Dr.-Ing. Georg Mauer

associated member

Institute of Energy and Climate Research
Forschungszentrum Jülich GmbH

Contact

Hub
  • How Hydrogen Admixture Changes Plasma Jet Characteristics in Spray Processes at Low Pressure
    Mauer, G.
    Plasma Chemistry and Plasma Processing 41 (2021)
    In plasma spraying, hydrogen is widely used as a secondary working gas besides argon. In particular under low pressure, there are strong effects on the plasma jet characteristics even by small hydrogen percentages. Under such conditions, fundamental mechanisms like diffusion and recombination are affected while this is less relevant under atmospheric conditions. This was investigated for argon–hydrogen mixtures by optical emission spectroscopy (OES). The small electron densities under the investigated low pressure conditions implied specific difficulties in the application of several OES-based methods which are discussed in detail. Adding hydrogen to the plasma gas effected an increased plasma enthalpy. Moreover, the jet expanded radially as the reactive part of the thermal conductivity was enhanced by recombination of atomic hydrogen so that the shock waves were less reflected at the cold jet rims. In the jet cores, the lowest temperatures were found for the highest hydrogen admixture because the energy consumption due to the dissociation of molecular hydrogen outbalanced the increase of the plasma enthalpy. Variations in the radial temperature profiles were related to the jet structure and radial thermal conductivity. The local hydrogen–argon concentration ratios revealed an accumulation of hydrogen atoms at the jet rims. Clear indications were found, that higher hydrogen contents promoted the fast recombination of electrons and ions. However, it is assumed that the transport properties of the plasma were hardly affected by this, since the electron densities and thus the ionization degrees were generally small due to the low pressure conditions. © 2020, The Author(s).
    view abstract10.1007/s11090-020-10143-6
  • Coatings with Columnar Microstructures for Thermal Barrier Applications
    Mauer, G. and Vaßen, R.
    Advanced Engineering Materials 22 (2020)
    Columnar-structured thermal barrier coatings (TBCs) manufactured by electron beam-physical vapor deposition (EB-PVD) are well known to exhibit high strain tolerance. However, as EB-PVD is a high-vacuum process, it is expensive. Suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD) are alternatives for the manufacture of similar microstructures. Herein, the state of the art of manufacturing columnar-structured TBCs by SPS and PS-PVD is outlined. Both processes have been investigated and further developed at Forschungszentrum Jülich for many years. The mechanisms leading to the formation of columnar-structured coatings are described and differentiated from EB-PVD. Examples are given for SPS and PS-PVD columnar microstructures and their life performance. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201900988
  • Cold Gas Spraying of Nickel-Titanium Coatings for Protection Against Cavitation
    Mauer, G. and Rauwald, K.-H. and Sohn, Y.J. and Weirich, T.E.
    Journal of Thermal Spray Technology (2020)
    Cavitation erosion is a sever wear mechanism that takes place in hydrodynamic systems. Examples are turbine vanes of hydropower plants or components of valves and pumps in hydraulic systems. Nickel-titanium shape memory alloys (NiTi) are attractive materials for cavitation-resistant coatings because of their pronounced intrinsic damping mitigating cavitation-induced erosion. In this work, NiTi coatings were produced by cold gas spraying. The phase transformation behaviors of the powder feedstock and the as-sprayed coatings were investigated. Regarding the obtained transformation temperatures, the measured substrate temperatures during spraying rule out that either the shape memory effect or the pseudoelasticity of NiTi could affect the deposition efficiency under the applied conditions of cold gas spraying. Another potential effect is stress-induced amorphization which could occur at the particle–substrate interfaces and impair particle bonding by stress relaxation. Moreover, also oxide formation can be significant. Thus, the presence of amorphous phases and oxides in the near-surface zone of particles bounced off after impact was investigated. Oxidation could be confirmed, but no indication of amorphous phase was found. Besides, also the evolution of local microstrains implies that the substrate temperatures affect the deposition efficiency. These temperatures were significantly influenced by the spray gun travel speed. © 2020, The Author(s).
    view abstract10.1007/s11666-020-01139-x
  • Cr2AlC MAX phase as bond coat for thermal barrier coatings: Processing, testing under thermal gradient loading, and future challenges
    Gonzalez-Julian, J. and Mauer, G. and Sebold, D. and Mack, D.E. and Vassen, R.
    Journal of the American Ceramic Society 103 (2020)
    Cr2AlC layers with thickness up to 100 µm were deposited by high-velocity-atmospheric plasma spray (HV-APS) on Inconel 738 substrates to analyze the potential of MAX phases as bond coat in thermal barrier coating systems (TBCs). The deposited Cr2AlC layers showed high purity with theoretical densities up to 93%, although some secondary phases were detected after the deposition process. On top of this MAX phase layer, a porous yttria-stabilized zirconia (YSZ) was deposited by atmospheric plasma spraying. The system was tested under realistic thermal loading conditions using a burner rig facility, achieving surface and substrate temperatures of 1400°C and 1050°C, respectively. The system failed after 745 cycles mainly for three reasons: (i) open porosity of the bond coat layer, (ii) oxidation of secondary phases, and (iii) inter-diffusion. Nevertheless, these results show a high potential of Cr2AlC and other Al-based MAX phases as bond coat material for high-temperature applications. Furthermore, future challenges to transfer MAX phases as eventual bond coat or protective layer are discussed. © 2019 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals, Inc. on behalf of American Ceramic Society (ACERS)
    view abstract10.1111/jace.16935
  • Crystal structure analysis and high-temperature phase transitions of complex rare-earth perovskite, La2(Al1/2MgTa1/2)O6
    Sohn, Y.J. and Mauer, G. and Roth, G. and Guillon, O. and Vaßen, R.
    Journal of the American Ceramic Society 103 (2020)
    In situ high-temperature powder X-ray diffraction analysis (HT-XRD) was carried out in the temperature range from 25°C-1430°C to investigate the crystal structure of double perovskites, La2(Al1/2MgTa1/2)O6 (LAMT) and its phase transitions. This complex perovskite is a promising candidate for application in thermal barrier coating systems. Rietveld analysis shows a rock-salt type ordering of the B-site cations in the monoclinic space group symmetry, P21/n at room temperature. Upon heating, a structural phase transition occurs at ~855°C, and the crystal structure becomes rhombohedral with the space group symmetry (Formula presented.). On further heating, LAMT transforms to the ideal cubic phase at ~1390°C with the space group symmetry (Formula presented.). Both of the structural phase transitions are completely reversible, and were confirmed through complementary differential scanning calorimetry and thermogravimetry measurements. With increasing temperature, the degree of the octahedral tilting decreases and the variance of the different B–O bond lengths is reduced, until in the cubic phase, no tilting is present, and almost equal B–O bond lengths are obtained. © 2019 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals, Inc. on behalf of American Ceramic Society (ACERS)
    view abstract10.1111/jace.16740
  • Thermal cycling performances of multilayered yttria-stabilized zirconia/gadolinium zirconate thermal barrier coatings
    Zhou, D. and Mack, D.E. and Bakan, E. and Mauer, G. and Sebold, D. and Guillon, O. and Vaßen, R.
    Journal of the American Ceramic Society 103 (2020)
    Gadolinium zirconate (Gd2Zr2O7, GZO) as an advanced thermal barrier coating (TBC) material, has lower thermal conductivity, better phase stability, sintering resistance, and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (YSZ, 6-8 wt%) at temperatures above 1200°C. However, the drawbacks of GZO, such as the low fracture toughness and the formation of deleterious interphases with thermally grown alumina have to be considered for the application as TBC. Using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS), double-layered YSZ/GZO TBCs, and triple-layered YSZ/GZO TBCs were manufactured. In thermal cycling tests, both multilayered TBCs showed a significant longer lifetime than conventional single-layered APS YSZ TBCs. The failure mechanism of TBCs in thermal cycling test was investigated. In addition, the CMAS attack resistance of both TBCs was also investigated in a modified burner rig facility. The triple-layered TBCs had an extremely long lifetime under CMAS attack. The failure mechanism of TBCs under CMAS attack and the CMAS infiltration mechanism were investigated and discussed. © 2019 The American Ceramic Society
    view abstract10.1111/jace.16862
  • Thermal Spray Processes for the Repair of Gas Turbine Components
    Fiebig, J. and Bakan, E. and Kalfhaus, T. and Mauer, G. and Guillon, O. and Vaßen, R.
    Advanced Engineering Materials 22 (2020)
    Gas turbine components are often operated in harsh conditions, which can lead to severe damage. As it is highly desirable from both an economical and an ecological point of view to restore these worn areas instead of manufacturing new components, repair technologies are of huge interest for companies supplying maintenance and overhaul of gas turbines. In this article, two thermal techniques are described that can be used for this application: cold gas spraying (CGS) and vacuum plasma spraying (VPS). The CGS process allows the deposition of metallic coatings with excellent mechanical properties; several examples including γ-TiAl, Inconel (IN) 718, and IN 738 are given. Essential for the deposition of high-performance coatings in CGS is to exceed the so-called critical velocity. This is discussed also with experimental findings. As a final topic, experiments that use VPS for the repair of single-crystal alloys are described. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201901237
  • Tuning the Microstructure and Thickness of Ceramic Layers with Advanced Coating Technologies Using Zirconia as an Example
    Guillon, O. and Dash, A. and Lenser, C. and Uhlenbruck, S. and Mauer, G.
    Advanced Engineering Materials 22 (2020)
    The properties of ceramic layers are not only related to the coating material but also—to a very high degree—the processing technology used. In particular, microstructure and thickness are key to the successful implementation of functional layers in application. This will be shown using yttria-stabilized zirconia (YSZ) as an example, a highly versatile compound with high fracture toughness, high chemical and thermal stability, high biological compatibility, and high oxygen ion conductivity. For each application, specific microstructures are required, which can only be obtained by suitable processing. Herein, coating technologies for layers with thicknesses spanning the nanometer range up to several hundred micrometers, and from full density to tailored open porosity are focused. Wet processing routes, thin-film deposition from the gas phase as well as thermal and plasma spraying are presented along with the resulting YSZ layers. © 2020 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH
    view abstract10.1002/adem.202000529
  • Cold spray deposition of Cr2AlC MAX phase for coatings and bond-coat layers
    Go, T. and Sohn, Y.J. and Mauer, G. and Vaßen, R. and Gonzalez-Julian, J.
    Journal of the European Ceramic Society 39 (2019)
    Highly pure Cr2AlC powders were synthesized and deposited for the first time by cold spray technology on stainless steel substrates. The Cr2AlC coatings were relative dense, up to 91%, and present high purity (> 98%) since only small traces of Cr2Al, Al2O3 and Cr2O3 were detected by XRD, SEM and EDX. The microstructure of the coatings is homogeneous, although some preferential orientation in the basal plane was observed by XRD pole figures. The adhesion between the coating and the substrate is strong, and compressive residual stresses up to 300 MPa in the coating were determined by XRD. Furthermore, a conventional YSZ Thermal Barrier Coating (TBCs) was deposited by Atmospheric Plasma Spray (APS) on top of the cold sprayed Cr2AlC coating in order to demonstrate the processing feasibility of Cr2AlC MAX phases as a bond-coat layer. © 2018 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2018.11.035
  • High-temperature materials for power generation in gas turbines
    Bakan, E. and Mack, D.E. and Mauer, G. and Vaßen, R. and Lamon, J. and Padture, N.P.
    Advanced Ceramics for Energy Conversion and Storage (2019)
    The chapter describes the different aspects of ceramic materials in gas turbines. The operation conditions such as high-pressure ratio and high temperatures result in improved efficiencies and make necessary the use of materials with high-temperature capability. In addition to the often used single-crystal alloys ceramic materials are discussed. Different bulk ceramics, for example, based on silicon nitride are described. A special focus is laid on ceramic matric composites, both oxide and nonoxide-based materials, which are of increasing interest for gas-turbine applications. In addition to the structural applications ceramics are also often used as coating material. Standard coating processes for protective coatings in gas turbines are described. Furthermore, thermal barrier coatings, a widely used coating system in gas turbines, and environmental barrier coatings as protective coatings for ceramic matrix composites are discussed in detail. Finally, also degradation and failure modes for the different high-temperature coating systems are the topics of this chapter. © 2020 Elsevier Ltd. All rights reserved.
    view abstract10.1016/B978-0-08-102726-4.00001-6
  • 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 on growth mechanisms of columnar structured YSZ coatings in Plasma Spray-Physical Vapor Deposition (PS-PVD)
    He, W. and Mauer, G. and Sohn, Y.J. and Schwedt, A. and Guillon, O. and Vaßen, R.
    Journal of the European Ceramic Society 39 (2019)
    By Plasma Spray-Physical Vapor Deposition (PS-PVD), major fractions of the powder feedstock can be evaporated so that the coating builds up mainly from vapor phase. In this work, the deposition mechanisms at different PS-PVD conditions were investigated. Depending on the plasma flow conditions and the substrate temperature, the columns in the coatings possess successively pyramidal, cauliflower, and lamellar shaped tops. In addition, the different microstructures show characteristic crystallographic textures, in which different in-plane and out-of-plane orientations were observed by pole figures. Based on investigations by electron back-scatter diffraction (EBSD), the overall coating growth process can be roughly divided into three subsequent stages: equiaxed growth, competitive growth, and preferential growth. Influences of diffusion and shadowing on final coating microstructure and orientation were discussed. The formation of equiaxed grains was proposed to be caused by high nucleation rates, which are probably induced by large undercooling and super-saturation at the beginning of deposition. The preferential growth orientation was preliminarily explained based on an evolutionary selection mechanism. © 2019 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2019.04.003
  • Lanthanum tungstate membranes for H 2 extraction and CO 2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition
    Ivanova, M.E. and Deibert, W. and Marcano, D. and Escolástico, S. and Mauer, G. and Meulenberg, W.A. and Bram, M. and Serra, J.M. and Vaßen, R. and Guillon, O.
    Separation and Purification Technology (2019)
    In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H 2 extraction and CO 2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H 2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La 28−x W 4+x O 54+δ , LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H 2 flux of ca. 0.4 ml/min cm 2 measured at 825 °C in 50 vol% H 2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H 2 separation membranes and it is well comparable with the H 2 flux reported for other newly developed dual phase cer-cer and cer-met membranes. © 2019 Elsevier B.V.
    view abstract10.1016/j.seppur.2019.03.015
  • Lanthanum tungstate membranes for H2 extraction and CO2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition
    Ivanova, M.E. and Deibert, W. and Marcano, D. and Escolástico, S. and Mauer, G. and Meulenberg, W.A. and Bram, M. and Serra, J.M. and Vaßen, R. and Guillon, O.
    Separation and Purification Technology 219 (2019)
    In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H2 extraction and CO2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La28−xW4+xO54+δ, LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H2 flux of ca. 0.4 ml/min cm2 measured at 825 °C in 50 vol% H2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H2 separation membranes and it is well comparable with the H2 flux reported for other newly developed dual phase cer-cer and cer-met membranes. © 2019 Elsevier B.V.
    view abstract10.1016/j.seppur.2019.03.015
  • Numerical Study on Particle–Gas Interaction Close to the Substrates in Thermal Spray Processes with High-Kinetic and Low-Pressure Conditions
    Mauer, G.
    Journal of Thermal Spray Technology 28 (2019)
    In thermal spray processes, the interaction between the gas jet and the particulate feedstock can affect the coating build-up mechanisms considerably. In particular under high-kinetic and low-pressure conditions, small particles are subjected to rapid deflection and velocity changes close to the substrate. In this work, numerical studies were carried out to investigate the interaction between gas and particles in the substrate boundary layers (BL). Typical conditions for suspension plasma spraying (SPS), plasma spray-physical vapor deposition (PS-PVD), and aerosol deposition (AD) were taken as a basis. Particular importance was attached to the consideration of rarefaction and compressibility effects on the drag force. Typical Stokes numbers for the different thermal spray processes were calculated and compared. Possible effects on the resulting coating build-up mechanisms and microstructure formation are discussed. The results show that just for larger particles in the SPS process the laminar flow attached to the particles begins to separate so that the drag coefficients have to be corrected. Furthermore, slip effects occur in all the investigated processes and must be considered. The comparison of calculated Stokes numbers with critical values shows that there is a disposition to form columnar microstructures or stacking effects depending on the particle size for PS-PVD and SPS, but not for AD. © 2018, ASM International.
    view abstract10.1007/s11666-018-0810-3
  • PS-PVD Processing of Single-Phase Lanthanum Tungstate Layers for Hydrogen-Related Applications
    Marcano, D. and Ivanova, M.E. and Mauer, G. and Sohn, Y.J. and Schwedt, A. and Bram, M. and Menzler, N.H. and Vaßen, R.
    Journal of Thermal Spray Technology 28 (2019)
    This work presents a systematic study of the lanthanum tungstate (LaWO) ceramic layers formation on porous metallic substrates as a function of the PS-PVD processing parameters including plasma characteristics, support type and temperature, as well as addition of O2 during the spraying. Through precise control of the PS-PVD parameters, a set of processing conditions were found that led to He gas-tight purely cubic LaWO layers with negligible secondary phase precipitations. Being dependent on process conditioning, the formation and evolution of the cubic La6−xWO12−δ (x = 0.3-0.6) as the main phase of functional importance and of the undesired secondary phases (La2O3 and La6W2O15) was strongly affected by the cation and oxygen stoichiometries. The rapid cooling of the feedstock at particle impact on the substrate led to the formation of highly La-saturated compositions which exhibited significant lattice expansion in comparison with conventionally processed LaWO and is considered beneficial in terms of material performance. And indeed, the H2 permeation performance of the PS-PVD processed LaWO ceramic layers shown earlier by our group was 0.4 ml/min∙cm2 at 825 °C for 60 µm thickness of the functional layer, the highest value reported for this type of proton conducting ceramics, so far. © 2019, ASM International.
    view abstract10.1007/s11666-019-00935-4
  • A TEM Investigation of Columnar-Structured Thermal Barrier Coatings Deposited by Plasma Spray-Physical Vapor Deposition (PS-PVD)
    Rezanka, S. and Somsen, C. and Eggeler, G. and Mauer, G. and Vaßen, R. and Guillon, O.
    Plasma Chemistry and Plasma Processing 38 (2018)
    The plasma spray-physical vapor deposition technique (PS-PVD) is used to deposit various types of ceramic coatings. Due to the low operating pressure and high enthalpy transfer to the feedstock, deposition from the vapor phase is very effective. The particular process conditions allow for the deposition of columnar microstructures when applying thermal barrier coatings (TBCs). These coatings show a high strain tolerance similar to those obtained by electron beam-physical vapor deposition (EB-PVD). But compared to EB-PVD, PS-PVD allows significantly reducing process time and costs. The application-related properties of PS-PVD TBCs have been investigated in earlier work, where the high potential of the process was described and where the good resistance to thermo-mechanical loading conditions was reported. But until now, the elementary mechanisms which govern the material deposition have not been fully understood and it is not clear, how the columnar structure is built up. Shadowing effects and diffusion processes are assumed to contribute to the formation of columnar microstructures in classical PVD processing routes. For such structures, crystallographic textures are characteristic. For PS-PVD, however, no crystallographic textures could initially be found using X-ray diffraction. In this work a more detailed TEM investigations and further XRD measurements of the columnar PS-PVD microstructure were performed. The smallest build units of the columnar TBC structure are referred to as sub-columns. The observed semi-single crystal structure of individual sub-columns was analyzed by means of diffraction experiments. The absence of texture in PS-PVD coatings is confirmed and elementary nucleation and growth mechanisms are discussed. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s11090-018-9898-y
  • Advanced crystallographic study of the columnar growth of YZS coatings produced by PS-PVD
    He, W. and Mauer, G. and Schwedt, A. and Guillon, O. and Vaßen, R.
    Journal of the European Ceramic Society 38 (2018)
    In the Plasma Spray-Physical Vapor Deposition (PS-PVD) process, columnar structured coatings are deposited mainly from the vapor phase due to the intensive evaporation of the feedstock powder. This paper highlights the application of electron backscatter diffraction (EBSD) for the characterization of columnar structured ceramic PS-PVD coatings. The growth processes of PS-PVD coatings could be elucidated, developing from small equiaxed crystals to large columnar crystals. Furthermore, the main effect of the torch swing on coating deposition could be the interruption of crystal growth and thus repeated nucleation. This may have a similar effect as slowly rotating the substrate in Electron Beam-Physical Vapor Deposition (EB-PVD). © 2017 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2017.12.054
  • Cold gas spraying of Ti-48Al-2Cr-2Nb intermetallic for jet engine applications
    Bakan, E. and Mauer, G. and Sohn, Y.J. and Schwedt, A. and Rackel, M.W. and Riedlberger, F. and Pyczak, F. and Peters, J.O. and Mecklenburg, M. and Gartner, T.M. and Vaßen, R.
    Surface and Coatings Technology (2018)
    The present article describes aspects of the cold gas spray processability of the intermetallic Ti-48Al-2Cr-2Nb (at. %) alloy, which is employed as a structural material in gas turbine engines. The effects of processing parameters, namely, gas pressure, gas temperature, spray distance, as well as the gas atomized feedstock particle size (d50 = 30 and 42 μm, respectively) and phase composition on deposition, were investigated. The results showed that when the highest available gas pressure (40 bar) and temperature (950 °C) were combined with a short spray distance (20 mm), well-adhering coatings could be deposited regardless of the investigated particle size. However, the maximum coating thickness could be achieved was about 30 μm with a deposition efficiency of 1%. Phase composition of the gas atomized feedstock was investigated with HT-XRD and according to the findings, heat treatment of the feedstock under vacuum was carried out. With this treatment, non-equilibrium, disordered α phase of the atomized powder was transformed into an α α2 and γ phase mixture. At the same time, an increase in the hardness and oxygen content of the powder was detected. Swipe test performed with the heat treated powder revealed no improvement in terms of deposition, in fact, the number of adhering particles on the substrate was decreased in comparison with that of the untreated powder. © 2018 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2018.11.092
  • Conditions for nucleation and growth in the substrate boundary layer at plasma spray-physical vapor deposition (PS-PVD)
    Mauer, G. and Vaßen, R.
    Surface and Coatings Technology (2018)
    Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition can come off not only from liquid splats but also from vapor phase. Moreover, there is the suggestion that also nano-sized clusters can be formed by homogeneous nucleation and contribute to deposition. In this work, the conditions for nucleation and growth of such nano-sized particles in the plasma flow around the substrate under PS-PVD conditions were investigated. A boundary layer kinetics model was coupled to an approach for homogeneous nucleation from supersaturated vapors and primary particle growth by condensation as well as secondary particle formation by coagulation. The results confirm the importance of the boundary layer on the substrate. However, since these particles are relatively small, their contribution to coating deposition is limited. Furthermore, microstructure or crystallographic orientations are unlikely to be affected by this cluster deposition. © 2018 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2018.06.086
  • Laser cladding of embedded sensors for thermal barrier coating applications
    Zhang, Y. and Mack, D.E. and Mauer, G. and Vaßen, R.
    Coatings 8 (2018)
    The accurate real-time monitoring of surface or internal temperatures of thermal barrier coatings (TBCs) in hostile environments presents significant benefits to the efficient and safe operation of gas turbines. A new method for fabricating high-temperature K-type thermocouple sensors on gas turbine engines using coaxial laser cladding technology has been developed. The deposition of the thermocouple sensors was optimized to provide minimal intrusive features to the TBC, which is beneficial for the operational reliability of the protective coatings. Notably, this avoids a melt pool on the TBC surface. Sensors were deposited onto standard yttria-stabilized zirconia (7-8 wt % YSZ) coated substrates; subsequently, they were embedded with second YSZ layers by the Atmospheric Plasma Spray (APS) process. Morphology of cladded thermocouples before and after embedding was optimized in terms of topography and internal homogeneity, respectively. The dimensions of the cladded thermocouple were in the order of 200 microns in thickness and width. The thermal and electrical response of the cladded thermocouple was tested before and after embedding in temperatures ranging from ambient to approximately 450 °C in a furnace. Seebeck coefficients of bared and embedded thermocouples were also calculated correspondingly, and the results were compared to that of a commercial standard K-type thermocouple, which demonstrates that laser cladding is a prospective technology for manufacturing microsensors on the surface of or even embedded into functional coatings. © 2018 by the authors.
    view abstract10.3390/coatings8050176
  • Monte Carlo simulation of column growth in plasma spray physical vapor deposition process
    Wang, P. and He, W. and Mauer, G. and Mücke, R. and Vaßen, R.
    Surface and Coatings Technology 335 (2018)
    Plasma spray-physical vapor deposition is used to produce columnar microstructure coatings under particular operating parameters. Simulations of the growth of columns were carried out through a two-dimensional Monte Carlo model. The modeling was performed using inclined vapor flux impinging onto a substrate due to shadowing effects. An incoming particle travels along a straight line and attaches itself to already deposited particles. Furthermore, the newly deposited particle will relax to a stable surrounding position along the incoming velocity direction. The modeling results predicted the linking of an oblique vapor flux and column orientation. The numerical simulations were validated in three ways. Firstly, the porosity of simulated columns was predicted and compared to that obtained in the experimental columnar microstructure of coatings. Secondly, the morphology of simulated structures is compared to that of experimental coatings produced by plasma spray physical vapor deposition. Finally, the simulated orientation of columns is compared to the experimental one. © 2017
    view abstract10.1016/j.surfcoat.2017.12.023
  • Systematic Investigation on the Influence of Spray Parameters on the Mechanical Properties of Atmospheric Plasma-Sprayed YSZ Coatings
    Mutter, M. and Mauer, G. and Mücke, R. and Guillon, O. and Vaßen, R.
    Journal of Thermal Spray Technology (2018)
    In the atmospheric plasma spray (APS) process, micro-sized ceramic powder is injected into a thermal plasma where it is rapidly heated and propelled toward the substrate. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so-called splats and eventually the coating. In this study, a systematic investigation on the influence of selected spray parameters on the coating microstructure and the coating properties was conducted. The investigation thereby comprised the coating porosity, the elastic modulus, and the residual stress evolution within the coating. The melting status of the particles at the impingement on the substrate in combination with the substrate surface condition is crucial for the coating formation. Single splats were collected on mirror-polished substrates for selected spray conditions and evaluated by identifying different types of splats (ideal, distorted, weakly bonded, and partially molten) and their relative fractions. In a previous study, these splat types were evaluated in terms of their effect on the above-mentioned coating properties. The particle melting status, which serves as a measure for the particle spreading behavior, was determined by in-flight particle temperature measurements and correlated to the coating properties. It was found that the gun power and the spray distance have a strong effect on the investigated coating properties, whereas the feed rate and the cooling show minor influence. © 2018 ASM International
    view abstract10.1007/s11666-018-0697-z
  • Correlation of splat morphologies with porosity and residual stress in plasma-sprayed YSZ coatings
    Mutter, M. and Mauer, G. and Mücke, R. and Guillon, O. and Vaßen, R.
    Surface and Coatings Technology 318 (2017)
    The Atmospheric Plasma Spray (APS) Process is a widely used technique for manufacturing ceramic coatings with unique properties for many applications, whereat the residual stresses within these coatings are an important factor affecting their performance. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so called splats. Understanding the mechanisms connecting these smallest structural elements with the overall coating properties enables the specific design of the coating process. In the present work, the correlation between coating porosity, Young's modulus, and residual stress with the particle melting was investigated. Therefore, single splats were collected on mirror-polished substrates and evaluated by identifying different types and their relative fractions under various spray conditions. Microscopic investigations were performed to obtain information on the spreading behavior of the different splat types to establish a relation to the coating properties. Furthermore, the particles were characterized by in-flight measurement of their temperature and the obtained results were evaluated in terms of their degree of melting. This was compared with the experimentally observed splat type amounts and finally correlated to the investigated coating properties. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.12.061
  • Diagnostics of Cold-Sprayed Particle Velocities Approaching Critical Deposition Conditions
    Mauer, G. and Singh, R. and Rauwald, K.-H. and Schrüfer, S. and Wilson, S. and Vaßen, R.
    Journal of Thermal Spray Technology (2017)
    In cold spraying, the impact particle velocity plays a key role for successful deposition. It is well known that only those particles can achieve successful bonding which have an impact velocity exceeding a particular threshold. This critical velocity depends on the thermomechanical properties of the impacting particles at impacting temperature. The latter depends on the gas temperature in the torch but also on stand-off distance and gas pressure. In the past, some semiempirical approaches have been proposed to estimate particle impact and critical velocities. Besides that, there are a limited number of available studies on particle velocity measurements in cold spraying. In the present work, particle velocity measurements were performed using a cold spray meter, where a laser beam is used to illuminate the particles ensuring sufficiently detectable radiant signal intensities. Measurements were carried out for INCONEL® alloy 718-type powders with different particle sizes. These experimental investigations comprised mainly subcritical spray parameters for this material to have a closer look at the conditions of initial deposition. The critical velocities were identified by evaluating the deposition efficiencies and correlating them to the measured particle velocity distributions. In addition, the experimental results were compared with some values estimated by model calculations. © 2017 ASM International
    view abstract10.1007/s11666-017-0596-8
  • Effect of test atmosphere composition on high-temperature oxidation behaviour of CoNiCrAlY coatings produced from conventional and ODS powders
    Huang, T. and Bergholz, J. and Mauer, G. and Vassen, R. and Naumenko, D. and Quadakkers, W.J.
    Materials at High Temperatures (2017)
    The oxidation behaviour of free-standing CoNiCrAlY coatings produced by low-pressure plasma spraying using conventional powder and oxide dispersion strengthened (ODS) powder containing 2 wt. % Al-oxide dispersion was investigated. Thermogravimetric experiments at 1100 °C in Ar-20%O2 and Ar-4%H2-2%H2O showed lower oxidation rates of the ODS than the conventional coating. In the latter material the scale growth was enhanced by extensive Y-incorporation of Y/Al-mixed oxide precipitates in the scale and apparently by Y-segregation to oxide grain boundaries. In the ODS coating the alumina dispersion bonded Y in the form of Y-aluminate thereby effectively suppressing scale ‘overdoping’. SEM/EBSD studies of all alumina scales revealed a columnar grain structure with the lateral grain size increasing approximately linearly with depth from the oxide/gas interface. For both coatings the alumina scale growth was slower in Ar–H2–H2O than in Ar–O2. The result is believed to be related to a lower oxygen potential gradient and to slower grain boundary diffusion in the scale forming in H2/H2O containing gas. © 2017 Informa UK Limited, trading as Taylor & Francis Group
    view abstract10.1080/09603409.2017.1389422
  • Effects of substrate roughness and spray-angle on deposition behavior of cold-sprayed Inconel 718
    Singh, R. and Rauwald, K.-H. and Wessel, E. and Mauer, G. and Schruefer, S. and Barth, A. and Wilson, S. and Vassen, R.
    Surface and Coatings Technology 319 (2017)
    In this study, Inconel 718 powder particles were successfully cold-sprayed on Inconel 718 substrate by using nitrogen gas for a repair application of aero engine components. The effects of substrate roughness and spray-angle on the deposition behavior of Inconel 718 particles were investigated. It has been found that the deposition behavior of Inconel 718 powder on Inconel 718 substrates is highly influenced by substrate surface roughness. Single powder particle interaction with substrates of different roughness showed that plastic deformation and interfacial material mixing is higher if powder particles interact with substrates of higher roughness. Consequently, substrates with lower roughness demonstrated many abnormalities (e.g. cracks, peeling-off) during the deposition. Substrate roughness is limited to influence the deposition efficiency of the first few layers. Besides, it is also observed that particle deformation and coating quality are significantly affected by the spray-angle because of asymmetric deformation of the particle due to additional tangential momentum. Moreover, it is observed that deposition efficiency is reduced with a decrease in spray-angle, while the coating porosity and coating roughness show an inverse trend. Furthermore, the effects of three input parameters, namely substrate preparation, spray angle and stand-off distance, on four outputs (responses), such as thickness, roughness, porosity and Vickers-hardness of the coating were studied using non-linear statistical regression analysis. The above mentioned four outputs were found to be significantly dependent on substrate preparation and spray-angle. © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.03.072
  • Erratum to: The 2016 Thermal Spray Roadmap (Journal of Thermal Spray Technology, (2016), 25, 8, (1376-1440), 10.1007/s11666-016-0473-x)
    Vardelle, A. and Moreau, C. and Akedo, J. and Ashrafizadeh, H. and Berndt, C.C. and Berghaus, J.O. and Boulos, M. and Brogan, J. and Bourtsalas, A.C. and Dolatabadi, A. and Dorfman, M. and Eden, T.J. and Fauchais, P. and Fisher, G. and Gaertner, F. and Gindrat, M. and Henne, R. and Hyland, M. and Irissou, E. and Jodoin, B. and Jordan, E.H. and Khor, K.A. and Killinger, A. and Lau, Y.-C. and Li, C.-J. and Li, L. and Longtin, J. and Markocsan, N. and Masset, P.J. and Matejicek, J. and Mauer, G. and McDonald, A. and Mostaghimi, J. and Sampath, S. and Schiller, G. and Shinoda, K. and Smith, M.F. and Syed, A.A. and Themelis, N.J. and Toma, F.-L. and Trelles, J.P. and Vassen, R. and Vuoristo, P.
    Journal of Thermal Spray Technology 26 (2017)
    We regret that the original article contained the following errors and omissions: • The following author was not listed: Bertrand Jodoin, Department of Mechanical Engineering, University of Ottawa, Ottawa, ON, Canada. • The caption for Fig. 2 on page 1378 should read, “Fig. 2 Cross section of single impacts of cold-sprayed Ti-6Al-4V particles on titanium substrate. Example demonstrates that metallurgical bonding by ASI occurs in the particle substrate interfaces providing higher strength as compared to the substrate material.” • The caption for Fig. 6 on page 1382 should read, “Fig. 6 Application roadmap of AD method.” Errors in Figure and Table Numbering • On page 1380, section 2.2.4 should read, “Medical applications such as ceramic coatings for dentures and artificial bones have also been studied (Ref 27) (Fig. 6).” It was incorrectly listed as “Fig. 7.” • On page 1381, section 2.2.5 should read, “As shown in Fig. 4, clear lattice images with crystal grains sized less than 10 nm across were observed.” It was incorrectly listed as “Fig. 1.” • On page 1383 in the first paragraph, the fifth sentence should read, “This method is envisaged to variably change the deposition principle by introducing the thermal effect of the conventional thermal spray process to the AD method such that the HAD method benefits from both of these complimentary technologies (Fig. 7).” It was incorrectly listed as “Fig. 4.” • On page 1402, section 3.3.1, in the second paragraph, the seventh sentence should read, “The different types or modes of wear are shown in Fig. 23.” It was incorrectly listed as “Fig. 1.” • On page 1422, at the end of the first paragraph, the text should read, “Table 2 summarizes the key applications and components where thermal spray technology is being used.” It was incorrectly printed as “Table 1.”. © 2016, ASM International.
    view abstract10.1007/s11666-017-0560-7
  • Excitation Temperature and Constituent Concentration Profiles of the Plasma Jet Under Plasma Spray-PVD Conditions
    He, W. and Mauer, G. and Vaßen, R.
    Plasma Chemistry and Plasma Processing 37 (2017)
    Plasma spray-physical vapor deposition (PS-PVD) is a promising technology to produce columnar structured thermal barrier coatings with excellent cyclic lifetime. The characteristics of plasma jets generated by standard plasma gases in the PS-PVD process, argon and helium, have been studied by optical emission spectroscopy. Abel inversion was introduced to reconstruct the spatial characteristics. In the central area of the plasma jet, the ionization of argon was found to be one of the reasons for low emission of atomic argon. Another reason could be the demixing so that helium prevails around the central axis of the plasma jet. The excitation temperature of argon was calculated by the Boltzmann plot method. Its values decreased from the center to the edge of the plasma jet. Applying the same method, a spurious high excitation temperature of helium was obtained, which could be caused by the strong deviation from local thermal equilibrium of helium. The addition of hydrogen into plasma gases leads to a lower excitation temperature, however a higher substrate temperature due to the high thermal conductivity induced by the dissociation of hydrogen. A loading effect is exerted by the feedstock powder on the plasma jet, which was found to reduce the average excitation temperature considerably by more than 700 K in the Ar/He jet. © 2017, Springer Science+Business Media, LLC.
    view abstract10.1007/s11090-017-9832-8
  • Impact of processing conditions and feedstock characteristics on thermally sprayed MCrAlY bondcoat properties
    Mauer, G. and Sebold, D. and Vaßen, R. and Hejrani, E. and Naumenko, D. and Quadakkers, W.J.
    Surface and Coatings Technology 318 (2017)
    One of the options to manufacture MCrAlY bondcoats (M. =Co, Ni) for thermal barrier coating systems is High Velocity Oxy-Fuel spraying (HVOF). In this work, particle diagnostics were applied to investigate the impact of processing conditions and feedstock characteristics on the relevant bondcoat properties. The results showed that compromises must be made on the oxygen/fuel ratio, spray distance, and particle size distribution to strike a balance between low oxidation and dense microstructures.These limitations initiated the development of the High Velocity Atmospheric Plasma Spray process (HV-APS) as a further alternative process. In this work, HV-APS process parameters were developed for a three cathode torch in combination with a 5. mm diameter high speed nozzle. A one-dimensional calculation of the expansion through this nozzle to atmospheric pressure yielded supersonic conditions with a Mach number of 1.84. The calculated plasma temperatures at the nozzle exit and in the expanded jet are 8400. K and slightly above 5200. K, respectively, which is low compared to conventional APS processes. A very fine powder with a median particle size of 18. μm was identified to be most suitable. Although the spray conditions were relatively cold, reasonable deposition efficiencies up to 61% and rather dense coatings were achieved using this feedstock. The as-sprayed porosity was ≈. 2% which was reduced by the subsequent vacuum heat treatment to <. 1%. The oxygen content determined by chemical analysis for a sample sprayed at a spray distance of 100. mm was 0.41. ±. 0.04. wt%.Moreover, reference samples were manufactured by Low Pressure Plasma Spraying (LPPS). The oxidation behavior was compared in isothermal and cyclic oxidation tests. The oxidation rates of the HV-APS coatings were found to be significantly lower than those of LPPS coatings. The thermally grown oxide scale showed less yttrium incorporation and better adherence in case of HV-APS. The latter is suggested to be related to a unique new distribution of Y-rich nano-sized oxide precipitates. The cyclic oxidation test confirmed the better oxidation resistance of the HV-APS coatings. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.08.079
  • Investigation of the resistance of open-column-structured PS-PVD TBCs to erosive and high-temperature corrosive attack
    Rezanka, S. and Mack, D.E. and Mauer, G. and Sebold, D. and Guillon, O. and Vaßen, R.
    Surface and Coatings Technology 324 (2017)
    In modern gas turbines, highly loaded components are internally cooled and furthermore covered with thermal barrier coatings (TBCs) to withstand the harsh operating conditions with temperatures exceeding the application limit of such coatings. Under realistic operating conditions, siliceous minerals, of a calcium-magnesium-aluminum-silicate (CMAS) composition, are ingested into the turbine and deposited on the TBCs. Besides erosion, this also leads to degradation by chemical interaction. The plasma spray-physical vapor deposition (PS-PVD) process is an advanced method for manufacturing TBCs, which fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). Due to the unique plasma conditions, coatings with columnar microstructures exhibiting high strain tolerance can be created. However, because of the high amount of open porosity the resistance of such structures to CMAS and erosion attack was expected to be low. In the present work, PS-PVD TBCs were investigated in a burner rig facility under thermal gradient cycling conditions and simultaneous CMAS attack. The interactions of the PS-PVD-deposited YSZ and the CMAS melt were studied by means of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) and compared to EB-PVD coatings. Additionally, the resistance of PS-PVD TBCs to erosion is compared to APS TBCs by means of room temperature tests according to ASTM G76-13. © 2017
    view abstract10.1016/j.surfcoat.2017.05.003
  • Investigations on the Nature of Ceramic Deposits in Plasma Spray–Physical Vapor Deposition
    He, W. and Mauer, G. and Gindrat, M. and Wäger, R. and Vaßen, R.
    Journal of Thermal Spray Technology 26 (2017)
    In Plasma Spray–Physical Vapor Deposition (PS-PVD) process, major fractions of the feedstock powder can be evaporated so that coatings are deposited mainly from the vapor phase. In this work, Computational Fluid Dynamics (CFD) results indicate that such evaporation occurs significantly in the plasma torch nozzle and even nucleation and condensation of zirconia is highly possible there. Experimental work has been performed to investigate the nature of the deposits in the PS-PVD process, in particular coatings from condensate vapor and nano-sized clusters produced at two spraying distances of 1000 mm and 400 mm. At long spraying distance, columns in the coatings have pyramidal tops and very sharp faceted microstructures. When the spraying distance is reduced to 400 mm, the tops of columns become relatively flat and a faceted structure is not recognizable. XRD patterns show obvious preferred orientations of (110) and (002) in the coatings sprayed at 400 mm but only limited texture in the coatings sprayed at 1000 mm. Meanwhile, a non-line of sight coating was also investigated, which gives an example for pure vapor deposition. Based on these analyses, a vapor and cluster depositions are suggested to further explain the formation mechanisms of high-quality columnar-structured PS-PVD thermal barrier coatings which have already shown excellent performance in cyclic lifetime test. © 2016, ASM International.
    view abstract10.1007/s11666-016-0513-6
  • Isothermal and cyclic oxidation behavior of free standing MCrAlY coatings manufactured by high-velocity atmospheric plasma spraying
    Hejrani, E. and Sebold, D. and Nowak, W.J. and Mauer, G. and Naumenko, D. and Vaßen, R. and Quadakkers, W.J.
    Surface and Coatings Technology 313 (2017)
    In the present paper the high temperature oxidation behavior of a free standing NiCoCrAlY coating produced by high-velocity atmospheric plasma spraying (HV-APS) is investigated and compared with that produced by conventional low pressure plasma spraying (LPPS). Isothermal thermogravimetric experiments at 1000 and 1100 °C in synthetic air revealed a lower oxidation rate of the HV-APS than the LPPS coating. Both coatings formed oxide scales based on alpha alumina, however, in the LPPS coating incorporation of coarse mixed Y/Al-oxide pegs into the scale occurred, increasing the oxidation rate by providing short circuit paths for oxygen diffusion probably due to higher diffusivities in the mixed oxide and/or along the interfaces between mixed oxide and alumina. In the HV-APS coatings most of the yttrium was tied-up in sub-μm Y-containing oxide particles and only minor amounts of mixed Y/Al oxide precipitates were found in the alumina surface scale. Cyclic air oxidation tests at 1100 °C revealed a lower oxidation rate and better scale adherence for the HV-APS coating. The results thus show that HV-APS is a promising method for the processing of MCrAlY coatings. The specific yttrium distribution in form of fine oxide precipitates in the HV-APS material prevents the formation of deleterious Y-rich oxide pegs and promotes formation of a slowly growing, protective alumina scale with excellent adherence. © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.01.081
  • Manufacturing of Composite Coatings by Atmospheric Plasma Spraying Using Different Feed-Stock Materials as YSZ and MoSi2
    Koch, D. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 26 (2017)
    Yttria-stabilized zirconia (YSZ) is the state-of-the-art material for the top coat of thermal barrier coatings. To increase the efficiency and lifetime of gas turbines, the integration of MoSi2 as a healing material was proposed. A new method of manufacture was explored in order to enable the spraying of a homogeneous mixed layer of YSZ and MoSi2. As the chemical and physical properties of these powders are very different, they require contrasting process conditions. Due to the evaporation of Si from MoSi2 at spraying conditions suitable for YSZ, more moderate conditions and a shorter time of flight are required for depositing MoSi2. At the same time, the spraying conditions still need to be sufficient for melting the YSZ particles in order to produce a coating. To obtain a homogeneous mixture, both conditions can be matched using an injection system that allows powder injection at two different locations of the plasma jet. Two-color pyrometry during flight (DPV-2000, Tecnar) was used to monitor the actual particle temperature. By optimizing the injection point for the MoSi2, a mixed coating was obtained without decomposition of the MoSi2, which has been analyzed by means of XRD and SEM. © 2017, ASM International.
    view abstract10.1007/s11666-017-0537-6
  • Manufacturing of high performance solid oxide fuel cells (SOFCs) with atmospheric plasma spraying (APS) and plasma spray-physical vapor deposition (PS-PVD)
    Marcano, D. and Mauer, G. and Vaßen, R. and Weber, A.
    Surface and Coatings Technology 318 (2017)
    In the present work, a metal supported SOFC half-cell was fabricated by means of plasma spray. As support, a Fe-Cr alloy with a porous structure was used. The anode and electrolyte were applied using atmospheric plasma spray (APS) and plasma spray-physical vapor deposition (PS-PVD), respectively. A standard Ni/YSZ (coat mix) powder was used for the anode and the cathode layer consisted of a screen-printed La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) non-sintered paste. The development of a thin, dense, gas-tight 8YSZ electrolyte was the key issue of this work. Analysis of microstructure, phases, and gas-tightness were carried out for various processing conditions. Different parameters were varied, such as: powder feed rate and carrier gas flow rate, robot speed, spraying distance and plasma gas composition. A partially reduced anode coating with 9% porosity and a gas-tight 26μm electrolyte layer were obtained. Such an assembly was air-tight and delivered a cell with an acceptable open circuit voltage (OCV) and an excellent performance of 1A/cm2 at 800C and 0.7V. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.10.088
  • Monitoring and Improving the Reliability of Plasma Spray Processes
    Mauer, G. and Rauwald, K.-H. and Mücke, R. and Vaßen, R.
    Journal of Thermal Spray Technology 26 (2017)
    Monitoring and improving of process reliability are prevalent issues in thermal spray technology. They are intended to accomplish specific quality characteristics by controlling the process. For this, implicit approaches are in demand to rapidly conclude on relevant coating properties, i.e., they are not directly measured, but it is assumed that the monitored variables are in fact suggestive for them. Such monitoring can be performed in situ (during the running process) instead of measuring coating characteristics explicitly (directly) and ex situ (after the process). Implicit approaches can be based on extrinsic variables (set from outside) as well as on intrinsic parameters (internal, not directly adjustable) having specific advantages and disadvantages, each. In this work, the effects of atmospheric plasma spray process variables are systemized in process schemes. On this basis, different approaches to contribute to improved process reliability are described and assessed paying particular attention to in-flight particle diagnostics. Finally, a new test applying spray bead analysis is introduced and first results are presented. © 2017, ASM International.
    view abstract10.1007/s11666-017-0559-0
  • Recent developments in plasma spray processes for applications in energy technology
    Mauer, G. and Jarligo, M.O. and Marcano, D. and Rezanka, S. and Zhou, D. and Vaßen, R.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    This work focuses on recent developments of plasma spray processes with respect to specific demands in energy technology. High Velocity Atmospheric Plasma Spraying (HV-APS) is a novel variant of plasma spraying devoted to materials which are prone to oxidation or decomposition. It is shown how this process can be used for metallic bondcoats in thermal barrier coating systems. Furthermore, Suspension Plasma Spraying (SPS) is a new method to process submicron-sized feedstock powders which are not sufficiently flowable to feed them in dry state. SPS is presently promoted by the development of novel torch concepts with axial feedstock injection. An example for a columnar structured double layer thermal barrier coating is given. Finally, Plasma Spray-Physical Vapor Deposition (PS-PVD) is a novel technology operating in controlled atmosphere at low pressure and high plasma power. At such condition, vaporization even of high-melting oxide ceramics is possible enabling the formation of columnar structured, strain tolerant coatings with low thermal conductivity. Applying different conditions, the deposition is still dominated by liquid splats. Such process is termed Low Pressure Plasma Spraying-Thin Film (LPPS-TF). Two examples of applications are gas-tight and highly ionic and electronic conductive electrolyte and membrane layers which were deposited on porous metallic substrates. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/181/1/012001
  • Yb2Si2O7 Environmental Barrier Coatings Deposited by Various Thermal Spray Techniques: A Preliminary Comparative Study
    Bakan, E. and Marcano, D. and Zhou, D. and Sohn, Y.J. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 26 (2017)
    Dense, crack-free, uniform, and well-adhered environmental barrier coatings (EBCs) are required to enhance the environmental durability of silicon (Si)-based ceramic matrix composites in high pressure, high gas velocity combustion atmospheres. This paper represents an assessment of different thermal spray techniques for the deposition of Yb2Si2O7 EBCs. The Yb2Si2O7 coatings were deposited by means of atmospheric plasma spraying (APS), high-velocity oxygen fuel spraying (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) techniques. The initial feedstock, as well as the deposited coatings, were characterized and compared in terms of their phase composition. The as-sprayed amorphous content, microstructure, and porosity of the coatings were further analyzed. Based on this preliminary investigation, the HVOF process stood out from the other techniques as it enabled the production of vertical crack-free coatings with higher crystallinity in comparison with the APS and SPS techniques in atmospheric conditions. Nevertheless, VLPPS was found to be the preferred process for the deposition of Yb2Si2O7 coatings with desired characteristics in a controlled-atmosphere chamber. © 2017, ASM International.
    view abstract10.1007/s11666-017-0574-1
  • Atomic layer deposition and high-resolution electron microscopy characterization of nickel nanoparticles for catalyst applications
    Dashjav, E. and Lipińska-Chwałek, M. and Grüner, D. and Mauer, G. and Luysberg, M. and Tietz, F.
    Surface and Coatings Technology 307 (2016)
    Ni nanoparticles (diameter <  10 nm) are deposited on Si and ceramic substrates of porous lanthanum-substituted strontium titanate/yttrium-stabilized zirconia (LST/YSZ) composites by a two-step process. First, NiO films are produced by atomic layer deposition at 200 °C using bis(methylcyclopentadienyl)nickel(II) (Ni(MeCp)2) and H2O as precursors. In the second step, the NiO films are reduced in H2 atmosphere at 400–800 °C. The size of the resulting Ni nanoparticles is controlled by the temperature. The largest particles with a diameter of about 7 nm are obtained at 800 °C. NiO film and Ni nanoparticles deposited on Si substrates are characterized by high-resolution electron microscopy. It was found that the Ni(MeCp)2 precursor reacts with the substrate, leading to the formation of NiSi2 precipitates beneath the surface of the Si wafer and amorphization of the surrounding area, resulting in a 10 nm thick top layer of the Si wafer. After reductive annealing, NiSi2 precipitates are preserved but Si recrystallizes and the amorphous NiO film transforms into crystalline Ni nanoparticles well distributed on the wafer surface. Process parameters were optimized for Si substrates and transfer of the process to ceramic LST/YSZ substrates is possible in principle. However, a much higher number of ALD cycles (1200 compared to 100 for Si) are necessary to obtain Ni nanoparticles of similar size and the number density of particles is lower than observed for Si substrates. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.08.074
  • Atomic-layer-controlled deposition of TEMAZ/O2-ZrO2 oxidation resistance inner surface coatings for solid oxide fuel cells
    Keuter, T. and Mauer, G. and Vondahlen, F. and Iskandar, R. and Menzler, N.H. and Vaßen, R.
    Surface and Coatings Technology 288 (2016)
    Solid oxide fuel cells (SOFCs) directly convert the chemical energy of fuels into electrical energy with high efficiency. Under certain conditions oxygen can diffuse to the Ni/8 mol% Y2O3-doped ZrO2 substrate of anode-supported SOFCs, then the nickel re-oxidizes, leading to cracks in the electrolyte and cell failure thus limiting the durability of SOFCs. In order to improve the stability of SOFCs with respect to oxidation, the inner surface of the porous substrate is coated with a ZrO2 oxidation resistance layer using atomic layer deposition (ALD) with the precursors tetrakis(ethylmethylamino)zirconium (TEMAZ) and molecular oxygen. This TEMAZ/O2-ZrO2 ALD process has not yet been reported in the literature and hence, the development of the process is described in this paper. The inner surface of the porous substrate is coated with ZrO2 and the film thickness is compared with theoretical predictions, verifying the ALD model. Furthermore, the coating depth can be estimated using a simple analytical equation. The ALD ZrO2 film protects the nickel in the substrate against oxidation for at least 17 re-oxidation/re-reduction cycles. The ZrO2 inner surface coating is a highly promising candidate for enhancing the resistance of SOFCs to re-oxidation because of the excellent oxidation resistance and good cycling stability of the film. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.01.026
  • Controlling the stress state of La1-xSrxCoyFe1-yO3-δ oxygen transport membranes on porous metallic supports deposited by plasma spray-physical vapor process
    Marcano, D. and Mauer, G. and Sohn, Y.J. and Vaßen, R. and Garcia-Fayos, J. and Serra, J.M.
    Journal of Membrane Science 503 (2016)
    La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF), deposited on a metallic porous support by plasma spray-physical vapor deposition (PS-PVD) is a promising candidate for oxygen-permeation membranes. However, after O2 permeation tests, membranes show vertical cracks leading to leakage during the tests. In the present work, one important feature leading to crack formation was identified. More specifically; membrane residual stress changes during thermal loading were found to be related to a phase transformation in the support. In order to improve the performance of the membranes, the metallic support was optimized by applying an appropriate heat treatment. The observed oxygen fluxes during permeation tests had infinite selectivity and were amongst the highest fluxes ever measured for LSCF membranes in the thickness range of 30μm, supported by LSCF porous substrates. © 2016 Elsevier B.V.
    view abstract10.1016/j.memsci.2015.12.029
  • Effect of Plasma Enthalpy on the Structure of La2Zr2O7 Coatings Prepared by Suspension Plasma Spraying
    Zotov, N. and Guignard, A. and Mauer, G. and Vaßen, R.
    Journal of the American Ceramic Society 99 (2016)
    La2Zr2O7 coatings, prepared by suspension plasma spraying, have been studied by X-ray diffraction (XRD) as a function of torch power. Rietveld refinements of high-resolution XRD data show that with increasing plasma temperature (as a result of the increasing torch power), the La2Zr2O7 coatings remain cation ordered but progressively anion disordered. © 2015 The American Ceramic Society.
    view abstract10.1111/jace.14058
  • Investigations on the Initial Stress Evolution During Atmospheric Plasma Spraying of YSZ by In Situ Curvature Measurement
    Mutter, M. and Mauer, G. and Mücke, R. and Vaßen, R. and Back, H.C. and Gibmeier, J.
    Journal of Thermal Spray Technology 25 (2016)
    The residual stresses within plasma-sprayed coatings are an important factor that can influence the lifetime as well as the performance in operation. The investigation of stresses evolving during deposition and post-deposition cooling for atmospheric plasma spraying of yttria-stabilized zirconia coatings using in situ measurement of the samples curvature is a powerful tool for identifying the factors that contribute to stress generation. Under various spray conditions, the first deposition pass leads to a significantly larger increase in samples curvature than the subsequent passes. It is shown in this work that the amount of curvature change at the onset of spraying is significantly influenced by the spray conditions, as well as by the substrate material. More information on the origin of this steep curvature increase at the onset of spraying was obtained by single splat experiments, which yielded information on the splat bonding behavior under various conditions. A comparison of the compressive yield strength for different substrate materials indicated the influence of substrate residual stress relaxation. Residual stress measurements using the incremental hole-drilling method and x-ray diffraction confirmed that the coating deposition affects the substrate residual stress level. The yield strength data were combined with the substrate near-surface temperature during deposition, obtained by finite element simulations, and with the measured residual stress-profile. This revealed that residual stress relaxation is the key factor for the initial curvature increase. © 2016, ASM International.
    view abstract10.1007/s11666-016-0398-4
  • The 2016 Thermal Spray Roadmap
    Vardelle, A. and Moreau, C. and Akedo, J. and Ashrafizadeh, H. and Berndt, C.C. and Berghaus, J.O. and Boulos, M. and Brogan, J. and Bourtsalas, A.C. and Dolatabadi, A. and Dorfman, M. and Eden, T.J. and Fauchais, P. and Fisher, G. and Gaertner, F. and Gindrat, M. and Henne, R. and Hyland, M. and Irissou, E. and Jordan, E.H. and Khor, K.A. and Killinger, A. and Lau, Y.-C. and Li, C.-J. and Li, L. and Longtin, J. and Markocsan, N. and Masset, P.J. and Matejicek, J. and Mauer, G. and McDonald, A. and Mostaghimi, J. and Sampath, S. and Schiller, G. and Shinoda, K. and Smith, M.F. and Syed, A.A. and Themelis, N.J. and Toma, F.-L. and Trelles, J.P. and Vassen, R. and Vuoristo, P.
    Journal of Thermal Spray Technology 25 (2016)
    Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications. © 2016, ASM International.
    view abstract10.1007/s11666-016-0473-x
  • The Role of Oxygen Partial Pressure in Controlling the Phase Composition of La1−xSrxCoyFe1−yO3−δ Oxygen Transport Membranes Manufactured by Means of Plasma Spray-Physical Vapor Deposition
    Marcano, D. and Mauer, G. and Sohn, Y.J. and Vaßen, R. and Garcia-Fayos, J. and Serra, J.M.
    Journal of Thermal Spray Technology 25 (2016)
    La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF) deposited on a metallic porous support by plasma spray-physical vapor deposition is a promising candidate for oxygen-permeation membranes. Ionic transport properties are regarded to depend on the fraction of perovskite phase present in the membrane. However, during processing, the LSCF powder decomposes into perovskite and secondary phases. In order to improve the ionic transport properties of the membranes, spraying was carried out at different oxygen partial pressures p(O2). It was found that coatings deposited at lower and higher oxygen partial pressures consist of 70% cubic/26% rhombohedral and 61% cubic/35% rhombohedral perovskite phases, respectively. During annealing, the formation of non-perovskite phases is driven by oxygen non-stoichiometry. The amount of oxygen added during spraying can be used to increase the perovskite phase fraction and suppress the formation of non-perovskite phases. © 2016, ASM International.
    view abstract10.1007/s11666-016-0383-y
  • Cavitation-resistant NiTi coatings produced by low-pressure plasma spraying (LPPS)
    Bitzer, M. and Rauhut, N. and Mauer, G. and Bram, M. and Vaßen, R. and Buchkremer, H.-P. and Stöver, D. and Pohl, M.
    Wear 328-329 (2015)
    Cavitation is a severe wear mechanism in technical applications where parts are in contact with rapidly flowing liquids. Examples are turbine blades in hydropower plants or pump components. Coating exposed surfaces with wear-resistant materials is an effective measure for extending lifetime in the case of cavitation attack. NiTi is an attractive material for such coatings considering its clearly pronounced damping behavior based on its pseudoelastic properties. A promising processing route for coating net-shaped components with NiTi is low-pressure plasma spraying (LPPS). In the present work, NiTi layers were produced by LPPS, starting from pre-alloyed NiTi powder. Cavitation resistance was investigated in relation to LPPS parameters, layer thickness and specific surface treatment. Increased cavitation resistance was demonstrated compared to UTP 730, an established cavitation protection material. The study was accompanied by comprehensive characterization of microstructure and phase transformation behavior of the NiTi coatings. © 2015 Elsevier B.V.
    view abstract10.1016/j.wear.2015.03.003
  • Cycling Performance of a Columnar-Structured Complex Perovskite in a Temperature Gradient Test
    Schlegel, N. and Sebold, D. and Sohn, Y.J. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 24 (2015)
    To increase the efficiency of turbines for the power generation and the aircraft industry, advanced thermal barrier coatings (TBCs) are required. They need to be long-term stable at temperatures higher than 1200 °C. Nowadays, yttria partially stabilized zirconia (YSZ) is applied as standard TBC material. But its long-term application at temperatures higher than 1200 °C leads to detrimental phase changes and sintering effects. Therefore, new materials have to be investigated, for example, complex perovskites. They provide high melting points, high thermal expansion coefficients and thermal conductivities of approx. 2.0 W/(m K). In this work, the complex perovskite La(Al1/4Mg1/2Ta1/4)O3 (LAMT) was investigated. It was deposited by the suspension plasma spraying (SPS) process, resulting in a columnar microstructure of the coating. The coatings were tested in thermal cycling gradient tests and they show excellent results, even though some phase decomposition was found. © 2015, ASM International.
    view abstract10.1007/s11666-015-0254-y
  • Effects of Feedstock Decomposition and Evaporation on the Composition of Suspension Plasma-Sprayed Coatings
    Mauer, G. and Schlegel, N. and Guignard, A. and Vaßen, R. and Guillon, O.
    Journal of Thermal Spray Technology 24 (2015)
    Emerging new applications and growing demands of plasma-sprayed coatings have initiated the development of new plasma spray processes. One of them is suspension plasma spraying (SPS). The use of liquid feedstock such as suspensions yields higher flexibility compared to the conventional atmospheric plasma spray processes as even submicron-to nano-sized particles can be processed. This allows achieving particular microstructural features, e.g., porous segmented or columnar-structured thermal barrier coatings. To exploit the potentials of such novel plasma spray processes, the plasma-feedstock interaction must be understood better. In this study, decomposition and evaporation of feedstock material during SPS were investigated, since particular difficulties can occur with respect to stoichiometry and phase composition of the deposits. Plasma conditions were analyzed by optical emission spectroscopy (OES). Experimental results are given, namely for gadolinium zirconate and for lanthanum strontium cobalt ferrite deposition. Moreover, the applied OES approach is validated by comparison with the simpler actinometry method. © 2015, ASM International.
    view abstract10.1007/s11666-015-0250-2
  • Modeling precursor diffusion and reaction of atomic layer deposition in porous structures
    Keuter, T. and Menzler, N.H. and Mauer, G. and Vondahlen, F. and Vaßen, R. and Buchkremer, H.P.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 33 (2015)
    Atomic layer deposition (ALD) is a technique for depositing thin films of materials with a precise thickness control and uniformity using the self-limitation of the underlying reactions. Usually, it is difficult to predict the result of the ALD process for given external parameters, e.g., the precursor exposure time or the size of the precursor molecules. Therefore, a deeper insight into ALD by modeling the process is needed to improve process control and to achieve more economical coatings. In this paper, a detailed, microscopic approach based on the model developed by Yanguas-Gil and Elam is presented and additionally compared with the experiment. Precursor diffusion and second-order reaction kinetics are combined to identify the influence of the porous substrate's microstructural parameters and the influence of precursor properties on the coating. The thickness of the deposited film is calculated for different depths inside the porous structure in relation to the precursor exposure time, the precursor vapor pressure, and other parameters. Good agreement with experimental results was obtained for ALD zirconiumdioxide (ZrO2) films using the precursors tetrakis(ethylmethylamido)zirconium and O2. The derivation can be adjusted to describe other features of ALD processes, e.g., precursor and reactive site losses, different growth modes, pore size reduction, and surface diffusion. © 2014 Author(s).
    view abstract10.1116/1.4892385
  • Modelling and proper evaluation of volumetric kinetics of hydrogen desorption by metal hydrides
    Drozdov, I.V. and Kochubey, V. and Meng, L. and Mauer, G. and Vaßen, R. and Stöver, D.
    International Journal of Hydrogen Energy 40 (2015)
    A simple model of the hydrogen desorption kinetics of metal hydrides is formulated and solved analytically. The particle surface reaction is assumed to be a rate-controlling-step. Then a volumetric measurement of hydrogen desorption process is evaluated on an example of wet ball milled magnesium hydride, and can be applied generally for any metal hydride. The solution reproduces the shape of experimental curves for desorption process. In the case of surface-controlled kinetics, a volumetric measurement requires a special evaluation of results, predicted by the solution of the model. An improved evaluation of the volumetric measurement of hydrogen desorption from magnesium hydride powders using the model has been demonstrated. © 2015 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2015.05.119
  • Novel opportunities for thermal spray by PS-PVD
    Mauer, G. and Jarligo, M.O. and Rezanka, S. and Hospach, A. and Vaßen, R.
    Surface and Coatings Technology 268 (2015)
    Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition takes place not only from liquid splats but also from nano-sized clusters and from the vapor phase. This offers new opportunities to obtain advanced microstructures and thus to comply with the growing demands on modern functional coatings. Thin and dense ceramic coatings as well as highly porous columnar structures can be achieved, offering novel opportunities for the application of thermal spray technology. This study describes process conditions, which are relevant for the formation of particular microstructures in the PS-PVD process. Following the structure of the process, the feedstock treatment close to the plasma source, plasma particle interaction in the open jet and the formation of coating microstructures on the substrate are covered. Calculated results on the plasma particle interaction under PS-PVD process conditions were found to be in good agreement with OES results and microstructural observations. They show that the feedstock treatment along the very first trajectory segment between injector and jet expansion plays a key role. Varying the plasma parameters, feedstock treatment can be controlled to a broad extent. Consequently, the manifold nature of the feedstock species arriving on the substrate enables to achieve various coating microstructures. As examples, application specific features of PS-PVD coatings are reported for strain-tolerant thermal barrier coatings as well as for gas-tight oxygen transport membranes with high mixed electronic-ionic conductivity. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.06.002
  • Porosity-Property Relationships of Plasma-Sprayed Gd2Zr2O7/YSZ Thermal Barrier Coatings
    Bakan, E. and Mack, D.E. and Mauer, G. and Mücke, R. and Vaßen, R.
    Journal of the American Ceramic Society 98 (2015)
    During the past decade, gadolinium zirconate (Gd<inf>2</inf>Zr<inf>2</inf>O<inf>7</inf>, GZO) has attracted interest as an alternative material to partially yttria-stabilized zirconia (YSZ) for thermal barrier coatings (TBCs). Despite the well-known benefits of GZO, such as lower thermal conductivity and superior temperature capability compared to YSZ, processing of GZO via atmospheric plasma spraying (APS) still remains a challenge. Here, we report on APS experiments which were performed to investigate the influence of processing on GZO microstructure and lifetime of GZO/YSZ double-layer TBCs. Different microstructures of GZO were produced and characterized in terms of porosity, stoichiometry, Young′s modulus, and their effects on the lifetime of YSZ/GZO double-layer TBCs were discussed. Particle diagnostics were utilized for the optimization of the process parameters with respect to different microstructures of GZO and stoichiometry. It was found that both cumulative porosity of GZO and pore size distribution, which alter the Young′s modulus significantly, govern the lifetime of double layers. In addition, it was shown that the deviation in GZO stoichiometry due to gadolinia evaporation in the investigated range does not display any critical effect on lifetime. © 2015 The American Ceramic Society.
    view abstract10.1111/jace.13611
  • Columnar-Structured Mg-Al-Spinel Thermal Barrier Coatings (TBCs) by Suspension Plasma Spraying (SPS)
    Schlegel, N. and Ebert, S. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 24 (2014)
    The suspension plasma spraying (SPS) process has been developed to permit the feeding of sub-micrometer-sized powder into the plasma plume. In contrast to electron beam-physical vapor deposition and plasma spray-physical vapor deposition, SPS enables the cost-efficient deposition of columnar-structured coatings. Due to their strain tolerance, these coatings play an important role in the field of thermal barrier coatings (TBCs). In addition to the cost-efficient process, attention was turned to the TBC material. Nowadays, yttria partially stabilized zirconia (YSZ) is used as standard TBC material. However, its long-term application at temperatures higher than 1200 °C is problematic. At these high temperatures, phase transitions and sintering effects lead to the degradation of the TBC system. To overcome those deficits of YSZ, Mg-Al-spinel was chosen as TBC material. Even though it has a lower melting point (~2135 °C) and a higher thermal conductivity (~2.5 W/m/K) than YSZ, Mg-Al-spinel provides phase stability at high temperatures in contrast to YSZ. The Mg-Al-spinel deposition by SPS resulted in columnar-structured coatings, which have been tested for their thermal cycling lifetime. Furthermore, the influence of substrate cooling during the spraying process on thermal cycling behavior, phase composition, and stoichiometry of the Mg-Al-spinel has been investigated. © 2014, ASM International.
    view abstract10.1007/s11666-014-0138-6
  • Gadolinium zirconate/YSZ thermal barrier coatings: Plasma spraying, microstructure, and thermal cycling behavior
    Bakan, E. and Mack, D.E. and Mauer, G. and Vaßen, R.
    Journal of the American Ceramic Society 97 (2014)
    Processing of Gd2Zr2O7 by atmospheric plasma spraying (APS) is challenging due to the difference in vapor pressure between gadolinia and zirconia. Gadolinia is volatilized to a greater extent than zirconia and the coating composition unfavorably deviates from the initial stoichiometry. Aiming at stoichiometric coatings, APS experiments were performed with a TriplexPro™ plasma torch at different current levels. Particle diagnostics proved to be an effective tool for the detection of potential degrees of evaporation via particle temperature measurements at these varied current levels. Optimized spray parameters for Gd2Zr2O7 in terms of porosity and stoichiometry were used to produce double-layer TBCs with an underlying yttria-stabilized zirconia (7YSZ) layer. For comparison, double layers were also deposited with relatively high torch currents during Gd2Zr2O7 deposition, which led to a considerable amount of evaporation and relatively low porosities. These coatings were tested in thermal cycling rigs at 1400°C surface temperature. Double layers manufactured with optimized Gd2Zr2O7 spray parameters revealed very good thermal cycling performance in comparison to standard 7YSZ coatings, whereas the others showed early failures. Furthermore, different failure modes were observed; coatings with long lifetime failed due to TGO growth, while the coatings displaying early failures spalled through crack propagation in the upper part of the 7YSZ layer. © 2014 The American Ceramic Society.
    view abstract10.1111/jace.13204
  • Improved thermal cycling durability of thermal barrier coatings manufactured by PS-PVD
    Rezanka, S. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 23 (2014)
    The plasma spray-physical vapor deposition (PS-PVD) process is a promising method to manufacture thermal barrier coatings (TBCs). It fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). The durability of PS-PVD manufactured columnar TBCs is strongly influenced by the compatibility of the metallic bondcoat (BC) and the ceramic TBC. Earlier investigations have shown that a smooth BC surface is beneficial for the durability during thermal cycling. Further improvements of the bonding between BC and TBC could be achieved by optimizing the formation of the thermally grown oxide (TGO) layer. In the present study, the parameters of pre-heating and deposition of the first coating layer were investigated in order to adjust the growth of the TGO. Finally, the durability of the PS-PVD coatings was improved while the main advantage of PS-PVD, i.e., much higher deposition rate in comparison to EB-PVD, could be maintained. For such coatings, improved thermal cycling lifetimes more than two times higher than conventionally sprayed TBCs, were measured in burner rigs at ~1250 C/1050 C surface/substrate exposure temperatures. © 2013 ASM International.
    view abstract10.1007/s11666-013-9971-2
  • MCrAlY bondcoats by high-velocity atmospheric plasma spraying
    Mauer, G. and Sebold, D. and Vaßen, R.
    Journal of Thermal Spray Technology 23 (2014)
    MCrAlY bondcoats (M = Co, Ni) are used to protect metallic substrates from oxidation and to improve adhesion of ceramic thermal barrier coatings for high temperature applications, such as in land-based and aviation turbines. Since MCrAlYs are prone to take up oxygen during thermal spraying, bondcoats often are manufactured under inert gas conditions at low pressure. Plasma spraying at atmospheric conditions is a cost-effective alternative if it would be possible to limit the oxygen uptake as well as to obtain sufficiently dense microstructures. In the present work, high-velocity spray parameters were developed for the TriplexPro 210 three-cathode plasma torch using MCrAlY powders of different particle size fractions to achieve these objectives. The aims are conflictive since the former requires cold conditions, whereas the latter is obtained by more elevated particle temperatures. High particle velocities can solve this divergence as they imply shorter time for oxidation during flight and contribute to coating densification by kinetic rather than thermal energy. Further aims of the experimental work were high deposition efficiencies as well as sufficient surface roughness. The oxidation behavior of the sprayed coatings was characterized by thermal gravimetric analyses and isothermal heat treatments. © 2013 ASM International.
    view abstract10.1007/s11666-013-0026-5
  • Plasma characteristics and plasma-feedstock interaction under PS-PVD process conditions
    Mauer, G.
    Plasma Chemistry and Plasma Processing 34 (2014)
    Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition takes place not only from liquid splats but also from nano-sized clusters and from the vapor phase. This offers new opportunities to obtain advanced microstructures and thus to comply with growing demands on modern functional coatings. In this study, different process conditions were investigated with regard to the application of the PS-PVD process for ceramic thermal barrier coatings. Plasma characteristics were calculated under chemical equilibrium conditions by minimizing the Gibbs energy. The plasma-feedstock interaction was modeled taking into account the particular conditions at very low pressure. Since the plasma is highly rarefied, the small feedstock particles are in the free molecular flow regime. Hence, continuum methods commonly used in fluid mechanics and heat transfer approaches with continuous boundary conditions are not appropriate; alternative methods based on the kinetic theory of gases are required. The experimental results confirm the predictions about the degree of vaporization made by such calculations. In particular, they show that the feedstock treatment mainly takes place within the very first trajectory segment between injector and jet expansion. © 2014 Springer Science+Business Media New York.
    view abstract10.1007/s11090-014-9563-z
  • Plasma spray physical vapor deposition of La1-x Sr x Co y Fe1-y O3-δ Thin-film oxygen transport membrane on porous metallic supports
    Jarligo, M.O. and Mauer, G. and Bram, M. and Baumann, S. and Vaßen, R.
    Journal of Thermal Spray Technology 23 (2014)
    Plasma spray physical vapor deposition (PS-PVD) is a very promising route to manufacture ceramic coatings, combining the efficiency of thermal spray processes and characteristic features of thin PVD coatings. Recently, this technique has been investigated to effectively deposit dense thin films of perovskites particularly with the composition of La0.58Sr 0.4Co0.2Fe0.8O3-δ (LSCF) for application in gas separation membranes. Furthermore, asymmetric type of membranes with porous metallic supports has also attracted research attention due to the advantage of good mechanical properties suitable for use at high temperatures and high permeation rates. In this work, both approaches are combined to manufacture oxygen transport membranes made of gastight LSCF thin film by PS-PVD on porous NiCoCrAlY metallic supports. The deposition of homogenous dense thin film is challenged by the tendency of LSCF to decompose during thermal spray processes, irregular surface profile of the porous metallic substrate and crack and pore-formation in typical ceramic thermal spray coatings. Microstructure formation and coating build-up during PS-PVD as well as the annealing behavior at different temperatures of LSCF thin films were investigated. Finally, measurements of leak rates and oxygen permeation rates at elevated temperatures show promising results for the optimized membranes. © 2013 ASM International.
    view abstract10.1007/s11666-013-0004-y
  • Plasma Spraying of Ceramics with Particular Difficulties in Processing
    Mauer, G. and Schlegel, N. and Guignard, A. and Jarligo, M.O. and Rezanka, S. and Hospach, A. and Vaßen, R.
    Journal of Thermal Spray Technology 24 (2014)
    Emerging new applications and growing demands of plasma-sprayed coatings initiate the development of new materials. Regarding ceramics, often complex compositions are employed to achieve advanced material properties, e.g., high thermal stability, low thermal conductivity, high electronic and ionic conductivity as well as specific thermo-mechanical properties and microstructures. Such materials however, often involve particular difficulties in processing by plasma spraying. The inhomogeneous dissociation and evaporation behavior of individual constituents can lead to changes of the chemical composition and the formation of secondary phases in the deposited coatings. Hence, undesired effects on the coating characteristics are encountered. In this work, examples of such challenging materials are investigated, namely pyrochlores applied for thermal barrier coatings as well as perovskites for gas separation membranes. In particular, new plasma spray processes like suspension plasma spraying and plasma spray-physical vapor deposition are considered. In some cases, plasma diagnostics are applied to analyze the processing conditions. © 2014, ASM International.
    view abstract10.1007/s11666-014-0149-3
  • Investigation and comparison of in-flight particle velocity during the plasma-spray process as measured by laser doppler anemometry and DPV-2000
    Mauer, G. and Vaßen, R. and Zimmermann, S. and Biermordt, T. and Heinrich, M. and Marques, J.-L. and Landes, K. and Schein, J.
    Journal of Thermal Spray Technology 22 (2013)
    Plasma spraying has become one of the most important thermal-spray technologies due to low operating costs, high deposition rates, and a high efficiency. It is especially suitable for producing coatings used to improve thermal, corrosion, and wear protection. The quality of coatings produced by thermal-spray processes are determined by particle characteristics, such as in-flight velocity, which can be investigated using various diagnostic systems. Velocity is a particularly relevant parameter for small particles, but it is difficult to measure. Hence, different velocity diagnostics must be validated for small injected particles. We compared the laser Doppler anemometry (LDA) system with the DPV-2000 system and measured the particle velocities of a F4 plasma torch. The results agreed well when the limited detectability of small particles by LDA was taken into account. © 2013 ASM International.
    view abstract10.1007/s11666-013-9940-9
  • Plasma spraying of efficient photoactive TiO2 coatings
    Mauer, G. and Guignard, A. and Vaßen, R.
    Surface and Coatings Technology 220 (2013)
    In TiO2 coatings for photocatalytic applications or dye-sensitized solar cells, anatase phase with specific rutile content is often preferred to achieve optimum activity. At appropriate process parameters, such phase composition can be obtained by suspension plasma spraying (SPS). However, immediately after deposition, partial transformation to rutile can take place if the substrate temperature is sufficient. Experimental results show that the phase composition has to be balanced with other coating characteristics like microstructure and deposition rate.Another approach to improve the photoactivity of TiO2 is nitrogen doping. It is known that such anionic dopant can create states within the band gap so as to reduce locally the energy barrier of the photoexited electron. Thus, the photoactivity in the range of visible light can be enhanced. TiN addition was investigated to introduce nitrogen in TiO2 coatings directly during SPS. First results are presented. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.08.042
  • Plasma-sprayed thermal barrier coatings: New materials, processing issues, and Solutions
    Mauer, G. and Jarligo, M.O. and Mack, D.E. and Vaßen, R.
    Journal of Thermal Spray Technology 22 (2013)
    Growing demands on thermal barrier coatings (TBCs) for gas turbines regarding their temperature and cyclic capabilities, corrosion resistance, and erosion performance have instigated the development of new materials and coating systems. Different pyrochlores, perovskites, doped yttria-stabilized zirconia, and hexaaluminates have been identified as promising candidates. However, processing these novel TBC materials by plasma spraying is often challenging. During the deposition process, stoichiometric changes, formation of undesired secondary phases or non-optimum amorphous contents, as well as detrimental microstructural effects can occur in particular. This article describes these difficulties and the development of process-related solutions by employing diagnostic tools. © 2013 ASM International.
    view abstract10.1007/s11666-013-9889-8
  • Process conditions and microstructures of ceramic coatings by gas phase deposition based on plasma spraying
    Mauer, G. and Hospach, A. and Zotov, N. and Vaßen, R.
    Journal of Thermal Spray Technology 22 (2013)
    Plasma spraying at very low pressure (50-200 Pa) is significantly different from atmospheric plasma conditions (APS). By applying powder feedstock, it is possible to fragment the particles into very small clusters or even to evaporate the material. As a consequence, the deposition mechanisms and the resulting coating microstructures could be quite different compared to conventional APS liquid splat deposition. Thin and dense ceramic coatings as well as columnar-structured strain-tolerant coatings with low thermal conductivity can be achieved offering new possibilities for application in energy systems. To exploit the potential of such a gas phase deposition from plasma spray-based processes, the deposition mechanisms and their dependency on process conditions must be better understood. Thus, plasma conditions were investigated by optical emission spectroscopy. Coating experiments were performed, partially at extreme conditions. Based on the observed microstructures, a phenomenological model is developed to identify basic growth mechanisms. © 2012 ASM International.
    view abstract10.1007/s11666-012-9838-y
  • Process development and coating characteristics of plasma spray-PVD
    Mauer, G. and Hospach, A. and Vaßen, R.
    Surface and Coatings Technology 220 (2013)
    Plasma spray physical vapor deposition (PS-PVD) was developed with the aim of depositing uniform and relatively thin coatings with large area coverage. At high power input (~. 150. kW) and very low pressure (~. 100. Pa) the plasma jet properties change considerably compared to conventional plasma spraying and it is even possible to evaporate the powder feedstock material enabling advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition (PVD). Moreover, the resulting microstructures are unique and can hardly be obtained by other processes.In this paper, plasma characteristics of different gas mixtures are investigated. The measurements and calculations provide indications of the growth modes and help to explain the resulting microstructures and coating chemistries. Coatings sprayed from different ceramic powders are discussed. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.08.067
  • Characteristics of ceramic coatings made by thin film low pressure plasma spraying (LPPS-TF)
    Hospach, A. and Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 21 (2012)
    The thin film low pressure plasma spray process (LPPS-TF) has been developed with the aim of efficient depositing uniform and thin coatings with large area coverage by plasma spraying. At high power input (∼150 kW) and very low pressure (∼100 Pa) the plasma jet properties change considerably and it is even possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition. In addition, the resulting microstructures are unique and can hardly be obtained by other processes. In this paper, microstructures made by LPPS-TF are shown and the columnar layer growth by vapor deposition is demonstrated. In addition to the ceramic materials TiO 2, Al2 O3 or MgAl2O4, the focus of the research was placed on partially yttria-stabilized zirconia. Variations of the microstructures are shown and discussed concerning potential coating applications. © ASM International.
    view abstract10.1007/s11666-012-9748-z
  • Decomposition of Ba(Mg 1/3Ta 2/3)O 3 perovskite during atmospheric plasma spraying
    Jarligo, M.O. and Mauer, G. and Sebold, D. and Mack, D.E. and Vaßen, R. and Stöver, D.
    Surface and Coatings Technology 206 (2012)
    Perovskite Ba(Mg 1/3Ta 2/3)O 3, BMT has promising bulk properties (thermal conductivity ~2W/m-K and coefficient of thermal expansion ~11×10 -6/K at 1473K) for thermal barrier coating (TBC) applications at high temperature. However, during atmospheric plasma spraying (APS), such material was found to lose constituents due to the differences of vapor pressures resulting to non-stoichiometric composition of deposited coatings. To investigate the extent of phase decomposition at spray distance and varying electric arc current, different feedstock powders were plasma sprayed into water and collected for chemical, microstructural and phase analyses. When the electric arc current was decreased from 500A to 300A, the decomposition of the powders was reduced and the microstructure of the deposited coatings was improved. The thermal cycling lifetime of the deposited coatings at ~1250°C surface temperature is also higher. © 2011 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2011.11.003
  • Deposition and characteristics of submicrometer-structured thermal barrier coatings by suspension plasma spraying
    Guignard, A. and Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 21 (2012)
    In the field of thermal barrier coatings (TBCs) for gas turbines, suspension plasma sprayed (SPS) submicrometer-structured coatings often show unique mechanical, thermal, and optical properties compared to conventional atmospheric plasma sprayed ones. They have thus the potential of providing increased TBC performances under severe thermo-mechanical loading. Experimental results showed the capability of SPS to obtain yttria stabilized zirconia coatings with very fine porosity and high density of vertical segmentation cracks, yielding high strain tolerance, and low Young's modulus. The evolution of the coating microstructure and properties during thermal cycling test at very high surface temperature (1400 °C) in our burner rigs and under isothermal annealing was investigated. Results showed that, while segmentation cracks survive, sintering occurs quickly during the first hours of exposure, leading to pore coarsening and stiffening of the coating. In-situ measurements at 1400 °C of the elastic modulus were performed to investigate in more detail the sintering-related stiffening. © ASM International.
    view abstract10.1007/s11666-012-9762-1
  • Deposition of La 1-xSr xFe 1-yCo yO 3-δ coatings with different phase compositions and microstructures by low-pressure plasma spraying-thin film (LPPS-TF) processes
    Zotov, N. and Hospach A. and Mauer G. and Sebold D. and Vaßen, R.
    Journal of Thermal Spray Technology 21 (2012)
    Perovskite-type materials with the general chemical formula A 1-xÁ xB́ 1-yB́ yO 3δ have received considerable attention as candidates for oxygen separation membranes. Preparation of La 1-xSr xFe 1-yCo yO 3-δ (LSFC) coatings by low-pressure plasma spraying-thin film processes using different plasma spray parameters is reported and discussed. Deposition with Ar-He plasma leads to formation of coatings containing a mixture of cubic LSFC perovskite, SrLaFeO4, FeCo, and metal oxides. Coatings deposited at higher oxygen partial pressures by pumping oxygen into the vacuum chamber contain more than 85% perovskite and only a few percent Fe32xCoxO4, and/or CoO. The microstructures of the investigated LSFC coatings depend sensitively on the oxygen partial pressure, the substrate temperature, the plasma jet velocities, and the deposition rate. Coatings deposited with Ar-rich plasma, relatively low net torch power, and with higher plasma jet velocities are most promising for applications as oxygen permeation membranes. © ASM International.
    view abstract10.1007/s11666-012-9768-8
  • Improving atmospheric plasma spraying of zirconate thermal barrier coatings based on particle diagnostics
    Mauer, G. and Sebold, D. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 21 (2012)
    Lanthanum zirconate (La 2Zr 2O7) has been proposed as a promising material for thermal barrier coatings. During atmospheric plasma spraying (APS) of La 2Zr 2O7 a considerable amount of a2O3 can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the La 2O 3-ZrO 2 system, in the composition range of pyrochlore structure, the stoichiometric La 2Zr 2O 7 has the highest melting point and other compositions are eutectic. APS experiments were performed with a TriplexPro&trade;-200 plasma torch at different power levels to achieve different degrees of evaporation and thus stoichiometry. For comparison, some investigations on gadolinium zirconate (Gd 2Zr 2O 7) were included, which is less prone to evaporation and formation of non-stoichiometry. Particle temperature distributions were measured by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected at specific torch input powers indicating evaporation and solidification processes. Based on this, process parameters can be defined to provide stoichiometric coatings that show good thermal cycling performance. © ASM International.
    view abstract10.1007/s11666-011-9706-1
  • Plasma Spray-PVD: Plasma characteristics and impact on coating properties
    Mauer, G. and Vaßen, R.
    Journal of Physics: Conference Series 406 (2012)
    Typical plasma characteristics of the plasma spray-physical vapour deposition (PS-PVD) process were investigated by optical emission spectroscopy. Electron temperatures were determined by Boltzmann plots while temperatures of the heavy species as well as electron densities were obtained by broadening analysis of spectral lines. The results show how the plasma properties and thermodynamic equilibrium conditions are affected by the admixture of hydrogen and the ambient chamber pressure. Some experimental examples of PS-PVD coatings demonstrate the impact on feedstock treatment and deposited microstructures.
    view abstract10.1088/1742-6596/406/1/012005
  • Study on the effects of wet ball milling and boron nitride additive on Li-N-H hydrogen storage system
    Du, L. and Mauer, G. and Vaßen, R.
    Energy Procedia 29 (2012)
    In this work, wet ball milling with Tetrahydrofuran (THF) was applied to activate the LiNH2+1.2LiH hydrogen storage system. Based on this, the effect of boron nitride additives was studied. Compared to dry ball milled samples, the wet ball milled material showed similar particle and crystallite size. While an additional 24 hours wet ball milling with fine milling ball process reduced the particle size obviously. The recyclability of this hydrogen storage system was enhanced significantly by an additive of 3 wt. % boron nitride, while particle and crystallite size were not influenced after milling. © 2012 Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2012.09.019
  • Columnar-structured thermal barrier coatings (TBCs) by thin film low-pressure plasma spraying (LPPS-TF)
    Hospach, A. and Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 20 (2011)
    The very low-pressure plasma Spray (VLPPS) process has been developed with the aim of depositing uniform and thin coatings with coverage of a large area by plasma spraying. At typical pressures of 100-200 Pa, the characteristics of the plasma jet change compared to conventional low-pressure plasma-spraying processes (LPPS) operating at 5-20 kPa. The combination of plasma spraying at low pressures with enhanced electrical input power has led to the development of the LPPS-TF process (TF = thin film). At appropriate parameters, it is possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This technique offers new possibilities for the manufacturing of thermal barrier coatings (TBCs). Besides the material composition, the microstructure is an important key to reduce thermal conductivity and to increase strain tolerance. In this regard, columnar microstructures deposited from the vapor phase show considerable advantages. Therefore, physical vapor deposition by electron beam evaporation (EB-PVD) is applied to achieve such columnar-structured TBCs. However, the deposition rate is low, and the line-of-sight nature of the process involves specific restrictions. In this article, the deposition of TBCs by the LPPS-TF process is shown. How the evaporation of the feedstock powder could be improved and to what extent the deposition rates could be increased were investigated. © 2010 ASM International.
    view abstract10.1007/s11666-010-9549-1
  • Current developments and challenges in thermal barrier coatings
    Mauer, G. and Vaßen, R.
    Surface Engineering 27 (2011)
    Some of the recent developments and challenges in thermal barrier coatings (TBC) are discussed. A wide flexibility of coating morphologies can be obtained by atmospheric plasma spraying (APS), and the size of microstructural features within the coatings is typically governed by that of the feedstock. By means of suspension plasma spraying (SPS), high segmentation crack densities can be obtained in TBCs while the porosity is maintained at a considerably higher level than by APS. Partially yttria stabilised zirconia (YSZ) with 6 to 8 wt-% yttria content is frequently used as TBC material due to its high thermal expansion coefficient and its relatively good fracture toughness. A strategy to improve the efficiency of gas turbines is abradable TBCs which are widely used in compressors and high-pressure stages. The infiltration of TBC microstructures by thin oxide layers applied by atomic layer chemical vapour deposition can yield improved sintering stability.
    view abstract10.1179/1743294411Y.0000000013
  • Enhanced characteristics of HVOF-sprayed MCrAlY bond coats for TBC applications
    Rajasekaran, B. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 20 (2011)
    This study is focused on the variation of the microstructures of different CoNiCrAlY bond coats sprayed by the high-velocity oxy-fuel (HVOF) process for thermal barrier coating (TBC) applications. Three different size fractions of the CoNiCrAlY bond coat powder have been considered for this investigation: AMDRY 9951 (5-37 μm), AMDRY 9954 (11-62 lm), and AMDRY 995C (45-75 lm). The influence of HVOF process parameters and process conditions have been studied in detail to achieve quality bond coats in terms of low porosity level, low oxygen content, and high surface roughness. The results have been promising and have shown that dense bond coats with low porosity can be achieved by HVOF spraying through the appropriate selection of powder size and process parameters. Importantly, HVOF bond coats appear to be competitive to VPS bond coats in terms of its oxygen content and high surface roughness. © ASM International.
    view abstract10.1007/s11666-011-9668-3
  • Improving powder injection in plasma spraying by optical diagnostics of the plasma and particle characterization
    Mauer, G. and Vaßen, R. and Stöver, D. and Kirner, S. and Marqués, J.-L. and Zimmermann, S. and Forster, G. and Schein, J.
    Journal of Thermal Spray Technology 20 (2011)
    Powder injection parameters such as gas flow, injection angle, and injector position strongly influence the particle beam and thus coating properties. The interaction of the injection conditions on particle properties based on DPV-2000 measurements using the single-cathode F4 torch is presented. Furthermore, the investigation of the plasma plume by emission computer tomography is described when operating the three-cathode TriplexPro™ torch. By this imaging technology, the three-dimensional shape of the radiating plasma jet is reproduced based on images achieved from three CCD cameras rotating around the plume axis. It is shown how the formation of the plasma jet changes with plasma parameters and how this knowledge can be used to optimize particle injection. © 2010 ASM International.
    view abstract10.1007/s11666-010-9577-x
  • Plasma and particle temperature measurements in thermal spray: Approaches and applications
    Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 20 (2011)
    Growing demands on the quality of thermally sprayed coatings require reliable methods to monitor and optimize the spraying processes. Thus, the importance of diagnostic methods is increasing. A critical requirement of diagnostic methods in thermal spray is the accurate measurement of temperatures. This refers to the hot working gases as well as to the in-flight temperature of the particles. This article gives a review of plasma and particle temperature measurements in thermal spray. The enthalpy probe, optical emission spectroscopy, and computer tomography are introduced for plasma measurements. To determine the in-flight particle temperatures mainly multicolor pyrometry is applied and is hence described in detail. The theoretical background, operation principles and setups are given for each technique. Special interest is attached to calibration methods, application limits, and sources of errors. Furthermore, examples of fields of application are given in the form of results of current research work. © 2010 ASM International.
    view abstract10.1007/s11666-010-9603-z
  • Review of new developments in suspension and solution precursor thermal spray processes
    Killinger, A. and Gadow, R. and Mauer, G. and Guignard, A. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 20 (2011)
    Thermal spray coatings from liquid feedstock such as suspensions and solution precursors have received increasing interest due to the unique coating properties obtainable by these processes. Several research groups are working on the basis of plasma as well as on high-velocity oxy-fuel approaches to manufacture advanced nanostructured and nanophased materials. These activities are reflected in various recent publications and conference presentations about feedstock preparation, equipment and process design, modeling techniques, in-process diagnostics, coating characterization, and emerging applications. This article will review these recent developments to give an up-to-date overview and to trace the current trends. © 2011 ASM International.
    view abstract10.1007/s11666-011-9639-8
  • Strategies to Improve the Reliability of Anode-Supported Solid Oxide Fuel Cells with Respect to Anode Reoxidation
    Ettler, M. and Menzler, N.H. and Mauer, G. and Tietz, F. and Buchkremer, H.P. and Stöver, D.
    Advances in Materials Science for Environmental and Nuclear Technology II 227 (2011)
    Solid oxide fuel cells (SOFCs) are highly efficient devices for converting the chemical energy of a fuel into electrical energy featuring high fuel flexibility. Forschungszentrum Julich has been developing the concept of a planar cell design based on a nickel/yttria-stabilized zirconia anode substrate for approximately twenty years. This development work covers the full spectrum ranging from fundamental research in materials science, processing and engineering issues, such as cell and stack design and construction aiming at stationary and mobile applications, up to cell and stack testing and system modeling. The main focus at present is the improvement of long-term stability and reliability for cells and stacks. This contribution will present the concept developed in Julich and the challenges and strategies involved in achieving better reliability here especially by the example of the problem of nickel reoxidation in the anode. Operating conditions causing nickel reoxidation may result in irreversible microstructural changes and macroscopic expansion of the anode, potentially leading to catastrophic cell failure. © 2011 The American Ceramic Society.
    view abstract10.1002/9781118144527.ch11
  • Atmospheric plasma spraying of high melting temperature complex perovskites for TBC application
    Jarligo, M.O. and MacK, D.E. and Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 19 (2010)
    High melting materials have always been very attractive candidates for materials development in thermal barrier coating (TBC) applications. Among these materials, complex perovskites with Ba(Mg 1/3Ta 2/3)O 3 and La(Al 1/4Mg 1/2T 1/4)O 3 compositions have been developed and deposited in TBC systems by atmospheric plasma spraying. Spray parameters were optimized and in-flight particle temperatures were recorded using Accuraspray-g3 and DPV 2000. Plasma sprayed coatings were found to undergo non-stoichiometric decomposition of components which could have contributed to early failure of the coatings. Particle temperature diagnostics suggest that gun power of ∼15 kW or lower where majority of the particles have already solidified upon impact to the substrate could probably prevent the decomposition of phases. Additionally, it has been found that the morphology of the powder feedstock plays a critical role during atmospheric plasma spraying of complex perovskites. © 2009 ASM International.
    view abstract10.1007/s11666-009-9377-3
  • Coating of high-alloyed, ledeburitic cold work tool steel applied by HVOF spraying
    Rajasekaran, B. and Mauer, G. and Vassen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Journal of Thermal Spray Technology 19 (2010)
    This study demonstrates the processing of a cold work tool steel (X220CrVMo13-4) coating using HVOF spraying. The coating formation was analyzed based on microstructure, phase, hardness, porosity, oxidation, and adhesion characteristics. An online diagnostic tool was utilized to find out the in-flight characteristics of powder such as temperature and velocity during the coating process to identify the influencing parameters to achieve dense cold work tool steel coatings with low oxidation. The influence of powder size, process parameters, and in-flight characteristics on the formation of cold work tool steel coatings was demonstrated. The results indicated that thick and dense cold work tool steel coatings with low oxidation can be obtained by the selection of appropriate powder size and process parameters. © ASM International.
    view abstract10.1007/s11666-009-9456-5
  • Development of cold work tool steel based-MMC coating using HVOF spraying and its HIP densification behaviour
    Rajasekaran, B. and Mauer, G. and Vaßen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Surface and Coatings Technology 204 (2010)
    The aim of the present study is to develop a Fe-based metal matrix composite (MMC) coating using high velocity oxy-fuel spraying (HVOF) process. A ledeburitic high alloyed cold work tool steel (X220CrVMo13-4) and NbC with an average size of 2μm at different volume fractions have been considered as metal matrix and hard particles respectively. MMC coatings were deposited on austenitic stainless substrates and the coatings were subsequently densified by hot isostatic pressing (HIP) with and without encapsulation. Microstructural analysis of the as-sprayed and HIPed coatings were characterized by SEM and XRD methods. Results showed that the feedstock preparation involving fine NbC was an influencing factor on the coating deposition. A relatively homogeneous dispersion of fine NbC up to 30. vol.% in cold work tool steel matrix was possible using optimized HVOF spraying. Besides, HVOF spraying and its subsequent HIP treatment induced significant microstructural and phase changes in the MMC coatings. The study showed the potential of HVOF spraying for the development of steel based MMC coatings and its subsequent densification can be achieved by HIP process with and without encapsulation. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.05.001
  • Process design and monitoring for plasma sprayed abradable coatings
    Steinke, T. and Mauer, G. and Vaßen, R. and Stöver, D. and Roth-Fagaraseanu, D. and Hancock, M.
    Journal of Thermal Spray Technology 19 (2010)
    Abradable coatings in compressor and high-pressure stages of gas turbines must provide specific hardness and porosity values to achieve an optimal cut-in of the blade tips. A fractional factorial experimental plan was designed to investigate the influence of the plasma spraying parameters argon flow rate, current, spraying distance and powder feed rate on these properties of magnesia spinel. Based on the results, magnesia spinel coatings with low (∼400 HV 0.5), medium (∼600 HV 0.5) and high hardness (∼800 HV 0.5) could be reliably manufactured. Further incursion rig tests confirmed the dependence of the rub-in behavior and abradability on the coating characteristics and process parameters, respectively. Process monitoring was also applied during plasma spraying of magnesia spinel abradables on batches of turbine components. The recorded particle characteristics and coating properties showed a good reproducibility of the spraying process. © ASM International.
    view abstract10.1007/s11666-010-9468-1
  • Process diagnostics in suspension plasma spraying
    Mauer, G. and Guignard, A. and Vaßen, R. and Stöver, D.
    Surface and Coatings Technology 205 (2010)
    Benefits and limitations of process diagnostics are investigated for the suspension plasma spraying of yttria-stabilized zirconia thermal barrier coatings. The methods applied were enthalpy probe measurements, optical emission spectroscopy, and in-flight particle diagnostic.It was proved that the plasma characteristics are not affected negatively by the injection of the ethanol based suspension since the combustion of species resulting from ethanol decomposition achieves a gain of plasma enthalpy. Furthermore, the conditions of the suspension injection into the plasma were found to be optimum as a significant content of evaporated powder material could be detected. Regarding the void content and segmentation crack density of the coatings, the in-flight particle diagnostic showed that the spray distance should be dimensioned in a way that the molten particles reach the substrates just before solidification starts. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.03.007
  • Suspension plasma spraying: Process characteristics and applications
    Vaßen, R. and Kaner, H. and Mauer, G. and Stöver, D.
    Journal of Thermal Spray Technology 19 (2010)
    Suspension plasma spraying (SPS) offers the manufacture of unique microstructures which are not possible with conventional powdery feedstock. Due to the considerably smaller size of the droplets and also the further fragmentation of these in the plasma jet, the attainable microstructural features like splat and pore sizes can be downsized to the nanometer range. Our present understanding of the deposition process including injection, suspension plasma plume interaction, and deposition will be outlined. The drawn conclusions are based on analysis of the coating microstructures in combination with particle temperature and velocity measurements as well as enthalpy probe investigations. The last measurements with the water cooled stagnation probe gives valuable information on the interaction of the carrier fluid with the plasma plume. Meanwhile, different areas of application of SPS coatings are known. In this paper, the focus will be on coatings for energy systems. Thermal barrier coatings (TBCs) for modern gas turbines are one important application field. SPS coatings offer the manufacture of strain-tolerant, segmented TBCs with low thermal conductivity. In addition, highly reflective coatings, which reduce the thermal load of the parts from radiation, can be produced. Further applications of SPS coatings as cathode layers in solid oxide fuel cells (SOFC) and for photovoltaic (PV) applications will be presented. © 2009 ASM International.
    view abstract10.1007/s11666-009-9451-x
  • Thick tool steel coatings using HVOF spraying for wear resistance applications
    Rajasekaran, B. and Mauer, G. and Vaßen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Surface and Coatings Technology 205 (2010)
    This paper deals with the processing of thick cold work tool steel coating using high velocity oxy-fuel (HVOF) spraying process. A full factorial experimental design was established to identify the influencing process parameters on the formation of dense coating with low oxidation. Microstructural analysis of the coating was carried out using optical, SEM and XRD techniques. Cold work tool steel coatings with a thickness up to 2 mm were developed on bond coated low carbon steel substrates for wear resistance evaluation. A pin on disc test was performed to examine the wear resistance of thick cold work tool steel coatings on different types and sizes of abrasive papers. The wear results were compared with the wear resistance of a standard high speed steel pin. The abrasive wear resistance of cold work tool steel coated pins was found to be superior against soft and fine abrasive papers than the standard high speed steel. Besides, the performance of the coated pins against hard and coarser abrasive papers was found to be similar to standard high speed steel. The study showed the potential of HVOF spraying on the development of thick cold work tool steel coatings for wear resistance applications. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.09.041
  • Thin and dense ceramic coatings by plasma spraying at very low pressure
    Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 19 (2010)
    The very low pressure plasma spray (VLPPS) process operates at a pressure range of approximately 100 Pa. At this pressure, the plasma jet interaction with the surrounding atmosphere is very weak. Thus, the plasma velocity is almost constant over a large distance from the nozzle exit. Furthermore, at these low pressures the collision frequency is distinctly reduced and the mean free path is strongly increased. As a consequence, at low pressure the specific enthalpy of the plasma is substantially higher, but at lower density. These particular plasma characteristics offer enhanced possibilities to spray thin and dense ceramics compared to conventional processes which operate in the pressure range between 5 and 20 kPa. This paper presents some examples of gas-tight and electrically insulating coatings with low thicknesses <50 μm for solid oxide fuel cell applications. Furthermore, plasma spraying of oxygen conducting membrane materials such as perovskites is discussed. © 2009 ASM International.
    view abstract10.1007/s11666-009-9416-0
  • ceramics

  • coatings

  • perovskite

  • physical vapor deposition

  • plasma jets

  • porous materials

  • thermal barrier coatings

  • thermal spraying

« back