlSiN tool coatings to influence the temperature and surface quality in friction-spinning processes
    Tillmann, W. and Fehr, A. and Stangier, D. and Dildrop, M. and Homberg, W. and Lossen, B. and Hijazi, D.
    Production Engineering 13 (2019)
    Friction-spinning is an incremental forming process, which is accompanied by complex thermal and mechanical loads in the tool and the formed part. To influence the process temperature, two main process parameters, i.e. the rotation speed and the feed rate, can be adapted. With the objective to improve the tool performance and the quality of the workpiece, this study focuses on a coating concept for friction-spinning tools made of high speed steel (HS6-5-2C, 1.3343). Atmospheric plasma sprayed (APS) Al 2 O 3 and ZrO 2 -8Y 2 O 3 coatings serve as a thermal insulator, while physically vapor deposited (PVD) Ti 16.7 Al 21.8 Si 7.9 N and Cr 18.7 Al 22.2 Si 7.5 N films are applied to increase the hardness of the tools. In addition, duplex coatings, combining the APS and PVD technique, are synthesized to influence both the heat transfer and the tribological properties of friction-spinning tools. Subsequently, all coated tools are tested in a friction-spinning process to form flanges made of AW-6060 (AlMgSi—3.3206) tube materials. The tool temperatures are determined in situ to investigate the impact of the tool coating on the process temperature. Compared to an uncoated tool, the alumina and zirconia coatings contribute to a reduction of the tool temperature by up to half, while the PVD films increase the hardness of the tool by 20 GPa. Furthermore, it is shown that the surface quality of thermally sprayed or PVD coated tools is directly related to the surface roughness of the resulting workpiece. © 2019, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00899-y
  • Combination of an atmospheric pressured arc reactor and a magnetron sputter device for the synthesis of novel nanostructured thin films
    Tillmann, W. and Kokalj, D. and Stangier, D. and Fu, Q. and Kruis, E.
    Thin Solid Films 689 (2019)
    Nanocomposite coatings are state of the art, nevertheless the possible material combinations are restricted. Therefore, this work demonstrates an approach to synthesize novel nanostructured thin films by producing the nanoparticles and thin films independently. An atmospheric-pressure transferred arc reactor is used to synthesize TiN nanoparticles. The device is linked to a magnetron sputter device with an aerodynamic lens system. The aerodynamic lens enables the in-situ introduction of the synthesized nanoparticles into the PVD chamber by compensating the pressure differences. In this study, the influence of the linkage on the properties of a CrN thin film as well as the chamber conditions (pressure, temperature, bias-voltage, and plasma) on the TiN nanoparticles are analyzed. The CrN thin film is only slightly affected by the incoming working gas of the nanoparticle reactor. The TiN nanoparticles reveal a crystallite size of 9.3 ± 2.3 nm and are successfully introduced into the PVD chamber as agglomerates with sizes of 0.04 μm2 and are then deposited onto substrates. It is shown that the particle distribution, agglomerate size, morphology, and crack behavior can be influenced by the chamber conditions. © 2019 Elsevier B.V.
    view abstract10.1016/j.tsf.2019.137528
  • Combining thermal spraying and magnetron sputtering for the development of Ni/Ni-20Cr thin film thermocouples for plastic flat film extrusion processes
    Tillmann, W. and Kokalj, D. and Stangier, D. and Schöppner, V. and Malatyali, H.
    Coatings 9 (2019)
    In the digitalization of production, temperature determination is playing an increasingly important role. Thermal spraying and magnetron sputtering were combined for the development of Ni/Ni-20Cr thin film thermocouples for plastic flat film extrusion processes. On the thermally sprayed insulation layer, AlN and BCN thin films were deposited and analyzed regarding their structural properties and the interaction between the plastic melt and the surfaces using Ball-on-Disc experiments and High-Pressure Capillary Rheometer. A modular tool, containing the deposited Ni/Ni-20Cr thin film thermocouple, was developed and analyzed in a real flat film extrusion process. When calibrating the thin film thermocouple, an accurate temperature determination of the flowing melt was achieved. Industrial type K sensors were used as reference. In addition, PP foils were produced without affecting the surface quality by using thin film thermocouples. © 2019 by the authors.
    view abstract10.3390/coatings9100603
  • Development and construction of AISI H11/ZrO2 joints for injection molding tools
    Tillmann, W. and Anar, N.B. and Manka, M. and Wojarski, L. and Lehmert, B.
    Welding in the World 63 (2019)
    Increasing demands in industrial applications and simultaneous efforts to provide long-lasting and cost-efficient tools in the injection molding industry lead to the use of metal–ceramic joints with the aim to combine the specific properties of both materials. Due to its high CTE, zirconium oxide (ZrO2) is used for the ceramic part and is joined with the tool steel AISI H11 (1.2343). In this work, suitable joining techniques with a low heat input and therefore a low thermal load are applied and characterized for the production of metal–ceramic composites. The selection of joining techniques is based on the boundary conditions during the injection molding process, in which the composites have to resist the temperature, pressure, as well as shear and tensile loads. Therefore, besides brazing, other joining processes such as gluing, screwing, shrinking, and clamping were analyzed as possible low temperature joining techniques for ceramic-metal-compounds. The best results for the tensile strengths with 90 MPa were achieved by a brazing process, carried out in vacuum with approximately 10−5 mbar, at a temperature of 920 °C for 5 min, using the commercially available brazing filler alloy CB4. © 2019, International Institute of Welding.
    view abstract10.1007/s40194-019-00800-6
  • Effect of the dwell time on the microstructure and tensile strength of vacuum-brazed tool steels using BNi-2 filler metal
    Tillmann, W. and Henning, T. and Boretius, M.
    Welding in the World (2019)
    Nickel-based brazing alloys are widely used to vacuum braze numerous base materials such as tool steels, heat resistant steels, austenitic steels, and nickel-based alloys. The formation of intermetallic phases like Ni3Si, Ni3B, or CrxBy can cause a significant embrittlement of the joint. A sufficient diffusion of the melting point depressants boron and silicon will avoid such phase formations and can be primarily affected by the temperature-time-cycle. The process parameters required to achieve an entire nickel solid solution microstructure can be thermodynamically predicted, but usually exceeds the specifications of the heat treatment of the base material by far or necessitate hardly practicable small brazing gaps. This research is focused on the microstructure and the properties of vacuum-brazed joints, using a lower process temperature compared to thermodynamically optimized brazing parameters of AISI H11/BNi-2 joints. In order to investigate the influence of the dwell time, the tool steels AISI H11 and AISI 420 were brazed at 1050 °C for 25 and for 90 min with a BNi-2 amorphous foil (50.8 μm). The extended dwell time has mainly led to a higher Fe/Ni ratio within the brazed metal. Therefore, the average tensile strength was improved from 668 to 1304 MPa for AISI H11 joints as well as from 815 to 1351 MPa for AISI 420 joints. Furthermore, the fracture path was located at the interface brazed metal/diffusion area and could be attributed to a high disparity of the microhardness as well as a weakening by Kirkendall porosity. © 2019, International Institute of Welding.
    view abstract10.1007/s40194-019-00734-z
  • Effects of AISI H11 surface integrity on the residual stresses and adhesion of TiAlN/substrate compounds
    Tillmann, W. and Grisales, D. and Stangier, D.
    Surface and Coatings Technology 357 (2019)
    The material of the substrate prior to the deposition of PVD coatings as well as its surface roughness and microstructure, hardness, and Young's modulus both on the surface as well as in the material matrix, and their residual stresses on the surface and in depth, have a high influence on the adhesion of the coating to the substrate. During metal forming processes, tools are exposed to high loads, which promote plastic deformation in the material of the tool and thus reduce its lifespan. In this regard, different substrate pre-treatments, allow modifying the hardness and toughness of the substrate, positively affecting the adhesion of the coating/substrate compound as well as the overall performance of the PVD coated tool. In this investigation, four different pre-treatments were performed on the hot work tool steel AISI HI I (X37CrMoV5-1). Plasma nitriding, two different heat treatments, and polishing sequences (austenitizing, quenching and double tempering) as well as a combination of these two processes (heat treatment prior plasma nitriding). Subsequently, TiAlN coatings with a 3 μm thickness were deposited onto the four differently treated substrates by means of Direct Current Magnetron Sputtering. Residual stresses of the substrate surface, before and after coating deposition were determined by means of X-ray diffraction. Additionally, residual stresses depth profiles of the steel after each pre-treatment prior to the deposition of the coatings were measured utilizing selective electropolishing of the substrate surface. To evaluate the adhesion of the TiAlN monolayers to the coatings, scratch tests and Rockwell C adhesion tests were performed on the coating/substrate compounds. The adhesion of the coatings, deposited on steel with lower residual stresses prior to the deposition and flatter residual stress gradients, was improved. © 2018 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2018.10.032
  • Effects of AlN and BCN thin film multilayer design on the reaction time of Ni/Ni-20Cr thin film thermocouples on thermally sprayed Al2O3
    Tillmann, W. and Kokalj, D. and Stangier, D. and Schöppner, V. and Malatyali, H.
    Sensors (Switzerland) 19 (2019)
    Thin film thermocouples are widely used for local temperature determinations of surfaces. However, depending on the environment in which they are used, thin film thermocouples need to be covered by a wear or oxidation resistant top layer. With regard to the utilization in wide-slit nozzles for plastic extrusion, Ni/Ni-20Cr thin film thermocouples were manufactured using directcurrent (DC) magnetron sputtering combined with Aluminiumnitride (AlN) and Boron- Carbonitride (BCN) thin films. On the one hand, the deposition parameters of the nitride layers were varied to affect the chemical composition and morphology of the AlN and BCN thin films. On the other hand, the position of the nitride layers (below the thermocouple, above the thermocouple, around the thermocouple) was changed. Both factors were investigated concerning the influence on the Seebeck coefficient and the reaction behaviour of the thermocouples. Therefore, the impact of the nitride thin films on the morphology, physical structure, crystallite size, electrical resistance and hardness of the Ni and Ni-20Cr thin films is analysed. The investigations reveal that the Seebeck coefficient is not affected by the different architectures of the thermocouples. Nevertheless, the reaction time of the thermocouples can be significantly improved by adding a thermal conductive top coat over the thin films, whereas the top coat should have a coarse structure and low nitrogen content. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/s19153414
  • Fatigue and corrosion fatigue behaviour of brazed stainless steel joints AISI 304L/BAu-4 in synthetic exhaust gas condensate
    Schmiedt-Kalenborn, A. and Lingnau, L.A. and Manka, M. and Tillmann, W. and Walther, F.
    Materials 12 (2019)
    As brazed stainless steel components in service often have to withstand cyclic loads in corrosive environments, the corrosion fatigue properties of brazed joints have to be characterised. Application-relevant corrosion fatigue tests in corrosive media are extremely rare for brazed joints and cyclic deformation curves are barely investigated. In this study, fatigue tests of brazed AISI 304L/BAu-4 joints were performed in air and synthetic exhaust gas condensate K2.2 according to VDA 230-214. The fatigue behaviour of the brazed joints was compared to properties of the austenitic base material. Strain, electrical, magnetic, temperature and electrochemical measurement techniques were applied within fatigue and corrosion fatigue tests to characterise the cyclic deformation and damage behaviour of the brazed joints. It was found that the fatigue strength of 397 MPa at 2 × 106 cycles was reduced down to 51% due to the superimposed corrosive loading. Divergent microstructure-related damage mechanisms were identified for corrosion fatigue loadings and fatigue loadings of specimens in the as-received and pre-corroded conditions. The investigations demonstrate the important role of corrosive environments for the mechanical performance of brazed stainless steel joints. © 2019 by the authors.
    view abstract10.3390/ma12071040
  • Improved adhesion of a-C and a-C:H films with a CrC interlayer on 16MnCr5 by HiPIMS-pretreatment
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Maus-Friedrichs, W. and Gustus, R. and Thomann, C.A. and Moldenhauer, H. and Debus, J.
    Surface and Coatings Technology 375 (2019)
    Ensuring a high adhesion of amorphous carbon films to steel substrates remains a challenging task, sustaining continuous research efforts to improve the adhesion strength. Besides the interlayer system and the substrate material, surface pretreatments have a significant impact on the adhesion behavior. Within this context, the influence of the High Power Impulse Magnetron Sputtering (HiPIMS) pretreatment on the adhesion of magnetron sputtered hydrogenfree (a-C) and hydrogenated (a-C:H) amorphous carbon films with a chromium carbide (CrC) interlayer on 16MnCr5 steel is investigated. The plasma treatment consisted of 30 min Ar ion etching as well as a sequential 5 min of HiPIMS-pretreatment with a Cr cathode. Subsequently this pretreatment was compared to a procedure without utilizing the HiPIMS technique. The impact of the HiPIMS-pretreatment on the structure of the film was systematically analyzed by taking the CrC interlayer as well as the entire film structure into consideration. The adhesion strength of the a-C and a-C:H films is significantly improved by the formation of a Cr HiPIMS-nanolayer in the substrate/film interface. In scratch tests, the critical load Lc3 for a total film delamination increases from 43 ± 4 to 59 ± 3 N and from 48 ± 2 to 64 ± 3 N for the a-C and a-C:H film. The improved adhesion behavior of the carbon films is ascribed to the increased adhesion of the CrC interlayer, which did not delaminate when scratched with a load up to 159 ± 18 N. Complementary Rockwell indentation tests reveal that the HiPIMS-pretreatment improves the adhesion class from HF6 to HF4 and from HF5 to HF3 for a-C and a-C:H. The enhanced adhesion is essential to exploit the properties of a-C and a-C:H films in applications with high loads. In conclusion, the HiPIMS-pretreatment has proven to be a promising technique to increase the adhesion strength of carbon films. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.07.076
  • Influence of Ag on antibacterial performance, microstructure and phase transformation of NiTi shape memory alloy coatings
    Momeni, S. and Tillmann, W.
    Vacuum 164 (2019)
    Shape memory binary NiTi and ternary NiTiAg coatings were deposited by means of magnetron sputtering technique. The results show how simultaneous sputtering of Ag can affect the microstructure, phase transformation behavior and antibacterial properties of NiTi coatings. © 2019 Elsevier Ltd
    view abstract10.1016/j.vacuum.2019.02.051
  • Influence of etching-pretreatment on nano-grained WC-Co surfaces and properties of PVD/HVOF duplex coatings
    Tillmann, W. and Hagen, L. and Stangier, D. and Krabiell, M. and Schröder, P. and Tiller, J. and Krumm, C. and Sternemann, C. and Paulus, M. and Elbers, M.
    Surface and Coatings Technology 374 (2019)
    The deposition of coatings by means of Physical Vapor Deposition (PVD)is an established process to enhance the lifetime and performance of carbide bulk tools. Although the effect of surface pretreatments on conventional WC-Co surfaces is well known, this investigation examines for the first time, how different surface pretreatments affect the surface integrity of thermally sprayed WC-Co substrates prior a subsequent PVD layer deposition and its resulting properties. Therefore, a WC-12Co feedstock with an average WC particle size of 100 nm was thermally sprayed on AISI M3 steel substrates using High Velocity Oxy-Fuel (HVOF)technique. Hereinafter, the HVOF sprayed WC-Co coatings were grounded and polished, thus serving as substrates for further surface pretreatments and the deposition of a CrAlN PVD hard coating by means of magnetron sputtering. To evaluate the influence of various surface pretreatments on the HVOF sprayed WC-Co coatings, several sequences such as heating, inert gas ion etching, metal ion etching, and High Power Impulse Magnetron Sputtering (HIPIMS)-etching were carried out. With respect to the subsequent PVD layer deposition, the results show that the pretreatment does neither affect the hardness nor Young's modulus of the CrAlN top layer. Yet, different effects on the WC-Co surface and PVD coating adhesion are observed. Inert gas ion etching leads to a faster removal of the carbides than of the Co-binder matrix. In contrast, metal ion etching provides a “micro-blasting” effect and removes the binder matrix as verified by Atomic Force Microscope (AFM)measurements. As a result, a decrease of the compressive residual stress state and an increase of the surface free energy are observed. With respect to HIPIMS-etching, a Cr-nanolayer was applied onto the WC-Co surface, which enhances the adhesion of the CrAlN top layer. Nevertheless, HRC Rockwell adhesion and scratch tests reveal a superior adhesion for samples pretreated with the metal ion etching. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.05.054
  • Influence of Introducing an Organic Pore-Forming Agent on the Porosity and Microstructure of Alumina Coatings Produced by the Atmospheric Plasma Spray Process
    Tillmann, W. and Khalil, O. and Abdulgader, M.
    Journal of Thermal Spray Technology 28 (2019)
    Controlling the amount of porosity that can be added to ceramic coatings produced by means of atmospheric plasma spraying has always been a challenging task, especially, when coatings are produced with high adhesion/cohesion strengths combined with higher deposition efficiency. However, coating porosity can be varied in a limited fashion between 2 and 25% by changing the process parameters. The use of pore-forming agent is, therefore, an adequate alternative to add porosity by using the optimal spraying parameters. In this work, baking flour was vertically inserted into the plasma plume at different positions and with different feed rates while depositing alumina particles, as the basic feedstock powder, using optimized spray parameters. According to the spray conditions used in this work, flour particles have successfully contributed to the obtained coating porosity; however, the effectiveness is higher when flour is inserted at about half of the spray distance and depends on the feed rate of the flour. In comparison with flour-free alumina coatings, the porosity of flour-alumina’s coatings increased by approx. 50 area% and 170 area% when the flour’s weight fraction was equal 10 and 20 wt.% of the feedstock powder, respectively. Adhesion/cohesion strengths, hardness and thickness of coatings were strongly affected. © 2019, ASM International.
    view abstract10.1007/s11666-019-00934-5
  • Influence of plasma nitriding pretreatments on the tribo-mechanical properties of DLC coatings sputtered on AISI H11
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Surface and Coatings Technology 357 (2019)
    The duplex treatment, consisting of plasma nitriding and the deposition of a DLC coating, was carried out on the hot-work tool steel AISI H11. The coating structure, composed of Cr-based interlayers and a hydrogenated carbon layer, was sputtered on non-nitrided, nitrided, as well as nitrided-repolished AISI H11 steel with an either annealed or quenched and tempered base condition to examine the influence of the pretreatment condition on the tribo-mechanical properties of the DLC coating. Besides the graded hardness profile, plasma nitriding leads to a roughness increase, which affects the microstructure as well as the mechanical properties of the DLC coating. The rougher surface favors a film growth of a carbon layer with larger cluster-like structures. As a result, these DLC coatings exhibit hardness values below 22 GPa, while the coating systems sputtered on substrates with smoother surfaces reach values of approximately 26 GPa and showed a good adherence. The heat treatment condition influences the load-bearing capacity of the nitrided substrate as the higher core hardness enhances the mechanical support of the coating and reaches the highest adhesion class HF1 in the Rockwell C tests. Due to the lower film adhesion and the low hardness of the DLC coatings sputtered on nitrided non-repolished AISI H11, high coefficients of frictions and wear coefficients of up to 0.59 and 3.19 ∗ 10−5 mm3/N∗m were determined in tribometer tests against WC/Co counterparts. In contrast, the nitrided repolished steel exhibits a low coefficient of friction of 0.12 as well as a low wear coefficient of 0.06 ∗ 10−5 mm3/N∗m. Therefore, a repolishing of the nitrided AISI H11 with quenched and tempered base condition ensures the highest load-bearing capability of the substrate as well as an improved friction and wear behavior of the DLC coating. © 2018 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2018.11.002
  • Influence of the deposition parameters on the texture and mechanical properties of magnetron sputtered cubic MoNx thin films
    Tillmann, W. and Kokalj, D. and Stangier, D.
    Materialia 5 (2019)
    Molybdenum nitride thin films are well known for their excellent tribological properties. In the Mo-N system different phases can be formed in dependence of the manufacturing process and the nitrogen content. Within this context, the focus is mainly on cubic γ-MoN0.5 and hexagonal δ-MoN thin films. In this study, metastable cubic MoN0.80 – 0.92 thin films in a B1-NaCl structure were deposited by means of DC magnetron sputtering, while the bias-voltage and the heating were varied. The influence of the deposition parameters on the morphology and topography was investigated with the aid of SEM. The physical structure was analyzed by means of XRD and the texture was examined by 2D GI-XRD using synchrotron radiation. Through nanoindentation the hardness and the Young's modulus were analyzed and correlated with the texture of the thin films. It was found out that the bias-voltage effects the orientation of the thin films and, accordingly, the hardness as well residual stresses. Shear stresses were found in the MoNx thin films in addition to the normal stress. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.mtla.2018.100186
  • Influence of the etching processes on the adhesion of TiAlN coatings deposited by DCMS, HiPIMS and hybrid techniques on heat treated AISI H11
    Tillmann, W. and Grisales, D. and Stangier, D. and Ben Jebara, I. and Kang, H.
    Surface and Coatings Technology 378 (2019)
    Ensuring a good adhesion of the coatings to the substrate is one of the key points during the manufacturing of machining and forming tools. The nature of the substrate material and the way it is pre-treated and cleaned before the deposition plays an important role in the adhesion of the coatings. The in-situ cleaning processes as plasma etching and metal ion etching have demonstrated to have an influence on the adhesion of different coating/substrate systems. The introduction of HiPIMS technologies for the in-situ cleaning and the deposition of PVD coatings throughout this technique has opened varied opportunities to improve the performance of the coated parts. Systematic and comparative analysis of the influence of plasma etching itself (PE), plasma etching and metal ion etching (PE + MIE), and plasma etching and HiPIMS etching (PE + HiPE) etching processes on the adhesion of the TiAlN to the hot work tool steel AISI H11 has been performed. Additionally, subsequent to the etching processes, TiAlN coatings have been deposited using DCMS, HiPIMS and hybrid (DCMS/HiPIMS) technologies. Residual stresses of the heat treated AISI H11 were evaluated before and after the different etching process as well as after coating's deposition. It has been shown that the etching process affects the growth direction and microstructure of the TiAlN coatings, especially of those deposited by DCMS and hybrid. For instance, DCMS and hybrid TiAlN coatings deposited after PE have the presence of the TiAlN (200) reflection, not evidenced on the coatings deposited after PE + MIE and PE + HiPE. Moreover, hybrid coatings on PE + HiPE have a preferential (220) growth orientation and a (111) orientation when deposited on PE and PE + MIE cleaned substrate. Finally, in order to evaluate the adhesion of the coatings to the substrate, both scratch test and Rockwell C indentation test were used. The coatings deposited on the mere plasma etched (PE) substrate turn out to be the coatings with the highest critical load Lc3 and the lowest HF standards (HF1–HF3). This behaviour is attributed to the conservation of the substrate's surface integrity and the no promotion of surface tensions that can act in detriment of the adhesion of the coating. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.125075
  • Influences of substrate pretreatments and Ti/Cr interlayers on the adhesion and hardness of CrAlSiN and TiAlSiN films deposited on Al 2 O 3 and ZrO 2 -8Y 2 O 3 thermal barrier coatings
    Tillmann, W. and Fehr, A. and Stangier, D. and Dildrop, M.
    Results in Physics 12 (2019)
    Using atmospheric plasma spraying (APS), ceramic Al 2 O 3 and ZrO 2 -8Y 2 O 3 thermal barrier coatings (TBC) were applied on AISI H11 (1.2343) and subsequently polished to serve as a substrate to magnetron sputter CrAlSi 7.5 N and TiAlSi 7.9 N films. The influences of polishing as well as plasma etching processes on the surface roughness and residual stresses of the TBCs were correlated with the adhesion of the metal nitride films. As metallic interlayers are typically used to metallize insulating substrates to increase the film adhesion, the effect of different Cr and Ti interlayer thicknesses (50–150 nm) on the CrAlSi 7.5 N and TiAlSi 7.9 N adhesion was examined. Despite tensile stresses in the TBCs, a duplex coating structure, consisting of Al 2 O 3 + Ti 100 nm /TiAlSi 7.9 N, generated a high adhesion (L c3 = 61.04 ± 2.36 N). In contrast to Cr, titanium interlayers are assumed to lead to a stress relaxation in the interface between the TBC and the PVD film. In general, using ZrO 2 -8Y 2 O 3 as a substrate resulted in a minor adhesion of all PVD film combinations, which is traced back to lattice mismatches between the substrate and the films. Moreover, the number of pores on the TBC surface is crucial for the adhesion and hardness of CrAlSi 7.5 N and TiAlSi 7.9 N. © 2019 The Authors
    view abstract10.1016/j.rinp.2019.02.048
  • Interaction effects of cathode power, bias voltage, and mid-frequency on the structural and mechanical properties of sputtered amorphous carbon films
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Bayer, M. and Moldenhauer, H. and Debus, J. and Schmitz, M. and Berges, U. and Westphal, C.
    Applied Surface Science 487 (2019)
    The Design of Experiments is a promising method to investigate the cause-effect relation between the mid-frequency magnetron sputtering parameters on the structural and mechanical properties of amorphous carbon (a-C) films. Based on the Central Composite Design, the cathode power of two graphite targets, bias voltage, and mid-frequency were simultaneously varied from 1500 to 4000 W, −100 to −200 V, and 20 to 50 kHz, respectively. The chemical bonding state was characterized using UV and visible Raman spectroscopy with excitation wavelengths of 266 and 532 nm. Corresponding measurements were performed by X-ray photoelectron spectroscopy (XPS) using synchrotron radiation. Additionally, hardness and elastic modulus of the sputtered a-C films were determined in nanoindentation tests. Multi-wavelength Raman spectroscopy identified an sp3 content below 20%, with most a-C films having an sp3 value in the range of 12 to 18%. The formation of sp3 bonded atoms is negatively influenced by a high cathode power and bias voltage, whereas the highest sp3 content is obtained with a-C films sputtered with a cathode power and bias voltage of 2750 W and −150 V. However, higher values of the cathode power and bias voltage result in a film delamination and decrease of the sp3 concentration. The bonding state affects the mechanical properties, as high hardness and elastic modulus result from a high sp3 content. Therefore, a targeted adjustment of cathode power and bias voltage is necessary to obtain a-C films with a high hardness. In contrast, the mid-frequency does not have a significant impact on the mechanical properties. In conclusion, the Central Composite Design has proven to be a suitable method to investigate the cause-effects of the sputtering parameters on the properties of the a-C film. © 2019
    view abstract10.1016/j.apsusc.2019.05.131
  • Internal Diameter Coating Processes for Bond Coat (HVOF) and Thermal Barrier Coating (APS) Systems
    Tillmann, W. and Schaak, C. and Hagen, L. and Mauer, G. and Matthäus, G.
    Journal of Thermal Spray Technology 28 (2019)
    Current developments in different industrial sectors show an increasing demand of thermally sprayed internal diameter (ID) coatings. The most recent research and development is mainly focused on commercial applications such as arc spraying (AS), atmospheric plasma spraying (APS), and plasma transferred wire arc spraying, especially for cylinder liner surfaces. However, efficient HVOF torches are meanwhile available for ID applications as well, but in this field, there is still a lack of scientific research. Especially, the compact design of HVOF-ID and APS-ID spray guns, the need of finer powders, and the internal spray situation leads to new process effects and challenges, which have to be understood in order to achieve high-quality coating properties comparable to outer diameter coatings. Thus, in the present work, the focus is on the ID spraying of bond coats (BC) and thermal barrier coatings (TBC) for high-temperature applications. An HVOF-ID gun with a N2 injection was used to spray dense BCs (MCrAlY) coatings. The TBCs (YSZ) were sprayed by utilizing an APS-ID torch. Initially, flat steel samples were used as substrates. The morphology and properties of the sprayed ID coating systems were investigated with respect to the combination of different HVOF and APS spray parameter sets. The results of the conducted experiments show that the HVOF-ID spray process with N2 injection allows to adjust the particle temperatures and speeds within a wide range. CoNiCrAlY bond coats with a porosity from 3.09 to 3.92% were produced. The spray distance was set to 53 mm, which leads to a smallest coatable ID of 133 mm. The porosity of the TBC ranged from 7.2 to 7.3%. The spray distance for the APS-ID process was set to 70 mm, which leads to a smallest coatable ID of 118 mm. © 2018, ASM International.
    view abstract10.1007/s11666-018-0781-4
  • Investigation of HVOF-ID spraying with WC-CoCr-155 μm feedstock powder
    Tillmann, W. and Schaak, C. and Hagen, L. and Dildrop, M.
    IOP Conference Series: Materials Science and Engineering 480 (2019)
    High velocity oxygen fuel (HVOF) spraying of WC-Co(Cr) with different chemical compositions, different powder size fractions, and different mean carbide sizes is a well-established research field for outer diameter (OD) applications. These coatings are typically applied as wear protective layers for different types of industries. Current demands for internal diameter (ID) coatings lead to a great interest in HVOF-ID spraying. This field of application necessitates a special spray gun equipment and spray powders with particle size fractions smaller than 20 μm. At the same time, the process control concerning both the spray gun configuration and the use of fine powders leads to new challenges which differ from those of OD HVOF spraying. In this study, HVOF-ID spraying using a WC-CoCr 86-10-4 (-155 μm) feedstock with a mean WC particle size of 400 nm is investigated with respect to the resulting coating properties. A statistical design of experiments (DoE) is utilized to enable a systematic analysis of various process parameter settings along with their interaction on the microstructural characteristics as well as the deposition efficiency (DE). Based on the results, a desirability-based multi-criteria optimization is carried out in order to produce adequate coating properties. The obtained knowledge about the spray system enables to realize dense WC-CoCr coatings with a porosity of approximately 1 %. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/480/1/012008
  • Investigation of LBM-processed bimodal powder mixtures of the nickel base alloy HX and WC–Co
    Schaak, C. and Kleszczynski, S. and Tillmann, W. and Witt, G.
    Progress in Additive Manufacturing 4 (2019)
    Laser beam melting (LBM) is an additive manufacturing technology (AM), which enables the production of individual, complex metal parts. The development of AM-specific materials is a necessary step to exploit the full technological potential and should be a useful contribution for the ongoing process of implementing, establishing, and realizing AM processes in industrial fields of application. Theoretically, the LBM process offers the opportunity to process any weldable metal powder feedstock. Since LBM parameters affect the solidification conditions directly, this process is predestinated for future material developments. In the present paper, first research results to process the conventional LBM powder (Hastelloy X; HX) that was blended with a fine WC–Co powder are presented. Due to its high temperature and corrosion resistance, HX is widely used in industrial AM applications. WC–Co was added to improve the wear and mechanical properties. In this study, the blended, bimodal powder feedstock was analyzed, and the influence of the key process parameters on the porosity, morphology, and grain structure was investigated. The aim was to obtain the first basic information concerning the behavior of such powder blends during LBM processing and the resulting material properties. In summary, processing powder blends is possible. Since an increased porosity was determined for a high WC–Co content, the process parameters need to be further improved. In addition, it was determined that the energy input during the powder melting process directly affects the distribution of WC–Co particles within the HX matrix. High-energy inputs lead to a degradation of the WC particles. Low-energy inputs foster a consistent embedding of WC–Co particles within the HX matrix. © 2018, Springer International Publishing AG, part of Springer Nature.
    view abstract10.1007/s40964-018-0058-y
  • Investigation of low-pressure cold-gas dynamic spraying of polyamide-12 (PA12) on steel surfaces
    Tillmann, W. and Zajaczkowski, J.F.
    IOP Conference Series: Materials Science and Engineering 480 (2019)
    Due to their good tribological properties and corrosive resistance, thermoplastic coatings are becoming increasingly important in industrial applications. Thermal spray technologies in general and flame spraying in particular belong to the most common application methods. However, the high thermal energy inevitably leads to the risk of thermal decomposition, especially in the case of materials such as polyamide-12, which have a small discrepancy between the melting and degradation temperatures. Furthermore, flame-sprayed coatings often are highly porous, reducing its effectiveness as a corrosive barrier. As an alternative approach, polyamide-12 coatings were applied to steel substrates using a conventional low-pressure cold-gas dynamic spray system. Various mixtures including ceramic powders (Al2O3) as well as different injection concepts were investigated and evaluated. The focus was put on the resulting coating morphology. Furthermore, the cold-gas spraying system was modified with a continuous temperature control for the gas temperature to enable a more precise adjustment based on the melting temperature of the material in order to reduce the risk of thermal degradation. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/480/1/012009
  • Investigation of the tribo-mechanical properties of sputtered a-C:Si films using design of experiments
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Diamond and Related Materials 91 (2019)
    Adding silicon to sputtered amorphous carbon films is a promising approach to enhance tribo-mechanical properties. Although most works analyse the influence of one specific deposition parameter, the Design of Experiments is a more suitable method to investigate the cause-effect relation between the parameters on the properties of silicon-containing amorphous carbon (a-C:Si) films. In a sputtering process, the cathode power of the silicon target and the bias voltage were simultaneously varied from 217 to 783 W and −83 to −197 V, based on the Central Composite Design. The modification of the chemical composition was evaluated by means of glow-discharge optical emission spectroscopy. To analyse the influence of the silicon content and bias voltage on the tribo-mechanical properties, the hardness and elastic modulus were determined by nanoindentation, while the friction and wear behaviour were investigated in tribometer tests utilizing 100Cr6 counterparts. The silicon content increases linearly from 8.6 to 31.7 at.-% with an increasing cathode power without any impact of the bias voltage. However, an interaction between the two parameters is observed for the mechanical properties. At a high Si content and high bias voltage, the a-C:Si films show a high hardness of 22.0 ± 1.0 GPa and a high elastic modulus of 231.7 ± 10.6 GPa. In contrast, the lowest hardness of 18.1 ± 0.7 GPa and lowest elastic modulus of 203.2 ± 9.6 GPa are obtained at a low Si content and low bias voltage. The lowest coefficient of friction of 0.083 ± 0.029 as well as the lowest wear coefficients of 0.094 ± 0.025 × 10−5 mm3/N × m were obtained by a-C:Si films with a small amount of silicon. It was determined that the tribo-mechanical properties of a-C:Si films are interactively influenced by the silicon content and bias voltage. In this context, the Central Composite Design is an efficient method to obtain fundamental knowledge concerning the interrelation between these parameters. © 2018 Elsevier B.V.
    view abstract10.1016/j.diamond.2018.11.014
  • Investigation of the tribofilm formation of HiPIMS sputtered MoSx thin films in different environments by Raman scattering
    Tillmann, W. and Wittig, A. and Stangier, D. and Thomann, C.-A. and Moldenhauer, H. and Debus, J. and Aurich, D. and Brümmer, A.
    Lubricants 7 (2019)
    Understanding the generation of third body particles and their contribution to the formation of tribofilms of MoSx thin films is still challenging due to a large number of influencing factors. Besides the structure of the as-deposited MoSx films, the environment and the conditions during the Ball-on-disk tests affect tribofilms and thus the friction. Therefore, the influence of the surface pressure and sliding velocity in air, argon and nitrogen environments on the generation of the third body particles and the tribofilm formation of randomly oriented MoSx films is investigated. A high surface pressure is one major factor to achieve low friction, especially under humid conditions, which is important considering the use in industrial applications, for example dry-running screw machines. However, the mechanisms leading to that frictional behavior are still affected by the surrounding environment. While low friction is caused by a more extensive tribofilm formation in air, in argon and nitrogen, large size third body particles dispensed all over the contact area contribute to a lower friction. Raman scattering reveal a different chemistry of these particles reflected in the absence of laser- or temperature-induced surface oxidation compared to the as-deposited film and the wear track. The Raman scattering results are discussed with respect to the wear particle size, its chemical reactivity and strain-induced bonding changes. © 2019 by the authors.
    view abstract10.3390/lubricants7110100
  • Investigations of the corrosion damage process of the brazed joint AISI 304L/BNi-2
    Tillmann, W. and Walther, F. and Manka, M. and Schmiedt, A. and Wojarski, L. and Eilers, A. and Reker, D.W.
    Results in Physics 12 (2019)
    One of the most frequently used material combinations to produce heat exchangers for automotive exhaust systems are vacuum brazed AISI 304L/BNi-2 components. In order to simulate the influence of condensate corrosion on such parts during service, the well-established test procedure for testing the resistance of metallic materials to condensate corrosion in exhaust gas, VDA 230-214, is used in this study. For a test duration of up to 6 weeks, miscellaneous corrosive mechanisms were observed and examined concerning their formation and progression. The corrosive attack starts at the diffusion zone on the base metal side due to the formation of chromium borides and the reduced nickel content. Between week 4 and 5, the greatest material removal was observed, which goes in line with a diameter reduction from 6.5 mm down to 5.8 mm of the load-bearing area. In this regard, the ultimate tensile strength drops down from 253.3 MPa to 147.8 MPa after 6 weeks of corrosive testing. © 2019 The Authors
    view abstract10.1016/j.rinp.2018.12.074
  • Local quasi-static and cyclic deformation behaviour of brazed AISI 304L/BAu-4 joints characterised by digital image correlation
    Schmiedt, A. and Manka, M. and Tillmann, W. and Walther, F.
    Welding in the World 63 (2019)
    For a reliable design of brazed components, the degradation of mechanical properties due to the corrosive attack by aggressive operating environments has to be considered. In this study, the effect of a condensate corrosion, which is performed according to VDA test sheet 230-214 up to 6 weeks, on the mechanical behaviour of brazed AISI 304L/BAu-4 stainless steel joints is investigated. A time-dependent reduction of the tensile and fatigue strength values down to 42% of the as-received condition is determined. As standard strain measurements are not appropriate to characterise the local strain distributions of heterogeneous material systems, the optical digital image correlation technique is used to evaluate the local quasi-static and cyclic deformation behaviour of the 50 μm wide brazing seam. A novel triggered image acquisition enables measurements in fatigue tests at a frequency of 10 Hz. The reduction of the virtual gauge length from 12.5 down to 0.5 mm leads to an increase of the total strain and ratcheting strain values, which is more pronounced for higher stresses and enhanced for pre-corroded brazed joints. For a microstructure-related analysis of the damage processes, scanning electron microscopy was used. © 2019, International Institute of Welding.
    view abstract10.1007/s40194-018-00693-x
  • Lubrication mechanisms of C-MoS 2 -Fe 2 O 3 (Fe 3 O 4 ) nano-composite lubricants at the rubbing interfaces of non-copper coated solid wires against the contact tube [Schmiermechanismus von C-MoS 2 -Fe 2 O 3 (Fe 3 O 4 ) Nanokomposit-Schmiermitteln an den Reibflächen von kupferfrei-beschichtetem Volldraht gegen den Kontaktschlauch]
    Li, Z.X. and Wan, Q. and Li, G.D. and Li, H. and Li, H.J. and Zhang, T.L. and Kim, H.J. and Tillmann, W.
    Materialwissenschaft und Werkstofftechnik 50 (2019)
    Graphite-MoS 2 -Fe 2 O 3 (Fe 3 O 4 ) nano-composite lubricating coatings were prepared on the surfaces of non-copper coated solid wires by a mechanical coating technique. The tribological behaviours of graphite-MoS 2 -Fe 2 O 3 (Fe 3 O 4 ) coatings at the rubbing interfaces of welding wires against the contact tube were investigated. The results demonstrate that the lubricating properties of graphite-Fe 3 O 4 coatings outperform the lubricating properties of graphite-Fe 2 O 3 coatings. The anti-wear performance of the contact tube is strengthened with increasing nano-MoS 2 contents. Layers of protective tribofilms are formed at the rubbing interfaces of welding wires against a contact tube by tribochemical reaction among lubricants. The tribofilms are composed of FeO, MoO 3 and FeMoO 4 with excellent lubricating properties. They can avoid direct contact of welding wires against the contact tube, thus decreasing contact tube wear. With the transition of the contact tube wear from mild to severe, the dominant wear mechanisms of contact tube change from fatigue peeling and oxidative wear to abrasive wear and arc ablation. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201700228
  • Microstructural and tribological properties of sputtered AlCrSiWN films deposited with segmented powder metallurgic target materials
    Tillmann, W. and Fehr, A. and Stangier, D.
    Thin Solid Films 687 (2019)
    When synthesizing magnetron sputtered films with a complex stoichiometry, integrating the desired coating constituents into one target material is favorable in order to avoid nanolaminar film depositions and to enable a homogenous film growth. In contrast to alloyed targets, segmented plug targets allow to merge elements with different physical properties in one target material. Two targets, amalgamating 20 and 48 hot-pressed 85.6Cr9.9Si4.5W (at. %) plugs, respectively, into a monolithic aluminum target were fabricated and employed in a direct current magnetron sputtering process to deposit AlCrSiWN films on high-speed steel (AISI M3:2, 1.3344). The cathode powers for the Al(CrSiW)20 and Al(CrSiW)48 targets were varied between 7.5 and 17.5 W/cm2 to analyze how differently composed targets and various cathode powers affect the microstructure and tribological properties of the sputtered films. The results revealed that the chemical composition as well as the thickness of the films strongly depend on the target setup. All AlCrSiWN films exhibited a Cr/Si/W ratio of approximately 84/11/6. The Cr and Al contents were dominant (19–29 at. %), while the Si and W contents varied between 2 and 3 at. %. Especially the Al/Cr ratio of the films is affected by the varying Al/CrSiW surface area ratio of the manufactured plug targets. Furthermore, the mechanical properties are significantly influenced by the Al/Cr ratio, which is responsible for a dense coating growth and the crystalline structure of the films. All AlCrSiWN films were (111) textured indicating a B1 (Al, Cr, W)N structure, which exhibited a finer crystalline growth with an increasing cathode power on the Al(CrSiW)20 target. Tribological analyses of the films against Al2O3 balls further revealed that thinner films resulted in a decreased wear coefficient. © 2019 Elsevier B.V.
    view abstract10.1016/j.tsf.2019.137465
  • Microstructural characteristics of high-feed milled HVOF sprayed WC-Co coatings
    Tillmann, W. and Hagen, L. and Stangier, D. and Paulus, M. and Tolan, M. and Sakrowski, R. and Biermann, D. and Freiburg, D.
    Surface and Coatings Technology 374 (2019)
    Over the last decade, great efforts have been undertaken in science and industry to provide WC-Co feedstock with nano-sized WC particles that significantly improves tribo-mechanical coating properties. For tribologically stressed surfaces, superior surface characteristics can be achieved by applying tailored surfaces, using novel technologies in the field of production engineering such as High-Feed Milling (HFM). For the first time, textured surface patterns were produced onto HVOF sprayed WC-Co coatings with nano-sized WC particles by implementing a HFM post process. In dependence to two different textures resulting from the HFM, the microstructural characteristics of the produced surfaces are analyzed. Confocal microscopy revealed the machinability of textured patterns onto a HVOF-sprayed WC-12Co hard coating, which comprise of nano-sized WC particles, by means of HFM technology. X-ray diffraction analyses confirmed the insertion of macro- and micro-scale residual stresses. The experiments showed a significant insertion of compressive residual stresses transverse to the feed direction, whereas the insertion of compresses residual stresses with its feed direction being less pronounced. It was found that the HFM post process leads to a refinement of the WC crystallite size and a distinct increase of its internal strain, which both can be attributed to plastic deformations during HFM. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.06.012
  • Porosity characterization and its effect on thermal properties of APS-sprayed alumina coatings
    Tillmann, W. and Khalil, O. and Abdulgader, M.
    Coatings 9 (2019)
    In the thermal spraying process, the porosity of ceramic coatings contributes directly to the efficiency of the thermal insulation. The size, shape, and distribution of the pores determine the level of both thermal and sintering resistance. In this work, three different atmospheric plasma sprayed (APS) alumina coatings were fabricated with the same spraying parameters using alumina powders with fine, medium, and coarse particle size. The microstructure of the obtained coatings was analyzed regarding the obtained total porosity, pore size, and pore shape. It was found that it is expedient to divide the pore size range into fine, medium, and large sizes. The shape was characterized with regard to the circularity aspect. In this way, all types of cracks can be considered as oblate pores and were included in the calculation of the total porosity. In the case of using fine feedstock powder, the densest coatings were produced among all coatings, and the fraction of fine pores and cracks are thereby substantially higher. However, the total porosity increases with increasing feedstock powder size. A connection was also made between thermal insulation and porosity fraction which includes fine pores and cracks. © 2019 by the authors.
    view abstract10.3390/coatings9100601
  • Powder metallurgic fabricated plug targets for the synthesis of AlCrSiWN multicomponent coating systems
    Tillmann, W. and Fehr, A. and Stangier, D.
    International Journal of Refractory Metals and Hard Materials 85 (2019)
    Monolithic and alloyed targets, conventionally produced by means of melt metallurgy, have been established as conventional target material designs for physical vapor depositions (PVD). However, integrating refractory metals into the sputtering material, leads to restrictions concerning the solubility and phase formation in the target compound. In this context, plug targets are commonly used to deposit multinary coatings with a desired chemical composition. However, producing plugs by means of melt metallurgy restricts the types and amounts of integrated elements. Since current PVD coating concepts aim at an extension of the functionality spectrum by element doping, new target concepts are required. The use of several monolithic targets is one method to produce coatings combining various elements within one coating. Yet depending on the target setup, this approach can result in a nanolaminar coating deposition. To circumvent this, a new production route, which ensures the integration of sintered CrSiW plugs in a monolithic aluminum target, is examined in this study. Two coating deposititons, each with an Al(CrSiW)20 and an Al(CrSiW)48 plug target, were performed by means of direct current (DC) magnetron sputtering and compared with a reference coating, which was synthesized using an AlCr20 target. The dense morphology of AlCrN was significantly changed to a more columnar structure due to slight additions of silicon and tungsten. High aluminum contents in AlCrN and AlCrSiWN, in turn, resulted in a distinct enhancement of the mechanical properties. © 2019 Elsevier Ltd
    view abstract10.1016/j.ijrmhm.2019.105081
  • Process of Molecular Dynamics Simulation of Nanojoining Processes [纳米连接过程的分子动力学模拟研究进展]
    Li, H. and Yuan, J. and Li, Z. and Tillmann, W. and Hu, A.
    Zhongguo Jixie Gongcheng/China Mechanical Engineering 30 (2019)
    Nano materials were widely used because of their unique structures and properties, and nano joining technology was also developed. In the field of nanofabrication, although there were many varieties of advanced microscopy methods, it was difficult to observe and analyze many phenomenons and behaviors of nano materials by experiments due to the restrictions of real-time observation. Molecular dynamics simulation within nanometer spatial scales and nanoseconds time scale allowed insight into molecular motions on an atomic scale since it was consistent with the time and spatial scales of many processes in nanofabrication. Therefore, it was feasible to use molecular dynamics simulation to simulate the properties of nanomaterials and the dynamic behavior of atoms in the nano joining processes. Herein, the progresses of molecular dynamics simulation of nano-scale materials, including nanomultilayers, nanowires, carbon nanotubes and nanoparticles in the nanojoining processes, as well as applications of these materials for joining of large size materials were reviewed. Meanwhile it is illustrated that the simulation results are in good agreement with the experimental ones. The existing problems, possible solutions and development trends were discussed. © 2019, China Mechanical Engineering Magazine Office. All right reserved.
    view abstract10.3969/j.issn.1004-132X.2019.04.016
  • Reduction of ejection forces in injection molding by applying mechanically post-treated CrN and CrAln PVD films
    Tillmann, W. and Stangier, D. and Lopes Dias, N.F. and Gelinski, N. and Stanko, M. and Stommel, M. and Krebs, E. and Biermann, D.
    Journal of Manufacturing and Materials Processing 3 (2019)
    In injection molding, the reduction of ejection forces is a process relevant aspect to improve the production rates. For this purpose, CrN and CrAlN films were sputtered on cylindrical and quadratic AISI H11 cores of an injection mold in order to investigate their influence on the resulting ejection forces to demold polypropylene test components. Within this context, the ejection forces of the PVD coated cores were compared to those of uncoated cores made of AISI H11. For both the cylindrical and quadratic cores, the as-deposited CrN and CrAlN films exhibit higher ejection forces than the uncoated cores due to the increase of the roughness profile after sputtering. It is known that the ejection forces are directly related to the surface roughness. In order to ensure comparable surface conditions to the uncoated surfaces, and to demonstrate the potential of PVD coated mold surfaces when reducing the ejection forces, the coated surfaces were mechanically post-treated to obtain a similar roughness profile as the uncoated cores. The combination of a PVD deposition and post-treatment ensures a significant reduction of the ejection forces by 22.6% and 23.7% for both core geometries. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
    view abstract10.3390/jmmp3040088
  • Resonant Raman scattering characterization of thermally annealed HiPIMS deposited MoSx coatings
    Moldenhauer, H. and Wittig, A. and Kokalj, D. and Stangier, D. and Brümmer, A. and Tillmann, W. and Debus, J.
    Surface and Coatings Technology 377 (2019)
    Raman spectroscopy is used to investigate the structural and tribological properties of HiPIMS sputtered MoSx thin films which were post-growth-annealed at different temperatures. The Raman scattering combined with X-ray diffraction determines a reduction in the residual strain within the MoSx layers with increasing annealing temperature. In the high-temperature annealed coatings a Raman signature at 40 cm−1 emerges, which results from a strengthening of the inter-layer van-der-Waals interaction. This observation indicates that the thermally annealed MoSx thin films become more resistant against shear forces, which is manifested in an increase of the coefficient of friction measured with a ball-on-disc tribometer. The coefficient of friction moreover decreases with lowering the sulfur/molybdenum ratio which, in turn, depends on the substrate and annealing temperatures. Furthermore, a Raman forbidden mode may be exploited to detect stacking faults within the sputtered coatings. Its observation is realized through resonant excitation of an MoS2 exciton at about 633 nm. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.124891
  • Statistical Analysis of the Lifetime of Diamond-Impregnated Tools for Core Drilling of Concrete
    Malevich, N. and Müller, C.H. and Kansteiner, M. and Biermann, D. and Ferreira, M. and Tillmann, W.
    Studies in Classification, Data Analysis, and Knowledge Organization (2019)
    The lifetime of diamond-impregnated tools for core drilling of concrete is studied via the lifetimes of the single diamonds on the tool. Thereby, the number of visible and active diamonds on the tool surface is determined by microscopical inspections of the tool at given points in time. This leads to interval-censored lifetime data if only the diamonds visible at the beginning are considered. If also the lifetimes of diamonds appearing during the drilling process are included, then the lifetimes are doubly interval-censored. We use a well-known maximum likelihood method to analyze the interval-censored data and derive a new extension of it for the analysis of the doubly interval-censored data. The methods are applied to three series of experiments which differ in the size of the diamonds and the type of concrete. It turns out that the lifetimes of small diamonds used for drilling into conventional concrete are much shorter than the lifetimes when using large diamonds or high-strength concrete. © Springer Nature Switzerland AG 2019.
    view abstract10.1007/978-3-030-25147-5_15
  • Structural and mechanical properties of carbon incorporation in DC/HiPIMS CrAlN coatings
    Tillmann, W. and Stangier, D. and Roese, P. and Shamout, K. and Berges, U. and Westphal, C. and Debus, J.
    Surface and Coatings Technology 374 (2019)
    Incorporating carbon into ternary nitride coatings to tune the mechanical and tribological properties of thin films is of great interest in order to improve the performance of tools and components. Especially, the approach to tailor CrAlN coatings by doping transition metals has been extensively studied in recent years. Nevertheless, the microstructural changes, induced by carbon incorporation, especially into Al-rich CrAlN coatings, are not yet fully understood. Thus, detailed investigations of the microstructure, performed by means of synchrotron radiation, using x-ray diffraction and x-ray photoelectron spectroscopy and Raman scattering with different laser excitation (355 nm and 532 nm), were conducted to understand the evolution of the mechanical properties of CrAlCN coatings depending on the carbon content. The results prove that an increasing carbon content significantly influences the microstructure, residual stresses, as well as the mechanical properties of the coatings. The presence of C[dbnd]C and C[dbnd]N bonds was proven by investigating the C 1s orbital. Furthermore, the increasing amount of carbon forms amorphous Cr[sbnd]C structures, which were detected by analyzing the Cr 3p orbital. These results were confirmed for the amorphous phases by Raman scattering additionally indicating the formation of nanocomposite structures due to the formation of carbon nano-onoin like structures. The investigations of the crystalline structure using XRD reveal the existence of a fcc structure for the CrAlN phase as well as small amounts of hexagonal AlN in the CrAlCN coating with the highest carbon content. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.06.053
  • Structure and mechanical properties of hafnium nitride films deposited by direct current, mid-frequency, and high-power impulse magnetron sputtering
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Tolan, M. and Paulus, M.
    Thin Solid Films 669 (2019)
    The structural properties of hafnium nitride films are mainly influenced by the deposition conditions, which are affected by the sputtering technique. A suitable use of the different sputtering modes allows to control the structural development of the films and thus to adjust the profile of the properties. NaCl-type hafnium nitride films were deposited using direct current magnetron sputtering (dcMS), mid-frequency magnetron sputtering (mfMS), and high-power impulse magnetron sputtering (HiPIMS). dcMS produces films with a columnar microstructure, whereas a fully-dense morphology is achieved by mfMS and HiPIMS. X-ray diffraction patterns show that films sputtered in dcMS mode have a (200) orientation, whereas mfMS and HiPIMS favor an orientation with the (111) plane parallel to the samples’ surface. mfMS leads to films with the largest crystal sizes and lowest stresses, which is ascribed to recrystallization mechanisms during the film growth. Hafnium films with an overstoichiometric composition show the highest hardness values. In this context, the dcMS-Hf49.8N50.2, mfMS-Hf50.4N49.6, and HiPIMS-Hf49.0N51.0 have a hardness of 28.2 ± 2.1, 32.4 ± 3.4, and 30.4 ± 3.1 GPa, respectively. In summary, the sputtering technique has a crucial role on the properties of the film and can be suitable used to adjust the structure and hardness of HfN films. © 2018 Elsevier B.V.
    view abstract10.1016/j.tsf.2018.10.035
  • Temperature-induced formation of lubricous oxides in vanadium containing iron-based arc sprayed coatings
    Tillmann, W. and Hagen, L. and Kokalj, D. and Paulus, M. and Tolan, M.
    Coatings 9 (2019)
    In the field of surface engineering, the use of self-lubricous coatings with the incorporation of vanadium represent a promising approach to reduce friction, thus contributing to the wear behavior. For vanadium containing hard coatings produced by means of thin film technology, the reduction in friction at elevated temperatures was repeatedly attributed to temperature-induced and tribo-oxidatively formed oxides which act as solid lubricant. Only very few studies focused on the tribological characteristics of vanadium containing arc sprayed coatings. In this study, the tribological characteristics of a vanadium containing iron-based arc sprayed deposit were investigated in dry sliding experiments under ambient conditions and different temperatures. Types of wear at the worn surfaces and counterparts were examined by means of electron microscopy and energy dispersive X-ray (EDX) spectroscopy. The speciation of vanadium in the superficial layer was determined using X-ray absorption near edge structure (XANES) spectroscopy. It was found that the vanadium-containing coating exhibited a distinctly reduction of the coefficient of friction above 450 °C which further decreased with increasing temperature. XANES spectroscopy indicated an increased oxidation state for the V component on the coating surface, suggesting the prevalence of specific vanadium oxides which promote a self-lubricating ability of the coating. © 2018 by the authors.
    view abstract10.3390/coatings9010018
  • The effect of machined surface conditioning on the coating interface of high velocity oxygen fuel (HVOF) sprayed coating
    Tilger, M. and Biermann, D. and Abdulgader, M. and Tillmann, W.
    Journal of Manufacturing and Materials Processing 3 (2019)
    Roughening the substrate surface is essential for thermal sprayed coatings. In this regard, sandblasting has established itself as an easy to use surface conditioning procedure. The quality of the obtained roughness depends on the conditions of the sandblasting material, adjusted parameters, and the kind of the process execution (manual or mechanical). These preconditions limit the reproducibility of the roughness obtained. Sandblasting causes residual compressive stress and may also lead to the inclusion of sand particles and notches in the roughened surface, which affects the interfacial properties of the coating, as well as the flexural strength of the coated parts. The hardness of the roughened surface plays, thereby, an important role. However, in order to reliably avoid these effects, microfinishing can be used as an alternative to generate a homogenous roughened substrate surface, control the induced residual stresses, and increase the reproducibility. In addition, the roughened surface pattern can be produced during the chip forming process of the to-be-coated parts. The utilization of the appropriate combination of machining processes and parameters should lead to the required surface pattern and thus to an enhanced coating adhesion and flexural strength of the coated part. The induced residual stresses and the quality of the obtained surface roughness have a significant influence on the coating adhesion and the lifespan of the coated parts. This paper aims to analyze, as a first step, the effect of the turning and microfinishing on the surface conditioning of the bearing steel 100Cr6 (AISI 52100). The investigation concludes by comparing the microfinished with the sandblasted surfaces with regard to the interface to and the adhesion of the WC–Co high velocity oxygen fuel (HVOF) sprayed coatings on them. Surface conditioning plays a decisive role by the induced residual stresses and the elimination of adhesion defects. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
    view abstract10.3390/jmmp3030079
  • Tribological performance of PVD film systems against plastic counterparts for adhesion-reducing application in injection molds
    Tillmann, W. and Dias, N.F.L. and Stangier, D. and Gelinski, N.
    Coatings 9 (2019)
    The deposition of physical vapor deposition (PVD) hard films is a promising approach to enhance the tribological properties of injection molds in plastic processing. However, the adhesion is influenced by the pairing of PVD film and processed plastic. For this reason, the friction behavior of different PVD films against polyamide, polypropylene, and polystyrene was investigated in tribometer tests by correlating the relation between the roughness and the adhesion. It was shown that the dispersive and polar surface energy have an impact on the work of adhesion. In particular, Cr-based nitrides with a low polar component exhibit the lowest values ranging from 65.5 to 69.4 mN/m when paired with the polar polyamide. An increased roughness leads to a lower friction due to a reduction of the adhesive friction component, whereas a higher work of adhesion results in higher friction for polyamide and polypropylene. Within this context, most Cr-based nitrides exhibited coefficients of friction below 0.4. In contrast, polystyrene leads to a friction-reducing material transfer. Therefore, a customized deposition of the injection molds with an appropriated PVD film system should be carried out according to the processed plastic. © 2019 by the authors.
    view abstract10.3390/COATINGS9090588
  • Tribomechanical behaviour of TiAlN and CrAlN coatings deposited onto AISI H11 with different pre-treatments
    Tillmann, W. and Grisales, D. and Stangier, D. and Butzke, T.
    Coatings 9 (2019)
    In the metalworking industry, different processes and applications require the utilisation of custom designed tools. The selection of the appropriated substrate material and its pre-treatment as well as the protective coating are of great importance in the performance and life time of forming tools, dies, punches and coated parts in general. TiAlN and CrAlN coatings have been deposited onto the hot work tool steel AISI H11 by means of Direct Current Magnetron Sputtering. Prior to the deposition, the steel substrate was modified by the implementation of three different pre-treatments: nitriding of the annealed substrate [Nitr.], heat treatment of the steel (quenching and double tempering) [HT] and nitridation subsequent to a heat treatment of the substrate [HT + Nitr.]. The purpose of this research is to obtain valuable information on the microstructural properties and tribomechanical behaviour of two of the most promising ternary transition metal nitride coatings, TiAlN and CrAlN, when deposited on the AISI H11 steel with different initial properties. The different pre-treatments performed to the steel prior to the deposition favour the tailoring during the design and construction of tools for specific applications. The microstructure, the adhesion and the wear resistance of TiAlN coatings were highly influenced by the substrate preparation. Contrarily, CrAlN results were more independent of the substrate preparation and no high influences were found. For instance, the adhesion of the TiAlN coating varied from 17 to 43 N for the coating deposited onto the HT + Nitr. substrate and the HT substrate respectively, while the lowest and highest adhesion of the CrAlN coating varied between 42 and 53 N for the HT and the HT + Nitr. respectively. Likewise, the wear coefficient of the CrAlN were ten times smaller than those found for the TiAlN coatings, presumably due to the presence of hex-AlN phases and the small differences on the Young's Modulus of the substrate and the CrAlN coatings. © 2019 by the author.
    view abstract10.3390/coatings9080519
  • Void distribution in a brazed cemented carbide steel joint analyzed by X-ray microscopy
    Yared, W. and Chen, C.-Y. and Sievers, N. and Tillmann, W. and Zielke, R. and Schimpfermann, M.
    Measurement: Journal of the International Measurement Confederation 141 (2019)
    Brazing is a relatively fast process that offers sufficient strength in the joint of dissimilar materials. Cemented carbides are often brazed onto steel components in order to improve the wear resistance of engineering tools. In the case of brazing such materials in an ambient atmosphere, a flux is necessary to improve the wetting of the liquid filler alloy on the surfaces. In some cases, the flux cannot be sufficiently removed from the small joint, thus forming voids during solidification. This phenomenon can greatly affect the integrity of the joint. Such voids are not adequately detectable by visual inspection or common nondestructive testing methods, such as ultrasonic scanning, acoustic emission testing, or thermography. In this study, X-ray microscopy is shown to provide adequate visualization and a quantitative analysis of the dispersion of voids within brazed components of cold work steel, 115CrV3, and cemented carbide, K10 (ISO 513). One of the challenging tasks when analyzing the aforementioned brazed materials is achieving a sufficiently high resolution within the joint gap, since the sample materials have similar X-ray absorption coefficients. Such high resolution was successfully achieved in this study by means of multiple scanning and image reconstruction techniques, such as beam filtering, dataset levelling, and noise removal. The voids on the 115CrV3-side are found to expand radially towards the edges of the specimen up to a maximum volume of 1.18E + 07 µm3. The same radial pattern was detected on the side of the K10, where the voids contracted in volume towards the center of the specimen. However, the K10-side was found to exhibit relatively larger voids with a maximum volume of 7.70E + 07 µm3, that is approximately seven times larger than that detected on the 115CrV3-side. © 2019 Elsevier Ltd
    view abstract10.1016/j.measurement.2019.04.045
  • An examination of interactions between temperature, pressure, sintering time, and Si/W ratio on the sintering behavior of CrSiW
    Tillmann, W. and Fehr, A. and Ferreira, M. and Stangier, D.
    International Journal of Refractory Metals and Hard Materials 73 (2018)
    A Statistic Design of Experiments (DoE) was implemented with the aim to systematically investigate sintering parameter interactions of hot pressed 80Cr10Si10W. By varying the temperature between 999 and 1200 °C at pressures ranging from 2 to 8 MPa for 6.6 to 23.4 min, an initial examination of the ternary system is realized. The overall objective of this study is to minimize the porosity and to foster a diffusion of the elements. The investigations revealed that high temperatures (&gt;1000 °C) and pressures (&gt;6 MPa) support the diffusion between chromium and silicon while tungsten particles accumulate at the grain boundaries of silicon. XRD analyses confirmed the existence of a c-CrSi3 phase. The setup of the DoE, which promises the highest densification, was subsequently used to examine the influence of the silicon and tungsten content on the porosity according to the pattern 80Cr(20-x)Si(x)W for 0 ≥ x ≥ 20 (in wt%). This approach generated the lowest porosity (3.2 ± 0.93 area%) for 80Cr20W and led to homogenous particle distributions. © 2018 Elsevier Ltd
    view abstract10.1016/j.ijrmhm.2018.02.012
  • Characterisation of the corrosion fatigue behaviour of brazed AISI 304L/BNi-2 joints in synthetic exhaust gas condensate
    Schmiedt, A. and Manka, M. and Tillmann, W. and Walther, F.
    Welding in the World 62 (2018)
    Exhaust gas recirculation (EGR) with brazed heat exchangers is commonly used for the reduction of NOx emissions of automotive diesel engines. Since the fatigue loading of brazed EGR coolers during the operation is superimposed by corrosive attack due to aggressive exhaust gases, the corrosion fatigue behaviour of brazed joints is relevant for the component design. In the present study, brazed AISI 304L/BNi-2 joints were cyclically tested in the synthetic exhaust gas condensate K2.2, using a corrosion cell, as well as in air after pre-corrosion acc. to VDA 230–214 with ageing durations of up to 6 weeks. A significant reduction of the fatigue strength at 2 × 106 cycles down to 43% was determined for superimposed and down to 22% for successive corrosion fatigue loading. A galvanostatic anodic polarisation of the specimens tested in situ was applied successfully for an efficient reproduction of the corrosion fatigue behaviour after the long-time and cost-intensive pre-corrosion. In this context, a novel test strategy with local strain measurements, using a newly developed extensometer, as well as electrical and electrochemical measurements was established for a precise corrosion fatigue assessment of the brazed joints. Microstructure-related corrosion fatigue damage mechanisms were evaluated using scanning electron microscopy. © 2018, International Institute of Welding.
    view abstract10.1007/s40194-018-0557-y
  • Control of the material flow in sheet-bulk metal forming using modifications of the tool surface
    Löffler, M. and Schulte, R. and Freiburg, D. and Biermann, D. and Stangier, D. and Tillmann, W. and Merklein, M.
    International Journal of Material Forming (2018)
    Sheet-bulk metal forming (SBMF) processes are characterized by a successive and/or simultaneous occurrence of different load conditions such as the stress and strain states. These conditions influence the material flow and often lead to a reduced geometrical accuracy of the produced components. To improve the product quality, a control of the material flow is required. One suitable approach, the local adaption of tribological conditions by means of surface modifications of tool or workpiece, so-called tailored surfaces. To control the material flow and thus to improve the component accuracy, methods to reduce and to increase friction are needed. The aim of this study is to determine requirements for necessary adaptions of the friction, the identification of tribological mechanisms for different types of tool-sided tailored surfaces as well as the verification of the effectiveness of these surface modifications to improve the results of a specific SBMF process. The numerical analysis of a combined deep drawing and upsetting process revealed that this process is characterized by two areas of varying tribological load conditions. Using a numerical analysis, the friction factor gradient between these two areas was identified as a main influencing factor on the material flow. Based on this finding, Chromium-based hard coatings for the reduction of the friction and high-feed milled surfaces for an increase of the friction were investigated regarding their frictional behaviour. The results of the ring-compression tests revealed that the carbon content and the post treatment of coated tool surfaces are relevant to reduce the friction. The increased profile depth of the milled surfaces was identified as the main influencing factor on the tribological behaviour of this kind of tailored surfaces. The effectiveness of both types of tailored surfaces was verified for the combined deep drawing and upsetting process. © 2018 Springer-Verlag France SAS, part of Springer Nature
    view abstract10.1007/s12289-018-1399-2
  • Development of a constitutive model for friction in bulk metal forming
    Lüchinger, M. and Velkavrh, I. and Kern, K. and Baumgartner, M. and Klien, S. and Diem, A. and Schreiner, M. and Tillmann, W.
    Lubricants 6 (2018)
    This paper presents a systematic procedure for the development of a constitutive model of friction with focus on the application in bulk metal forming simulations. The empirically based friction model describes friction as a function of sliding distance and the most relevant friction influencing parameters. The latter were determined by means of designed experiments. An optimal friction model is obtained as a trade-offbetween model accuracy and complexity by using stepwise nonlinear regression and a modified version of the Akaike information criterion. Within this study, the procedure is applied to determine a friction model for tube drawing. However, the same approach can also be used for modeling friction of any other bulk metal forming process. © 2018 by the author.
    view abstract10.3390/lubricants6020042
  • Effect of Hf on the microstructure, mechanical properties, and oxidation behavior of sputtered CrAlN films
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Vacuum 154 (2018)
    Al-rich CrAlN films with a varying Hf content between 0 and 11.6 at.-% were synthetized by dc magnetron sputtering. The structural changes in the morphology and topography caused by Hf were evaluated by scanning electron microscopy. In addition, the phase composition of the Hf-alloyed CrAlN films was determined utilizing X-ray diffraction. The hardness and indentation modulus were determined using nanoindentation. In order to evaluate the effect of Hf on the oxidation behavior, the films were tempered in ambient air at 800 °C and subsequently analyzed. The incorporation of Hf leads to a morphological change from a fully-dense structure with small columns to a structure with more pronounced columns, which are visible as larger column tops on the surface. The Al-rich CrAlHfN films consist of the Wurtzite structure of AlN. A hardness decrease from 22.2 to 18.6 GPa is observed with an increasing Hf content and ascribed to the structural changes. When exposed to higher temperatures, the Hf-alloyed CrAlN films form an oxide layer, whose thickness is affected by the Hf content. An improved oxidation resistance is already achieved by a small amount of 2.0 at.-% of Hf. © 2018 Elsevier Ltd
    view abstract10.1016/j.vacuum.2018.05.015
  • Experimental and numerical analyses of residual stresses induced by tube drawing
    Vollert, F. and Lüchinger, M. and Schuster, S. and Simon, N. and Gibmeier, J. and Kern, K. and Schreiner, M. and Tillmann, W.
    Journal of Strain Analysis for Engineering Design 53 (2018)
    Lightweight constructions are used to fulfil the ever-increasing demands regarding fuel efficiency and carbon dioxide emission in transportation industries. In order to reduce weight, technical components made of solid materials are often replaced by tubular structures. Under service conditions, the components are frequently exposed to cyclic loads. Hence, residual stresses that are induced by manufacturing processes can have a significant impact on service life. In this work, the focus is on tube manufacturing processes, precisely cold tube sinking and fixed plug drawing. Both processes induce characteristic residual stress states, which are important to assess the mechanical integrity and load-carrying capacity of tubular components during service. The aim of this article is to examine the residual stress depth distribution for medium-carbon steel tubes manufactured by cold tube sinking and fixed plug drawing. The residual stresses are measured by means of the Sachs method and the hole-drilling method, respectively. The measured results are compared to finite element simulations of the tube drawing process. It is shown that the residual stress obtained with the different experimental methods and the numerical simulations are consistent. Furthermore, it is shown that the residual stresses can be significantly reduced when a plug is used in the drawing process. © 2018, IMechE 2018.
    view abstract10.1177/0309324718770339
  • Frequency-dependent fatigue and corrosion fatigue assessment of brazed AISI 304L/BNi-2 joints in air and synthetic exhaust gas condensate
    Schmiedt, A. and Lücker, L. and Manka, M. and Tillmann, W. and Walther, F.
    Fatigue and Fracture of Engineering Materials and Structures 41 (2018)
    Since brazed components are often cyclically loaded in corrosive environments, the corrosion fatigue behaviour of the joints has to be investigated. Fatigue tests of brazed AISI 304L/BNi-2 joints, relevant for exhaust gas heat exchangers, were performed with specimens in the as-received condition and after pre-corrosion according to VDA 230-214. Additionally, the superimposed corrosion fatigue loading in an exhaust gas condensate was realised using a corrosion cell. Corrosion-induced and deformation-induced microstructural changes were metallographically evaluated. The influence of the test frequency from 1 to 150 Hz on the cyclic deformation and damage behaviour was characterised. In a statistical analysis, the fatigue strength of 210 MPa at 2·107 cycles was determined for the as-received condition with a 50% failure probability. The pre-corrosion as well as the superimposed loading lead to a reduction of the fatigue strength down to 22%. A novel test strategy is suitable for precise fatigue and corrosion fatigue assessments. © 2018 Wiley Publishing Ltd.
    view abstract10.1111/ffe.12902
  • In Situ Acoustic Monitoring of Thermal Spray Process Using High-Frequency Impulse Measurements
    Tillmann, W. and Walther, F. and Luo, W. and Haack, M. and Nellesen, J. and Knyazeva, M.
    Journal of Thermal Spray Technology 27 (2018)
    In order to guarantee their protective function, thermal spray coatings must be free from cracks, which expose the substrate surface to, e.g., corrosive media. Cracks in thermal spray coatings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology and coating defects were inspected using light microscopy on metallographic cross sections. Additionally, the resulting crack patterns were imaged in 3D by means of x-ray microtomography. © 2017, ASM International.
    view abstract10.1007/s11666-017-0673-z
  • In situ analysis of damage evolution in an Al/ Al 2O 3 MMC under tensile load by synchrotron X-ray refraction imaging
    Nellesen, J. and Laquai, R. and Müller, B.R. and Kupsch, A. and Hentschel, M.P. and Anar, N.B. and Soppa, E. and Tillmann, W. and Bruno, G.
    Journal of Materials Science 53 (2018)
    The in situ analysis of the damage evolution in a metal matrix composite (MMC) using synchrotron X-ray refraction radiography (SXRR) is presented. The investigated material is an Al alloy (6061)/10 vol% Al 2O 3 MMC after T6 heat treatment. In an interrupted tensile test the gauge section of dog bone-shaped specimens is imaged in different states of tensile loading. On the basis of the SXRR images, the relative change of the specific surface (proportional to the amount of damage) in the course of tensile loading was analyzed. It could be shown that the damage can be detected by SXRR already at a stage of tensile loading, in which no observation of damage is possible with radiographic absorption-based imaging methods. Moreover, the quantitative analysis of the SXRR images reveals that the amount of damage increases homogeneously by an average of 25% with respect to the initial state. To corroborate the experimental findings, the damage distribution was imaged in 3D after the final tensile loading by synchrotron X-ray refraction computed tomography (SXRCT) and absorption-based synchrotron X-ray computed tomography (SXCT). It could be evidenced that defects and damages cause pronounced indications in the SXRCT images. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s10853-017-1957-x
  • In situ analysis utilizing electrical resistance measurements when brazing cemented carbides
    Tillmann, W. and Sievers, N. and Schmidt, A. and Timmer, C.
    Materialpruefung/Materials Testing 60 (2018)
    Usually it is not possible to observe a high temperature brazing process such as joining cemented carbides to steel with copper as filler metal. The process takes place in a vacuum chamber at high temperatures and an analysis of the joint formed is only possible in a subsequent step. In this investigation, a novel setup has been introduced to allow for an in situ measurement of the brazing process via its electrical resistance. For this, a 4wiretechnique has been applied to the specimen to precisely measure the low resistance of the joint within a range of µΩ. By choosing two different holding times at a process temperature of 1150 °C and by analyzing the ensuing joint structures, it was possible to correlate the signal measured with the on-going diffusion and bonding process. Finally, the authors have revealed that the applied current of the measurement system had a distinct effect on the joint structure formed, which could be used additionally to improve the quality of the joint. © Carl Hanser Verlag, München Materials Testing.
    view abstract10.3139/120.111165
  • Influence of condensate corrosion on tensile and fatigue properties of brazed stainless steel joints AISI 304L/BNi-2 for automotive exhaust systems [Einfluss von Kondensatkorrosion auf die Zug- und Ermüdungseigenschaften der Edelstahllötverbindungen 1.4307/Ni 620 für Automobilabgassysteme]
    Schmiedt, A. and Manka, M. and Tillmann, W. and Walther, F.
    Materialwissenschaft und Werkstofftechnik 49 (2018)
    Materials and brazed joints for automotive exhaust systems have to resist the corrosive nature of aggressive exhaust gases as well as static and cyclic loads. In the present study, the influence of condensate corrosion according to VDA 230–214, with an ageing duration of up to 6 weeks, on the tensile and fatigue properties of stainless steel AISI 304L and brazed AISI 304L/BNi-2 joints is investigated. In relation to the as-received condition, the ultimate tensile strength is decreased down to 58 % and a reduction of the fatigue strength at 2⋅106 cycles down to 22 % is determined for brazed specimens, pre-corroded for 6 weeks. In contrast to the brazed stainless steel joints, the condensate corrosion does not influence the tensile properties of the AISI 304L base material. Stress concentrations at the corrosion-dependent circumferential grooves at the brazing seam are evaluated by stress intensity factors, which are well appropriate to characterise the fatigue behaviour depending on the corrosion condition. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201800007
  • Influence of Cr-Content on the thermoelectric and mechanical properties of NiCr thin film thermocouples synthesized on thermally sprayed Al2O3
    Tillmann, W. and Kokalj, D. and Stangier, D. and Schöppner, V. and Benis, H.B. and Malatyali, H.
    Thin Solid Films 663 (2018)
    Ni/Ni-Cr thin film thermocouples were deposited by means of DC magnetron sputtering for the application in plastic extrusion dies. In order to ensure the application of the thermocouples on complex surfaces, a thermally sprayed Al2O3 layer was used between the substrate and the thin films for thermal insulation and electrical isolation. The study includes the deposition, characterization, calibration, and validation of thin film thermocouples. The influence of the composition (10 at.-%, 20 at.-% and 30 at.-% Cr) was analyzed concerning mechanical and electrical properties. It was found out that the electrical resistance increases with the temperature and the Cr-content due to a decreasing crystallite size. By means of a scratch-tester and nanoindentation, a higher scratch load and hardness were observed for larger Cr-contents on the thermally sprayed Al2O3 coating. Despite the composition, the influence of the size as well as the thickness of the thermocouples were analyzed regarding the Seebeck coefficient. Independent from the size and Cr-content, all thermocouples feature a comparable Seebeck coefficient of about 40 μV/K. This high coefficient is similar to that of wire thermocouples, showing a low number of defects in the thin films. The ceramic coating features a thermal insulation of about 20 °C at 200 °C, allowing a more accurate measurement of the ambient medium. © 2018 Elsevier B.V.
    view abstract10.1016/j.tsf.2018.08.023
  • Influence of interlayer on the properties of Kovar / DM308 laser welded joint [中间层对可伐合金4J29/钼组玻璃DM308激光焊接接头结合性能的影响]
    Jia, L. and Li, Z. and Li, H. and Tillmann, W.
    Hanjie Xuebao/Transactions of the China Welding Institution 39 (2018)
    Kovar alloy and DM308 glass were bonded with YLS-6000 fiber laser. The effect of interlayer on the strength, interface structure and interface bonding mechanism was investigated. The diffusion behavior of interface elements is analyzed. The results showed that the Mo-Mn-Ni interlayer can reduce the cracks at the edge of the joint,and Ni2O3-MnO2-B2O3 interlayer can reduce the number of micro cracks and the number of bubbles on the glass side. The weakest part of the 4J29 / DM308 laser welding joint is near the glass side, which breaks through the bubble. The biggest shear strength of Mo-Mn-Ni interlayer joints is 10.96 MPa. The biggest shear strength of Ni2O3-MnO2-B2O3interlayer joints is 13.46 MPa. The thickness of Ni2O3-MnO2-B2O3 interlayer transition layer is about 30-40 μm. There is obvious dendrite growth in the transition layer. The results of joint interface XRD phase analysis show that the interface transition layer is FeSiO3 and Fe7SiO10. It is belong to Fe and Si cubic complex oxides. The results of EDS analysis showed that Fe, Co, Ni, Al, Na and Si elements had diffused in the whole interface region. The interfacial bonding mainly depends on chemical reaction and element diffusion. © 2018, Editorial Board of Transactions of the China Welding Institution, Magazine Agency Welding. All right reserved.
    view abstract10.12073/j.hjxb.2018390268
  • Investigation on the oxidation behavior of AlCrVxN thin films by means of synchrotron radiation and influence on the high temperature friction
    Tillmann, W. and Kokalj, D. and Stangier, D. and Paulus, M. and Sternemann, C. and Tolan, M.
    Applied Surface Science 427 (2018)
    Friction minimization is an important topic which is pursued in research and industry. In addition to the use of lubricants, friction-reducing oxide phases can be utilized which occur during. These oxides are called Magnéli phases and especially vanadium oxides exhibit good friction reducing properties. Thereby, the lubrication effect can be traced back to oxygen deficiencies. AlCrN thin films are being used as coatings for tools which have to withstand high temperatures. A further improvement of AlCrN thin films concerning their friction properties is possible by incorporation of vanadium. This study analyzes the temperature dependent oxidation behavior of magnetron sputtered AlCrVN thin films with different vanadium contents up to 13.5 at.-% by means of X-ray diffraction and X-ray absorption near-edge spectroscopy. Up to 400 °C the coatings show no oxidation. A higher temperature of 700 °C leads to an oxidation and formation of Magnéli phases of the coatings with vanadium contents above 10.7 at.-%. Friction coefficients, measured by ball-on-disk test are correlated with the oxide formation in order to figure out the effect of vanadium oxides. At 700 °C a decrease of the friction coefficient with increasing vanadium content can be observed, due to the formation of VO2, V2O3 and the Magnéli phase V4O7. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.09.029
  • Microstructural Characteristics and Residual Stresses in Arc-Sprayed Cermet Coatings Using Different Carbide Grain Size Fractions
    Tillmann, W. and Hagen, L. and Luo, W. and Chehreh, A.B.
    Journal of Thermal Spray Technology 27 (2018)
    Different studies have emphasized the technological relevance of residual stresses in engineered surfaces, such as thermally sprayed coatings, and their effect on the fracture and fatigue behavior. In arc-sprayed coatings, the microstructural characteristics and resulting residual stresses are determined primarily by the inherent process characteristics and feedstock material. With the scope of this work, a study on the residual stress field in coatings formed by an arc spraying process for different electric parameter settings has been carried out using different wire configurations. Thus, iron-based cored wires with different grain-sized tungsten carbides as filler material were used as feedstock. The coatings are mainly composed of eutectic carbides, and eta carbides such as M6C, M12C, or M23C and some iron-rich oxide phases, as well as characterized by an inhomogeneous, lamellar microstructure. The results demonstrated that the magnitude of the residual stresses in the coating depends on the carbide grain size fraction used as filling for cored wires. A smaller carbide grain size leads to a more pronounced dissolution of eutectic carbides, resulting in the formation of eta carbides, which in turn is accompanied by decreased tensile residual stresses across the coating. With respect to the spray parameter settings, reduced tensile residual stresses are observed when an increased voltage is applied, which can be attributed to phase evolution phenomena during spraying and thermal effects on the substrate-coating system. © 2018, ASM International.
    view abstract10.1007/s11666-018-0745-8
  • Optimization of the deposition parameters of Ni-20Cr thin films on thermally sprayed Al2O3 for sensor application
    Tillmann, W. and Kokalj, D. and Stangier, D.
    Surface and Coatings Technology 344 (2018)
    Ni-Cr (80–20 at.-%) thin films were synthesized by means of DC magnetron sputtering on thermally sprayed Al2O3 insolating coatings. In view of the application as a thin-film thermocouple, the ceramic layer provides an electrical as well a thermal insulation. The thin film must feature a good adhesion as well as a good electrical conductivity to be suitable as a thin-film thermocouple. Design of Experiments was utilized to determine if deposition parameters including the bias-voltage, heating power, and chamber pressure have a significant influence regarding critical scratch load, hardness to Young's modulus ratio, and electrical resistance. It was found out that the bias-voltage is the most important influencing factor. In order to extract the significant parameters, the morphology and topography were analyzed by means of SEM. The structural properties were obtained by means of XRD on an atomic level, using synchrotron radiation. Mechanical properties, such as the hardness and Young's modulus, as well the scratch load were evaluated using nanoindentation and a scratch-tester. To verify the findings, an optimized thin film was deposited, which showed an increased layer adhesion and a significantly lower electrical resistance. © 2018 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2018.03.029
  • Research Progress on the Effect of Welding Process on Weld Geometry and Joint Properties of Stainless Steel Sheet [新型焊接工艺对不锈钢薄板焊缝成形及接头性能影响的研究进展]
    Li, Z. and Wang, N. and Tillmann, W.
    Materials China 37 (2018)
    Stainless steel sheet, as one kind of energy-saving materials with superior performance, has been widely applied to national production, and has broad application prospects. Based on the importance of stainless steel sheet, studying the welding process is of great significance to promote the development of stainless steel sheet industry. In this paper, the effect of welding process on weld geometry and joint properties of stainless steel sheets are reviewed. Weld geometry of laser welding joint is better than that of arc welding joint for stainless steel sheet welding. Good weld geometry can be obtained through appropriate shielding gas method and waveform control. High energy beam welding and friction stir welding can optimize the joint mechanical properties due to grain refinement. The corrosion sensitivity of joint by welding process of low heat input falls off. The shielding gas flow rate and external magnetic field have strong positive effect on corrosion resistance. © 2018, The Editorial Board of Materials China. All right reserved.
    view abstract10.7502/j.issn.1674-3962.2018.03.11
  • Spray pattern analysis in TWAS using photogrammetry and digital image correlation
    Tillmann, W. and Rademacher, H.G. and Hagen, L. and Abdulgader, M. and El Barad'Ei, M.
    IOP Conference Series: Materials Science and Engineering 373 (2018)
    In terms of arc spraying processes, the spray plume characteristic is mainly affected by the flow characteristic of the atomization gas at the nozzle inlet and intersection point of the wire tips, which in turn affect the particle distribution at the moment of impact when molten spray particles splash onto the substrate. With respect to the route of manufacturing of near net-shaped coatings on complex geometries, the acquisition of the spray patterns is pressingly necessary to determine the produced coating thickness. Within the scope of this study, computer fluid dynamics (CFD) simulations were carried out to determine the distribution of spray particles for different spray parameter settings. The results were evaluated by three-dimensional spray spot analyses using an optical measurement based on photogrammetry and digital image correlation. The optical measurement represents a promising and much faster candidate to measure spray patterns compared to the tactile measurement system but with an equal accuracy. For given nozzle configurations and spray parameter settings, numerous spray patterns were examined to their shape factors, demonstrating the potential of an online analysis, which encompasses a "fast sample loop" and a data processing system to generate a three-dimensional surface of the spray spot profile. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/373/1/012014
  • Tensile and fatigue assessments of brazed stainless steel joints using digital image correlation
    Schmiedt, A. and Jaquet, S. and Manka, M. and Tillmann, W. and Walther, F.
    MATEC Web of Conferences 165 (2018)
    For mechanical tests of brazed stainless steel joints, the local deformation behaviour within the small area of the brazing seam is a major concern, because local strains cannot be detected with standard mechanical extensometers. The current study allows a fundamental comprehension of the gauge length influence on the strain measurements of brazed joints with smooth and notch-containing surfaces, under quasi-static and cyclic loadings. Therefore, the optical measurement technique of digital image correlation (DIC) is used within tensile and fatigue tests of brazed AISI 304L/BAu-4 joints in an as-received and pre-corroded condition. A triggered image acquisition of the DIC system is successfully applied to evaluate the local ratcheting fatigue behaviour in the area of the brazing seam at a frequency of 10 Hz. The gauge length influence, analysed in the range of 0.5 to 12.5 mm, is more pronounced with increasing tensile and fatigue stresses and is significantly enhanced for notch-containing surfaces. Instrumented load increase tests with strain, electrical, magnetic and temperature measuring techniques have proven to be appropriate to estimate fatigue properties of the brazed joints with a deviation of 4%. Fatigue and corrosion fatigue damage mechanisms are evaluated by using scanning electron microscopy with secondary and back-scattered electron detectors. © The Authors, published by EDP Sciences, 2018.
    view abstract10.1051/matecconf/201816506003
  • Tribo-mechanical properties of CrC/a-C thin films sequentially deposited by HiPIMS and mfMS
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Surface and Coatings Technology 335 (2018)
    A sequence of high power impulse magnetron sputtering (HiPIMS) and mid-frequency magnetron sputtering (mfMS) was carried out to deposit a chromium carbide (CrC) interlayer and an amorphous carbon (a-C) top layer. The deposition of the interlayer by HiPIMS results in a higher adhesion strength and hence affects the properties of the a-C layer. The mechanical and tribological properties of a group of CrC/a-C films consisting of CrC with growing C content are investigated. In this context, single CrC and CrC/a-C films were systematically analyzed in order to evaluate the influence of the CrC interlayer on the properties of CrC/a-C. A higher amount of C changes the morphology of CrC films from a columnar to fully dense microstructure. The hardness decreases from 13.8 to 12.3 GPa with a growing C content, but the H/E- and H3/E2-ratios increase to 0.073 and 0.068 GPa, respectively. In contrast to the CrC interlayers, the CrC/a-C film systems are marked by a higher hardness of up to 19.8 GPa. The H/E- and H3/E2-ratios are also significantly higher with values of 0.090 and 0.155 GPa when compared to the CrC single films. The CrC layers exhibit the best adhesion class HF1 in Rockwell tests and a maximum critical load Lc3 of 41 N in scratch tests. The adhesion strength of CrC/a-C is strongly affected by the CrC interlayers; as generally similar failure mechanisms are observed for both film systems. The friction behavior of the CrC/a-C films is only influenced by the a-C top layer. The high adhesion strength of CrC prevents delamination failures when tribologically loading CrC/a-C films. © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.12.035
  • Vacuum brazing of 316L stainless steel based on additively manufactured and conventional material grades
    Tillmann, W. and Henning, T. and Wojarski, L.
    IOP Conference Series: Materials Science and Engineering 373 (2018)
    Many industrial applications require components with an increasing geometric complexity and specific material properties. Furthermore, the production costs and the affordable production time have to be minimized in order to ensure competitiveness. These divergent objectives are difficult to achieve with a single manufacturing technology. Therefore, joining of selective laser melted (SLM) complex shaped parts to conventionally produced high-volume components provides a high potential. The current investigation focuses on vacuum brazing conventionally manufactured to non-hipped SLM generated AISI 316L stainless steel. Cylindrical samples (o 14 mm) were brazed using a B-Ni2 foil (50 μm) at 1050 °C for 30 minutes in vacuum (&lt; 4.5•10-5 mbar) and directly cooled down to room temperature with 4 bar overpressure to prevent the formation of chromium carbides within the base material. It could be proven that the brazing quality is extremely sensitive to even marginal porosities (&lt; 0.2 %) and/or oxide inclusions of the SLM microstructure. Therefore, the fracture mirror in SLM/conventional steel brazements was at the joint braze/SLM steel interface, leading to a joint strength of 317.4 MPa. This corresponds to only 67.4 % of the joint strength obtained with conventional steel, where the fracture propagated through the diffusion area. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/373/1/012023
  • A Study on the Tribological Behavior of Vanadium-Doped Arc Sprayed Coatings
    Tillmann, W. and Hagen, L. and Kokalj, D. and Paulus, M. and Tolan, M.
    Journal of Thermal Spray Technology 26 (2017)
    The formation of thin reactive films in sliding contacts under elevated temperature provides enhanced tribological properties since the formation of Magnéli phases leads to the ability of self-lubricating behavior. This phenomenon was studied for vanadium-doped coating systems which were produced using CVD and PVD technology. Vanadium-containing arc sprayed coatings were not widely examined so far. The aim of this study was to characterize Fe-V coatings deposited by the Twin Wire Arc Spraying process with respect to their oxidation behavior at elevated temperatures and to correlate the formation of oxides to the tribological properties. Dry sliding experiments were performed in the temperature range between 25 and 750 °C. The Fe-V coating possesses a reduced coefficient of friction and wear coefficient (k) at 650 and 750 °C, which were significant lower when compared to conventional Fe-based coatings. The evolution of oxide phases was identified in situ by x-ray diffraction for the investigated temperature range. Further oxidation of (pre-oxidized) arc sprayed Fe-V coatings, as verified by differential thermal analysis and thermo-gravimetric analysis, starts at about 500 °C. © 2017 ASM International
    view abstract10.1007/s11666-017-0524-y
  • Adjustment of friction by duplex-treated, bionic structures for Sheet-Bulk Metal Forming
    Tillmann, W. and Stangier, D. and Lopes-Dias, N.-F. and Biermann, D. and Krebs, E.
    Tribology International 111 (2017)
    Bionic structures were applied on hardened AISI M3:2 and duplex-treated by means of plasmanitriding. Subsequently, a thin hard CrAlN coating was deposited on the substrate. The tribological properties of the structure systems were investigated in the full contact area, using identical friction partners (steel DP 600) of the forming process. The tribological behavior was analyzed in fundamental laboratory tests, using a modified tribometer and subsequently evaluated in application-orientated adapted ring-compression tests. Bionic structures are suitable to locally adjust the friction condition and proved to be an appropriate method to control the material flow of sheet metals in bulk forming operations. However, process related micro burrs lead to a roughness increase during subsequent treatments, thus increasing the friction and adhesive effects. © 2017 Elsevier Ltd
    view abstract10.1016/j.triboint.2017.02.037
  • Analysis of the ductility dip cracking in the nickel-base alloy 617mod
    Eilers, A. and Nellesen, J. and Zielke, R. and Tillmann, W.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    While testing steam leading power plant components made of the nickel-base alloy A617mod at elevated temperatures (700 °C), ductility dip cracking (DDC) was observed in welding seams and their surroundings. In order to clarify the mechanism of crack formation, investigations were carried out on welded specimens made of A617mod. Interrupted tensile tests were performed on tensile specimens taken from the area of the welding seam. To simulate the conditions, the tensile tests were conducted at a temperature of 700 °C and with a low strain rate. Local strain fields at grain boundaries and inside single grains were determined at different deformation states by means of two-dimensional digital image correlation (DIC). Besides the strain fields, local hardnesses (nanoindentation), energy dispersive X-Ray spectroscopy (EDX), and electron backscatter diffraction (EBSD) measurements were performed. Besides information concerning the grain orientation, the EBSD measurement provides information on the coincidence site lattice (CSL) at grain boundaries as well as the Schmid factor of single grains. All results of the analysis methods mentioned above were correlated and compared to each other and related to the crack formation. Among other things, correlations between strain fields and Schmid factors were determined. The investigations show that the following influences affect the crack formation: orientation of the grain boundaries to the direction of the loading, the orientation of the grains to each other (CSL), and grain boundary sliding. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/181/1/012020
  • Duplex stainless steel fabricated by selective laser melting - Microstructural and mechanical properties
    Hengsbach, F. and Koppa, P. and Duschik, K. and Holzweissig, M.J. and Burns, M. and Nellesen, J. and Tillmann, W. and Tröster, T. and Hoyer, K.-P. and Schaper, M.
    Materials and Design 133 (2017)
    In the scope of the present study, microstructural and mechanical characterizations of duplex stainless steel UNS S31803 processed by selective laser melting (SLM) are conducted. The findings shed light on the phase arrangement evolving in the as-built condition and in several heat-treated conditions. In the as-built condition, austenite formation is almost suppressed due to process-related high cooling rates. Therefore, several heat treatments ranging from 900 °C to 1200 °C for 5 min each were performed in order to adjust to the desired austenitic-ferritic microstructure. Results generated by transmission electron microscopy (TEM) reveal a high dislocation density induced during SLM fabrication, such that a recrystallized microstructure prevails after the heat treatment. Tensile tests display the severe impact of the heat treatment on the resulting mechanical response. The nearly complete ferritic as-built specimens obtain a higher ultimate tensile strength and a reduced elongation at fracture compared to the heat-treated specimens. © 2017 Elsevier Ltd
    view abstract10.1016/j.matdes.2017.07.046
  • Effect of flux-cored ingredients and welding parameters on arc stability of ENiCrMo3T0-4 wire
    Wang, H. and Li, Z. and Li, G. and Tillmann, W. and Kim, H.J.
    Hanjie Xuebao/Transactions of the China Welding Institution 38 (2017)
    The effect of flux-cored ingredients and welding parameters on arc stability of ENiCrMo3T0-4 flux-cored wires were investigated by Hanover Analyzer. It was found that with the ratio of CaO/(SiO2+TiO2) decreased in flux-cored ingredients, the lower the basicity of melt slag were, the better the welding arc stability were. Compared with standard deviation of voltage and current, coefficient of voltage, current and burning time variability, standard deviation of short-circuiting and burning time, coefficient of short-circuiting time variability can reflect welding arc stability more accurately. In the range of normal welding process parameters, the arc stability became more stable with the welding parameters increase. © 2017, Editorial Board of Transactions of the China Welding Institution, Magazine Agency Welding. All right reserved.
    view abstract10.12073/j.hjxb.20160412003
  • Embedment of eutectic tungsten carbides in arc sprayed steel coatings
    Tillmann, W. and Hagen, L. and Kokalj, D.
    Surface and Coatings Technology 331 (2017)
    Tungsten carbide reinforced deposits have already evolved into a predominant coating system in order to protect stressed surfaces against wear. Among thermal spraying processes, due to a high deposition rate, arc spraying is a promising process to manufacture cost-saving, wear resistant coatings. However, inherent process characteristics prevailing in arc spraying as well as the utilization of tungsten carbides, as a filling for cored wires, could lead to undesirable phase evolutions, which in turn provoke the degradation of the mechanical properties. The embedment of tungsten carbides into the surrounding metallic matrix is affected by metallurgical interactions with molten spray particles. Within the scope of this study, an external injection of tungsten carbides was applied in order to analyze the embedment of tungsten carbides in arc sprayed low alloyed steel. Accordingly, metallographic investigations were carried out, which address the reactive layer at the interface of embedded tungsten carbides to the surrounded iron-based matrix. Microstructural characteristics such as mechanical properties and phase composition were scrutinized by means of nanoindentation, energy dispersive X-ray spectroscopy, and X-ray diffraction, respectively. It was found that the embedment of tungsten carbides, which have been externally injected into the arc burning zone, differs from that obtained from deposits produced with the use of cored wire with tungsten carbide as filling. Thus, externally injected tungsten carbides are less inclined to form eta carbides due to dissolution, which again results in differences in the mechanical properties across the reactive layer. © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.10.044
  • Feasibility study of fluxless brazing cemented carbides to steel
    Tillmann, W. and Sievers, N.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    One of the most important brazing processes is the joints between cemented carbides and steel for the tool industry such as in rotary drill hammers or saw blades. Even though this technique has already been used for several decades, defects in the joint can still occur and lead to quality loss. Mostly, the joining process is facilitated by induction heating and the use of a flux to enhance the wetting of the filler alloy on the surface of the steel and cemented carbide in an ambient atmosphere. However, although the use of flux enables successful joining, it also generates voids within the joint, which reduces the strength of the connection while the chemicals within the flux are toxic and polluting. In this feasibility study, a fluxless brazing process is used to examine the joint between cemented carbides and steel for the first time. For this, ultrasound is applied during induction heating to enable the wetting between the liquid filler metal and the surfaces of the cemented carbide and steel. The ultrasound generates cavitations within the liquid filler metal, which remove the oxides from the surface. Several filler metals such as a silver based alloy Ag449, pure Zn, and an AlSi-alloy were used to reduce the brazing temperature and to lower the thermal residual stresses within the joint. As a result, every filler metal successfully wetted both materials and led to a dense connection. The ultrasound has to be applied carefully to prevent a damage of the cemented carbide. In this regard, it was observed that single grains of the cemented carbide broke out and remained in the joint. This positive result of brazing cemented carbides to steel without a flux but using ultrasound, allows future studies to focus on the shear strength of these joints as well as the behavior of the thermally induced residual stresses. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/181/1/012007
  • FEM study of analyzing the electrical resistance of brazed joint by the 4-wire technique for quality assurance
    Tillmann, W. and Sievers, N. and Henning, T. and Jakimenko, D.
    Measurement: Journal of the International Measurement Confederation 104 (2017)
    The validation of a brazed work piece requires lot of operating experiences in order to assure a sufficient joint quality for a reliable manufacturing process. In most cases, a quality assurance is only performed by destructive methods to measure the strength of the joint. Conventional non-destructive testing methods such as computer tomography, ultrasonic inspection, and magnetic particle inspection as well as other available optical and thermal methods are commonly not suitable for brazing applications in mass production. This is partly due to the fact that these methods are very expensive or even highly limited when it comes to the detection of critical imperfections. Hence, there is a high demand to investigate alternative non-destructive testing methods. The main focus of this feasibility study is the examination of the electrical resistance behavior of the brazed joint. In this regard, different positions of defects within the filler metal as well as different contact positions for the current supply and voltage measurement of the 4-wire technique are investigated by means of FEM-analyses. A key aspect is the evaluation of the particular voltage distribution within the brazed work pieces as a function of these variations and to generate significant potential differences for the measurement of brazed joints. The results will contribute the design of the resistance measurements on various component geometries to enable a precise quality inspection. © 2017
    view abstract10.1016/j.measurement.2017.03.015
  • High-quality cutting edge preparation of micromilling tools using wet abrasive jet machining process
    Krebs, E. and Wolf, M. and Biermann, D. and Tillmann, W. and Stangier, D.
    Production Engineering (2017)
    The cutting edge preparation is a common process in the production chain of cemented carbide macro tools. It is used to reduce failures resulting from grinding and to generate a specific cutting edge geometry that is appropriate for the application of the cutting tool. The adhesion of a subsequently applied coating is also increased due to the rounded and more regular shape of a prepared cutting edge. Even though cutting edge preparation is able to significantly increase the life of macro tools, it is not state of the art in the production of micro tools since common preparation processes have not been developed and established for this case of application. Within the investigations, the feasibility of the wet abrasive jet machining process for the preparation of micromilling tools is analysed. For this purpose, the preparation process is refined which allows an effective reduction of the defects and a successful adjustment of different rounding sizes of the cutting edge in a relatively short preparation time. In addition, a high-quality statistical model is achieved to describe the interdependency of the process parameters. In conclusion, TiAlN layers are applied on the rounded cutting edges by a PVD-process without obstructive droplets. © 2017 German Academic Society for Production Engineering (WGP)
    view abstract10.1007/s11740-017-0787-7
  • Hot isostatic pressing of IN718 components manufactured by selective laser melting
    Tillmann, W. and Schaak, C. and Nellesen, J. and Schaper, M. and Aydinöz, M.E. and Hoyer, K.-P.
    Additive Manufacturing 13 (2017)
    Selective laser melting and other additive manufacturing (AM) techniques have recently attracted substantial interest of both researchers and the processing industry. The freedom of design leads to completely new possibilities for constructions and, thus, to entirely new products. In the selective laser melting (SLM) process, the components are produced layer-wise using a laser beam. SLM is a powder bed based AM process and is characterized by the complete melting of the utilized powder material. Employing SLM, complex three-dimensional parts and light weight structures can be produced directly from 3D CAD data. However, although SLM is a very promising technology, there are still challenges to solve. In the present study, a close look is taken at the porosity. Under cyclic loading, pores can act as stress raisers and lead to premature crack initiations, which reduce the fatigue strength of the material. Hot isostatic pressing (HIP) offers the possibility to reduce the porosity. HIP combines high pressure and high temperature to produce materials with superior properties. The influence of the HIP process parameters on the density and microstructure of IN718 SLM components is investigated by means of micro X-ray computed tomography and scanning electron microscopy. The results of the experiments show that the majority of pores can be densified by means of HIP. On the other hand, some pores cannot be densified. The reason for this is seen in entrapped argon gas from the SLM process. © 2016 Elsevier B.V.
    view abstract10.1016/j.addma.2016.11.006
  • Improvement strategies for the formfilling in incremental gear forming processes
    Sieczkarek, P. and Wernicke, S. and Gies, S. and Tekkaya, A.E. and Krebs, E. and Wiederkehr, P. and Biermann, D. and Tillmann, W. and Stangier, D.
    Production Engineering 11 (2017)
    Incremental Sheet-Bulk Metal Forming offers an innovative and flexible approach for the manufacturing of gears. An insufficient formfilling of the generated gearing, especially of the first tooth formed, is observed. Aiming for a formfilling improvement of the first tooth element, three influencing factors were investigated. First, the prevailing friction is analyzed and a possibility for its adjustment is offered by a tailored adaption of the tool surface topographies. These were manufactured by micromilling, EDM and polishing processes and partially covered by CrAlN PVD-coatings. Based on ring-compression tests, which were performed to determine the resulting friction conditions, the analyzed topographies were transferred onto real tool surfaces and used in the incremental gear forming process. Second, the influence on the formfilling of the blank cutting process and the resulting sheet edge properties were investigated. The third aspect to enhance the formfilling of the gear elements was the modification of the process strategy of the incremental forming process. Due to different conditions for the initial and the following indentations, a preforming operation was investigated in order to realize a similar material flow for all indentations. With the combination of the best parameters regarding the tool surface, the blank cutting process and the forming strategy, an improvement of the formfilling of the first formed gear element by up to 33% and for the following gears by up to 13% was achieved. © 2017 German Academic Society for Production Engineering (WGP)
    view abstract10.1007/s11740-017-0764-1
  • Influence of bias voltage and sputter mode on the coating properties of TiAlSiN: Einfluss der Biasspannung und des Sputtermodus auf die Schichteigenschaften von TiAlSiN
    Tillmann, W. and Dildrop, M.
    Materialwissenschaft und Werkstofftechnik 48 (2017)
    Silicon offers promising opportunities to improve the characteristics of thin coatings. By adding silicon to TiAlN, the oxidation resistance as well as the tribological properties can be increased and improved. To analyze the influence of the silicon content on the coating properties of TiAlSiN, it is necessary to keep the ratio of the other coating elements constant by using the right target configuration. Within this study, TiAlSiN coatings were deposited on hot work steel AISI H11 by using magnetron sputtering (Cemecon CC800/9 sinox ML). This steel was previously plasma nitrided to increase the hardness and hence the carrying load of the substrate, avoiding shell egg effect during the analysis. Different sputter modes were used to analyze the possibility to produce TiAlSiN by utilizing a pure low conductive silicon target. The bias voltages were systematically varied to see their influence on the structure and chemical compositions of the coating which were investigated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Furthermore, the roughness of the surface of the coatings was measured by an optical three-dimensional surface analyzer. The results of this study serve as a basis for further investigations regarding the variation of the silicon content of TiAlSiN coatings. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201600731
  • Influence of PVD-duplex-treated, Bionic Surface Structures on the Wetting Behavior for Sheet-Bulk Metal Forming Tools
    Tillmann, W. and Stangier, D. and Lopes Dias, N.F.
    Journal of Bionic Engineering 14 (2017)
    Bionic surface structures, inspired by the flora, were developed for Sheet-Bulk Metal Forming (SBMF) in order to locally control the friction condition by adjusting the wetting behavior. Five bionic structures were micromilled on ASP®2023 in annealed as well as hardened and tempered conditions. Subsequently, the structured surfaces were plasma-nitrided and coated with a CrAlN thin film. The influence of the treatment method on the structural geometry was investigated with the aid of a scanning electron microscope and 3D-profilometer. The wetting behaviors of water and deep drawing oil (Berufluid ST6007) on bionic surfaces were evaluated using contact angle measurements. The resulting micro-milled structures exhibit an almost identical shape as their bionic models. However, the roughness of the structured surfaces is influenced by the microstructure. The combination of plasma-nitriding and Physical Vapor Deposition (PVD) leads to an increase in roughness. All bionic structures possess higher contact angles than that of the unstructured surfaces when wetted by water. This can be explained by the fact that the structural elevations block the spreading. When the bionic surfaces are wetted by deep drawing oil, the lubricant spreads in the structural cavities, leading to smaller contact angles. Furthermore, the anisotropy of the structure has an influence on the wetting behavior. © 2017 Jilin University
    view abstract10.1016/S1672-6529(16)60418-3
  • Influence of Si content on mechanical and tribological properties of TiAlSiN PVD coatings at elevated temperatures
    Tillmann, W. and Dildrop, M.
    Surface and Coatings Technology 321 (2017)
    TiN- and CrN-based binary or ternary coatings have been used for many years in order to extend the service life of machining tools. The increasing demands in the metalworking industry require more efficient coating systems. According to recent studies, silicon offers promising opportunities to positively influence the characteristics of thin titanium or chromium-based coatings. The nanocomposite TiAlSiN presents a high hardness and a fine grain structure. Furthermore, by adding silicon, the oxidation resistance as well as the tribological properties can be increased and improved. In this study, TiAlSiN coatings with different Si contents (0–10.9 at%) were produced by means of magnetron sputtering. In order to test the possibility to sputter pure, low conductive silicon targets, different sputter and bias modes were tested. The ratios of the other coating elements were kept constant while varying the silicon content inside the PVD coatings. Nitrided steel samples (AISI H11) were used as substrate materials. The influence of the Si content on the tribomechanical properties of TiAlSiN were analyzed. The analyses focused on the coatings with a silicon content of 5–10 at% due to the change of the coating morphology within this range. The coating morphology and different chemical compositions of the silicon-doped coatings were investigated by means of scanning electron microscopy and EDX analyses. Phase analyses were conducted and residual stresses were measured by means of X-ray diffraction. The hardness and Young's modulus of the PVD coatings were investigated using nanoindentation. Furthermore, scratch tests were performed in order to characterize the adhesion between the substrate and the coating. Finally, high temperature tribometer tests were executed to determine the wear resistance of the TiAlSiN coatings at room temperature as well as at elevated temperatures (500 °C, 800 °C). © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.05.014
  • Influence of the WC grain size on the properties of PVD/HVOF duplex coatings
    Tillmann, W. and Stangier, D. and Hagen, L. and Schröder, P. and Krabiell, M.
    Surface and Coatings Technology 328 (2017)
    Duplex coatings are an innovative approach to further improve cemented carbide surfaces in terms of tribo-mechanical properties by depositing a thin Physical Vapor Deposition (PVD) layer on top of the High Velocity Oxygen Fuel (HVOF) sprayed coating. Within this investigation, AISI M3 steel substrates were primarily coated with three different WC-12Co feedstock powders by means of HVOF spraying. The agglomerate size of all powders was constant (− 45 + 15 μm), the WC grain size was varied from conventional (carbide size 2.5 μm) over fine (carbide size 1 μm) to nanosized (carbide size 0.1 μm) carbides. Subsequently, the samples were polished prior to the deposition of a CrAlN PVD coating by means of magnetron sputtering. The aim is to evaluate the influence of the WC grain size on the CrAlN/WC-12Co duplex coating properties. It was shown that a smaller carbide grain size influences the microstructure of the CrAlN coating and causes a finer crystalline structure. Furthermore, higher compressive residual stresses occur in the duplex interlayer and the top layer when using nanosized carbides. Moreover, nanosized carbides in the HVOF coating are favorable in terms of layer adhesion, which was analyzed by means of HRC indent test and scratch test. Further enhancements are the increase in hardness and the reduction in roughness which was directly influenced by the WC grain size. Considering the obtained results, it is to conclude that a reduction of the interlayer WC grain size improves the performance of CrAlN/WC-12Co duplex coatings. © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.08.064
  • Investigation of the influence of the vanadium content on the high temperature tribo-mechanical properties of DC magnetron sputtered AlCrVN thin films
    Tillmann, W. and Kokalj, D. and Stangier, D. and Paulus, M. and Sternemann, C. and Tolan, M.
    Surface and Coatings Technology 328 (2017)
    The forming of high-strength steels or new aluminum alloys leads to a steady increase of the load of tools and coatings. One approach is to positively influence the manufacturing process by using thin solid films with self-lubricating features, provided by oxides at high temperatures with low decohesion energies. For the purpose of this study, AlCrN provides the matrix, while vanadium oxides are used to enhance the frictional and wear properties. However, it is not yet clear which minimum amount of vanadium has to be incorporated in DC magnetron sputtered AlCrN coatings to improve the tribological behavior. Therefore, in this study, AlCrVN coatings are synthesized with an increasing vanadium content by means of reactive DC magnetron sputtering. Additionally, a vanadium-free AlCrN coating is used as reference for the tribo-mechanical investigations. The coatings were synthesized up to a vanadium content of 13.5 at.-% and no phase change could be detected by means of x-ray diffraction. Moreover, no hexagonal AlN phase, which reduces the mechanical properties and the oxidation resistance, was formed. In contrast to the vanadium-free coating, the hardness of the coatings containing vanadium is slightly reduced. The coating with the smallest vanadium content shows the highest hardness of all analyzed coatings. A heat treatment at 400 °C does not lead to any significant changes with respect to mechanical properties, but at 700 °C hardness, modulus of elasticity and critical load decreased for all coatings, indicating a significant change in mechanical properties. The ball-on-disc test at room-temperature, 400 °C, and 700 °C shows the highest wear coefficient for the coating with the lowest vanadium content, due to the poor adhesion of the coating, although this coating shows the highest H/E-ratio. © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.08.046
  • Investigation on the Tribological Behavior of Arc-Sprayed and Hammer-Peened Coatings Using Tungsten Carbide Cored Wires
    Tillmann, W. and Hagen, L. and Schröder, P.
    Journal of Thermal Spray Technology 26 (2017)
    Due to their outstanding properties, WC-W2C iron-based cermet coatings are widely used in the field of wear protection. Regarding commonly used WC-W2C reinforced coating systems, it has been reported that their tribological behavior is mainly determined by the carbide grain size fraction. Although the manufacturing route for arc-sprayed WC-W2C cermet coatings is in an advanced state, there is still a lack of knowledge concerning the performance of cored wires with tungsten carbides as filling material and their related coating properties when post-treatment processes are used such as machine hammer peening (MHP). A major objective was to characterize WC-W2C FeCMnSi coatings, deposited with different carbide grain size fractions as a filling using cored wires, with respect to their tribological behavior. Moreover, deposits derived from cored wires with a different amount of hard phases are investigated. According to this, polished MHP surfaces are compared to as-sprayed and polished samples by means of metallographic investigations. With the use of ball-on-disk and dry rubber wheel tests, dry sliding and rolling wear effects on a microscopic level are scrutinized. It has been shown that the MHP process leads to a densification of the microstructure formation. For dry sliding experiments, the MHP coatings obtain lower wear resistances, but lower coefficients of friction than the conventional coatings. In view of abrasion tests, the MHP coatings possess an improved wear resistance. Strain hardening effects at the subsurface area were revealed by the mechanical response using nanoindentation. However, the MHP process has caused a cracking of embedded carbides, which favor breakouts, leading to advanced third-body wear. © 2016, ASM International.
    view abstract10.1007/s11666-016-0447-z
  • Progress of Nano-materials in Non-copper Coated Solid Wires
    Li, Z. and Cao, X. and Tillmann, W.
    Beijing Gongye Daxue Xuebao/Journal of Beijing University of Technology 43 (2017)
    Non-copper coated solid wires are the future development trends for gas shielded solid wires. The research progress and application of nano-materials in non-copper coated solid wire were reviewed. It was concluded that the surface nano-coating of non-copper surface with corrosion resistance by increasing self-corrosion potential. The arc stability is improved by refining molten droplets and accelerating transition frequency. The nano-additives surface with coefficient and wear scar diameter of tribo-pair through several lubrication mechanisms on the rubbing interfaces such as physical adsorption or chemical reaction film, “micro ball” and “micropolishing”, thus providing some theoretical guidance to decrease contact tip wear for non-copper coated solid wires. © 2017, Editorial Department of Journal of Beijing University of Technology. All right reserved.
    view abstract10.11936/bjutxb2016110013
  • Research Progress of Laser Bonding Process and Joint Properties of Non-metallic Materials
    Li, Z. and Jia, L. and Li, H. and Tillmann, W. and Li, C.
    Beijing Gongye Daxue Xuebao/Journal of Beijing University of Technology 43 (2017)
    The research progress of the laser beam welding process and the properties of joints between glass, ceramic, polymer, bulk metallic glass (bulk metallic glass, BMG) and metal were reviewed in this paper. The effects of laser welding power, welding speed, defocusing amount and the pressure on the performance of the joint were analyzed. The influence of femtosecond and nanosecond laser on the shear strength of metal-glass joints was compared with each other. The bonding mechanism and key factors influencing the reliable bonding of glass and metal, ceramic and metal & ceramic and polymer were also explored. The cracking problems of the high boron silicon glass on the surface of were solved by adding the intermediate layer. The pre-oxidation of metal surface can promote interface bonding and improve joints performance. At the same time, the weld preheat and the postweld heat treatment were adopted to reduce stress and cracks. © 2017, Editorial Department of Journal of Beijing University of Technology. All right reserved.
    view abstract10.11936/bjutxb2016050063
  • Robot based deposition of WC-Co HVOF coatings on HSS cutting tools as a substitution for solid cemented carbide cutting tools
    Tillmann, W. and Schaak, C. and Biermann, D. and Aßmuth, R. and Goeke, S.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    Cemented carbide (hard metal) cutting tools are the first choice to machine hard materials or to conduct high performance cutting processes. Main advantages of cemented carbide cutting tools are their high wear resistance (hardness) and good high temperature strength. In contrast, cemented carbide cutting tools are characterized by a low toughness and generate higher production costs, especially due to limited resources. Usually, cemented carbide cutting tools are produced by means of powder metallurgical processes. Compared to conventional manufacturing routes, these processes are more expensive and only a limited number of geometries can be realized. Furthermore, post-processing and preparing the cutting edges in order to achieve high performance tools is often required. In the present paper, an alternative method to substitute solid cemented carbide cutting tools is presented. Cutting tools made of conventional high speed steels (HSS) were coated with thick WC-Co (88/12) layers by means of thermal spraying (HVOF). The challenge is to obtain a dense, homogenous, and near-net-shape coating on the flanks and the cutting edge. For this purpose, different coating strategies were realized using an industrial robot. The coating properties were subsequently investigated. After this initial step, the surfaces of the cutting tools were ground and selected cutting edges were prepared by means of wet abrasive jet machining to achieve a smooth and round micro shape. Machining tests were conducted with these coated, ground and prepared cutting tools. The occurring wear phenomena were analyzed and compared to conventional HSS cutting tools. Overall, the results of the experiments proved that the coating withstands mechanical stresses during machining. In the conducted experiments, the coated cutting tools showed less wear than conventional HSS cutting tools. With respect to the initial wear resistance, additional benefits can be obtained by preparing the cutting edge by means of wet abrasive jet machining. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/181/1/012011
  • Self-sharpening-effect of nickel-diamond coatings sprayed by HVOF
    Tillmann, W. and Brinkhoff, A. and Schaak, C. and Zajaczkowski, J.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    The durability of stone working and drilling tools is an increasingly significant requirement in industrial applications. These tools are mainly produced by brazing diamond metal matrix composites inserts to the tool body. These inserts are produced by sintering diamonds and metal powder (e.g. nickel). If the wear is too high, the diamonds will break out of the metal matrix and other diamonds will be uncovered. This effect is called self-sharpening. But diamonds are difficult to handle because of their thermal sensitivity. Due to their high thermal influence, manufacturing costs, and complicate route of manufacturing (first sintering, then brazing), there is a great need for alternative production methods for such tools. One alternative to produce wear-resistant and self-sharpening coatings are thermal spray processes as examined in this paper. An advantage of thermal spray processes is their smaller thermal influence on the diamond, due to the short dwelling time in the flame. To reduce the thermal influence during spraying, nickel coated diamonds were used in the HVOF-process (high velocity oxygen fuel process). The wear resistance was subsequently investigated by means of a standardized ball-on-disc test. Furthermore, a SEM (scanning electron microscope) was used to gain information about the wear-mechanism and the self-sharpening effect of the coating. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/181/1/012016
  • Spray Characteristics and Tribo-Mechanical Properties of High-Velocity Arc-Sprayed WC-W2C Iron-Based Coatings
    Tillmann, W. and Hagen, L. and Kokalj, D.
    Journal of Thermal Spray Technology (2017)
    In terms of arc-sprayed coatings, the lamellar coating microstructure is mainly affected by the atomization behavior of the molten electrode tips. When using compressed air, oxide formations occur during atomization, across the particle-laden spray plume and when the molten droplets splash onto the substrate. Within the scope of this study, the potential of a high-velocity arc-spraying process due to elevated atomization gas pressures and its effect on the spray and coating characteristics was analyzed using a cast tungsten carbide (CTC)-reinforced FeCMnSi cored wire. Since the atomization behavior corresponds with the electrode phenomena, the power spectrum and the droplet formation were observed during spraying. The tribo-mechanical properties of CTC-FeCMnSi coatings were examined in dry sliding experiments and indentation tests. In addition, adhesion tests and metallographic investigations were carried out to analyze the bonding strength, cohesive behavior, and lamellar microstructure. The occurrence of oxide phases was evaluated by x-ray diffraction and electron microscopy. Moreover, the oxygen content was determined by using glow discharge optical emission spectroscopy as well as energy-dispersive x-ray spectroscopy. With respect to elevated atomization gas pressures, a dense microstructure with improved adhesion to the substrate and reduced surface roughness was observed. Dry sliding experiments revealed an advanced wear behavior of specimens, when using above average increased atomization gas pressures. Analytic methods verified the existence of oxide phases, which were generated during spraying. A significant change of the extent and type of oxides, when applying an increased flow rate of the atomization gas, cannot be observed. Besides an enhanced coating quality, the use of increased atomization gas pressure exhibited good process stability. © 2017 ASM International
    view abstract10.1007/s11666-017-0605-y
  • Tailoring microstructure, mechanical and tribological properties of NiTi thin films by controlling in-situ annealing temperature
    Momeni, S. and Biskupek, J. and Tillmann, W.
    Thin Solid Films 628 (2017)
    Magnetron sputtered near equiatomic NiTi thin films were deposited on Si (100) and hot work tool steel substrates. The deposited thin films were in-situ annealed at four different temperatures viz., 80 °C, 305 °C, 425 °C, and 525 °C. The effect of the in-situ annealing temperature on the microstructure of the film, the morphology, as well as mechanical and tribological properties was studied using X-ray diffraction, synchrotron diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, ball-on-disc, scratch test, and three dimensional optical microscopy. The obtained results revealed how the variation of in-situ annealing temperature affects the crystallization, microstructure evolution, as well as mechanical and tribological properties of NiTi thin films. © 2017
    view abstract10.1016/j.tsf.2017.02.052
  • Tribological characteristics of tungsten carbide reinforced arc sprayed coatings using different carbide grain size fractions
    Tillmann, W. and Hagen, L. and Schröder, P.
    Tribology in Industry 39 (2017)
    Tungsten carbide reinforced coatings play an important role in the field of surface engineering to protect stressed surfaces against wear. For thermally sprayed coatings, it is already shown that the tribological properties get mainly determined by the carbide grain size fraction. Within the scope of this study, the tribological characteristics of iron based WC-W2C reinforced arc sprayed coatings deposited using cored wires consisting of different carbide grain size fractions were examined. Microstructural characteristics of the produced coatings were scrutinized using electron microscopy and x-ray diffraction analyses. Ball-on-disk test as well as Taber Abraser and dry sand rubber wheel test were employed to analyze both the dry sliding and the abrasive wear behavior. It was shown that a reduced carbide grain size fraction as filling leads to an enhanced wear resistance against sliding. In terms of the Taber Abraser test, it is also demonstrated that a fine carbide grain size fraction results in an improved wear resistant against abrasion. As opposed to that, a poorer wear resistance was found within the dry sand rubber wheel tests. The findings show that the operating mechanisms for both abrasion tests affect the stressed surface in a different way, leading either to microcutting or microploughing. © 2017 Published by Faculty of Engineering.
    view abstract10.24874/ti.2017.39.02.03
  • Using a standard pin-on-disc tribometer to analyse friction in a metal forming process
    Velkavrh, I. and Lüchinger, M. and Kern, K. and Klien, S. and Ausserer, F. and Voyer, J. and Diem, A. and Schreiner, M. and Tillmann, W.
    Tribology International 114 (2017)
    Tribological conditions in bulk metal forming processes are characterized by high contact pressures and large relative movements at high velocities between the tool and the workpiece. In addition, significant surface enlargements and elevated temperatures can occur. It is very challenging to reproduce such contact conditions on pin-on-disc tribometers. In this study, a method for simulating the contact conditions of a metal forming process on a pin-on disc tribometer is presented. Special test samples have been designed in order to achieve high contact pressures and surface enlargements with a homogeneous distribution of these quantities within the contact zone. It is shown that the test method delivers reproducible results and enables the detection of small changes of the contact conditions. Furthermore, microstructural changes of the real contact surface can also be simulated, which was verified by SEM imaging. © 2017 Elsevier Ltd
    view abstract10.1016/j.triboint.2017.04.052
  • Composite cavitation resistant PVD coatings based on NiTi thin films
    Momeni, S. and Tillmann, W. and Pohl, M.
    Materials and Design 110 (2016)
    As a protective coating, TiCN hard PVD coating was deposited on magnetron sputtered NiTi thin films under various coating architectures. The microstructure, composition, mechanical properties, tribological performance as well as the cavitation resistance of deposited coatings were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nanoindentation, ball–on-disc, scratch test, three dimensional (3D) optical microscopy, and the standard ultrasonic cavitation test (ASTM G 32). The obtained results revealed that under a specific coating architecture, the superelasticity of NiTi can be combined with high hardness and wear resistance of TiCN coatings. As a consequence of the combination of these properties, the composite NiTi based coatings are capable of presenting remarkable cavitation resistance and tribological performance. © 2016 Elsevier Ltd
    view abstract10.1016/j.matdes.2016.08.054
  • Effect of Residual Stress on the Wear Resistance of Thermal Spray Coatings
    Luo, W. and Selvadurai, U. and Tillmann, W.
    Journal of Thermal Spray Technology 25 (2016)
    The wear resistance of thermal spray coatings mainly depends on coating properties such as the microstructure, hardness, and porosity, as well as on the residual stress in the coating. The residual stress is induced by a variety of influences e.g., temperature gradients, difference of the thermal expansion coefficient of the coating/substrate materials, and the geometry of the components. To investigate the residual stress, the impulse excitation technique was employed to measure the Young’s and shear moduli. The residual stress was determined by the hole-drilling method and x-ray diffraction. Pin-on-Disk and Pin-on-Tube tests were used to investigate the wear behavior. After the wear tests, the wear volume was measured by means of a 3D-profilometer. The results show that the value of the residual stress can be modified by varying the coating thickness and the substrate geometry. The compressive stress in the HVOF-sprayed WC-Co coatings has a significant positive influence on the wear resistance whereas the tensile stress has a negative effect. © 2015, ASM International.
    view abstract10.1007/s11666-015-0309-0
  • Experimental and numerical analysis of tribological effective surfaces for forming tools in Sheet-Bulk Metal Forming
    Kersting, P. and Gröbel, D. and Merklein, M. and Sieczkarek, P. and Wernicke, S. and Tekkaya, A.E. and Krebs, E. and Freiburg, D. and Biermann, D. and Weikert, T. and Tremmel, S. and Stangier, D. and Tillmann, W. and Matthias, S. and Reithmeier, E. and Löffler, M. and Beyer, F. and Willner, K.
    Production Engineering 10 (2016)
    Sheet-Bulk Metal Forming (SBMF) allows the manufacture of complex parts with integrated functional form elements, such as teeth and thickened areas. Therefore, bulk forming operations are applied to sheets with initial thicknesses of 2 or 3 mm. The design and functionality of the tools are as important as the process itself. Therefore, the working group "Tools" of the Transregional Collaborative Research Centre on Sheet-Bulk Metal Forming (CRC/TR73) focuses on the optimization of the technical tool design. By varying topographies or applying tailored coatings, the friction behavior is changed to achieve a better form filling and to reduce process forces during the forming operations. In this paper, the potential of different tailored surfaces is validated by simulations and experimental studies. The tribological behavior of 14 surface microstructures is evaluated using a half-space model in order to select structures suitable for application. Those were characterized experimentally by ring-compression and pin-extrusion tests. The determined friction factors were used in a forming simulation to predict the form filling of small cavities in a flow forming operation. Furthermore, special attention is paid to the utilization of the anisotropic behavior of specific structures. The results were validated by an incremental gear forming process. © 2016, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-015-0651-6
  • Influence of Machine Hammer Peening on the Tribological Behavior and the Residual Stresses of Wear Resistant Thermally Sprayed Coatings
    Rausch, S. and Wiederkehr, P. and Biermann, D. and Zabel, A. and Selvadurai, U. and Hagen, L. and Tillmann, W.
    Procedia CIRP 45 (2016)
    WC-W2C iron based cermet coatings are widely used in the field of wear protection. In surface engineering, machine hammer peening (MHP) is a novel surface treatment technology, which enhances the surface properties, especially for surfaces in tribological contact. In this study, the wear behavior of peened WC-W2C FeCrCMnSi arc sprayed coatings is characterized and compared to conventional coatings under as-sprayed conditions. The resulting strain hardening effects were measured by mechanical response using nanoindentation. In addition, residual stresses at the surfaces were determined using X-ray diffraction and the sin2ψ method. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.
    view abstract10.1016/j.procir.2016.02.059
  • Influence of PVD-coating technology and pretreatments on residual stresses for sheet-bulk metal forming tools
    Tillmann, W. and Stangier, D. and Denkena, B. and Grove, T. and Lucas, H.
    Production Engineering 10 (2016)
    Residual stresses in the substrate material are significantly influencing the performance of PVD-coated parts and tools which are exposed to high forces. Especially for forming operations, such as sheet-bulk metal forming, during which normal contact pressures of 1.4 GPa can occur, the reduction of friction and, at the same time, the wear protection by means of thin Cr-based coatings are essential. To ensure a long service life of forming tool and tool coating, each step of the substrate pre-treatment, as well as the magnetron-sputtering process, has to be coordinated and compatible. Therefore, polished as well as nitrided samples consisting of high-speed steel (AISI M3:2) are exposed to a sequence of plasma-based pre-treatments prior to depositing a CrAlN coating. Hardness and Young’s modulus of the substrate and the coating are analysed by means of nanoindentation. To determine the adhesion between coating and substrate, scratch tests are conducted and analysed using a scanning electron microscope. For each step, the residual stresses are determined using sin2ψ measurements, which are correlated to the mechanical properties. A plasma-nitriding process before the CrAlN coating induces high compressive residual stresses into the sample subsurface and at the same time increases the hardness of the surface. This results in higher critical loads during the scratch tests and therefore a better adhesion of the coating on the substrate. © 2015, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-015-0653-4
  • Investigation and optimization of the tribo-mechanical properties of CrAlCN coatings using Design of Experiments
    Tillmann, W. and Stangier, D. and Schröder, P.
    Surface and Coatings Technology 308 (2016)
    The control of friction as well as its adaption is essential for forming operations. Thin hard coatings have a significant influence on the performance of production processes and the service life of tools, especially for Sheet-Bulk Metal Forming (SBMF) processes with high contact normal stresses and issues concerned with the filling of filigree functional elements. To handle these challenges, the CrAlCN coating system is generated by means of bipolar-pulsed reactive magnetron sputtering, using Design of Experiments. A Central Composite Design is selected to investigate the cathode power, bias voltage, as well as the reactive gas flow composition (nitrogen and acetylene). The aim is to evaluate the correlations and the interaction of the investigated process parameters on the mechanical as well as the tribological behavior of the CrAlCN coating, and to develop models to obtain the desired coating properties. The generated coatings show a clear dependency on the selected process parameters. An increased acetylene flow leads to a reduction of the mechanical properties (hardness and Young's modulus) as well as a decreased adhesion of the CrAlCN coating. In contrast to the influence of the acetylene flow, a higher negative bias voltage leads to improved mechanical properties in the context of wear resistant thin films. The tribological properties revealed that the coefficient of friction is related to the chemical composition of the coating which can, on the one hand, be adjusted by the acetylene flow and, on the other hand, by the cathode power. The optimized CrAlCN coating was deposited onto forming punches and the friction was evaluated using DC04 and DP600 specimens in an adapted ring-compression test. In comparison to polished and heat-treated ASP®2023 steel (62 HRC) and a CrAlN reference coating, the developed coating shows a significant reduction of friction due to the carbon incorporation. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.07.110
  • Investigation of the brazing characteristics of a new iron-based brazing filler metal
    Tillmann, W. and Wojarski, L. and Manka, M. and Trelenberg, A.
    Welding in the World 60 (2016)
    High temperature applications of new class of iron-based filler metals provide brazements with high corrosion resistance and mechanical properties. These brazements are cost-effective alternative to those made of the conventional brazing alloys. However, a wiser usage demands a deeper understanding of the wetting as well as gap filling behavior in conjunction with the resulting microstructure, which is mainly influenced by the applied brazing cycles. Therefore, this paper presents results of the investigation of specific brazing fundamentals for the new iron-based brazing alloy Fe-24Cr-20Ni-10Cu-7P-5Mn-5Si. Followed by DTA/DSC measurements, the spreading and gap filling behavior were examined by using stainless steel AISI 304 as base material. In wetting tests and wedge-gap experiments, the influence of the applied brazing temperature and the dwell time were investigated for vacuum brazing processes. The resulting microstructure was evaluated using a scanning electron microscope (SEM), equipped with an energy dispersive X-ray spectroscopy (EDS). Finally, strength tests were conducted in order to determine the influence of the brazing parameters on the mechanical properties of the joint. © 2016, International Institute of Welding.
    view abstract10.1007/s40194-016-0346-4
  • Investigation of the self-healing sliding wear characteristics of NiTi-based PVD coatings on tool steel
    Momeni, S. and Tillmann, W.
    Wear 368-369 (2016)
    Excellent damping capacity and superelasticity of the bulk NiTi shape memory alloy (SMA) makes it a suitable material of choice for tools in machining process as well as tribological systems. Although thin film of NiTi SMA has a same damping capacity as NiTi bulk alloys, it has a poor mechanical properties and undesirable tribological performance. This study aims at eliminating these application limitations for NiTi thin films. In order to achieve this goal, NiTi thin films were magnetron sputtered as an interlayer between reactively sputtered hard TiCN coatings and hot work tool steel substrates. The microstructure, composition, crystallographic phases, mechanical and tribological properties of the deposited thin films were analyzed by using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. It was found that under a specific coating architecture, the superelasticity of NiTi inter-layer can be combined with high hardness and wear resistance of TiCN protective layers. The obtained results revealed that the thickness of NiTi interlayers is an important factor controlling mechanical and tribological performance of bilayer composite coating systems. © 2016 Elsevier B.V.
    view abstract10.1016/j.wear.2016.08.004
  • Investigation of the tribological properties of high-feed milled structures and Cr-based hard PVD-coatings
    Tillmann, W. and Stangier, D. and Laemmerhirt, I.-A. and Biermann, D. and Freiburg, D.
    Vacuum 131 (2016)
    Sheet-bulk metal forming is an innovative approach to manufacture complex components which, due to secondary design features, have integrated as well as extended functions. A complex tribological load collective, that has not yet been sufficiently investigated, results from the high degree of deformation, high contact normal stresses of up to 3 GPa with a simultaneously high relative movement of the material, as well as the preference for local adaption of the material flow. Surface structures are a promising approach to locally adapt and control the material flow. Within this context, the present contribution investigates the influence as well as the interdependencies of three high-feed milled deterministic surface structures, combined with Cr-based PVD-hard coatings (CrAlN and CrAlCN), on the friction and wear behavior. The results show that CrAlN as well as CrAlCN-coatings significantly enhance the tribological property profile of the structured surfaces in comparison to hardened ASP2023. Despite the possibility to produce near net-shaped coatings, the wear resistance of the structures is increased by Cr-based hard materials, whereas the uncoated structures are destroyed. All three chosen surface structures show adherent material transfer of the 100Cr6 balls during the sliding contact. Especially the abrasive wear caused by the coated asperities of the structures leads to plowing of the 100Cr6 counter body resulting in debris on the CrAlN and CrAlCN surface. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.vacuum.2016.05.024
  • Model based optimization of a statistical simulation model for single diamond grinding
    Herbrandt, S. and Ligges, U. and Ferreira, M.P. and Kansteiner, M. and Biermann, D. and Tillmann, W. and Weihs, C.
    Computational Statistics (2016)
    We present a model for simulating normal forces arising during a grinding process in cement for single diamond grinding. Assuming the diamond to have the shape of a pyramid, a very fast calculation of force and removed volume can be achieved. The basic approach is the simulation of the scratch track. Its triangle profile is determined by the shape of the diamond. The approximation of the scratch track is realized by stringing together polyhedra. Their sizes depend on both the actual cutting depth and an error implicitly describing the material brittleness. Each scratch track part can be subdivided into three three-dimensional simplices for a straightforward calculation of the removed volume. Since the scratched mineral subsoil is generally inhomogeneous, the forces at different positions of the workpiece are expected to vary. This heterogeneous nature is considered by sampling from a Gaussian random field. To achieve a realistic outcome the model parameters are adjusted applying model based optimization methods. A noisy Kriging model is chosen as surrogate to approximate the deviation between modelled and observed forces. This deviation is minimized and the results of the modelled forces and the actual forces from conducted experiments are rather similar. © 2016 Springer-Verlag Berlin Heidelberg
    view abstract10.1007/s00180-016-0669-z
  • On the microstructural and mechanical properties of post-treated additively manufactured Inconel 718 superalloy under quasi-static and cyclic loading
    Aydinoz, M. E. and Brenne, F. and Schaper, M. and Schaak, C. and Tillmann, W. and Nellesen, J. and Niendorf, T.
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing 669 (2016)
    Inconel 718 specimens were additively manufactured via selective laser melting (SLM) and subjected to different post-process heat treatments. Performance under monotonic and cyclic loading in the low-cycle fatigue (LCF) regime was investigated at room temperature (RT). Hot isostatic pressing (HIP) was employed in order to reduce the porosity typically present after SLM. Prior functional encapsulation with a Ni-20Cr coating applied by cathodic arc deposition (Arc-PVD) was applied to achieve full densification. The results show that after Arc-PVD and HIP, the mechanical properties are not only affected by reduced porosity but also by substantial microstructural changes. Precipitates, i.e. Laves phase, already present in the as-built condition can be dissolved by solution heat treatment. HIP leads to recrystallization and thus significantly changes microstructural appearance. Elongated grains and sub-micron sized cell structures stemming from SLM processing are eliminated by HIP. Aging leads to evolution of strengthening gamma '' precipitates. Concurrently, delta phase forms upon HIP and aging. In comparison to aged conditions that were not subjected to HIP, microstructure upon HIP and aging results in improved ductility under monotonic loading, however, concomitantly deteriorates fatigue properties at RT. (C) 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.msea.2016.05.089
  • Optimization of a simulation for inhomogeneous mineral subsoil machining
    Herbrandt, S. and Weihs, C. and Ligges, U. and Ferreira, M. and Rautert, C. and Biermann, D. and Tillmann, W.
    Studies in Classification, Data Analysis, and Knowledge Organization (2016)
    For the new generation of concrete which enables more stable constructions, we require more efficient tools. Since the preferred tool for machining concrete is a diamond impregnated drill with substantial initial investment costs, the reduction of tool wear is of special interest. The stochastic character of the diamond size, orientation, and position in sintered segments, as well as differences in the machined material, justifies the development of a statistically motivated simulation. In the simulations presented in the past, workpiece and tool are subdivided by Delaunay tessellations into predefined fragments. The heterogeneous nature of the ingredients of concrete is solved by Gaussian random fields. Before proceeding with the simulation of the whole drill core bit, we have to adjust the simulation parameters for the two main components of the drill, diamond and metal matrix, by minimizing the discrepancy between simulation results and the conducted experiments. Due to the fact that our simulation is an expensive black box function with stochastic outcome, we use the advantages of model based optimization methods. © Springer International Publishing Switzerland 2016.
    view abstract10.1007/978-3-319-25226-1_41
  • Progress on effect of processes and microelements on liquation cracking of weld heat-affected zone of nickel-based alloy
    Li, Z. and Wang, H. and Li, Y. and Kim, H.J. and Tillmann, W.
    Jixie Gongcheng Xuebao/Journal of Mechanical Engineering 52 (2016)
    Nickel-based alloy with excellent high temperature resistance and corrosion resistance, has been widely applied to aerospace, nuclear power and offshore oil industry. Based on the importance of nickel-based alloy, studying the liquation cracking of weld heat-affected zone (HAZ) is of great significance to resolve the above problem. Welding process has a direct influence on welding performance of nickel-based alloy, so it is one of the important factors to lead to HAZ liquation cracking. By heat treatment processes, the susceptibility to liquation cracking can be reduced because of expected microstructure and properties obtained. Meanwhile, the microelements in base metal affect the non-equilibrium segregation behavior of grain boundaries, so it can leads to the generation of HAZ liquation cracking. The progress on liquation cracking of weld heat-affected zone of nichel-based alloy are reviewed from welding processes, heat treatment processes and micro-elements, and the test method of liquation cracking is summarized. The future research trends are prospected. © 2016 Journal of Mechanical Engineering.
    view abstract10.3901/JME.2016.06.037
  • Residual Analysis in Generalized Function-on-Scalar Regression for an HVOF Spraying Process
    Kuhnt, S. and Rehage, A. and Becker-Emden, C. and Tillmann, W. and Hussong, B.
    Quality and Reliability Engineering International 32 (2016)
    The coating of materials plays an important role in various fields of engineering. Essential properties such as wear protection can be improved by a suitable coating technique. One of these techniques is high-velocity oxygen-fuel spraying. A drawback of high-velocity oxygen-fuel spraying is that it lacks reproducibility due to effects which are hard to measure directly. However, coating powder particles are observable over time during their flight towards the material and contain valuable information about the state of the process. Because of their smooth nature, measures of temperature and velocity can be assumed as target variables in generalized function-on-scalar regression. We propose methods to perform residual analysis in this framework aiming at the detection of individual residual functions which deviate from the majority of residuals. These methods help to detect anomalies in the process and hence improve the estimators. Functional target variables result in functional residuals whose analysis is barely explored. One reason might be that ordinary residual plots should be inspected at each observed point in time. We circumvent this infeasible procedure by the use of functional depths that help to identify unusual residuals and thereby gain deeper insight of the data-generating process. In a simulation study, we find that a good depth for detecting trend outliers is the h-modal depth as long as the link function is chosen correctly. In case of shape outliers rFUNTA pseudo-depth performs well. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.
    view abstract10.1002/qre.2018
  • The detachment behavior of polycarbonate on thin films above the glass transition temperature
    Tillmann, W. and Hagen, L. and Hoffmann, F. and Dildrop, M. and Wibbeke, A. and Schöppner, V. and Resonnek, V. and Pohl, M. and Krumm, C. and Tiller, J.C. and Paulus, M. and Sternemann, C.
    Polymer Engineering and Science 56 (2016)
    When producing mono-axially stretched films made of amorphous polycarbonate, a self-reinforcement is generated due to the stretching process. This leads to an increase of the strength and stiffness. The mono-axial stretching process is conducted at temperatures above the glass transition temperature, whereas better mechanical properties are obtained at higher stretching temperatures. However, the film tends to adhere to the rolls, especially at temperatures from 10°C above the glass transition temperature. The rolls of the mono-axial stretching unit are made of an induction hardened and polished quenched and tempered steel 1.7225 – 42CrMo4. This work reports on the investigation of the detachment behavior of polycarbonate on different coatings as a function of the temperature and contact time. The main intention is to find a suitable coating on which the polycarbonate film adheres only slightly at temperatures clearly exceeding the glass transition temperature. POLYM. ENG. SCI., 56:786–797, 2016. © 2016 Society of Plastics Engineers. © 2016 Society of Plastics Engineers
    view abstract10.1002/pen.24307
  • Tribological development of TiCN coatings by adjusting the flowing rate of reactive gases
    Tillmann, W. and Momeni, S.
    Journal of Physics and Chemistry of Solids 90 (2016)
    TiCN coatings were deposited by means of direct current magnetron sputtering of Ti targets in presence of N2 and C2H2 reactive gases. The microstructure, composition, mechanical and tribological properties of the deposited thin films were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. The obtained results presents a reproducible processing route for tailoring microstructure, mechanical and tribological behavior of TiCN coatings by controlling flowing rate of the reactive gases. © 2015 Elsevier Ltd.
    view abstract10.1016/j.jpcs.2015.11.009
  • Tribological measures for controlling material flow in sheet-bulk metal forming
    Löffler, M. and Andreas, K. and Engel, U. and Schulte, R. and Groebel, D. and Krebs, E. and Freiburg, D. and Biermann, D. and Stangier, D. and Tillmann, W. and Weikert, T. and Wartzack, S. and Tremmel, S. and Lucas, H. and Denkena, B. and Merklein, M.
    Production Engineering 10 (2016)
    Sheet-bulk metal forming (SBMF) is characterized by successive and/or simultaneous occurrence of quite different load conditions regarding stress and strain states. These conditions significantly influence the material flow and thus the geometrical accuracy of the components. To improve the product quality a control of the material flow is required. An appropriate approach is given by locally adapted tribological conditions due to surface modifications of tool and workpiece, so-called tailored surfaces. Within the present study different methods to adapt the surfaces are presented and investigated with respect to their tribological effectiveness in SBMF. In a first step, requirements regarding necessary adaptions of the friction values for two SBMF processes are numerically defined. Based on the requirements different tailored surfaces are presented and analyzed regarding their tribological influence. Finally, the potential of surface modifications to improve SBMF processes is shown. © 2016, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-016-0695-2
  • Wear behavior of tribologically optimized tool surfaces for incremental forming processes
    Sieczkarek, P. and Wernicke, S. and Gies, S. and Tekkaya, A.E. and Krebs, E. and Wiederkehr, P. and Biermann, D. and Tillmann, W. and Stangier, D.
    Tribology International 104 (2016)
    The mechanical wear behavior of forming tools is the limiting factor during an incremental gear-forming process. These forming tools with a simply shaped geometry are exposed to high forming forces. Additionally, the necessary workpiece chambering, which is characteristic for this incremental process restricts the dimensioning of the tools. Thereby, the geometrical design of the forming tools is limited, which leads to a decreased lifetime. Functional structures on the tool surfaces can influence the occurring loading and wear behavior by reducing the contact area, the supply of lubricant pockets, and by a controlled influence and adjustment of the occurring material flow. For the extension of the tool's lifetime, different surface concepts and combinations with CrAlN PVD-coatings are investigated. To offer conditions with a high tool load, the investigations are focused on an incremental gear forming process with a simple one-wedge forming tool. The results show abrasive and adhesive wear characteristics, as well as outbreaks, and crack formations. The crack propagation on the flank leads to a chipping of the tool tip, hence limiting the tool life. Compared to the reference tool, a surface structure combined with a PVD-coating provides a significant increase of the tool life of 84%. © 2016 Elsevier Ltd
    view abstract10.1016/j.triboint.2016.08.028
  • X-ray diffraction (XRD)-studies on the temperature dependent interface reactions on hafnium, zirconium, and nickel coated monocrystalline diamonds used in grinding segments for stone and concrete machining
    Tillmann, W. and Tolan, M. and Pinho Ferreira, M. and Paulus, M. and Becke, M. and Stangier, D.
    Materialwissenschaft und Werkstofftechnik 47 (2016)
    Diamond impregnated metal matrix composites are the state of the art solution for the machining of mineral materials. The type of interface reactions between the metal matrix and diamond surface has an essential influence on the tool performance and durability. To improve the diamond retention, the diamonds can be coated by physical vapour deposition with metallic materials, which enforce interface reactions. Hence, this paper focuses on the investigation of the interfacial area on metal-coated monocrystalline diamonds. Hafnium and zirconium, both known as carbide forming elements, are used as coating materials. The third coating, which is used to determine its catalytic influences when applied as a physical vapour deposition (PVD)-layer, is nickel. Additionally, the coated diamond samples were heat-treated to investigate the starting point of the formation of new phases. X-ray diffraction-analyses revealed the assumed carbide formation on hafnium and zirconium coated samples. The formation temperature was identified between 800 °C and 1000 °C for hafnium and zirconium coatings. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201600713
  • Analyses of the spreading kinetics of AgCuTi melts on silicon carbide below 900°C, using a large-chamber SEM
    Tillmann, W. and Pfeiffer, J. and Sievers, N. and Boettcher, K.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 468 (2015)
    Studies of the spreading kinetics of molten metals on ceramic substrates provide valuable information about the processes determining the interaction between these two materials. Based on experimental and theoretical works, two determining wetting types have been proposed for a variety of high-temperature systems - the reaction limited wetting and the diffusion-limited wetting. In the scope of this contribution an AgCuTi-SiC-system is investigated, which, despite of its high relevance for joining applications, has not yet gained much attention in fundamental works.The experiments were carried out on a heating stage placed in a large-chamber SEM. This novel method, allows high-resolution imaging and a precise observation of the changes of the liquid drop. Images taken every 5. s were used to measure the wetted surface as a function of time at temperatures between 830 and 890. °C.The AgCuTi-melt exhibits different spreading stages during the wetting process. It was shown that, at the beginning of the wetting, the spreading kinetics is determined by the reaction rate at the triple line. After reaching a distinct temperature-dependent angle, the titanium diffusion becomes the main limiting factor. © 2014 Elsevier B.V.
    view abstract10.1016/j.colsurfa.2014.12.039
  • Comparison of NiTi thin films sputtered from separate elemental targets and Ti-rich alloy targets
    Tillmann, W. and Momeni, S.
    Journal of Materials Processing Technology 220 (2015)
    The kind of sputtering targets can adversely affect the microstructure, phase transformation behavior, mechanical and tribological properties of near equi-atomic NiTi thin films. This new finding was systematically investigated by employing comprehensive characterization and analysis techniques, including X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), nanoindentation, ball-on-disc, and three dimensional (3D) optical microscopy. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2015.01.014
  • Copper-based nanostructured coatings for low-temperature brazing applications
    Lehmert, B. and Janczak-Rusch, J. and Pigozzi, G. and Zuraw, P. and La Mattina, F. and Wojarski, L. and Tillmann, W. and Jeurgens, L.P.H.
    Materials Transactions 56 (2015)
    This feasibility study demonstrates the possibility to apply nanostructured filler materials for novel low-temperature brazing applications by exploiting the size-dependent melting behavior of metals and alloys when confined to the nano-scale regime. As an example, a copper-based nanostructured brazing filler is presented, which allows metal brazing of coated Ti-6Al-4V components at 750°C, much below the bulk melting point of copper (1083°C). The copper-based nanostructured brazing fillers can be produced in the form of coatings and free-standing brazing foils. The nano-confinement of Cu is abrogated after brazing and, consequently, the brazed joints can be operated well above their reduced brazing temperatures. © 2015 The Japan Institute of Metals and Materials.
    view abstract10.2320/matertrans.MI201419
  • Effect of the addition of low rare earth elements (lanthanum, neodymium, cerium) on the biodegradation and biocompatibility of magnesium
    Willbold, E. and Gu, X. and Albert, D. and Kalla, K. and Bobe, K. and Brauneis, M. and Janning, C. and Nellesen, J. and Czayka, W. and Tillmann, W. and Zheng, Y. and Witte, F.
    Acta Biomaterialia 11 (2015)
    Rare earth elements are promising alloying element candidates for magnesium alloys used as biodegradable devices in biomedical applications. Rare earth elements have significant effects on the high temperature strength as well as the creep resistance of alloys and they improve magnesium corrosion resistance. We focused on lanthanum, neodymium and cerium to produce magnesium alloys with commonly used rare earth element concentrations. We showed that low concentrations of rare earth elements do not promote bone growth inside a 750 μm broad area around the implant. However, increased bone growth was observed at a greater distance from the degrading alloys. Clinically and histologically, the alloys and their corrosion products caused no systematic or local cytotoxicological effects. Using microtomography and in vitro experiments, we could show that the magnesium-rare earth element alloys showed low corrosion rates, both in in vitro and in vivo. The lanthanum- and cerium-containing alloys degraded at comparable rates, whereas the neodymium-containing alloy showed the lowest corrosion rates. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actbio.2014.09.041
  • Functional encapsulation of laser melted Inconel 718 by Arc-PVD and HVOF for post compacting by hot isostatic pressing
    Tillmann, W. and Schaak, C. and Nellesen, J. and Schaper, M. and Aydinöz, M.E. and Niendorf, T.
    Powder Metallurgy 58 (2015)
    In this study, the Selective Laser Melting (SLM) technology was used to manufacture flat specimens from Inconel 718 powder. The SLM process parameters have amajor impact on the microstructure aswell as on the mechanical properties of the fabricated specimens. Despite using optimized processing parameters, defects like pores cannot be completely avoided. These pores act as stress raisers and lead to premature crack initiation under cyclic loading, eventually reducing the fatigue strength of the material. Hot Isostatic Pressing (HIP) offers the possibility to eliminate the porosity and thus to increase the fatigue performance of the material. HIP combines high pressure and high temperature to produce materials with superior properties. Unfortunately, open porosity, i.e. open pores on the surface, can prevent full densification. In the present work, SLM flat specimens were encapsulated by means of Cathodic Arc Deposition (Arc-PVD) and High Velocity Oxygen Fuel Spraying (HVOF) to seal open pores. For this purpose, different encapsulation materials were investigated with a focus on materials offering additional functions such as an improved high temperature corrosion resistance or applicability as a bond coat for thermal barrier coatings. © 2015 Institute of Materials, Minerals and Mining.
    view abstract10.1179/0032589915Z.000000000250
  • In-situ annealing of NiTi thin films at different temperatures
    Tillmann, W. and Momeni, S.
    Sensors and Actuators, A: Physical 221 (2015)
    Magnetron sputtered NiTi thin films are usually sputtered at ambient temperature and need a post-annealing treatment to promote crystallization and obtain shape memory effect. However, this treatment could adversely affect the microstructure as well as the morphology of the film. Within this study, NiTi thin films were generated by annealing during the sputtering process. The effect of the sputtering temperature on the morphology of the film, the composition, and shape memory behavior was studied using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), and differential scanning calorimetry (DSC). © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.sna.2014.10.034
  • Influence of in-situ and postannealing technique on tribological performance of NiTi SMA thin films
    Tillmann, W. and Momeni, S.
    Surface and Coatings Technology 276 (2015)
    Magnetron sputtered NiTi thin films were crystallized through two convenient techniques: (i) postannealing and (ii) in-situ annealing during the deposition. The annealing parameters (temperature and time) were kept constant by employing each technique. The microstructure, morphology, phase transformation behavior, mechanical and tribological properties of these thin films were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), 4-point probe resistivity measurement, nanoindentation test, pin-on-disc, scratch test and three dimensional (3D) optical microscopy. The results show how postannealing and in-situ annealing techniques can differently affect properties of NiTi thin films in spite of employing similar annealing temperature and time. © 2015 Elsevier B.V..
    view abstract10.1016/j.surfcoat.2015.07.012
  • Influence of surface modifications on friction, using high-feed milling and wear resistant PVD-coating for sheet-metal forming tools
    Biermann, D. and Freiburg, D. and Hense, R. and Tillmann, W. and Stangier, D.
    Key Engineering Materials 639 (2015)
    Increasing technological requirements, as well as the demand for an efficient production demands high performance materials and enhanced manufacturing processes. The development of a new manufacturing process, sheet-bulk metal forming (SBMF), is one approach to produce lightweight forming parts with an increased number of functional properties while, at the same time, combining the advantages of sheet and bulk metal forming. For SBMF processes, the specific adjustment of the friction between tool and workpiece for a specifically designed material flow, which is called tailored friction, is of great importance. The reduction of friction is essential in order to ensure a homogeneous forming zone. However, a higher friction can be used to control the material flow to increase the local thickness of the work piece for additional functional integration. This paper shows the development of surface structures for SBMF tools by means of high-feed milling. Process parameters like the tilt angle or the feed are varied to influence the surface parameters of the structures, which results in different tribological properties of the forming tool. The structured surfaces are subsequently coated with a wear resistant CrAlN coating, processed by a magnetron-sputtering process (PVD) to enhance the lifetime and performance of the forming tool. Finally, a ring compressing test is used to investigate the tribological behavior of the coated structures. © 2015 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.639.275
  • Influence of the handling parameters on residual stresses of HVOF-sprayed WC-12Co coatings
    Selvadurai, U. and Hollingsworth, P. and Baumann, I. and Hussong, B. and Tillmann, W. and Rausch, S. and Biermann, D.
    Surface and Coatings Technology 268 (2015)
    In this research work HVOF WC-12Co coatings were deposited on C45 steel (1.0503) substrates. Unfavorable residual stresses can lead to delamination and spallation and thus have to be avoided by optimizing the handling parameters. This study investigates the influence of the following handling parameters: substrate temperature, number of overruns, stand-off distance, track pitch, and gun velocity on the residual stress and hardness. Additionally, the effect of post-treating the coating by means of surface grinding was determined. For the HVOF spraying experiments, fine agglomerated and sintered WC-12Co powders (2-10. μm) with WC particles in the submicron range (400. nm) were chosen. It was determined that alterations of these handling parameters had significant effects on the residual stress and the hardness. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.11.055
  • Mechanical and microstructural analysis of ultrasonically assisted induction-brazed TiAl6V4 joints
    Tillmann, W. and Zimpel, M. and Dias, N.F.L. and Pfeiffer, J. and Wojarski, L. and Xu, Z.
    Welding in the World 59 (2015)
    This paper focuses on the process of ultrasonically assisted induction brazing with regard to titanium brazing. The titanium alloy TiAl6V4 was brazed using an aluminum-based filler alloy (AlMg2.5Cr0.3). It was apparent that the layer thickness of the brazing foil as well as the brazing temperature and the intensity of the ultrasound are significant influencing factors of the combined brazing process and microstructure. It is the aim of this paper to draw conclusions from the microstructural and mechanical investigations of the brazed joint about the process parameters, which are crucial for the properties and quality of the joint. The evaluation of the microstructure of the joint was conducted by means of metallographic investigations and results obtained by means of scanning electron microscopy. Besides mechanical microhardness measurements, strength investigations were conducted in order to evaluate the quality of the joint. Furthermore, the results of conventional vacuum brazing processes were correlated in order to be able to better facilitate and understand the adapted induction brazing process. © 2015, International Institute of Welding.
    view abstract10.1007/s40194-015-0260-1
  • Progress of hot cracking of weld metal and strength \& toughness of weld joint for nickel-based alloy
    Li, Z.-X. and Whang, H. and Tillmann, W. and Li, W. and Zhang, T.-L. and Li, Y.
    Beijing Gongye Daxue Xuebao/Journal of Beijing University of Technology 41 (2015)
    The progress of the hot cracking of weld metal and strength & toughness of weld joint for nickel-based alloy was reviewed in this paper. It concludes that 1) the susceptibility of ductility-dip cracking to weld metal can be decreased by adding Nb, Al, Ti and Ce alloying elements in filler wire. The formation of S, P, Si, Bi low melting point compounds can be prevented by adding Mn and Mg. Besides, the cracking resistance of weld metal can be improved by controlling the ratio of TiO2, SiO2, ZrO2 and MnO2 slagging agents in the flux; 2) by adopting appropriate welding process, the hot cracking of weld metal can be effectively avoided. The influence of new welding process, such as laser beam welding and electron beam welding, to the hot cracking of weld metal will be the hit issue of future study; 3) using the post weld heat treatment of solid solution and double aging, the strength and toughness of weld joint can be improved significantly. The tensile strength of weld joint can be improved by adding Al, Ti, Nb, Mo and Mn in filler. However, the impact toughness of weld joint will be decreased with the mass fraction of Mn increased in the filler wire. ©, 2015, Beijing University of Technology. All right reserved.
    view abstract10.11936/bjutxb2015030023
  • Progress on surface pretreatment and its effects on the joint strength in dissimilar materials joining
    Li, Z.-X. and Tie, M. and Li, H. and Tillmann, W.
    Beijing Gongye Daxue Xuebao/Journal of Beijing University of Technology 41 (2015)
    This paper reviews the research status of surface pretreatment and the effects of surface state on joint strength in dissimilar materials joining. Some conclusions are as follows: 1) Depositing an active metal can form well solid solubility with the base material or the filler metal in dissimilar materials joining, which can enhance the interface reaction and improve the joint strength; 2) The activity of surface elements is promoted and the diffusion ability is enlarged, the hard brittle phase in the joint is reduced and the joint strength is improved by perfecting the physical, chemical or mechanical properties of the base material. ©, 2015, Beijing University of Technology. All right reserved.
    view abstract10.11936/bjutxb2014050064
  • The Apparent Contact Angle and Wetted Area of Active Alloys on Silicon Carbide as a Function of the Temperature and the Surface Roughness: A Multivariate Approach
    Tillmann, W. and Pfeiffer, J. and Wojarski, L.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 (2015)
    Despite the broad field of applications for active filler alloys for brazing ceramics, as well as intense research work on the wetting and spreading behavior of these alloys on ceramic surfaces within the last decades, the manufactured joints still exhibit significant variations in their properties due to the high sensitivity of the alloys to changing brazing conditions. This increases the need for investigations of the wetting and spreading behavior of filler alloys with regard to the dominating influences combined with their interdependencies, instead of solely focusing on single parameter investigations. In this regard, measurements of the wetting angle and area were conducted at solidified AgCuTi and CuSnTi alloys on SiC substrates. Based on these measurements, a regression model was generated, illustrating the influence of the brazing temperature, the roughness of the faying surfaces, the furnace atmosphere, and their interdependencies on the wetting and spreading behavior of the filler alloys. It was revealed that the behavior of the melts was significantly influenced by the varied brazing parameters, as well as by their interdependencies. This result was also predicted by the developed model and showed a high accuracy. © 2015, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-015-2938-9
  • Thermally sprayed finestructured WC-12Co coatings finished by ball burnishing and grinding as an innovative approach to protect forming tools against wear
    Tillmann, W. and Hollingsworth, P. and Baumann, I. and Hiegemann, L. and Weddeling, C. and Tekkaya, A.E. and Rausch, S. and Biermann, D.
    Surface and Coatings Technology 268 (2015)
    The forming of high-strength steel sheets offers novel possibilities to produce lightweight structural parts with a high stiffness for the aircraft and automotive industries. However, the employment of such sheet materials leads to intense wear and thus reduces the lifetime of forming tools. At the same time, the requirements concerning their performance, their geometrical complexity, and their shape accuracy have been significantly increased. To counteract this problem, the tools are either treated by different thermo-chemical processes (e.g. hardening, nitriding) or are coated using thin film techniques such as PVD or CVD. In this study, thermal spraying is presented as a cost efficient and more flexible approach to protect the surface of forming tools against wear. For this purpose, planar samples as well as cylindrical deep drawing dies were coated by means of the high velocity oxy-fuel (HVOF) flame spraying technique, utilizing a fine WC-12Co powder (agglomerate size 2-10. μm) with a carbide size of 400. nm. Prior to the coating operation, a comprehensive parameter optimization was performed based on the statistical design of experiments (DOE) to achieve coatings with improved mechanical and tribological properties. The planar samples were used to ascertain the sliding and rolling wear behavior within two standardized test methods (Ball-on-Disc and Taber Abraser tests). The coated dies were smoothened by ball burnishing as well as grinding and afterwards evaluated within the deep drawing of high strength (HC380LA) steel sheets. In addition, the results were compared to those achieved with an uncoated conventional cold work steel die, which is commonly employed for this operation. In contrast to the cold work steel, both coated dies, the ball burnished as well as the ground die, showed a significantly better wear performance after the forming of 10,000 parts. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.06.039
  • Tribological investigation of bionic and micro-structured functional surfaces
    Tillmann, W. and Stangier, D. and Hagen, L. and Biermann, D. and Kersting, P. and Krebs, E.
    Materialwissenschaft und Werkstofftechnik 46 (2015)
    An established concept adjusting tribological properties and for increasing the wear resistance is presented by coatings. In addition to the material adaption of surfaces, there are efforts of applying structures on tool active parts in order to allow a further adjustment on the property profile. For this reason, the presented article investigates the influence of bionic and technologically textured surfaces on the friction and wear behavior with and without near-net shaped wear-resistant PVD coatings. Based on the example of nature, a honeycombed surface structure discovered on the head of scarab beetles as well as a dimple structure optimized for the manufacturing time were transferred on HSS steel by means of micro-milling. The analyses focus on the influence of the surface structures, the effects of PVD coatings and their interactions on the friction and wear behavior. The investigations show that the tribological properties depend on each surface structure and the material pairing. Both the technological and the bionic structures show a reduction of the friction coefficient in combination with the material pairing 100Cr6 and WCCo compared to polished samples. Furthermore, it is shown that the CrAlN coating has no influence on the friction behavior, but rather leads to the desired increase in the wear resistance. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201500434
  • Tribological performance of near equiatomic and Ti-rich NiTi shape memory alloy thin films
    Tillmann, W. and Momeni, S.
    Acta Materialia 92 (2015)
    Near equiatomic and Ti-rich NiTi shape memory alloy thin films were magnetron sputtered with the same processing parameters and thickness of 3 μm. The microstructure, composition, shape memory behavior, mechanical and tribological properties of the deposited thin films were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. The obtained results clearly show how the crystallization evolution and precipitation formation of these two sets of thin films can drastically influence their mechanical and tribological performances. © 2015 Acta Materialia Inc. All rights reserved.
    view abstract10.1016/j.actamat.2015.04.006
  • Wear behavior of bio-inspired and technologically structured HVOF sprayed NiCrBSiFe coatings
    Tillmann, W. and Hagen, L. and Stangier, D. and Laemmerhirt, I.-A. and Biermann, D. and Kersting, P. and Krebs, E.
    Surface and Coatings Technology 280 (2015)
    Surface modification by means of textured structures can largely enhance the tribological and wear behavior of components and tools under various environmental conditions. Continuous developments in machining processes, such as the micromilling technology, can be used to manufacture fine-scaled structures on hardened steel tool surfaces. Thus, the adjusted friction behavior, which can affect the tendency of a material to adhere to the surface, is compensated by the small number of contact points between the friction partner and the surface. Accordingly, anisotropic friction properties of such structures can lead to a locally different wear behavior. In this study, a NiCrBSiFe self-fluxing alloy is thermally sprayed onto specimens made of AISI M2 high-speed steel (HSS). Technological and bionic surface structures were applied on thermally sprayed and laser remelted substrates. Based on ball-on-disk tests, the coefficient of friction was determined and compared for different high velocity oxy fuel (HVOF) sprayed NiCrBSiFe coatings and surface textures. These experiments show that functional structures can reduce the coefficient of friction. The bio-inspired surface shows a friction reduction of approximately 35% compared to the as-sprayed and polished sample, and a reduction of 25% when compared to the remelted and smoothened surface. Moreover, the analyzed surface conditions lead to a different wear behavior than the bio-inspired structure, which possesses areas with a reduced oxidational wear and less adhesion when compared to the other surface conditions. © 2015 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2015.08.055
  • 3D µCT and SEM Analysis of Resolidified Tips of Cored Wires Used in Twin-Wire Arc Spraying
    Nellesen, J. and Abdulgader, M. and Tillmann, W. and Beckmann, F.
    Journal of Thermal Spray Technology 24 (2014)
    In twin-wire arc spraying (TWAS), the in-flight particles are atomized from a melting bath which generates an inhomogeneous spraying plume. This inhomogeneity is due to the fact that these particles are generated by the impingement of fast continuous flowing air upon the melting tips of electrically conductive wires. This work aims to contribute to the understanding of the initiation of such particles in the TWAS process. For this purpose, cored wires filled with W-rich particles were sprayed. After interrupting the TWAS process, the tips of these cored wires were imaged by 3D µCT and scanning electron microscopy in order to analyze how the filling powder interacts with the melted part of the sheath. The analysis of the 3D tomograms shows that the resolidified melting bath of the cored wires is interspersed with both spherical and irregular-shaped W-rich particles. This irregular shape suggests a partial melting of the W-rich particles. © 2014, ASM International.
    view abstract10.1007/s11666-014-0169-z
  • Adapting of the background-oriented schlieren (BOS) technique in the characterization of the flow regimes in thermal spraying processes
    Tillmann, W. and Abdulgader, M. and Rademacher, H.G. and Anjami, N. and Hagen, L.
    Journal of Thermal Spray Technology 23 (2014)
    In thermal spraying technique, the changes in the in-flight particle velocities are considered to be only a function of the drag forces caused by the dominating flow regimes in the spray jet. Therefore, the correct understanding of the aerodynamic phenomena occurred at nozzle out let and at the substrate interface is an important task in the targeted improvement in the nozzle and air-cap design as well as in the spraying process in total. The presented work deals with the adapting of an innovative technique for the flow characterization called background-oriented Schlieren. The flow regimes in twin wire arc spraying (TWAS) and high velocity oxygen fuel (HVOF) were analyzed with this technique. The interfering of the atomization gas flow with the intersected wires causes in case of TWAS process a deformation of the jet shape. It leads also to areas with different aero dynamic forces. The configurations of the outlet air-caps in TWAS effect predominantly the outlet flow characteristics. The ratio between fuel and oxygen determine the dominating flow regimes in the HVOF spraying jet. Enhanced understanding of the aerodynamics at outlet and at the substrate interface could lead to a targeted improvement in thermal spraying processes. © 2013 ASM International.
    view abstract10.1007/s11666-013-0018-5
  • Analysis of stress gradients in physical vapour deposition multilayers by X-ray diffraction at fixed depth intervals
    Fischer, G. and Selvadurai, U. and Nellesen, J. and Sprute, T. and Tillmann, W.
    Journal of Applied Crystallography 47 (2014)
    The objective of this article is to develop and apply a model for the design and evaluation of X-ray diffraction experiments to measure phase-specific residual stress profiles in multilayer systems. Using synchrotron radiation and angle-dispersive diffraction, the stress measurements are performed on the basis of the sin2ψ method. Instead of the traditional Ω or χ mode, the experiments are carried out by a simultaneous variation of the goniometer angles χ, Ω and φG to ensure that the penetration and information depth and the measuring direction φ remain unchanged when the polar angle ψ is varied. The applicability of this measuring and evaluation strategy is demonstrated by the example of a multilayer system consisting of Ti and TiAlN layers, alternately deposited on a steel substrate by means of physical vapour deposition. © 2014 International Union of Crystallography.
    view abstract10.1107/S1600576713030951
  • Deposition of superelastic composite NiTi based films
    Tillmann, W. and Momeni, S.
    Vacuum 104 (2014)
    In recent years, NiTi shape memory alloys (SMA) thin films have been widely used as promising high-performance materials in the field of biomedical and microelectromechanical (MEMS) systems. However, there are still important problems such as their unsatisfactory mechanical and tribological properties including a limited hardness and wear resistance. This study aimed at deposition of layered composite thin films made of NiTi and TiCN thin films on Si (100) substrate by means of DC magnetron sputtering. Subsequently, microstructures, mechanical properties and shape memory behavior of these bilayers were investigated using Nanoindentation, X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results of this study confirmed that the presence of TiCN layer on NiTi thin film modifies its mechanical properties while maintaining the shape memory effects. The initial findings of this research work are suggestive of the potential for fabrication of self-healed composite NiTi based films. © 2014 Published by Elsevier Ltd.
    view abstract10.1016/j.vacuum.2013.12.010
  • Development and characterization of B4C reinforced detonation-sprayed Al coatings
    Tillmann, W. and Hollingsworth, P.S. and Fischer, G. and Nellesen, J. and Beckmann, F.
    Journal of Thermal Spray Technology 23 (2014)
    Because of their excellent properties aluminum and its alloys cover a wide range of applications especially in the lightweight construction sector. In order to reach a higher strength and wear resistance metal matrix composites (MMCs) are used. Typically Al MMCs are manufactured by casting or extrusion processes. The disadvantage of these production routes is a cost-intensive and time-consuming finishing in terms of grinding and milling. The technique of thermal spraying provides the possibility to coat aluminum parts with MMCs close to their final shape. In addition to the shape accuracy the ductility and toughness of the coated parts are generally higher compared to extruded or casted parts. This study describes the development of detonation-sprayed boron carbide reinforced aluminum coatings on aluminum (EN AW 5754) substrates. The optimization of the coatings was focused on a homogeneous coating structure, a low coating porosity, a high deposition efficiency, a high number of embedded carbides, and a small percentage of oxides. In continuous tensile tests the influence of the MMC coating on the tensile strength was determined. Furthermore, the tensile strength was investigated in a discontinuous tensile test step by step. The different stages of deformation were analyzed by using micro computed tomography. This method enables the observation of tensile specimens in 3D, and consequently the site and moment of crack formation. © 2013 ASM International.
    view abstract10.1007/s11666-013-0036-3
  • Effects of bleaching agents on surface roughness of filling materials
    Markovic, L. and Jordan, R.A. and Glasser, M.-C. and Arnold, W.H. and Nebel, J. and Tillmann, W. and Ostermann, T. and Zimmer, S.
    Dental Materials Journal 33 (2014)
    The aim of this study was to use a non-tactile optical measurement system to assess the effects of three bleaching agents' concentrations on the surface roughness of dental restoration materials. Two composites (Grandio, Venus) and one glass ionomer cement (Ketac Fil Plus) were used in this in vitro study. Specimens were treated with three different bleaching agents (16% and 22% carbamide peroxide (Polanight) and 38% hydrogen peroxide (Opalescence Boost)). Surface roughness was measured with an optical profilometer (Infinite Focus G3) before and after the bleaching treatment. Surface roughness increased in all tested specimens after bleaching treatment (p<0.05). Our in vitro study showed that dental bleaching agents influenced the surface roughness of different restoration materials, and the restoration material itself was shown to have an impact on alteration susceptibility. There seemed to be no clinical relevance in case of an optimal finish.
    view abstract10.4012/dmj.2012-217
  • Examination of the porosity in Reactive Air Brazed joints by ultrasonic testing
    Tillmann, W. and Sievers, N. and Pfeiffer, J. and Wojarski, L. and Zielke, R. and Poenicke, A. and Schilm, J.
    Advanced Engineering Materials 16 (2014)
    Reactive air brazing (RAB) has been developed as a method to join ceramics and steel, using CuO as a reactive agent that interacts with the surface of the ceramic, enabling a wetting by the molten filler metal. A major benefit of this method is the fact that the joining process can be carried out in an ambient atmosphere, in contrast to active brazing processes, which need to be performed in a vacuum furnace. In the past, several investigations were conducted to improve the mechanical bonding properties for both methods. A sealed gas-tightness is also important for innovative applications such as solid oxide fuel cells. In this regard, the reduction of porosity, which is necessary in order to achieve reproducible joints with a long lifetime, presents a challenge. Therefore, it is necessary to conduct fundamental analyses of the driving forces and mechanisms of the formation of voids in the interfacial area to guarantee a reliable joint quality. In this study, the authors used an ultrasonic testing method in the realm of the immersion technique to evaluate the porosity in brazements produced with varying process parameters. A major goal was to assess the influence of the cooling stage on the pore formation. The advantage of this non-destructive method is the possibility to scan the entire joint area using just one scan. The investigations were flanked by SEM analyses on different cross-sections. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/adem.201400055
  • Global progress on welding consumables for HSLA steel
    Zhang, T. and Li, Z. and Young, F. and Jin Kim, H. and Li, H. and Jing, H. and Tillmann, W.
    ISIJ International 54 (2014)
    This paper reviews the global progress on welding consumables for high strength low alloy steel. The numerous aspects, such as the toughness and cleanliness of weld metal, the new removal mechanisms of impurity elements and the crack resistance of weld metal, are discussed. To meet increasing environment requirements, the fumes and life cycle assessment of welding consumables are also discussed. Finally, future trends in the development of welding consumables for high strength low alloy steel are pointed out. © 2014 ISIJ.
    view abstract10.2355/isijinternational.54.1472
  • In Situ Wear Test on Thermal Spray Coatings in a Large Chamber Scanning Electron Microscope
    Luo, W. and Tillmann, W. and Selvadurai, U.
    Journal of Thermal Spray Technology 24 (2014)
    Currently, the determination of the mass loss is usually used for a quantitative evaluation of wear tests, while the analysis of wear tracks is utilized for a qualitative evaluation of wear. Both evaluation methods can only be used after the wear testing process and their results only present the final outcome of the wear test. However, the changes during the wear test and the time-dependent wear mechanisms are of great interest as well. A running wear test in a large chamber scanning electron microscope (SEM) offers the first opportunity to observe the wear process in situ. Different wear mechanisms, such as the adhesive, abrasive wear, surface fatigue and tribochemical reaction, can be recorded with high magnification. Within this research, a special pin-on-disk testing device is designed for a vacuum environment. Using this device, arc-sprayed NiCrBSi coatings and high-velocity-oxygen-fuel-sprayed WC-12Co coatings were tested in a large chamber SEM with Al2O3 ceramic balls as wear counterparts. During the wear testing, different wear mechanisms were determined and the processes were recorded in short video streams. © 2014, ASM International.
    view abstract10.1007/s11666-014-0163-5
  • Influence of nitriding parameters on the tribological properties and the adhesion of Ti- and Cr-based multilayer designs
    Tillmann, W. and Dildrop, M. and Sprute, T.
    Surface and Coatings Technology 260 (2014)
    The growing competitive pressure in the industry makes the reduction of process costs increasingly important. One possibility to decrease the costs is to increase the lifetime of the tools by applying wear resistant and friction reduced PVD coatings. Especially in the field of hot forming, tools are exposed to high mechanical and thermal loads. The special requirements of these tools can be met by multilayer systems such as Ti/TiAlN or Cr/CrAlN, which provide properties such as a high hardness, sufficient heat resistance, and smooth surfaces. However, varying hardnesses and different thermoelastic properties between the multilayer systems and the substrate can lead to the delamination and spallation of the layers. Using an upstream plasma nitriding process, it is possible to produce a hardness gradient between the basis material and the coatings, thus ensuring a sufficient supporting effect for the coating. In this study for the first time a plasma nitriding process was integrated into the PVD coating device (Cemecon CC800/9 sinox ML) to realize a duplex coating process. The nitriding parameters were successively varied and the influence of the parameters on the material performance in the nitriding zone was examined. The morphology and structure of the nitriding zone were examined using scanning electron microscopy (SEM), energy dispersive scattering and X-ray diffractometry (XRD).Afterwards, the adhesion of the applied PVD layers was investigated in dependency on the nitriding parameters. The PVD layers were deposited on tool steel substrates AISI H11 (1.2343) by means of magnetron sputtering. The mechanical and tribological properties as well as the adhesion of these coating systems were studied using a scratch tester, a nanoindenter, and a ball-on-disc test. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.09.017
  • Influence of substrate pre-treatments on residual stresses and tribo-mechanical properties of TiAlN-based PVD coatings
    Sprute, T. and Tillmann, W. and Grisales, D. and Selvadurai, U. and Fischer, G.
    Surface and Coatings Technology 260 (2014)
    Residual stresses in the substrate and in the PVD coating have a significant influence on the coating adhesion and lifespan of machining as well as forming tools. Therefore, the understanding and control of the system's residual stresses will lead to a better performance of the coated components. Moreover, although investigations were conducted in the field of stress analysis of PVD coatings, they do not focus on interdependencies of residual stresses in the substrate and in the coating. In this investigation, three different metallographically prepared substrates were used. SiC grinding, diamond grinding, and SiC grinding and plasma nitriding preparations were selected, due to the substantial differences in their final residual stress states. Additionally, a Ti/TiAlN multilayer coating and a reference TiAlN monolayer were deposited on each pre-treated substrate. Their initial and final residual stress states were measured by means of X-ray diffraction. In addition to the residual stress analyses, tribo-mechanical tests, such as nano-indentation, ball-on-disc, and scratch tests were performed in order to correlate the results with these residual stress states. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.08.075
  • Metal matrix composites deposition in twin wire arc spraying utilizing an external powder injection composition
    Tillmann, W. and Abdulgader, M. and Hagen, L. and Nellesen, J.
    Journal of Thermal Spray Technology 23 (2014)
    The powder injection parameters, the location of the injection port, as well as the metal matrix composites are important features, which determine the deposition efficiency and embedding behavior of hard materials in the surrounding matrix of the twin wire arc-spraying process. This study investigates the applicability of external powder injection and aims to determine whether the powder injection parameters, the location, and the material combination (composition of the matrix as well as hard material) need to be specifically tailored. Therefore, the position of the injection port in relation to the arc zone was altered along the spraying axis and perpendicular to the arc. The axial position of the injection port determines the thermal activation of the injected powder. An injection behind the arc, close to the nozzle outlet, seems to enhance the thermal activation. The optimal injection positions of different hard materials in combination with zinc-, nickel- and iron-based matrices were found to be closer to the arc zone utilizing a high-speed camera system. The powder size, the mass of the particle, the carrier gas flow, and the electric insulation of the hard material affect the perpendicular position of the radial injection port. These findings show that the local powder injection, the wetting behavior of particles in the realm of the molten pool as well as the atomization behavior of the molten pool all affect the embedding behavior of the hard material in the surrounded metallic matrix. Hardness measurement by means of nanoindentation and EDX analysis along transition zones were utilized to estimate the bonding strength. The observation of a diffusion zone indicates a strong metallurgical bonding for boron carbides embedded in steel matrix. © 2013 ASM International.
    view abstract10.1007/s11666-013-0044-3
  • On- and offline detection of structural breaks in thermal spraying processes
    Borowski, M. and Rudak, N. and Hussong, B. and Wied, D. and Kuhnt, S. and Tillmann, W.
    Journal of Applied Statistics 41 (2014)
    We investigate and develop methods for structural break detection, considering time series from thermal spraying process monitoring. Since engineers induce technical malfunctions during the processes, the time series exhibit structural breaks at known time points, giving us valuable information to conduct the investigations. First, we consider a recently developed robust online (also real-time) filtering (i.e. smoothing) procedure that comprises a test for local linearity. This test rejects when jumps and trend changes are present, so that it can also be useful to detect such structural breaks online. Second, based on the filtering procedure we develop a robust method for the online detection of ongoing trends. We investigate these two methods as to the online detection of structural breaks by simulations and applications to the time series from the manipulated spraying processes. Third, we consider a recently developed fluctuation test for constant variances that can be applied offline, i.e. after the whole time series has been observed, to control the spraying results. Since this test is not reliable when jumps are present in the time series, we suggest data transformation based on filtering and demonstrate that this transformation makes the test applicable. © 2013 © 2013 Taylor & Francis.
    view abstract10.1080/02664763.2013.860957
  • Reactive transient liquid phase bonding of ceramic to steel using Zr-Cu-Zr- and Zr-Ni-Cu-Zr-interlayers for high temperature applications
    Tillmann, W. and Pfeiffer, J. and Wojarski, L. and Indacochea, J.-E.
    Materialwissenschaft und Werkstofftechnik 45 (2014)
    Joints manufactured by transient liquid phase bonding feature comparable properties as diffusion weldements, but considerably lower process temperatures and pressures have to be applied. The liquid phase, which is hereby used, occurs due to interdiffusion between the base and/or the filler materials at a constant temperature, which lies below the melting temperature of the substrates. An essential requirement for this diffusion-based melting is that the involved materials have low melting alloy-constitution areas, such as eutectics. The aim of the study, presented in this contribution, is to evaluate an approach, in which an active transient liquid is created by suitable interlayers, in order to facilitate the wetting of ceramics. The potential of this attempt will be illustrated on zirconia/stainless-steel-joints for high temperature applications, such as solid oxide fuel cells. In such applications, the used materials have to withstand harsh conditions, e.g. high operating temperatures, oxidizing or reducing environments, which represent a demanding challenge for joining technologies, even at the latest state of research. In this study interlayers, consisting of Zirconium, as the active element, in combination with Copper and/or Nickel, have been investigated. These systems exhibit a wide range of alloy-constitutions with low melting temperatures, which can be used for the formation of the transient liquid phase. For the application of the interlayers, physical vapor deposition as well as 75 μm-thick Nickel-foils have been used. The joining was carried out in high vacuum with changing holding times and temperatures. Additionally, the ratio of the thickness of the used interlayers was changed. Results of microstructural investigations, nano-hardness measurements of the joining area as well as shear strength and fractography are presented. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201400267
  • Sliding and rolling wear behavior of hvof-sprayed coatings derived from conventional, fine and nanostructured WC-12Co powders
    Tillmann, W. and Baumann, I. and Hollingsworth, P.S. and Hagen, L.
    Journal of Thermal Spray Technology 23 (2014)
    Fine structured and nanostructured materials represent a promising class of feedstock for future applications, which has also attracted increasing interest in the thermal spray technology. Within the field of wear protection, the application of fine structured or nanostructured WC-Co powders in the High Velocity Oxy-Fuel flame spraying technique (HVOF) provides novel possibilities for the manufacturing of cermet coatings with improved mechanical and tribological characteristics. In this study the tribological behavior of HVOF sprayed coatings derived from conventional, fine and nanostructured WC-12Co powders under sliding and rolling wear are investigated and the results are compared to C45 steel (Mat.-No. 1.0503). In addition, sliding and rolling wear effects on a microscopic level are scrutinized. It has been shown that under optimized spray conditions the corresponding fine and nanostructured WC-12Co coatings are able to obtain higher wear resistances and lower friction coefficients than the conventional coatings. This can be attributed to several scaling effects of the microstructure and to the phase evolution of the coating, which are discussed. © 2013 ASM International.
    view abstract10.1007/s11666-013-0038-1
  • Studying the Effect of the Air-Cap Configuration in Twin-Wire Arc-Spraying Process on the Obtained Flow Characteristics Using Design of Experiment Oriented Fluid Simulation
    Tillmann, W. and Abdulgader, M. and Anjami, N. and Hagen, L.
    Journal of Thermal Spray Technology 24 (2014)
    The computational fluid dynamics approach is adopted in this work using the design of experiments to reveal the effect of the air-cap configurations on the obtained gas velocity, the shear stresses, the high velocity zone, and the convergence of the obtained spraying plume in the twin-wire arc-spraying process. The parameters, which are revealed to optimize the air-cap configuration, are the throat diameter, the convergence angle of the throat inlet, the throat length, and the distance between the throat outlet and the intersection point of the approaching wires. The throat length is dependent upon the other configuration parameters. Outlet gas velocity, the turbulence in the flow, and the exerted shear stresses at wire tips are directly affected by the dominating flow regimes near the intersection point of the approaching wires. The presence of wires and the contact tips in the gas flow has enormous impact on the obtained flow characteristics. Air-cap throat diameter and the distance between throat outlet and intersection point determine the shape and length of the obtained high velocity zone in the spraying plum. © 2014, ASM International.
    view abstract10.1007/s11666-014-0183-1
  • The influence of multilayer design on residual stress gradients in Ti/TiAlN systems
    Selvadurai, U. and Tillmann, W. and Fischer, G. and Sprute, T.
    Materials Science Forum 768-769 (2014)
    In this research work, Ti/TiAlN multilayers of various designs were deposited on substrates pretreated by nitriding and etching procedures. The influence of the multilayer design on residual stress depth profiles was systematically analyzed for multilayers with different Titanium interlayer thicknesses. The depth-dependency of stress was measured by a modified sin2ψ method, using various defined gracing incident angles and measuring angles that ensure constant penetration depths. The residual stresses were investigated by synchrotron X-ray diffraction (SXRD) at the HASYLAB at DESY in Hamburg, Germany. SXRD allows a phase specific stress evaluation of the ceramic and metallic layers of the multilayer systems and the adjacent substrate region. This investigation reveals an influence of the Ti layer thickness on the values and the slope of the residual stress profile in ceramic TiAlN layers. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.768-769.264
  • TLP brazing of aluminum to steel using PVD-deposited interlayer
    Wojarski, L. and Tillmann, W.
    Welding in the World 58 (2014)
    The demand for hybrid material concepts is steadily growing, and especially dissimilar joints between aluminum and steel are, due to their wide dissemination, of major importance. The main obstacle for the fabrication of aluminum-steel joints using thermal processes is the embrittlement of the fusion area. In order to prevent direct contact between aluminum and iron and thus to suppress the formation of brittle iron aluminides, 3-μm thick diffusion barrier coatings, consisting of Ni or Ti, were applied onto the steel surface. Pure copper with a thickness of 3 and 6 μm, respectively, was used as a filler material, and the samples were brazed in a TLP process in a vacuum at 580 °C at varying dwell times (10...50 min). The samples brazed with Ni diffusion barriers showed a considerable formation of Fe2Al 5 even at low dwell times. Furthermore, additional complex ternary phase bands have generated due to the existence of diffusion barrier elements and were detected in the interfacial area. The application of Ti showed a significant decrease of iron aluminides, and no Fe2Al5 could be detected at low dwell times, resulting in a shear strength of 42 MPa for the optimized parameters. © 2014 International Institute of Welding.
    view abstract10.1007/s40194-014-0143-x
  • Tribological and mechanical properties of Ti/TiAlN/TiAlCN nanoscale multilayer PVD coatings deposited on AISI H11 hot work tool steel
    Al-Bukhaiti, M.A. and Al-Hatab, K.A. and Tillmann, W. and Hoffmann, F. and Sprute, T.
    Applied Surface Science 318 (2014)
    A new [Ti/TiAlN/TiAlCN]5 multilayer coatings were deposited onto polished substrate AISI H11 (DIN 1.2343) steel by an industrial magnetron sputtering device. The tribological performance of the coated system was investigated by a ball-on-disk tribometer against 100Cr6 steel and Al2O3 balls. The friction coefficients and specific wear rates were measured at various normal loads (2, 5, 8, and 10 N) and sliding velocities (0.2, 0.4, and 0.8 m/s) in ambient air and dry conditions. The phase structure, composition, wear tracks morphologies, hardness, and film/substrate adhesion of the coatings were characterized by light-microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), 3D-surface analyzer, nanoindentation, and scratch tests. Results showed that the deposited coatings showed low wear rates in the scale of 10-15 m3/N m, low friction coefficients against 100Cr6 and Al2O3 balls in the range of 0.25-0.37, and good hardness in the range of 17-20 GPa. Results also revealed that the friction coefficients and disc wear rates decrease and increase, respectively with the increase in normal load and sliding velocity for both coating/Al2O3 and coating/100Cr6 sliding system. Compared with the uncoated-H11 substrate, the deposited coating exhibited superior tribological and mechanical properties. The dominant wear mechanism was abrasive wear for coating/Al2O3 pair, while for coating/100Cr6 pair, a combination of mild adhesive wear, severe adhesive wear, and abrasive wear (extensive plowing) were the dominant wear mechanisms at different applied normal loads. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apsusc.2014.03.026
  • Wear analysis of thermal spray coatings on 3D surfaces
    Tillmann, W. and Luo, W. and Selvadurai, U.
    Journal of Thermal Spray Technology 23 (2014)
    Even though the application of thermal spray coatings on complex geometries gained a greater interest in the last decade, the effect of different geometrical features on the wear behavior is still ill-defined. In this study, the wear resistance of FTC-FeCSiMn coated 3D surfaces was investigated. The wear test was carried out by means of two innovative testing procedures. The first test is a Pin-on-Tubes test where the rotating motion is realized by a lathe chuck. The specimens in the second test were fixed on the table and a robot arm operated the pin. This wear test was applied on specimens with concave or convex surfaces. The residual stresses, which were determined by means of an incremental hole-drilling method, show a dependency on the substrate geometry. The obtained stresses were put in relation to the different radii. After the wear test, a 3D-profilometer determined the wear volume and the sections of the coatings were characterized by a scanning electron microscope. The results indicate that the wear resistance is strongly influenced by the geometry of the substrate. © 2013 ASM International.
    view abstract10.1007/s11666-013-9983-y
  • 3D analysis of micro-deformation in VHCF-loaded nodular cast iron by μCT
    Fischer, G. and Nellesen, J. and Anar, N.B. and Ehrig, K. and Riesemeier, H. and Tillmann, W.
    Materials Science and Engineering A 577 (2013)
    The impact of very high cycle fatigue (VHCF) load conditions on the microstructure of specimens consisting of nodular cast iron is analyzed by means of micro-computed tomography (μCT) utilizing both monochromatic synchrotron radiation and polychromatic X-ray tube radiation. Using 3D μCT, the microstructure in the region of the smallest cross-sections of shouldered round specimens is imaged in different stages of the VHCF loading. By digital image correlation (DIC) of these tomograms strain fields are analyzed three-dimensionally. Strain levels in the range of a few percent were detected. It is proven that a localization of strain allows to predict the site of the crack which precedes and induces the macroscopic failure of the specimens. © 2013 Elsevier B.V.
    view abstract10.1016/j.msea.2013.04.057
  • A study of mechanical and tribological properties of self-lubricating TiAlVN coatings at elevated temperatures
    Tillmann, W. and Momeni, S. and Hoffmann, F.
    Tribology International 66 (2013)
    There are several manufacturing processes, in which the employment of solid lubricants is limited. In addition, ecological damage and higher production costs are further consequences of using such solid lubricants. This work aims at using the great potential of thin film technology to deposit adaptive, self-lubricating coatings as an alternative to conventional solid lubricants. Using magnetron sputtering process several titanium aluminum vanadium nitride coatings (TiAlVN) were developed in this study. These quaternary coatings possess the ability of forming lubricious oxides, known as Magnéli phases, at elevated temperatures, which significantly reduces the friction coefficient and surface wear. © 2013 Elsevier Ltd.
    view abstract10.1016/j.triboint.2013.06.007
  • Biocompatibility of rapidly solidified magnesium alloy RS66 as a temporary biodegradable metal
    Willbold, E. and Kalla, K. and Bartsch, I. and Bobe, K. and Brauneis, M. and Remennik, S. and Shechtman, D. and Nellesen, J. and Tillmann, W. and Vogt, C. and Witte, F.
    Acta Biomaterialia 9 (2013)
    Biodegradable magnesium-based alloys are very promising materials for temporary implants. However, the clinical use of magnesium-based alloys is often limited by rapid corrosion and by insufficient mechanical stability. Here we investigated RS66, a magnesium-based alloy with extraordinary physicochemical properties of high tensile strength combined with a high ductility and a homogeneous grain size of ∼1 μm which was obtained by rapid solidification processing and reciprocal extrusion. Using a series of in vitro and in vivo experiments, we analyzed the biodegradation behavior and the biocompatibility of this alloy. In vitro, RS66 had no cytotoxic effects in physiological concentrations on the viability and the proliferation of primary human osteoblasts. In vivo, RS66 cylinders were implanted into femur condyles, under the skin and in the muscle of adult rabbits and were monitored for 1, 2, 3, 4 and 8 weeks. After explantation, the RS66 cylinders were first analyzed by microtomography to determine the remaining RS66 alloy and calculate the corrosion rates. Then, the implantation sites were examined histologically for healing processes and foreign body reactions. We found that RS66 was corroded fastest subcutaneously followed by intramuscular and bony implantation of the samples. No clinical harm with transient gas cavities during the first 6 weeks in subcutaneous and intramuscular implantation sites was observed. No gas cavities were formed around the implantation site in bone. The corrosion rates in the different anatomical locations correlated well with the local blood flow prior to implantation. A normal foreign body reaction occurred in all tissues. Interestingly, no enhanced bone formation could be observed around the corroding samples in the condyles. These data show that RS66 is biocompatible, and due to its interesting physicochemical properties, this magnesium alloy is a promising material for biodegradable implants. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actbio.2013.02.015
  • Brazing of cutting materials
    Tillmann, W. and Elrefaey, A. and Wojarski, L.
    Advances in Brazing: Science, Technology and Applications (2013)
    An important brazing application is the production of cutting and machining tools. This chapter provides an overview of brazing alloys for the joining of cutting materials. Process-related problems and solutions are presented and discussed. High strength values can be achieved with the right filler alloy and joint design.Most cemented carbides consist of a hard material such as tungsten carbide and a metallic binder. Due to the high carbide content, wetting and bonding reactions have to be monitored. Addition of active elements can help to induce a wetting reaction. Brazing ceramic cutting materials requires the use of active filler alloys to achieve a wetting reaction. The chapter reviews the alloys required. A joint can be weakened by interfacial effects or mismatches in physical properties. Employing the right filler metal with an adapted brazing process can significantly improve joint quality. Stress calculations based on the finite element method contribute to better joint design with reduced stress levels. © 2013 Woodhead Publishing Limited. All rights reserved.
    view abstract10.1533/9780857096500.3.423
  • Carbon reactivity of binder metals in diamond-metal composites - Characterization by scanning electron microscopy and X-ray diffraction
    Tillmann, W. and Ferreira, M. and Steffen, A. and Rüster, K. and Möller, J. and Bieder, S. and Paulus, M. and Tolan, M.
    Diamond and Related Materials 38 (2013)
    Diamond tooling is a successfully used technique in machining of very hard materials such as minerals and concrete. The type and strength of bonding between the diamond grains, that are mainly responsible for the machining process (e.g. cutting or grinding), and the metallic binder phase is directly linked to the tools quality. Therefore it is of interest to investigate the carbon reactivity of commonly used binder materials. This paper reports about the investigation of the interfacial area between diamonds and one-component metallic binder matrices. As matrix material pure chromium, cobalt, copper, iron, and nickel was used. After the sintering process the diamonds were extracted from the metallic matrix and analyzed by scanning electron microscopy and X-ray diffraction. The morphology of the diamond surface was investigated and a phase analysis was done. These experimental studies support the hypothesis that the carbon reactivity of transition metals is linked to their d-orbital electron configuration. © 2013 Elsevier B.V.
    view abstract10.1016/j.diamond.2013.07.002
  • High temperature wettability of multicomponent CrAlSiN and TiAlSiN coatings by molten glass
    Chang, Y.-Y. and Cheng, C.-M. and Liou, Y.-Y. and Tillmann, W. and Hoffmann, F. and Sprute, T.
    Surface and Coatings Technology 231 (2013)
    The phenomenon of glass-to-mold sticking is a major problem for industrial glass forming processes. Ternary TiAlN coatings attracted considerable industrial interest because of their excellent tribological performance and high oxidation resistance at high temperatures. Recently, multicomponent CrAlSiN and TiAlSiN coatings have been developed in order to gain high hardness and good thermal stability at temperatures exceeding 800. °C. In this study, CrAlSiN, TiAlSiN and AlTiN coatings were deposited on tungsten carbide substrates by using a cathodic-arc deposition system with lateral rotating arc cathodes. Titanium, chromium and AlSi (12. at.% of Si) cathodes were used for the deposition of CrAlSiN and TiAlSiN coatings. All the deposited CrAlSiN, TiAlSiN and AlTiN coatings showed a B1-NaCl crystal structure. The deposited CrAlSiN and TiAlSiN coatings exhibited nanocrystalline structure and possessed hardness as high as 35-37. GPa after annealing at 700. °C in air. The wettability of the CrAlSiN, TiAlSiN and AlTiN coated tungsten carbides by molten glass at temperatures between 300. °C and 700. °C in controlled air under 1.6. Pa was measured by using an improved sessile drop method. The CrAlSiN showed a low oxidation rate and a non-wetting characteristic superior to TiAlSiN and AlTiN coatings. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.04.050
  • Improvement of wear resistant thermally sprayed coatings by microfinishing
    Biermann, D. and Goeke, S. and Tillmann, W. and Nebel, J.
    CIRP Annals - Manufacturing Technology 62 (2013)
    To increase the wear resistance of tribologically highly stressed slide faces, thermally sprayed coatings based on tungsten carbide are widely used. In addition to the technological progress in powder metallurgy and coating technology, an improved preparation of the substrate surface increases the fatigue resistance of the coated workpieces. Surfaces machined by short-stroke honing instead of the commonly used abrasive blasting process, show a significantly enhanced interface between the surface and the coating. In addition, the use of diamond as hard cutting material enables the wear resistant coating to be honed and a high surface quality to be generated. © 2013 CIRP.
    view abstract10.1016/j.cirp.2013.03.023
  • In vitro and in vivo evaluation of biodegradable, open-porous scaffolds made of sintered magnesium W4 short fibres
    Bobe, K. and Willbold, E. and Morgenthal, I. and Andersen, O. and Studnitzky, T. and Nellesen, J. and Tillmann, W. and Vogt, C. and Vano, K. and Witte, F.
    Acta Biomaterialia 9 (2013)
    A cytocompatible and biocompatible, degradable, open-porous, mechanically adaptable metal scaffold made of magnesium alloy W4 melt-extracted short fibres was fabricated by liquid phase sintering. Cylindrical samples (3 × 5 mm) of sintered W4 short fibres were evaluated under in vitro (L929, HOB, eudiometer, weight loss) and in vivo conditions (rabbits: 6 and 12 weeks). The in vitro corrosion environment (e.g., temperature, flow, composition of corrosion solution, exposure time) significantly influenced the corrosion rates of W4 scaffolds compared with corrosion in vivo. Corrosion rates under cell culture conditions for 72 h varied from 1.05 to 3.43 mm y-1 depending on the media composition. Corrosion rates measured in eudiometric systems for 24 h were ∼24-27 times higher (3.88-4.43 mm y-1) than corrosion in vivo after 6 weeks (0.16 mm y-1). Moreover, it was found that the cell culture media composition significantly influences the ionic composition of the extract by selectively dissolving ions from W4 samples or their corrosion products. A pilot in vivo study for 6 and 12 weeks demonstrated active bone remodelling, no foreign body reaction and no clinical observation of gas formation during W4 scaffold implantation. Long-term in vivo studies need to be conducted to prove complete degradation of the W4 scaffold and total replacement by the host tissue. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actbio.2013.03.035
  • Influence of bias voltage on residual stresses and tribological properties of TiAlVN-coatings at elevated temperatures
    Tillmann, W. and Sprute, T. and Hoffmann, F. and Chang, Y.-Y. and Tsai, C.-Y.
    Surface and Coatings Technology 231 (2013)
    The extension of tool life is a crucial goal for heat resistant forming tools. Therefore, the industry is interested to reduce the friction and wear for these tools. The employment of metals, polymeric composites, and ceramics as solid lubricants increases the production as well as maintenance costs. Thus, the thin film technology and especially new self-lubricating coatings will become increasingly important. Titanium aluminum vanadium nitride as a self-lubricating coating has a high potential to improve the tribological behavior of heat resisting tool surfaces and has good mechanical properties such as a high hardness (more than 40GPa). In this study, TiAlVN coatings were deposited on HS6-5-2C high speed steel substrates by using a magnetron sputtering system. After annealing at 650°C, a V2O5 (Magnéli phase) which adds self-lubricating qualities to the coatings could be detected in the TiAlVN layer. Due to the influence of adhesive and cohesive damage processes, resulting from the residual stress behavior in the layer close to the substrate area, it is critical to measure residual stresses in order to increase the wear resistance. In addition to the phase analyses, residual stress measurements were investigated by means of x-ray diffractometry as well. An experimental method, based on the traditional sin2ψ-method and utilizing a grazing-incidence diffraction geometry was used in order to enhance the irradiation volume of thin film samples. This resulted in a higher intensity for high-angle Bragg peaks than for the Bragg-Brentano geometry. Furthermore, the mechanical and tribological properties of the TiAlVN coatings were characterized at elevated temperatures. The required results were provided by a high temperature ball-on-disk device and a nanoindenter. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.03.012
  • Influence of fine powder feedstock (-10 + 2 μm) on the HVOF spraying characteristics, coating morphology, and properties of WC-CoCr 86-10-4
    Tillmann, W. and Nebel, J. and Piotrowski, W.
    Journal of Thermal Spray Technology 22 (2013)
    The use of fine feedstock powder can extend the feasibility and scope of HVOF coatings to new fields of applications. Especially for the purpose of near-net-shape coatings, these powders facilitate homogeneous layer morphologies, and smooth coating surfaces. However, the small particle sizes also lead to several challenges. One major issue is the in-flight behavior which is distinctly affected by the low mass and relatively large specific surface of the particles. In this paper, the in-flight and coating characteristics of WC-CoCr 86-10-4 (-10 + 2 μm) were investigated. It was determined that the fine powder feedstock shows a high sensitivity to the gas flow, velocity, and temperature of the spray jet. Because of their low mass inertia, their velocity, for example, is actually influenced by local pressure nodes (shock diamonds) in the supersonic flow. Additionally, the relatively large specific surface of the particles promotes partial overheating and degradation. Nevertheless, the morphological and mechanical properties of the sprayed layer are hardly affected. In fact, the coatings feature a superior surface roughness, porosity, hardness, and wear resistance. © 2012 ASM International.
    view abstract10.1007/s11666-012-9832-4
  • Influence of parameter variations on WC-Co splat formation in an HVOF process using a new beam-shutter device
    Tillmann, W. and Hussong, B. and Priggemeier, T. and Kuhnt, S. and Rudak, N. and Weinert, H.
    Journal of Thermal Spray Technology 22 (2013)
    The formation of single splats is the foundation for any thermal spray coating. Therefore, this study focuses on the investigation of single splat morphologies to determine the influence of spray parameters on the morphological distribution of particles inside the flame. A new method to create a footprint of a spray jet with an extremely short exposure time was used. The resulting field of splats enabled the assignment of each splat to its radial position in the spray jet. The footprints were analyzed and the quantities and morphologies of the splats were correlated to particle in-flight measurements and coating properties. A strong correlation between the particle velocity, the percentage of the so-called pancake-like splats, and the porosity of the coating could be revealed. The influence of the particle temperature was found to be of minor importance to the splat form and the porosity of the coatings. Still, the particle temperature had a good correlation with the coating hardness and the dissolving of the WC. Measurements of the splat size in different areas of the footprints revealed that the percentage of splats larger than 40 μm in diameter was generally higher in the center of the footprint than in the outer regions. © 2013 ASM International.
    view abstract10.1007/s11666-012-9881-8
  • Influence of the spray angle on the properties of HVOF sprayed WC-Co coatings using (-10 + 2 μm) fine powders
    Tillmann, W. and Baumann, I. and Hollingsworth, P. and Laemmerhirt, I.-A.
    Journal of Thermal Spray Technology 22 (2013)
    The application of fine powders in thermal spray technology represents an innovative approach to apply dense and smooth near-net shape coatings on tools with complex geometry. However, this aim can only be achieved as long as the influence of the handling parameters of the spray process, such as the spray angle, is sufficiently understood. In this study, the influence of the spray angle on the deposition rate as well as on the coating properties (microhardness, roughness, and porosity) of HVOF-sprayed, fine-structured coatings are investigated. A fine, agglomerated, and sintered WC-12Co powder (agglomerate size: 2-10 μm, WC-particle Fisher sub-sieve size = 400 nm) was used as feedstock material. It has been shown that HVOF spraying of fine powders is less susceptible to an alteration of the spray angle than most other thermal spray processes such as plasma- or arc-spraying. The reduction of the spray angle results in a decrease in the deposition rate, while no significant degradation of the coating properties is found up to 30. However, at spray angles below 30 the coating strength is negatively affected by the formation of pores and cracks. © 2013 ASM International.
    view abstract10.1007/s11666-013-9882-2
  • Innovative tools to improve incremental bulk forming processes
    Sieczkarek, P. and Kwiatkowski, L. and Tekkaya, A.E. and Krebs, E. and Kersting, P. and Tillmann, W. and Herper, J.
    Key Engineering Materials 554-557 (2013)
    Sheet-bulk metal forming is an innovative process with a high potential to generate load-adapted parts with high precision. Bulk forming processes of sheet metals especially require high process forces, resulting in an intense contact pressure and, thus, in a very high abrasive and adhesive wear. As a method to reduce or avoid these common wear phenomena, even hardened or coated tool surfaces are not sufficient. The objective of this paper is to show an improvement of the tool resistance during an incremental forming process by an adapted tool design and the application of structured tool surfaces combined with coatings. For the tool surface the structure of the scarabaeus beetle serves as the basis for a bionic structure. This structure was manufactured by micromilling. Despite the high hardness of the tool material and the complex geometry of the forming tools, very precise patterns were machined successfully using ball-end milling cutters. The combination of bionic structures with coating techniques like physical vapor deposition (PVD) on plasma nitrided tool surfaces is very promising. In this work, the influence of process parameters (workpiece material, lubrication, tool design, stepwise infeed) on the tool resistance during the forming operation was analyzed experimentally. The results of the optimized forming tools were compared to conventional, unstructured, uncoated, and only plasma nitrided forming tools. The different tools were applied to 2 mm thick metal sheets made of aluminum (AlMg3) and steel (nonalloy quality steel DC04). As a result, the process forces could be reduced by a modified shape and surface of the tools. Thus, the lifetime of the tools can be enhanced. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.1490
  • Joining of aluminium sheets by combined solid state and TLP bonding processes
    Lee, E. and Quintana, O. and Indacochea, J.E. and Wojarski, L. and Pfeiffer, J. and Tillmann, W.
    Science and Technology of Welding and Joining 18 (2013)
    Our effort is to develop an innovative process to join Al sheets at moderate temperatures (200-300uC) by accumulative roll bonding (ARB) combined with transient liquid phase bonding utilising Ga. Solid state joining of Al is limited by the surface Al2O3 that hampers the Al atom diffusion despite the intimate contact between the metal sheets. Mechanical brushing of the Al2O3 improves bonding. Our approach uses ARB to break the brittle Al2O3 by roll bonding two sheets. Strips of 1100-Al were cut, cleaned coated with a thin film of Ga and then preheated before rolling. The Ga coated Al strips roll bonded at 200uC showed greatest strengths, but processing at 300uC resulted in lower strengths with evidence of liquid metal embrittlement. The undamaged Al2O3 prevents liquid Ga from contact with the Al metal. © 2013 Institute of Materials, Minerals and Mining.
    view abstract10.1179/1362171812Y.0000000087
  • Measurement of the Young's modulus of thermal spray coatings by means of several methods
    Tillmann, W. and Selvadurai, U. and Luo, W.
    Journal of Thermal Spray Technology 22 (2013)
    Thermally sprayed coatings are usually defined by their hardness, porosity, roughness, and wear resistance. Even though the Young's modulus is an essential property, which describes the mechanical behavior of the coated components during their use, only few efforts have been made in the past to determine this property. The most common measurement methods of the Young's modulus of thermally sprayed coatings are tensile tests, bending tests, and nanoindentations. During the tensile and bending tests a sliding of the splats can occur due to the laminar structure of the thermally sprayed coatings, influencing the measurement value. When using the nanoindentation test, only the elastic behavior of some splats can be determined because of a minimal measuring volume. However, the Young's modulus of thermally sprayed coatings can also be determined by means of a resonant method, called impulse excitation technique. In this paper, the values of the Young's moduli of thermally sprayed coatings, measured by several methods, are compared with each other and correlated to the microstructure of the coatings, investigated by means of scanning electron microscopy. © 2012 ASM International.
    view abstract10.1007/s11666-012-9855-x
  • New coating systems for temperature monitoring in turning processes
    Biermann, D. and Kirschner, M. and Pantke, K. and Tillmann, W. and Herper, J.
    Surface and Coatings Technology 215 (2013)
    High temperature loads in cutting processes can cause high tool wear and damages in the subsurface zone of the workpiece. Especially, the interaction between different cutting parameters affects the thermal loads in the cutting zone. Hence, the knowledge of temperatures in cutting processes is an important factor, and it is the main focus of current investigations. Therefore, this paper deals with an in-process monitoring system for the resulting temperatures in a turning process. In contrast to the investigations performed hitherto, this research deals with a new tool sensor system for temperature measurement. This sensor system is realized by a PVD coating of a Nickel and a Nickel-Chromium layer on the rake face of cutting inserts. On the junction points of this layer system, three thermocouples are deposited. The development of the coating system and the resulting measurement is shown. Additionally, the results are discussed in comparison to thermal imaging system and conventional thermocouples. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.08.086
  • Preliminary study for flaw detection in nodular cast iron by cyclic loading and thermography
    Tillmann, W. and Feng, X. and Fischer, G. and Nellesen, J.
    NDT and E International 56 (2013)
    The objective of this paper is to deduce thermographic parameters from infrared (IR) picture series which indicate the presence of bulk defects in nodular cast iron. For this task, specimens manufactured with and without artificial defects are sinusoidally loaded in the range of the fatigue limit for a short period. During this time, the IR picture series were recorded with a high-speed IR camera. After a picture alignment, the time-dependency of the average gray values in picture subsets was analyzed by discrete Fourier transform (DFT) to get the magnitude and phase shift of the fundamental frequency of the local IR radiation power. The results of the experiments and FE simulations indicate consistently that the volume defects can be detected on the basis of the average and standard deviation of the phase shift distributions on the surface of the specimens. © 2013 Elsevier Ltd.
    view abstract10.1016/j.ndteint.2013.01.017
  • Quality prediction of twin wire Arc sprayed coatings using acoustic Emission Analysis
    Tillmann, W. and Abdulgader, M. and Wang, G. and Zielke, R.
    Journal of Thermal Spray Technology 22 (2013)
    In this work, acoustic emission analysis is utilized in the twin wire arc spraying (TWAS) process to study the influence of the adjustable process parameters on the simultaneously obtained acoustic signals at the nozzle and at the substrate. The amplitude of recorded signals at the substrate was in general much higher than those recorded at the nozzle. At the substrate side, the amplitude of emitted acoustic signals is dependent on feedstock materials and is higher when using solid wires. The acoustic signals were recorded at the spraying gun for different gas pressures without arc ignition (as dry runs) in order to reveal the effect of the arc on the emitted acoustic signals. A correlation between controllable parameters, the acoustic signals, and the obtained in-flight particle characteristics was observed. This work contributes to the online control of TWAS processes and is one of many proposed publications in the research field of the conducted acoustic emission analysis. © 2012 ASM International.
    view abstract10.1007/s11666-012-9861-z
  • Trade-off analysis considering tomogram quality and performance of a parallel computing hardware realization of Katsevich's reconstruction algorithm
    Nellesen, J. and Weichert, F. and Timm, C. and Kerbitz, R.A. and Tillmann, W.
    IEEE Transactions on Nuclear Science 60 (2013)
    In this paper, the trade-off between the target variables tomogram quality and reconstruction time of an implementation of Katsevich's exact reconstruction algorithm for helical cone-beam computed tomography (CT) is analyzed. This is accomplished by means of an OpenCL-based, parallel and portable realization of Katsevich's algorithm. The detailed examination is carried out on a phantom object which is composed of cubes with different attenuation coefficients. For this phantom object, X-ray cone-beam projections are simulated with different helix trajectory parameters. From the simulated projection data sets 3D tomograms are reconstructed. The impacts of the helix parameters on the tomogram quality and the reconstruction time are measured. Moreover, the speedup in dependence of the number of computing units on the OpenCL device is examined. To sum up the results: the tomogram quality increases monotonically with ascending number of projections while keeping the helix pitch fixed. A linear relationship between the overall runtime and the number of projections exists. The trade-off analysis between the tomogram quality and the reconstruction time proves that an optimum is reached if the criteria for the lateral and vertical sampling of the 3D space are fulfilled. © 1963-2012 IEEE.
    view abstract10.1109/TNS.2013.2265660
  • Tribological comparison of different surface topographies coated with chromium aluminum nitride
    Tillmann, W. and Herper, J. and Laemmerhirt, I.-A.
    Materialwissenschaft und Werkstofftechnik 44 (2013)
    The reduction of friction and wear is one important goal for the extension of the tool life in many industrial applications. The forming and cutting industries in particular, are very interested in new techniques and surface coating characteristics that will improve the tribological behavior of the tools. Biomimetics is a very promising approach using biological surfaces or phenomena to optimize the properties of engineering components. The "Lotus Effect" is the most famous example, whereby surfaces are made water- and dirt-repellent. Taking a closer look at nature, it can be noticed that many different natural surfaces have perfectly adapted to their environment in order to meet the respective requirements. Extending the use of natural and biological effects to improve the material performance, it was discovered that the skin of many insects has an excellent frictional behavior and thus the potential to be transferred onto technical surfaces. In this paper, the surface structure of a dung beetle (db) was investigated. The main objective was to combine nature-adapted surface patterns with wear-resistant near-netshape PVD-coatings (PVD=physical vapour deposition), in order to improve the tribological properties of a tool surface. The shell of the beetle served as a pattern for the structure of the surfaces. A substrate, composed of high speed steel material, was structured by means of milling prior to the deposition of a chromium aluminum nitride multilayer coating system, using a magnetron sputtering process. The mechanical and tribological properties of the structured and coated surfaces were compared by means of nanoindentation, ball-on-disc-testing, and scanning electron microscopy. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201300183
  • Vacuum brazing titanium using thin nickel layer deposited by PVD technique
    Elrefaey, A. and Wojarski, L. and Janczak-Rusch, J. and Tillmann, W.
    Materials Science and Engineering A 565 (2013)
    In this study, the evolution of the interfacial microstructure, hardness distribution, and the joint strength of vacuum brazed commercially pure titanium were evaluated. A thin nickel layer, with different thicknesses, was deposited by PVD technique to serve as the brazing filler metal. Test joints were processed at temperatures of 910°C and 960°C using a soaking time of 15 and 90min. The experimental results showed that sound joints with a good wetting quality as well as lack of pores and cracks can be achieved at a brazing temperature of 960°C. A Ti2Ni intermetallic compound was formed at the interfacial area at a soaking time of 15min and with a deposition rate of 90AH which was detrimental to the joint mechanical properties. Meanwhile, at a soaking time of 90min, intermetallic compound was not detected and the diffusion of nickel was completed at all deposition rates which improve the shear strength of the joints. © 2012 Elsevier B.V..
    view abstract10.1016/j.msea.2012.12.028
  • Wire composition: Its effect on metal disintegration and particle formation in twin-wire arc-spraying process
    Tillmann, W. and Abdulgader, M.
    Journal of Thermal Spray Technology 22 (2013)
    The wire tips in twin-wire arc-spraying (TWAS) are heated in three different zones. A high-speed camera was used to observe the melting behavior, metal breakup, and particle formation under different operating conditions. In zone (I), the wire tips are melted (liquidus metal) and directly atomized in the form of smaller droplets. Their size is a function of the specific properties of the molten metal and the exerting aerodynamic forces. Zone (II) is directly beneath zone (I) and the origin of the extruded metal sheets at the wire tips. The extruded metal sheets in the case of cored wires are shorter than those observed while using solid wires. In this study, the effects of adjustable parameters and powder filling on melting behavior, particle formation, and process instability were revealed, and a comparison between solid and cored wires was made. The findings can improve the accuracy of the TWAS process modeling. © 2012 ASM International.
    view abstract10.1007/s11666-012-9870-y
  • Analysis of Crack Patterns in Drying Corn Starch by in-situ Radiography and X-ray Computer Tomography
    Crostack, H.-A. and Nellesen, J. and Fischer, G. and Hofmann, M. and Rademacher, H.-G. and Tillmann, W.
    Experimental Mechanics 52 (2012)
    The formation of crack patterns in drying starch-water slurries is studied by means of in-situ radiography (measuring of the crack front velocity) and X-ray microtomography as an example of crack patterns driven by inhomogeneous shrinkage. The tomograms show the 3D crack networks forming columns with polygonal cross-sections. After crack initiation, the average crack spacing increases with growing depth, even if the crack front velocity is constant. A constant velocity is obtained by maintaining a constant evaporation rate using a feedback control. When the crack front has propagated at a constant evaporation rate over a distance of some millimeters, the average crack spacing approaches a stable value which depends on crack front velocity according to a power law. This relationship is compared to corresponding results of other authors and model predictions. The increase of crack spacing before stable values are achieved, is interpreted as a result of successive crack front instabilities. © 2012 Society for Experimental Mechanics.
    view abstract10.1007/s11340-011-9581-1
  • Correlation between tribological properties, sp 2/sp 3-ratio and H-content of low-wear diamond-like carbon (DLC) layers
    Vogli, E. and Hoffmann, F. and Bartis, E. and Oehrlein, G.S. and Tillmann, W.
    Materials Science Forum 706-709 (2012)
    It has been established that hardness and density of diamond-like carbon (DLC) layers can be raised by increasing ion energy during deposition, decreasing H-content and by increasing sp 3-fraction. To confirm differences in hydrogen content of hydrogen containing and hydrogen free DLC films deposited at different bias voltages, layers were etched in oxygen atmosphere in a capacitively coupled plasma device. By employing real-time ellipsometry measurements, the Hcontent of the hydrogen containing a-C:H layers were estimated by determining the optical constants n and k (n-real part and k-imaginary part of the refractive index). In addition, DLC layers were analyzed by X-ray photoelectron spectroscopy to estimate the ratio of sp 2- and sp 3- hybridization. The mechanical and tribological properties of the coatings were evaluated by means of nanoindentation and ball-on-disc-tests. Finally correlations between these properties, H-content and sp 3/sp 2-ratio were obtained in an effort to explain different tribological behaviors of DLC-layers. © 2012 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.706-709.2596
  • Deposition of hard and adherent TiBCN films for cutting tools applications
    Tillmann, W. and Bejarano, G. and Hoffmann, F.
    Physica Status Solidi (A) Applications and Materials Science 209 (2012)
    Metal cutting tools having wear resistant and chemically stable ceramic coatings are in many applications superior in performance to uncoated tools. Titanium boron carbon nitride (TiBCN) is a hard material particularly suitable as a protective coating for cutting tools due to its excellent properties, such as a high hardness and high wear and corrosion resistance, among other. TiBCN films were grown on Si (100) and high speed steel substrates by means of reactively pulsed DC magnetron sputtering technique. Two B 4C- and two Ti-targets, to which a pulsed DC voltage of middle frequency was applied, were used for the deposition of TiBCN. A chromium layer was first deposited to obtain a better adhesion of TiBCN to the substrates. The mechanical properties of these coatings deposited under different N 2 contents were investigated. The substrates were biased through a medium frequency power supply. The bias voltage value was -90 V for all coatings. The total film thickness was maintained at approximately 2 μm. The hardness of the coatings increased with reduced nitrogen content, while the adhesion decreased from 40.8 to 24.2 N, and the wear rate increased from 0.154 to 0.744 × 10 -16 m 3/N.m, the latter probably caused by the low content of the self-lubricating amorphous matrix of our coatings. However, the sample deposited by a nitrogen gas flow of 60 sccm presented a wear rate of four orders of magnitude smaller than the uncoated sample. The deposition method presented in this work seems very promising for the manufacture of TiBCN coatings. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201228130
  • EBSD-Orientation analysis of monocrystalline diamonds used for diamond metal composites - Influence of sample preparation
    Tillmann, W. and Biermann, D. and Weihs, C. and Ferreira, M. and Rautert, C. and Raabe, N.
    Materialwissenschaft und Werkstofftechnik 43 (2012)
    This paper focuses on a new field of application for the EBSD-technique. Generally, EBSD-mappings are performed on different metal alloys used for quality assurance and to get information about the microstructure regarding grain orientation, grain size and distribution. In contrast, the orientation determination of monocrystalline diamond grains with an EBSD system is not a conventional method. Thus, this work describes the EBSD testing sequence in detail and illustrates the preparation of orientation data for a statistical design. Furthermore, dependencies of the sample preparation, alignment to the detector, and the analyzed position on the diamond on the quality of the Kikuchi-patterns, respectively on the indexing rates, have been scrutinized. Finally, the orientation obtained of each tested diamond sample has been utilized in a statistical design to show a direct influence of the crystal orientation on the wear behavior of the diamond grains. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201200954
  • Evaluation of corrosion performance of titanium/steel joint brazed by Cu-based filler metal
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Journal of Materials Engineering and Performance 21 (2012)
    Furnace vacuum brazing has been employed to join commercially pure titanium and low carbon steel using copper-based filler metal with the composition of Cu-10.6Mn-1.9Ni, at.%. Three different brazing temperatures 930, 970, and 1000 °C and a holding time of 15 min were studied and evaluated. The corrosion behavior of the joint in 0.1 M sulfuric acid was investigated using immersion and electrochemical tests. Measurements of corrosion potential, corrosion current density, corrosion rate, polarization resistance, weight loss, and morphology of corrosion attack were used in this study. Experimental results showed that severe corrosion attack of the steel side at the interfacial area is clearly observed. Despite the difference in corrosion rate values obtained by electrochemical and weight loss measurements, the trend of results was identical to a large extent. Corrosion resistance of the joint showed a general tendency to increase with rising brazing temperature. The lowest corrosion rate was obtained for the couple bonded at 1000 °C. Meanwhile, at the lowest joining temperature of 930 °C, corrosion rate showed a higher value. The results of joints corrosion resistance were attributed to the difference in microstructure features and chemical analysis. © ASM International.
    view abstract10.1007/s11665-012-0149-8
  • Improved tool surfaces for incremental bulk forming processes of sheet metals
    Sieczkarek, P. and Kwiatkowski, L. and Tekkaya, A.E. and Krebs, E. and Biermann, D. and Tillmann, W. and Herper, J.
    Key Engineering Materials 504-506 (2012)
    Sheet-bulk metal forming is a process used to manufacture load-adapted parts with high precision. However, bulk forming of sheet metals requires high forces, and thus tools applied for the operational demand have to withstand very high contact pressures, which lead to high wear and abrasion. The usage of conventional techniques like hardening and coating in order to reinforce the surface resistance are not sufficient enough in this case. In this paper, the tool resistance is improved by applying filigree bionic structures, especially structures adapted from the Scarabaeus beetle to the tool's surface. The structures are realized by micromilling. Despite the high hardness of the tool material, very precise patterns are machined successfully using commercially available ball-end milling cutters. The nature-adapted surface patterns are combined with techniques like plasma nitriding and PVD-coating, leading to a multilayer coating system. The effect of process parameters on the resistance of the tools is analyzed experimentally and compared to a conventional, unstructured, uncoated, only plasma nitrided forming tool. Therefore, the tools are used for an incremental bulk forming process on 2 mm thick metal sheets made of aluminum. The results show that the developed methodology is feasible to reduce the process forces and to improve the durability of the tools.© (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.975
  • Influence of handling parameters on coating characteristics in order to produce near-net-shape wear resistant coatings
    Tillmann, W. and Krebs, B.
    Journal of Thermal Spray Technology 21 (2012)
    The present study investigates the influence of spray torch handling parameters such as the spray angle, spray distance, track pitch, and gun velocity on the deposition rate and the microstructure of atmospheric plasma sprayed WC-12Co coatings as well as twin wire arc sprayed WSC-Fe coatings. Similarities as well as fundamental differences in the sensitivity of the two spray processes, regarding changes in handling parameters are discussed, using results of light microscopic analyses. Both coating systems show distinct changes of the deposition rate when varying the handling parameters. An empirical model could be determined to describe the coating deposition. This model enables an optimization of path planning processes by reducing the number of optimization loops. However, the coatings show visible changes in the microstructure, which have to be taken into consideration in order to guarantee the production of high quality coatings. © ASM International.
    view abstract10.1007/s11666-012-9735-4
  • Investigation of PLC band nucleation in AA5754
    Feng, X. and Fischer, G. and Zielke, R. and Svendsen, B. and Tillmann, W.
    Materials Science and Engineering A 539 (2012)
    The purpose of the present work is the experimental investigation of the nucleation of PLC deformation bands in the aluminium alloy AA5754. The PLC bands are investigated using both mechanical methods and infrared (IR) thermography. The latter employs a high-speed IR camera which captures local changes of radiated power resulting from mechanical dissipation and heating due to the nucleation of PLC bands. The resulting IR images are used to determine spatio-temporal power field variations via image subtraction. Furthermore, band trajectories obtained from the IR images are used to study possible correlations between the spatio-temporal evolution of stress and radiated power in the specimens and PLC band development. © 2012 Elsevier B.V.
    view abstract10.1016/j.msea.2012.01.082
  • Microtomographic analysis of splat formation and layer build-up of a thermally sprayed coating
    Tillmann, W. and Nellesen, J. and Abdulgader, M.
    Journal of Thermal Spray Technology 21 (2012)
    Thermal spraying is a material processing technique, which is based on the combination of thermal and kinetic energy. The used feedstock is melted in a hot flame. The melt is atomized and accelerated by means of atomization or process gases. As the formed particles hit a pre-treated substrate they are rapidly solidified and consolidate to form splats. The splats pile one-on-top-of-other forming lamellas creating the final coating. In the work presented here a combination of cored wire (WC as filling powder) and massive wire (copper) were simultaneously sprayed using the twin wire arc spraying process. 3D micro tomography was used in order to gain knowledge about splat formation and layer build-up. Due to the high attenuation coefficient of tungsten in comparison with copper and carbon, tungsten-rich particles and splats can easily be spotted in the tomogram of the coating layer. It turns out that besides irregular formed flat splats also ball-shaped particles exist in the coating layer which suggests that the spherical particles impacted on the substrate in an un-molten state. By 3D data processing tungsten-rich particles were visualized to analyze their spatial distributions and to quantify their geometric parameters. This work aims at contributing to the understanding of spraying processes. © ASM International.
    view abstract10.1007/s11666-012-9737-2
  • Particle size distribution of the filling powder in cored wires: Its effect on arc behavior, in-flight particle behavior, and splat formation
    Tillmann, W. and Abdulgader, M.
    Journal of Thermal Spray Technology 21 (2012)
    The filling powder, as a part of the feedstock in cored wires, directly influences the particle formation, in-flight particle behavior, the coating microstructure, and consequently the behavior of the desired coating, produced by twin wire arc spraying (TWAS). In this work, the effect of the particle size distribution of the filling powder in cored wires was studied. The process parameters were changed for different intervals of particle size distributions. Arc fluctuations were measured and found to be higher at smaller particle sizes. The in-flight particles showed a higher velocity when powders with smaller grain sizes were used and higher particle temperature when bigger grain sizes were used. The splats tended to form a regular disk shape in the case of smaller grain sizes. This investigation studied the important effect of using cored wires and the filling powders grain sizes on the TWAS process. © ASM International.
    view abstract10.1007/s11666-012-9769-7
  • Preliminary investigation on brazing performance of Ti/Ti and Ti/steel joints using copper film deposited by PVD technique
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Journal of Materials Engineering and Performance 21 (2012)
    Microstructural aspects and bonding characteristics of vacuum brazed Ti/Ti and Ti/steel were investigated. A thin-copper film, with different thicknesses, was deposited on the brazed metals by physical vapor deposition technique to serve as a brazing filler metal. Test joints were processed at a temperature of 910°C and 15 min holding time. The resultant joints were characterized to determine the brittle intermetallic compound in the interfacial layer and the shear strength of the joints were tested. Our preliminary experimental results showed that sound joints with a good wetting quality, lack of pores and cracks can be achieved. Intermetallic phases such as Ti 2Cu, TiCu, FeTi, and Fe 2Ti were predicted from the chemical analyses. The Ti/Ti joints achieved a higher shear strength than the Ti/steel joints and there is a tendency for the tension shear strength to increase when a thick Cu-deposited layer is used. © ASM International.
    view abstract10.1007/s11665-012-0121-7
  • Wear analysis of rolled and ground iron-based-alloy-coatings with and without hard material reinforcement
    Tillmann, W. and Luo, W. and Tekkaya, A.E. and Franzen, V.
    Materialwissenschaft und Werkstofftechnik 43 (2012)
    In this research work the wear behavior of thermal sprayed wear resistant coatings, which are finished by incremental roller burnishing and by grinding in order to smooth the surface, are analyzed by means of the Pin-on-Disc test. Two different arc sprayed coatings WSC-FeCSiMn and FeCrBSiMn are compared to each other. At first the microstructure of the smoothed coatings were characterized by investigation of the topography and morphology. After that the wear behavior was analyzed with two different counterparts made of stainless steel and ceramic. In order to determine the different wear mechanisms the wear traces have been investigated by scanning electron microscope. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201200852
  • Analysis of the mechanical properties of an arc sprayed WC-FeCSiMn coating: Compression, bending, and tension behavior
    Tillmann, W. and Nebel, J.
    Journal of Thermal Spray Technology 20 (2011)
    This paper is concerned with the elastic and plastic forming behavior of arc sprayed WC-FeCSiMn coatings. The mechanical properties were investigated by indentation, bending, and tensile tests. These were performed on coated mild steel substrates as well as spark eroded and ground freestanding coatings with different geometries. The results of the indentation, bending, and tensile tests were evaluated concerning the coating microstructure, element, and pore distribution, as well as the local microhardness. The critical role of pores and inhomogeneities within the sprayed coating was examined in detail. Micro- and macrocracking were investigated by scanning electron microscopy after the indentation and tensile tests. It was figured out that the WC-FeCSiMn coating featured a distinctive brittle behavior. During the bending and tension tests, brittle forced fracture of the layer appeared almost without plastic deformations. A significant difference was determined between the compression and tensile performance of the coating. For instance, the Young's modulus for compression strains was measured to be approximately 60% higher than the tension case. © 2010 ASM International.
    view abstract10.1007/s11666-010-9567-z
  • Analysis of the mechanical properties of an arc-sprayed WC-FeCSiMn coating: Nanoindentation and simulation
    Tillmann, W. and Klusemann, B. and Nebel, J. and Svendsen, B.
    Journal of Thermal Spray Technology 20 (2011)
    The characterization of thermal-sprayed coatings is often limited to microstructural analysis to evaluate the coatings morphology. Indentation is commonly used to determine the mechanical properties of different kinds of engineering materials. However, due to the complex structure of thermal-sprayed coatings, few results have been obtained so far. In this article, experimental nanoindentation tests and simulation results are compared. The experimental indentation tests show scattering in the force-deformation data due to the complex structure of the arc-sprayed coating which is investigated by means of an indentation test simulation. Based on the results for single constituent parts of the coating, the Young's modulus as well as further mechanical properties are identified. A general procedure is presented to predict the effective mechanical properties based on the microstructure, porosity, chemical composition, and properties of the coating after thermal spraying. © 2010 ASM International.
    view abstract10.1007/s11666-010-9550-8
  • Erratum: Particle behavior during the arc spraying process with cored wires (Journal of Thermal Spray Technology DOI: 10.1007/s11666-008-9272-3)
    Tillmann, W. and Vogli, E. and Abdulgader, M. and Gurris, M. and Kuzmin, D. and Turek, S.
    Journal of Thermal Spray Technology 20 (2011)
    view abstract10.1007/s11666-008-9288-8
  • Hydrogen quantification of magnetron sputtered hydrogenated amorphous carbon (a-C:H) coatings produced at various bias voltages and their tribological behavior under different humidity levels
    Tillmann, W. and Hoffmann, F. and Momeni, S. and Heller, R.
    Surface and Coatings Technology 206 (2011)
    Hydrogenated amorphous carbon (a-C:H) films have extraordinary tribological properties under dry conditions since the C-atoms at the surface are hydratized and not available for any bonding with the opposing material. Under wet conditions water molecules are weakly absorbed by the a-C:H-coatings so the interaction between the coating surface and the tribological counterpart changes to a dipole-like interaction which is disadvantageous for the tribological performance. According to this, the hydrogen-content plays an important role in the wear and friction behavior of diamond-like carbon (DLC) coatings under different humid conditions.This work focuses on the quantification of the hydrogen content of differently bias a-C:H top layered coating systems and their influence on the tribological behavior under different humidity conditions. By means of a magnetron sputter device DLC-coating systems with an a-C:H-top layer have been deposited at bias voltages between -75 and -200. V. In order to quantify the hydrogen content of the layers Nuclear Reaction Resonance Analysis (NRRA) was used. In combination with the results of the tribological tests under different humid conditions using a ball-on-disk-tester, correlations between the hydrogen content, the bias voltage and the wear and friction performance were made. A clear relationship between the bias voltage and the hydrogen content has been observed, since the values decrease consistently from 27.2. at.% at -75. V to a minimum of 19.9. at.% at -200. V bias voltage. Furthermore the different humidity levels show a strong influence on the tribological performance, while the bias voltage effects mainly the wear and friction results of the samples tested under wet conditions. © 2011 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2011.07.089
  • Influence of DLC Layer on the Wear Behavior of Unhardened Steel Moulds
    Tillmann, W. and Vogli, E. and Gathen, M. and Momeni Sp, S.
    Friction, Wear and Wear Protection: International Symposium on Friction, Wear and Wear Protection 2008 Aachen, Germany (2011)
    view abstract10.1002/9783527628513.ch28
  • Influence of groove characteristics on strength of form-fit joints
    Weddeling, C. and Woodward, S.T. and Marré, M. and Nellesen, J. and Psyk, V. and Tekkaya, A.E. and Tillmann, W.
    Journal of Materials Processing Technology 211 (2011)
    Joining by electromagnetic forming (EMF) is an innovative method to connect, e.g. extruded aluminum profiles to lightweight frame structures without heating or penetrating the profile. This article describes the joining of extruded aluminum profiles by electromagnetic forming, taking into account the process characteristics and the joint design. Forming is initiated by a magnetic impulse of high energy density, such that material with high electrical conductivity is deformed by Lorentz forces. With this high-speed forming process high-strength joints can be manufactured as interference fits, form fits, impact welded joints or a combination of these types. The focus of this paper is on form-fit joints of extruded aluminum profiles for lightweight frame structures. Based on fundamental technological investigations the parameters which specifically affect the strength of the joints were identified and analyzed. Throughout these experiments the groove geometry was varied size and shape wise. The influence of the acting magnetic pressure and the charging energy on the joint strength was also analyzed for the various groove geometries. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2010.08.004
  • Influence of Ti/TiAlN-multilayer designs on their residual stresses and mechanical properties
    Vogli, E. and Tillmann, W. and Selvadurai-Lassl, U. and Fischer, G. and Herper, J.
    Applied Surface Science 257 (2011)
    In this research work, Ti/TiAlN multilayers of various designs were deposited onto substrates pretreated by different etching procedures. The influence of multilayer design and substrate pretreatment on multilayers adhesion, hardness, wear and friction coefficients was systematically analyzed and correlated with residual stresses of these multilayers as well as with residual stresses on the coating-near substrate region, which were analyzed by synchrotron X-ray diffraction at HZB-BESSYII. These investigations show that the adhesion can be improved by a specific etching procedure, which cause increased compressive stress in the coating-near the substrate region. Additionally, it was found, that the multilayer with the thickest ceramic layers has the highest hardness and the lowest wear coefficients as well as the lowest compressive residual stress within studied multilayers. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apsusc.2011.05.013
  • Sliding wear behaviour of diamond-like carbon (DLC) coatings deposited on plasma nitrided steels
    Tillmann, W. and Momeni, S. and Hoffmann, F.
    International Journal of Materials Research 102 (2011)
    A hydrogen-free DLC (diamond-like carbon) coating was deposited with a bias voltage of 150 V on various high and low alloy tool steels to study the effect of the pre-treatment of the steel substrate on the wear behaviour of the DLC coating in sliding contact with uncoated counterparts. The morphology and mechanical properties of the DLC coating as well as the effect of plasma nitriding on the surface roughness and the hardness of the steels were studied in order to perform a correlation with the results of tribology tests. It could be concluded from the results that the plasma nitriding of the high alloy tool steel X210CrW12 leads to a significant decrease in the wear and friction coefficient of the DLC coating. Furthermore, it was found that plasma nitriding of the steel results in a decrease in the wear of uncoated counterparts as well. Finally, the wear mechanisms and failure of DLC coatings deposited on various steels were compared with each other and discussed analytically. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.110547
  • Brazing of titanium to steel with different filler metals: Analysis and comparison
    Elrefaey, A. and Tillmann, W.
    Journal of Materials Science 45 (2010)
    Evaluations of vacuum brazed commercially pure titanium and low-carbon steel joints using one copper-based alloy (Cu-12Mn-2Ni) and two silver-based braze alloys (Ag-34Cu-2Ti, Ag-27.25Cu-12.5In-1.25Ti) have been studied. Both the interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate the joint quality. The optical and scanning electron microscopic results showed that all the filler metals interact metallurgically with steel and titanium, forming different kinds of intermetallic compounds (IMC) such as CuTi, Cu2Ti, Cu4Ti3, and FeTi. The presence of IMC (interfacial reaction layers) at the interfacial regions strongly affects the shear strength of the joints. Furthermore, it was found that the shear strength of brazed joints and the fracture path strongly depend on the thickness of the IMC. The maximum shear strength of the joints was 113 MPa for the specimen brazed at 750 °C using an Ag-27.25Cu-12.5In-1.25Ti filler alloy. © 2010 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-010-4357-z
  • Desirability-based multi-criteria optimisation of HVOF spray experiments
    Kopp, G. and Baumann, I. and Vogli, E. and Tillmann, W. and Weihs, C.
    Studies in Classification, Data Analysis, and Knowledge Organization (2010)
    The reduction of the powder grain size is of key interest in the thermal spray technology to produce superfine structured cermet coatings. Due to the low specific weight and a high thermal susceptibility of such fine powders, the use of appropriate process technologies and optimised process settings are required. Experimental design and the desirability index are employed to find optimal settings of a high velocity oxygen fuel (HVOF) spraying process using fine powders (2-8μm). The independent factors kerosene, hydrogen, oxygen, gun velocity, stand-off distance, cooling pressure, carrier gas and disc velocity are considered in a 12-run Plackett-Burman Design, and their effects on the deposition efficiency and on the coating characteristics microhardness, porosity and roughness are estimated. Following an examination of possible 2-way interactions in a 25-1 fractional-factorial design, the three most relevant factors are analysed in a central composite design. Derringer's desirability function and the desirability index are applied to find optimal factor settings with respect to the above characteristics. All analyses are carried out with the statistics software "R". The optimisation of the desirability index is done using the R-package "desiRe". © 2010 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/978-3-642-10745-0-90
  • Desirability-based multi-criteria optimization of HVOF spray experiments to manufacture fine structured wear-resistant 75Cr 3C 2-25(NiCr20) coatings
    Tillmann, W. and Vogli, E. and Baumann, I. and Kopp, G. and Weihs, C.
    Journal of Thermal Spray Technology 19 (2010)
    Thermal spraying of fine feedstock powders allow the deposition of cermet coatings with significantly improved characteristics and is currently of great interest in science and industry. However, due to the high surface to volume ratio and the low specific weight, fine particles are not only difficult to spray but also show a poor flowability in the feeding process. In order to process fine powders reliably and to preserve the fine structure of the feedstock material in the final coating morphology, the use of novel thermal spray equipment as well as a thorough selection and optimization of the process parameters are fundamentally required. In this study, HVOF spray experiments have been conducted to manufacture fine structured, wear-resistant cermet coatings using fine 75Cr 3C 2-25(Ni20Cr) powders (-8 + 2 μm). Statistical design of experiments (DOE) has been utilized to identify the most relevant process parameters with their linear, quadratic and interaction effects using Plackett-Burman, Fractional-Factorial and Central Composite designs to model the deposition efficiency of the process and the majorly important coating properties: roughness, hardness and porosity. The concept of desirability functions and the desirability index have been applied to combine these response variables in order to find a process parameter combination that yields either optimum results for all responses, or at least the best possible compromise. Verification experiments in the so found optimum obtained very satisfying or even excellent results. The coatings featured an average microhardness of 1004 HV 0.1, a roughness Ra = 1.9 μm and a porosity of 1.7%. In addition, a high deposition efficiency of 71% could be obtained. © 2009 ASM International.
    view abstract10.1007/s11666-009-9383-5
  • Development of temperature sensor thin films to monitor turning processes
    Tillmann, W. and Vogli, E. and Herper, J. and Biermann, D. and Pantke, K.
    Journal of Materials Processing Technology 210 (2010)
    Increasing demands on the cutting process require a fundamental analysis concerning the design as well as the material selection for cutting insert and its wear protection. Apart from monitoring the wear and the cutting forces, the knowledge of the developed temperatures during cutting process is essential and necessary. In this work, an innovative technology was employed to measure in situ the temperature development during cutting process. The measurement was based on the Seebeck-effect. Coating adhesion was systematically analyzed and optimized by varying the pretreatment conditions. Furthermore, the design of masks was enhanced and finally turning experiments were carried out to scrutinize the efficiency of deposited temperature sensors in cutting tests. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2010.01.013
  • Effect of plasma treatment on adhesion of DLC layers to steels
    Tillmann, W. and Vogli, E. and Momeni, S.
    Materials Science Forum 638-642 (2010)
    Diamond like carbon layers play a key role in industrial applications. However the layers quality deteriorates often due to insufficient interfacial adhesion. In this research work a prior plasma treatment of steel substrates was employed aimed to improve the interfacial adhesion of DLC-layers to steel substrates. Three different kinds of steels were employed and their microstructures as well as their compositions before and after plasma treatment were analyzed. The interfacial adhesion of DLC layers on the non-nitrided and nitrided steels was observed and the influence of the steel microstructure on the interfacial adhesion was studied. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/MSF.638-642.812
  • Improvement of press dies used for the production of diamond composites by means of DUPLEX-PVD-coatings
    Tillmann, W. and Vogli, E. and Momeni, S.
    Surface and Coatings Technology 205 (2010)
    In the machining of hard materials such as glass or stone, cemented carbides have been recently replaced by diamond tools, consisting of a metallic carrier, on to which diamond segments are brazed. One of the most economic ways for the production of diamond segments is the cold compaction of the mixture of a metallic powder and diamond particles. Due to a highly abrasive sliding contact between diamond particles and the die walls, the wear rate of the press dies is very high. As a result of a low lifetime of the press dies, they must be replaced in short time periods. To avoid the costly and time-consuming substitution of the press dies, in this work PVD-coatings were deposited on the inner surface of the pre-plasma nitrided press dies (DUPLEX treatment). Thereby, various high and low alloy tool steels were treated by means of plasma nitriding process. Subsequently, a nanocomposite TiAlN coating (nc-TiAlN) was deposited by means of a high ionization magnetron sputtering device on nitrided and non-nitrided steel substrates. The mechanical and tribological properties of these coating systems were studied by means of several standard tests such as nanoindentation, ball-on-disc and scratch test. The most wear resistant coating system was chosen to employ on the inner surface of the press dies. The wear resistance of the press dies developed in this study was tested under real loading condition during compaction of the mixture of diamond particles and cobalt powder. It was revealed that employing plasma nitrided tool, steels coated with nanocomposite TiAlN decreases the wear rate of the press dies up to 76%. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.08.048
  • Influence of substrate nitriding on adhesion, friction and wear resistance of DLC (diamond-like carbon)-coatings
    Tillmann, W. and Vogli, E. and Hoffmann, F. and Kemdem, P.
    Key Engineering Materials 438 (2010)
    Since diamond like carbon layers feature excellent mechanical and tribological behavior under defined environmental circumstances, they are well established in a wide field of industrial and automotive applications in the last decade. However, the pretreatment of the substrate plays also an important role in supporting and enforcing the excellent properties of the coatings. This work analyses the effect of the plasma nitrided cold working steel substrate (80CrV2) on the adhesion, friction and wear resistance of DLC-coatings and compares it to the performance of DLC-coatings applied on a non-hardened substrate material. Therefore the grinded and polished specimens were nitrogen-hardened in an Arc-PVD (Physical Vapor Deposition)-device before the DLC-coating was applied in a Magnetron Sputter-PVD-process. In order to measure the hardness of the thin film coating, a nanoindenter was used. The adhesion was tested with a scratch tester and the wear resistance was measured by using a Ball-on-disc-tester. A 3D-profilometer and a SEM (Scanning Electron Microscope) were utilized to analyze the scratches and wear tracks on the samples. With these results correlations between the substrate nitriding and the mechanical and tribological performance of the DLC-coating were made. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.438.211
  • Influence on diamonds during the spraying of diamond-bronze abrasive coatings
    Tillmann, W. and Vogli, E. and Nebel, J. and Buck, V. and Reuter, S.
    Journal of Thermal Spray Technology 19 (2010)
    Detonation spraying provides the opportunity to produce diamond grinding tools for the machining of stone, cement, and concrete. Especially the atmospheric conditions of the spraying process yield in a high production flexibility. However, during detonation spraying, the oxygenic atmosphere as well as the thermal and kinetic energy have an impact on the processed diamond. Despite its importance for the tools' performance, the influence of the spraying process on the superabrasive diamond is predominantly unknown. The potential decrease of the diamond durability and strength due to degradation effects during the production of sprayed diamond-CuSn 85/15 composites has not yet been determined. X-ray diffraction and Raman spectroscopy were used to verify thermally initiated surface reactions of the sprayed diamonds after exposure to the spraying process. Additionally, reference measurements on the degradation of diamonds in oxidizing and inert conditions were carried out to compare the spraying results. Differential thermal and thermogravimetric analyses were employed. To validate the mechanical properties of the diamond superabrasives, friability tests and fracture force tests were performed. It was found that under optimized detonation spraying conditions the thermal and mechanical impact remains low enough to ensure a good reliability of the processed diamonds. The diamond crystal structure endured the spraying process without detectable graphitization or oxidation. Deterioration indicators were not observed in SEM micrographs, x-ray diffraction patterns or Raman spectra. Furthermore, a high durability and strength of the sprayed diamonds were confirmed by mechanical testing. © 2009 ASM International.
    view abstract10.1007/s11666-009-9418-y
  • Investigation on corrosion of titanium/steel brazed joint
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Materialwissenschaft und Werkstofftechnik 41 (2010)
    Furnace vacuum brazing has been employed to join commercially pure titanium alloy and low carbon steel using a silver-based filler metal with a composition of Ag-Cu34-Ti2 (wt%). Three different brazing temperatures (850°C, 880°C, 930°C) and two holding times (5 and 15 min) were applied and evaluated. The corrosion behavior of the joints in 0.1 M sulfuric acid was investigated using immersion and electrochemical tests. Measurements of corrosion potential, corrosion current density, corrosion rate, polarization resistance, weight loss, and morphology of corrosion attack were used in this study. The results indicated that a severe corrosion attack at the interfacial area of the steel side took place. Despite the difference in corrosion rate values obtained by electrochemical and weight loss measurements, the trend of the results was identical to a large extent. The corrosion resistance of the joint showed a general tendency to increase with an increasing brazing temperature and holding time. Therefore, the joints produced at a temperature of 930°C and a holding time of 15 min produced the best result concerning the corrosion behavior. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201000684
  • Nanostructured bionic PVD-coatings for forming tools
    Tillmann, W. and Vogli, E. and Herper, J. and Haase, M.
    Key Engineering Materials 438 (2010)
    It is very important to minimize wear and friction in forming processes in order to avoid adhesion between work piece and tool. For the realization of these requirements, the PVD-coating system CrAlN was deposited and tested on substrates made from high speed steel 1.3343 by means of a reactive sputter process. The coatings were deposited as single- and multilayers with a metallic Crinterlayer. Prior to realizing the appropriate coating design, the substrates were pre-structured. For this purpose natural surfaces were used as a pattern and tested in this research work. The skin of an insect serves as a model and its fine structures were reproduced on the substrate surface by milling. The generated specimens were analyzed with a scanning electron microscope, nanoindenter and ball-on-disc tester to compare the performance of the coating systems. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.438.41
  • Supervision and measuring of particle parameters during the wire-arc spraying process with the diagnostic systems accuraspray-g3 and LDA (laser-doppler-anemometry)
    Zimmermann, S. and Vogli, E. and Kauffeldt, M. and Abdulgader, M. and Krebs, B. and Rüther, B. and Landes, K. and Schein, J. and Tillmann, W.
    Journal of Thermal Spray Technology 19 (2010)
    Due to low cost of operation, high deposition rates and efficiency, wire arc spraying has become one of the most important thermal spray technologies, especially as a tool for coatings used to improve corrosion and wear protection. In order to obtain high-quality coatings, the flow characteristics of the atomizing gas have to be optimized. Thus, the nozzle design as well as the properties of the gas used need to be adjusted and the resulting particle parameters have to be quantified. Employing the Accurasprayg3 system in combination with Laser Doppler Anemometry (LDA), the particle size distribution and velocity have been measured for a wide range of parameters, including different materials, different gas pressures and nozzles resulting in design suggestions and offering the possibility to compare the two different diagnostic systems. © ASM International.
    view abstract10.1007/s11666-009-9466-3
  • Surface modification of press dies used in a powder metallurgical production process
    Tillmann, W. and Vogli, E. and Momeni, S.
    Advanced Engineering Materials 12 (2010)
    Cold compaction in a press die and subsequent sintering of diamond particles, homogeneously distributed in a metallic powder as matrix, is one of the most economic ways for the production of diamond composites, used widely for cutting very hard materials like stone and glass. Owing to the very high hardness of diamonds, the wear of the press die is considerably high and because of a short life time, press tools must be substituted regularly. Recently, through plasma nitriding process and deposition of thin solid films, the wear resistance of the press dies has been significantly increased. This work aims at the investigation of the influence of roughness, friction coefficient, and hardness of the inner surface of various dies, which have been modified in different ways, on the physical properties of the compacted diamond segments. It was evidenced that improving the mechanical and tribological properties of the die surface leads to an increase of the hardness and density of the diamond composites produced. Several novel PVD coatings have been employed in order to improve the wear resistance of the press dies employed for the production of diamond composites. This study aims to investigate the relation between the surface modification of dies and the most important physical properties of the diamond composites compacted. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201000146
  • The correlation between the coating quality and the moving direction of the twin wire arc spraying gun
    Tillmann, W. and Vogli, E. and Abdulgader, M.
    Journal of Thermal Spray Technology 19 (2010)
    Asymmetric melting behavior of the electrodes is a process-related feature of the twin wire arc spraying (TWAS) technique since the heating of the negative wire is different from that of the positive wire. The asymmetric melting behavior, particle crossover, irregular plume shape, and last but not least the arc voltage fluctuations affect the spraying jet on the whole and lead to an inhomogeneous plume. To investigate the effect of inhomogeneous spraying plume on coating characteristics, coatings were produced by moving the spraying gun in different directions, with respect to the electrodes. The porosity, micro-cracks, hardness, thickness, and adhesion strength of the sprayed coatings were measured and brought in correlation with the gun moving direction. In this study, two different wire types were investigated in order to find out the effect of the spraying gun moving direction on the coating quality. © 2009 ASM International.
    view abstract10.1007/s11666-009-9452-9
  • Toward process optimization in laser welding of metal to polymer
    Tillmann, W. and Elrefaey, A. and Wojarski, L.
    Materialwissenschaft und Werkstofftechnik 41 (2010)
    The joining technology of dissimilar lightweight materials between metals and polymer is essential for realizing cars with hybrid structures and for other engineering applications. These types of joints are still difficult to generate and their behaviour is not fully understood. Laser welding offers specific process advantages over conventional technologies, such as short process times, while providing optically and qualitatively valuable weld seams and imposing minimal thermal stress. Furthermore, the process is compatible with automation. This paper summarizes the efforts to attain suitable joint strengths with the stainless steel plate type S30400 and a Polyethylene Terephtalate Glycol (PETG) plastic sheet. The study considers the optimization of two important process parameters, namely laser power, and welding speed. Microstructure features, test of tensile shear strength, investigation of the fracture location, and morphology were used to evaluate the joint performance. The result indicates that there is an optimum value for laser power, which achieves a sufficient melting and heat transfer to the joint without decomposing the plastic sheet and hence, enables to obtain high joint strength. Moreover, a low welding speed is preferable in most combinations of welding parameters since it achieves an adequate melting and wetting of the polymer to the steel surface. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201000674
  • Wear-protective cermet coatings for forming tools
    Tillmann, W. and Vogli, E. and Baumann, I. and Krebs, B. and Nebel, J.
    Materialwissenschaft und Werkstofftechnik 41 (2010)
    Two innovative thermal spray techniques have been investigated to manufacture wear protective coatings for forming tools. A Twin Arc Spray process (TWAS), with an AS-850 cored wire containing fused tungsten carbides (FTC-FeCCrMnSi), and a High Velocity Oxy-Fuel (HVOF) spray technique, with fine WC12Co (18 μm) and Cr3C225(Ni20Cr) (28 μm) feedstock powders were employed. Coating characterizations regarding the hardness, roughness, porosity, deposition rate and deposition efficiency were conducted. In addition, the wear and friction behavior of all coatings were analyzed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201000647
  • X-ray diffraction residual stress analysis on PVD-multilayer coatings
    Tillmann, W. and Selvadurai-Laßl, U. and Vogli, E. and Fischer, G. and Hoffmann, F.
    Materialwissenschaft und Werkstofftechnik 41 (2010)
    Hard and wear resistant thin layers provides significant improvements in tools employed in manufacturing industry and are recently of great interest to increase as well as to enhance the tools? performance and lifetime. Ceramic PVD-layers already feature a high hardness combined with a high abrasive wear resistance. However, such layers possess only a limited lifetime due to their low toughness. To increase the toughness as well as the durability of such layers multilayer systems are steadily developed. This research work presents the first results of the influence of the steel substrate pretreatments, multilayer designs and coating process parameters on the properties of two innovative PVD metal-ceramic multilayers, Ti/TiAIN and Cr/CrAIN. It was obviously that the substrate pretreatment and the multilayer design have a large influence on the layer properties and residual stresses. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201000630
  • coatings

  • joining

  • mechanical properties

  • physical vapor deposition

  • scanning electron microscopy

  • sintering

  • thermal spraying

  • thin films

  • tribology

  • x-ray diffraction

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