Prof. Dr. Robert Vaßen

Materials Synthesis and Processing
Forschungszentrum Jülich GmbH

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  • Columnar Thermal Barrier Coatings Produced by Different Thermal Spray Processes
    Kumar, N. and Gupta, M. and Mack, D.E. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology (2021)
    Suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD) are the only thermal spray technologies shown to be capable of producing TBCs with columnar microstructures similar to the electron beam-physical vapor deposition (EB-PVD) process but at higher deposition rates and relatively lower costs. The objective of this study was to achieve fundamental understanding of the effect of different columnar microstructures produced by these two thermal spray processes on their insulation and lifetime performance and propose an optimized columnar microstructure. Characterization of TBCs in terms of microstructure, thermal conductivity, thermal cyclic fatigue lifetime and burner rig lifetime was performed. The results were compared with TBCs produced by the standard thermal spray technique, atmospheric plasma spraying (APS). Bondcoats deposited by the emerging high-velocity air fuel (HVAF) spraying were compared to the standard vacuum plasma-sprayed (VPS) bondcoats to investigate the influence of the bondcoat deposition process as well as topcoat–bondcoat interface topography. The results showed that the dense PS-PVD-processed TBC had the highest lifetime, although at an expense of the highest thermal conductivity. The reason for this behavior was attributed to the dense intracolumnar structure, wide intercolumnar gaps and high column density, thus improving the strain tolerance and fracture toughness. © 2021, The Author(s).
    view abstract10.1007/s11666-021-01228-5
  • Determining interface fracture toughness in multi layered environmental barrier coatings with laser textured silicon bond coat
    Wolf, M. and Kakisawa, H. and Süß, F. and Mack, D.E. and Vaßen, R.
    Coatings 11 (2021)
    In the high temperature combustion atmosphere inside of aircraft turbines, the currently used ceramic matrix composites require a protective environmental barrier coating (EBC) to mitigate corrosion of the turbine parts. Besides thermomechanical and thermochemical properties like matching thermal expansion coefficient (CTE) and a high resistance against corrosive media, mechanical properties like a high adhesion strength are also necessary for a long lifetime of the EBC. In the present work, the adhesion between an air plasma sprayed silicon bond coat and a vacuum plasma sprayed ytterbium disilicate topcoat was aimed to be enhanced by a laser surface structuring of the Si bond coat. An increase in interface toughness was assumed, since the introduction of structures would lead to an increased mechanical interlocking at the rougher bond coat interface. The interface toughness was measured by a new testing method, which allows the testing of specific interfaces. The results demonstrate a clear increase of the toughness from an original bond coat/topcoat interface (8.6 J/m2) compared to a laser structured interface (14.7 J/m2). Observations in the crack propagation indicates that the laser structuring may have led to a strengthening of the upper bond coat area by sintering. Furthermore, in addition to cohesive failure components, adhesive components can also be observed, which could have influenced the determined toughness. © 2021 by the authors.
    view abstract10.3390/coatings11010055
  • Influence of substrate removal method on the properties of free-standing ysz coatings
    Vaßen, R. and Bakan, E. and Schwartz-Lückge, S.
    Coatings 11 (2021)
    Thermally sprayed ceramic coatings are often tested as free-standing layers to investigate different properties such as thermal expansion coefficient, thermal conductivity, sintering, mechanical behavior, corrosion resistance, gas tightness, or electrical properties. In this paper, four different substrate removal methods were used to obtain free-standing YSZ coatings. At first, spraying on a steel substrate and subsequent dissolution of the substrate-coating interface by hydrochloric acid (HCl) was used. Second, the steel substrate was removed by applying an electrical field via electrochemical corrosion of the surface of the substrate. In a third method, the coating was sprayed on a salt (NaCI) interlayer, which was removed later by dissolution in water. At last, the coating was sprayed on a graphite substrate and the substrate was removed by heat treatment. After the preparation of free-standing coatings, these were characterized using scanning electron microscopy, mercury porosimetry, indentation tests, and room temperature three-point bending tests, which allowed the determination of Young’s modulus and viscosity. The results revealed measurable differences in coating properties as a result of the substrate removal methods, i.e., HCl method led to higher porosity and lower modulus in the YSZ coating. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/coatings11040449
  • Injection molding and near-complete densification of monolithic and al2o3 fiber-reinforced ti2alc max phase composites
    Badie, S. and Gabriel, R. and Sebold, D. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Materials 14 (2021)
    Near-net shape components composed of monolithic Ti2AlC and composites thereof, containing up to 20 vol.% Al2O3 fibers, were fabricated by powder injection molding. Fibers were homogeneously dispersed and preferentially oriented, due to flow constriction and shear-induced velocity gradients. After a two-stage debinding procedure, the injection-molded parts were sintered by pressureless sintering at 1250 °C and 1400 °C under argon, leading to relative densities of up to 70% and 92%, respectively. In order to achieve near-complete densification, field assisted sintering technology/spark plasma sintering in a graphite powder bed was used, yielding final relative densities of up to 98.6% and 97.2% for monolithic and composite parts, respectively. While the monolithic parts shrank isotropically, composite assemblies underwent anisotropic densification due to constrained sintering, on account of the ceramic fibers and their specific orientation. No significant increase, either in hardness or in toughness, upon the incorporation of Al2O3 fibers was observed. The 20 vol.% Al2O3 fiber-reinforced specimen accommodated deformation by producing neat and well-defined pyramidal indents at every load up to a 30 kgf (~294 N). © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14133632
  • Mechanism for breakaway oxidation of the Ti2AlC MAX phase
    Badie, S. and Sebold, D. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Acta Materialia 215 (2021)
    The good oxidation resistance of MAX phases up to temperatures around 1200 °C can be compromised for long exposure due to the breakaway of the protective alumina layer. Herein, we unveil a mechanism of breakaway oxidation of the Ti2AlC MAX phase, identifying the main trigger and the solutions to avoid it. It is caused by excessive rumpling of the oxide scale on surfaces with arithmetical mean roughness (Ra) > 3 µm and constitutes a key factor in subsequent consumption of Ti2AlC. First, the oxide scale experienced rumpling due to significant radial stresses generated at the Ti2AlC/oxide interface. Second, scale blistering resulted from substantial buckling due to the evolution of in-plane stresses and lateral lengthening. Third, blister collapse and exposure of the underlying Al-depleted Ti2AlC surface led to rapid ingress of oxygen and oxide/substrate interface recession. The self-healing ability of Ti2AlC has been restrained and breakaway oxidation kinetics following a linear trend have been initiated. Similarly, breakaway oxidation was observed on micro-damaged surfaces. A mixed oxide layer with high porosity mainly composed of rutile titanium dioxide (TiO2) promptly formed on these surfaces, gradually consuming the base Ti2AlC material. © 2021
    view abstract10.1016/j.actamat.2021.117025
  • Microstructure and phase composition evolution of silicon-hafnia feedstock during plasma spraying and following cyclic oxidation
    Bakan, E. and Sohn, Y.J. and Vaßen, R.
    Acta Materialia 214 (2021)
    In this work, silicon–hafnia (Si-HfO2, 80/20 mol. %) feedstock was plasma sprayed for Environmental Barrier Coating bond coat application. In the as-sprayed coating, hafnium disilicide (HfSi2), HfO2 tetragonal (t), and cubic (c) phases with a total volume of ~20 % were detected together with Si and HfO2 monoclinic (m). The temperature-dependent evolution of these phases was analyzed and paired with microstructural observations. It was found that above 700 °C, HfSi2 oxidizes and HfO2 (t) and (c) transforms into (m) polymorph. Up to this temperature, as-sprayed coating showed a non-linear expansion behavior. Estimated volume expansion at ~750 °C was 3.6 % based on dilatometry measurement. The primary and secondary mechanisms leading to the expansion in the coating were identified as oxidation of HfSi2 and polymorphic phase transitions in HfO2, respectively. As a consequence of the volume expansion, the coating was extensively cracked during cyclic oxidation and hence not protective anymore. After 100 h at 1300 °C, the volume fraction of oxidation product SiO2 was significant in the coating (0.34), while HfO2 was largely consumed (0.1) in the formation of HfSiO4 (0.56). This result suggested that reversible α↔β phase transitions in SiO2-cristobalite could be another factor contributing to the cracking in the coating during cyclic oxidation. © 2021 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2021.117007
  • Path to single-crystalline repair and manufacture of Ni-based superalloy using directional annealing
    Kalfhaus, T. and Schaar, H. and Thaler, F. and Ruttert, B. and Sebold, D. and Frenzel, J. and Steinbach, I. and Theisen, W. and Guillon, O. and Clyne, T.W. and Vassen, R.
    Surface and Coatings Technology 405 (2021)
    Advanced methods for the repair of single-crystalline (SX) Ni-based superalloys are of special interest for the gas turbine industry. Polycrystalline repair approaches show promising results, while the repair of SX materials is still challenging. Directional annealing experiments resulted in large columnar grains by imposing thermal gradients at the abnormal grain growth temperature of a specific Ni-based superalloy. A numerical model of the Bridgman process is applied to provide an insight into the temperature evolution during zone annealing of the Vacuum-Plasma-Spray (VPS) repair coatings with the aim of promoting grain growth from the SX substrate. The results presented here suggest that this is a promising approach to repair or manufacture SX turbine blades. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.126494
  • Processing and oxidation response of Cr2AlC MAX-phase composites containing ceramic fibers
    Go, T. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Open Ceramics 6 (2021)
    Three different ceramic matrix composites (CMCs) were produced using Cr2AlC as a matrix, and carbon, SiC and Al2O3 short fibers as a secondary phase. Cr2AlC powders were synthesized by solid-state reaction, followed by mixing with the fibers, and full densification using a field-assisted sintering technique. Of the three different fiber types, Carbon fibers reacted strongly with Cr2AlC, while the reaction with SiC fibers was more limited and alumina fibers didn't show any reaction. Oxidation tests of the monolithic Cr2AlC and the composites were performed by thermogravimetric analysis. An alumina layer formed at 1000 ​°C on every sample, well attached and worked as a good oxidation barrier. Under realistic conditions using a burner rig for cyclic oxidation at 1200 ​°C for 500 cycles, the oxidation resistance of the alumina fiber CMC is good, as no defects or degradation are visible and the alumina layer is well attached. © 2021 The Author(s)
    view abstract10.1016/j.oceram.2021.100090
  • Synthesis, sintering, and effect of surface roughness on oxidation of submicron Ti2AlC ceramics
    Badie, S. and Dash, A. and Sohn, Y.J. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Journal of the American Ceramic Society 104 (2021)
    Submicron Ti2AlC MAX phase powder was synthesized by molten salt shielded synthesis (MS3) using a Ti:Al:C molar ratio of 2:1:0.9 at a process temperature of 1000°C for 5 hours. The synthesized powder presented a mean particle size of ~0.9 µm and a purity of 91 wt. % Ti2AlC, containing 6 wt. % Ti3AlC2. The Ti2AlC powder was sintered by pressureless sintering, achieving a maximal relative density of 90%, hence field-assisted sintering technology/spark plasma sintering was used to enhance densification. The fine-grained microstructure was preserved, and phase purity of Ti2AlC was unaltered in the latter case, with a relative density of 98.5%. Oxidation was performed at 1200°C for 50 hours in static air of dense monolithic Ti2AlC with different surface finish, (polished, ground and sandblasted) which resulted in the formation of an approx. 8 µm thin aluminum oxide (Al2O3) layer decorated with titanium dioxide (rutile, TiO2) colonies. Surface quality had no influence on Al2O3 scale thickness, but the amount and size of TiO2 crystals increased with surface roughness. A phenomenon of rumpling of the thermally grown oxide (TGO) was observed and a model to estimate the extent of deformation is proposed. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American
    view abstract10.1111/jace.17582
  • Unique performance of thermal barrier coatings made of yttria-stabilized zirconia at extreme temperatures (>1500°C)
    Vaßen, R. and Mack, D.E. and Tandler, M. and Sohn, Y.J. and Sebold, D. and Guillon, O.
    Journal of the American Ceramic Society 104 (2021)
    Yttria-stabilized zirconia (YSZ) has been for several decades the state of the art material for thermal barrier coating (TBC) applications in gas turbines. Although the material has unique properties, further efficiency improvement by increasing the temperature is limited due to its maximum temperature capability of about 1200°C. Above this temperature the deposited metastable tetragonal (t´) phase undergoes a detrimental phase transformation as well as enhanced sintering. Both processes promote the failure of the coatings at elevated temperatures and this early failure has been frequently observed in gradient tests. In this paper, we now experimentally shown for the first time that under typical cycling conditions not the time at elevated temperatures leads to the reduced lifetime but the transient cooling rates. If cooling rates were reduced to 10K/s, TBC systems could be operated in a burner rig at a surface temperature well above 1500°C without showing a lifetime reduction. The explanation of these astonishing findings is given by the evaluation of energy release rate peaks during fast transient cooling in combination with the phase evolution during cooling with the used cooling rates. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)
    view abstract10.1111/jace.17452
  • An investigation on burner rig testing of environmental barrier coatings for aerospace applications
    Bakan, E. and Mack, D.E. and Lobe, S. and Koch, D. and Vaßen, R.
    Journal of the European Ceramic Society (2020)
    In this study, burner rig testing of Si/Yb2Si2O7 environmental barrier coating protected SiC-based ceramic matrix composites was conducted. Tests were performed at standard conditions as well as with liquid water injection to the flame. Furthermore, the influence of the impingement angle of the flame (45° vs. 90°) on water vapor corrosion was explored. Gas flow rates were adapted in each test to adjust 1250 °C at the sample surface. The comparison of test results showed that water injection advances the corrosion of the Yb2Si2O7 topcoat and the impingement angle affects the size and shape of the corroded area on the sample surface. © 2020 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2020.06.016
  • 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
  • Compressive creep of SiC whisker/Ti3SiC2 composites at high temperature in air
    Dash, A. and Malzbender, J. and Dash, K. and Rasinski, M. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Journal of the American Ceramic Society 103 (2020)
    The compressive creep of a SiC whisker (SiCw) reinforced Ti3SiC2 MAX phase-based ceramic matrix composites (CMCs) was studied in the temperature range 1100-1300°C in air for a stress range 20-120 MPa. Ti3SiC2 containing 0, 10, and 20 vol% of SiCw was sintered by spark plasma sintering (SPS) for subsequent creep tests. The creep rate of Ti3SiC2 decreased by around two orders of magnitude with every additional 10 vol% of SiCw. The main creep mechanisms of monolithic Ti3SiC2 and the 10% CMCs appeared to be the same, whereas for the 20% material, a different mechanism is indicated by changes in stress exponents. The creep rates of 20% composites tend to converge to that of 10% at higher stress. Viscoplastic and viscoelastic creep is believed to be the deformation mechanism for the CMCs, whereas monolithic Ti3SiC2 might have undergone only dislocation-based deformation. The rate controlling creep is believed to be dislocation based for all the materials which is also supported by similar activation energies in the range 650-700 kJ/mol. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)
    view abstract10.1111/jace.17323
  • Correlation of Microstructure and Properties of Cold Gas Sprayed INCONEL 718 Coatings
    Vaßen, R. and Fiebig, J. and Kalfhaus, T. and Gibmeier, J. and Kostka, A. and Schrüfer, S.
    Journal of Thermal Spray Technology (2020)
    In the cold gas spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray is an ideal process for repair applications. In this study, INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas. The deposited coatings with different thicknesses were characterized using electron microscopy techniques to study grain refinement and precipitates in the coating. In addition, depth-resolved residual stress measurements have been performed by the incremental hole drilling method. The residual stress depth profiles in the coatings indicate compressive residual stresses of several hundred MPa which are hardly influenced by the coating thickness. In addition, large compressive stress levels are found in surface-near regions of the substrate due to the grit blasting process. Furthermore, a post-heat treatment analysis was performed to investigate its influence on residual stresses and bonding strength. These findings are used to develop a consistent explanation of the dependence of strength values on thickness. © 2020, The Author(s).
    view abstract10.1007/s11666-020-00988-w
  • 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
  • Degradation of zirconia in moisture
    Cao, X. and Vassen, R. and Wang, J. and Zou, B. and Li, S. and Hui, Y. and Yuan, J. and Song, W. and Cao, Q. and Jiang, J. and Deng, L. and Dong, S.
    Corrosion Science 176 (2020)
    Zirconia especially stabilized by Y2O3 (YSZ) is an important and popularly used material. We have observed the degradation of YSZ in moisture, and have also revealed the degradation mechanism which could be related to the following three chemical processes: (1) reaction of water with the residual Y2O3 which is not incorporated into the crystal lattice of ZrO2; (2) corrosion of YSZ by water; (3) depletion of Y3+ from the crystal lattice by water molecule due to oxygen vacancies. A core/shell structure of ZrO2/Y2O3 in YSZ crystalline grains has been proved. Calcination of YSZ above 1100 °C can prevent the degradation. © 2020 Elsevier Ltd
    view abstract10.1016/j.corsci.2020.109038
  • Development progress of coating first wall components with functionally graded W/EUROFER layers on laboratory scale
    Emmerich, T. and Qu, D. and Ghidersa, B.-E. and Lux, M. and Rey, J. and Vaßen, R. and Aktaa, J.
    Nuclear Fusion 60 (2020)
    In the course of developing functionally graded tungsten/steel-layer systems as protective coatings for the first wall (FW) of future fusion reactors, an overview of the results attained so far is given. This includes the determined parameters for creating such systems by vacuum plasma spraying on a laboratory scale and the achieved material properties determined in previous works. To realize the coating of future full scale FWs as well, the coating process is adapted to larger coating areas in the form of mock-ups. For such components, special attention needs to be paid to the challenges of the limited temperature window during coating to achieve good coating adhesion, whilst avoiding exceeding the tempering temperature of the steel. One successfully coated mock-up is also exposed to fusion-relevant heat loads in HELOKA (Helium Loop Karlsruhe) to evaluate the coating system behavior and verify its durability. Finally, for even larger components the coating design and process are further optimized, supported by finite element simulations. © 2020 EURATOM.
    view abstract10.1088/1741-4326/aba336
  • High-velocity water vapor corrosion of Yb-silicate: Sprayed vs. sintered body
    Bakan, E. and Kindelmann, M. and Kunz, W. and Klemm, H. and Vaßen, R.
    Scripta Materialia 178 (2020)
    The water vapor corrosion of Yb-silicates is of interest to their application as environmental barrier coatings in gas turbine technology. In this study, densified samples from the Yb-silicate powder, as well as plasma-sprayed free-standing Yb-silicate coating were tested at a high-velocity steam rig (T = 1400 °C, v = 90 m/s, PH2O = 0.19 atm) for microstructural comparison. After the test, the measured weight losses of the coatings were larger than that of the densified sample. At the same time, the thicknesses of the corroded scales at the coating surfaces were found to be thicker than that of the sintered sample by a factor of two. © 2019
    view abstract10.1016/j.scriptamat.2019.12.019
  • Improved Adhesion of Different Environmental Barrier Coatings on Al2O3/Al2O3-Ceramic Matrix Composites
    Gatzen, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Advanced Engineering Materials 22 (2020)
    In high-temperature combustion atmospheres, well-adhering environmental barrier coatings (EBCs) are required to protect the underlying ceramic matrix composites (CMCs) from corrosion. Herein the adhesion mechanisms of three different coatings produced by atmospheric plasma spraying (APS) on an Al2O3/Al2O3-CMC are investigated. In particular, the influence of surface structuring by laser ablation prior to coating production is investigated. Y2O3, yttria-stabilized zirconia (YSZ), and Gd2Zr2O7 are chosen as potential EBCs. The coating adhesion on CMC-substrates with and without surface structuring is analyzed by furnace cycling, pull-adhesion tests, and burner-rig tests with gradient. Special interest is paid to the interactions at the coating–substrate interface before and after heat treatment and their effect on the coating adhesion and lifetime. Two different adhesion mechanisms are found: adhesion promoted by chemical reaction and adhesion promoted by mechanical interlocking. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.202000087
  • In situ investigation of atmospheric plasma-sprayed Mn–Co–Fe–O by synchrotron X-ray nano-tomography
    Grünwald, N. and Lhuissier, P. and Salvo, L. and Villanova, J. and Menzler, N.H. and Guillon, O. and Martin, C.L. and Vaßen, R.
    Journal of Materials Science 55 (2020)
    Applying atmospherically plasma-sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) protective coatings on interconnector steels minimized the chromium-related degradation within solid oxide fuel cell stack-tests successfully. Post-test characterization of the coatings disclosed a severe microstructural and phase evolution. A self-healing of micro-cracks, the formation and agglomeration of small pores, the occurrence of a dense spinel layer at the surface and a strong elemental de-mixing were reported in ex situ experiments. In the present publication, we prove for the first time these mechanisms by tracking the microstructure in situ at a single APS coating using synchrotron X-ray nano-tomography at the European Synchrotron Radiation Facility. Therefore, a 100-µm-long cylindrical sample with a diameter of 123 µm was cut from an APS-MCF free-standing layer and measured within a high-temperature furnace. All microstructural changes mentioned above could be verified. Porosity measurements reveal a decrease in the porosity from 9 to 3% during the annealing, which is in good accordance with the literature. Additionally, a partial detachment of an approximately 5-µm-thick layer at the sample surface is observed. The layer is dense and does not exhibit any cracks which are penetrating the layer. This kind of shell is assumed to be gastight and thus protecting the bulk from further oxidation. © 2020, The Author(s).
    view abstract10.1007/s10853-020-04916-9
  • Influence of Process Parameters on the Aerosol Deposition (AD) of Yttria-Stabilized Zirconia Particles
    Mishra, T.P. and Singh, R. and Mücke, R. and Malzbender, J. and Bram, M. and Guillon, O. and Vaßen, R.
    Journal of Thermal Spray Technology (2020)
    Aerosol deposition (AD) is a novel deposition process for the fabrication of dense and rather thick oxide films at room temperature. The bonding of the deposited ceramic particles is based on a shock-loading consolidation, resulting from the impact of the ceramic particles on the substrate. However, the deposition mechanism is not fully understood. In addition, many technical challenges have been observed for achieving a successful deposition of the oxides with higher efficiency. In this work, the influence of different processing parameters on the properties of the deposited layer is studied. Proof of concept was done using 8 mol.% yttria-stabilized zirconia (8YSZ) powder as starting material. The window of deposition with respect to carrier gas flows for successful deposition was identified. The influence of this carrier gas flow, the substrate materials and the carrier gas species on the coating thickness, interface quality and coating microstructure was systematically investigated. The derived mechanical characteristics revealed an unexpected behavior related to a gradient microstructure. This study supports understanding of the mechanism of room-temperature impact consolidation and its effect on the mechanical properties of the deposited layer. © 2020, ASM International.
    view abstract10.1007/s11666-020-01101-x
  • Influence of rf plasma jet surface treatment on wetting behavior of yttria stabilized zirconia sps coatings
    Komarov, P. and Jech, D. and Čelko, L. and Pijáková, B. and Zhou, D. and Vaßen, R.
    Defect and Diffusion Forum 405 DDF (2020)
    According to the value of water contact angle (WCA), the surfaces can be roughly defined as hydrophilic (with WCA less than 90°) or as hydrophobic (with WCA higher than 90°). Water wetting behavior plays important role and surfaces with special wettability (hydrophobic-superhydrophobic; hydrophilic-superhydrophilic) can be used both in the daily life (solar cells, smartphones, car windows, etc.) and in the industry (corrosion resistance, self-cleaning, anti-icing properties, etc.). Nowadays, the development of hydrophobic surface treatment that may be applied in the industry is very interesting topic. Therefore, it was decided to estimate the influence of radiofrequency (RF) plasma jet in atmosphere on wetting behavior of ceramic plasma sprayed coatings. As the initial material for surface treatment, yttria stabilized zirconia suspension plasma sprayed coatings were used. The influence of RF plasma jet on suspension plasma sprayed coatings was estimated on both hydrophilic and hydrophobic surfaces, and resulted water contact angle and free surface energy of modified samples were measured by sessile droplet method. Microstructure, phase composition and topography investigation were carried out by means of light microscopy, X-ray diffraction techniques and non-contact profilometry. © 2020 Trans Tech Publications Ltd, Switzerland
    view abstract10.4028/www.scientific.net/DDF.405.423
  • Performance of wear resistant MCrAlY coatings with oxide dispersion strengthening
    Bolelli, G. and Vorkötter, C. and Lusvarghi, L. and Morelli, S. and Testa, V. and Vaßen, R.
    Wear 444-445 (2020)
    Aiming to devise suitable materials for sliding wear protection at high temperature, aluminium oxide-dispersion strengthened (ODS) CoNiCrAlY coatings were manufactured by vacuum plasma spraying (VPS). Feedstock materials were ball-milled powders with 2, 10 and 30 wt% Al2O3 content. The ball-on-disc sliding wear behaviour of the coatings was tested at 750 °C against an Al2O3 counterpart, and compared to a pure CoNiCrAlY coating (obtained from a commercial feedstock not subjected to ball milling) and to an uncoated Ni-base superalloy. Sliding wear rates decrease from the uncoated superalloy (≈3 × 10−5 mm3/(N·m)) to the pure CoNiCrAlY coating (≈2 × 10−5 mm3/(N·m)) and to the ODS ones, with the notable exception of the 10 wt% Al2O3-containing sample. Analyses of worn samples indicate that pure CoNiCrAlY is subject to severe adhesive wear, mitigated by the formation of a thick (>1 μm) “glaze” layer via compaction and (probable) sintering of tribo-oxidized debris particles. Addition of Al2O3 particles to the CoNiCrAlY matrix can either enhance or worsen the “glaze” stability. Specifically, a coating strengthened with 30 wt% Al2O3 provides an especially good mechanical support to the “glaze”. This produces beneficial effects resulting in a particularly low wear rate of ≈3 × 10−6 mm3/(N·m). © 2019 Elsevier B.V.
    view abstract10.1016/j.wear.2019.203116
  • Performance of YSZ and Gd2Zr2O7/YSZ double layer thermal barrier coatings in burner rig tests
    Vaßen, R. and Bakan, E. and Mack, D. and Schwartz-Lückge, S. and Sebold, D. and Jung Sohn, Y. and Zhou, D. and Guillon, O.
    Journal of the European Ceramic Society 40 (2020)
    Double layer thermal barrier coatings (TBCs) consisting of a Gd2Zr2O7 (GZO) top and an ytrria stabilized zirconia (YSZ) interlayer have been tested in a burner rig facility and the results compared to the ones of conventional YSZ single layers. In order to gain insight in the high temperature capability of the alternative TBC material, high surface temperatures of up to 1550 °C have been chosen while keeping the bond coat temperature similar. It turned out that the performance of all systems is largely depending on the microstructure of the coatings especially reduced porosity levels of GZO being detrimental. In addition, it was more difficult in GZO than in YSZ coatings to obtain highly porous and still properly bonded microstructures. Another finding was the reduced lifetime with increasing surface temperatures, the amount of reduction is depending on the investigated system. The reasons for this behavior are analyzed and discussed in detail. © 2019 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2019.10.021
  • Phase Transformation-Induced Changes in Microstructure and Residual Stresses in Thermally Sprayed MnCoFeO4 Protective Coatings
    Back, H.C. and Gibmeier, J. and Vaßen, R.
    Journal of Thermal Spray Technology 29 (2020)
    The contribution comprises the investigation of the microstructure and residual stresses in thermally sprayed Mn1.0Co1.9Fe0.1O4.0 (MCF) protective coatings for interconnectors of SOFC stacks, deposited on ferritic steel Crofer 22 APU via atmospheric plasma spraying (APS). The coatings are designated to prevent Cr evaporation during high operation temperature of the SOFCs. The local microstructure, pore distributions and pore shapes, phase fractions, micro-hardness, Youngs’ modulus and residual stresses through the coating thickness were characterized in as-sprayed state and compared with longtime (10-100 h) heat-treated samples (700 and 850 °C). The results show that the long-term thermal aging treatment causes a successive high sintering of the coatings characterized by a reduction in pore density, by phase transformation from the metastable rock salt structure that gradually transformed to a spinel structure and by a slight relaxation of the process-induced tensile residual stresses in the coating. For SOFC application of the MCF coating, this indicates an improvement in the coatings integrity. During operation, a self-repair proceeds leading to dense and gas-proof coatings, while the mechanical properties are mainly retained. © 2020, The Author(s).
    view abstract10.1007/s11666-020-00997-9
  • Resistance of pure and mixed rare earth silicates against calcium-magnesium-aluminosilicate (CMAS): A comparative study
    Wolf, M. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Journal of the American Ceramic Society 103 (2020)
    Rare earth silicate environmental barrier coatings (EBCs) are state of the art for protecting SiC ceramic matrix composites (CMCs) against corrosive media. The interaction of four pure rare earth silicate EBC materials Yb2SiO5, Yb2Si2O7, Y2SiO5, Y2Si2O7 and three ytterbium silicate mixtures with molten calcium-magnesium-aluminosilicate (CMAS) were studied at high temperature (1400°C). The samples were characterized by SEM and XRD in order to evaluate the recession of the different materials after a reaction time of 8 hours. Additionally, the coefficient of thermal expansion (CTE) was determined to evaluate the suitability of Yb silicate mixtures as EBC materials for SiC CMCs. Results show that monosilicates exhibit a lower recession in contact with CMAS than their disilicate counterparts. The recession of the ytterbium silicates is far lower than the recession of the yttrium silicates under CMAS attack. Investigation of the ytterbium silicate mixtures exposes their superior resistance to CMAS, which is even higher than the resistance of the pure monosilicate. Also their decreased CTE suggests they will display better performance than the pure monosilicate. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)
    view abstract10.1111/jace.17328
  • Short SiC fiber/Ti3SiC2 MAX phase composites: Fabrication and creep evaluation
    Dash, A. and Malzbender, J. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Journal of the American Ceramic Society 103 (2020)
    The compressive creep of silicon carbide fiber reinforced Ti3SiC2 MAX phase with both fine and coarse microstructure was investigated in the temperature range of 1000-1300°C. Comparison of only steady-state creep was done to understand the response of fabricated composite materials toward creep deformation. It was demonstrated that the fibers are more effective in reducing the creep rates for the coarse microstructure by an increase in activation energy compared to the variant with a finer microstructure, being partly a result of the enhanced creep rates for the microstructure with larger grain size. Grain boundary sliding along with fiber fracture appears to be the main creep mechanism for most of the tested temperature range. However, there are indications for a changed creep mechanism for the fine microstructure for the lowest testing temperature. Local pores are formed to accommodate differences in strain related to creeping matrix and predominantly elastically deformed fibers during creep. Microstructural analysis was done on the material before and after creep to understand the deformation mechanics. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)
    view abstract10.1111/jace.17337
  • Thermal barrier coatings with novel architectures for diesel engine applications
    Uczak de Goes, W. and Markocsan, N. and Gupta, M. and Vaßen, R. and Matsushita, T. and Illkova, K.
    Surface and Coatings Technology 396 (2020)
    The increased demands for higher efficiency and environmentally friendly diesel engines have led to a continuous search for new coating processing routes and new ceramic materials that can provide the required properties when applied on engine components such as pistons and exhaust manifolds. Although successful in gas turbine applications, thermal barrier coatings (TBCs) produced by suspension plasma spraying (SPS) processes have not been employed so far in the automotive industry. This work aims to achieve a better understanding of the role of thermal conductivity and thermal effusivity on the durability of SPS TBCs applied to pistons of diesel engines. Three different coating architectures were considered for this study. The first architecture was yttria-stabilized zirconia (YSZ) lamellar top coat deposited by APS (Atmospheric Plasma Spray) and used as a reference sample in this study. The second architecture was a columnar SPS top coat of either YSZ or gadolinium zirconate (GZO) while the third architecture was an SPS columnar top coat, “sealed” with a dense sealing layer deposited on the top coat. Two types of sealing layers were used, a metallic (M) or a ceramic thermal spray layer (C). Laser Flash Analysis (LFA) was used to determine the thermal conductivity and thermal effusivity of the coatings. Two different thermal cyclic tests were used to test the TBCs behavior under cyclic thermal loads. Microstructure analysis before and after the thermal cyclic tests were performed using SEM in different microstructures and materials. The thermal cyclic test results were correlated with coatings microstructure and thermophysical properties. It was observed that the columnar coatings produced by SPS had an enhanced service life in the thermal cyclic tests as compared to the APS coatings. © 2020 The Authors
    view abstract10.1016/j.surfcoat.2020.125950
  • 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 fatigue behavior of functionally graded W/EUROFER-layer systems using a new test apparatus
    Emmerich, T. and Vaßen, R. and Aktaa, J.
    Fusion Engineering and Design 154 (2020)
    In future fusion reactors tungsten coatings shall protect First Wall components, made of reduced activation ferritic martensitic steel, against the plasma, because of tungsten's favourable thermo-mechanical properties and low sputtering yield. Functionally graded material layers implemented between the coating and the steel substrate, compensate the difference in the coefficient of thermal expansion. By using the vacuum plasma spraying technique several layer systems were successfully produced and tested, among other aspects, in regard to their thermal fatigue behaviour up to 500 thermal cycles in a vacuum furnace. However, higher numbers of thermal cycles are anticipated for future fusion reactors and, therefore, a less time consuming approach for thermal fatigue testing is required. Hence, a new testing apparatus with induction heating and inert gas cooling was built and first thermal fatigue experiments with up to 5000 cycles were carried out on different functionally graded tungsten/steel layers systems. The subsequent investigations of these samples show that the layer systems are stable for the applied number of thermal cycles and their properties are solely determined during their respective coating processes. © 2020
    view abstract10.1016/j.fusengdes.2020.111550
  • 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
  • A constitutive model for the sintering of suspension plasma-sprayed thermal barrier coating with vertical cracks
    Lv, B. and Mücke, R. and Zhou, D. and Fan, X. and Wang, T.J. and Guillon, O. and Vaßen, R.
    Journal of the American Ceramic Society 102 (2019)
    The degradation of mechanical properties due to sintering is one of the major issues during high temperature service of thermal barrier coating system for advanced gas turbines. In this study, a constitutive model was developed by the variational principle, based on the experimentally observed microstructure features of suspension plasma-sprayed thermal barrier coatings. The constitutive model was further implemented in finite element analysis software, in order to investigate the effect of vertical cracks. The evolution of microstructure during sintering, coating shrinkage and mechanical degradation were predicted. The numerical predictions of Young's modulus were generally in agreement with experimental results. Furthermore, the effect of vertical cracks on the strain tolerance and sintering resistance were discussed. It was confirmed that the introduction of vertical cracks contributed to the improvement of both properties. © 2019 The American Ceramic Society
    view abstract10.1111/jace.16491
  • Architecture designs for extending thermal cycling lifetime of suspension plasma sprayed thermal barrier coatings
    Zhou, D. and Mack, D.E. and Gerald, P. and Guillon, O. and Vaßen, R.
    Ceramics International 45 (2019)
    Suspension plasma spraying (SPS) as a relatively new spraying technology has great potential on depositing high performance thermal barrier coatings (TBCs). In some cases, however, columnar SPS TBCs show premature failure in thermal cycling test. To explain the reasons of such failure, a failure mechanism for columnar SPS TBCs was proposed in this work. The premature failure of TBCs might be related to the radial stresses in the vicinity of top coat/bond coat interface. These radial stresses were introduced by the thermal misfit and the roughness of bond coat. According to this mechanism, two architecture designs of SPS TBCs were applied to improve the thermal cycling lifetime. One was a double layered top coat design with a lamellar atmospheric plasma sprayed (APS) sub-layer and a columnar SPS top-layer. The other one was a low roughness bond coat design with a columnar SPS top coat deposited on a low roughness bond coat which was grinded before the spraying. With both designs, lifetimes of SPS TBCs were significantly extended. Especially, a lifetime even better than conventional APS TBCs was achieved with the double layered design. © 2019 Elsevier Ltd and Techna Group S.r.l.
    view abstract10.1016/j.ceramint.2019.06.065
  • 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
  • Cr 2 AlC MAX phase foams by replica method
    Karimi, S. and Go, T. and Vaßen, R. and Gonzalez-Julian, J.
    Materials Letters 240 (2019)
    Highly pure Cr 2 AlC foams with high pore interconnectivity were processed by replica method, infiltrating commercial polyurethane foams with 20 and 30 pore per inch (ppi). Content of solid, dispersant and thickener was adjusted to achieve slurries with shear-thinning behavior. Cr 2 AlC foams were successfully obtained after two consecutive infiltrations, followed by two thermal processes, first in air to burn-out the polymeric foams and then in argon to consolidate the porous structure. The developed foams (cylinders with 40 mm diameter and 20 mm height) present high potential as component for heat exchangers and volumetric solar receivers. © 2019 Elsevier B.V.
    view abstract10.1016/j.matlet.2019.01.026
  • Effect of processing on high-velocity water vapor recession behavior of Yb-silicate environmental barrier coatings
    Bakan, E. and Sohn, Y.J. and Kunz, W. and Klemm, H. and Vaßen, R.
    Journal of the European Ceramic Society 39 (2019)
    The purpose of this research was to investigate the corrosion behavior of the low/high Yb2SiO5 containing Yb2Si2O7 coatings under high-velocity water vapor flow. To that end, Yb2Si2O7 and Si coatings were deposited by atmospheric plasma spraying on SiC substrates. The corrosion tests were performed in the burned natural gas under atmospheric pressure, with a gas flow velocity of 100 m/s at 1200 °C for 200 h. After the test, a 25 μm thick, porous corrosion layer at the surface of the Yb2Si2O7 rich coatings was found accompanied with a mass reduction, whereas samples with Yb2SiO5 rich coatings showed mass gain indicating the greater resistance of Yb2SiO5 against water vapor. A comparison of the Yb2Si2O7 rich coatings tested in this work and Yb2Si2O7 bulk samples tested in an earlier work at the same facility revealed significantly different recession rates. Possible mechanisms responsible for this distinct behavior are discussed in the manuscript. © 2018 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2018.11.048
  • Emergence and impact of Al2TiO5 in Al2O3-TiO2 APS coatings
    Richter, A. and Berger, L.-M. and Conze, S. and Sohn, Y.J. and Vaßen, R.
    IOP Conference Series: Materials Science and Engineering 480 (2019)
    Despite numerous studies and decades of industrial application, there is still a lack of understanding about the formation and the impact of aluminum titanate (Al2TiO5) in Al2O3-TiO2 thermal spray coatings. Especially the influence of the feedstock powder characteristics on the phase composition has only crudely been investigated so far. Therefore, in this work we have characterized commercial fused and crushed Al2O3-TiO2 feedstock powders: Three of them containing 13 wt.% TiO2 and three containing 40 wt.% TiO2. The effect of the varying phase compositions of the powders and their relevance on the deposition efficiency, the phase compositions, the porosity, and the hardness of the respective APS coatings is described in detail. While detrimental to the mechanical properties of 40 wt.% TiO2 coatings, we have found an enhancement of the hardness for 13 wt.% TiO2 coatings with a high Al2TiO5/Al6Ti2O13 content in the feedstock powder. Furthermore, it was found that Al2TiO5 may reform during APS when sprayed from an Al2TiO5-free powder. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/480/1/012007
  • Evolution of porosity, crack density, and CMAS penetration in thermal barrier coatings subjected to burner rig testing
    Mack, D.E. and Laquai, R. and Müller, B. and Helle, O. and Sebold, D. and Vaßen, R. and Bruno, G.
    Journal of the American Ceramic Society 102 (2019)
    Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources has been a persistent issue since many years. In this study, state of the art electron microscopy was correlated with X-ray refraction techniques to elucidate the intrusion of CMAS into the porous structure of atmospheric plasma sprayed (APS) TBCs and the formation and growth of cracks under thermal cycling in a burner rig. Results indicate that the sparse nature of the infiltration as well as kinetics in the burner rig are majorly influenced by the wetting behavior of the CMAS. Despite the obvious attack of CMAS on grain boundaries, the interaction of yttria-stabilized zirconia (YSZ) with intruded CMAS has no immediate impact on structure and density of internal surfaces. At a later stage the formation of horizontal cracks is observed in a wider zone of the TBC layer. © 2019 The American Ceramic Society
    view abstract10.1111/jace.16465
  • 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
  • Impact of Al2O3-40 wt.% TiO2 feedstock powder characteristics on the sprayability, microstructure and mechanical properties of plasma sprayed coatings
    Richter, A. and Berger, L.-M. and Sohn, Y.J. and Conze, S. and Sempf, K. and Vaßen, R.
    Journal of the European Ceramic Society 39 (2019)
    Atmospheric plasma sprayed (APS) Al2O3-TiO2 coatings have found a wide range of industrial application due to their favorable properties, combined with low costs and a high availability. However, the detailed effect of the phase composition and the element distribution of the feedstock powders on the coating properties and the spraying process have only crudely been investigated so far. Here the impact of aluminum titanate (Al2TiO5) on the microstructural features and mechanical properties of Al2O3-40 wt.% TiO2 APS coatings is demonstrated by investigating the detailed phase composition and the distribution of aluminum and titanium in three fused and crushed feedstock powders and the respective coatings. Thereby, a direct influence of Al2TiO5 content on the deposition efficiency, the porosity, the elastic modulus, and the hardness of the coatings is revealed. The results emphasize the need for a more detailed specification of commercial Al2O3-TiO2 feedstock powders to ensure a high reliability of the coating properties. © 2019 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2019.08.026
  • Influence of different annealing atmospheres on the mechanical properties of freestanding MCrALY bond coats investigated by micro-tensile creep tests
    Giese, S. and Neumeier, S. and Bergholz, J. and Naumenko, D. and Quadakkers, W.J. and Vaßen, R. and Göken, M.
    Metals 9 (2019)
    The mechanical properties of low-pressure plasma sprayed (LPPS) MCrAlY (M = Ni, Co) bond coats, Amdry 386, Amdry 9954 and oxide dispersion strengthened (ODS) Amdry 9954 (named Amdry 9954 + ODS) were investigated after annealing in three atmospheres: Ar–O2, Ar–H2 O, and Ar–H2 –H2 O. Freestanding bond coats were investigated to avoid any influence from the substrate. Miniaturized cylindrical tensile specimens were produced by a special grinding process and then tested in a thermomechanical analyzer (TMA) within a temperature range of 900–950◦ C. Grain size and phase fraction of all bond coats were investigated by EBSD before testing and no difference in microstructure was revealed due to annealing in various atmospheres. The influence of annealing in different atmospheres on the creep strength was not very pronounced for the Co-based bond coats Amdry 9954 and Amdry 9954 + ODS in the tested conditions. The ODS bond coats revealed significantly higher creep strength but a lower strain to failure than the ODS-free Amdry 9954. The Ni-based bond coat Amdry 386 showed higher creep strength than Amdry 9954 due to the higher fraction of the β-NiAl phase. Additionally, its creep properties at 900◦ C were much more affected by annealing in different atmospheres. The bond coat Amdry 386 annealed in an Ar–H2 O atmosphere showed a significantly lower creep rate than the bond coat annealed in Ar–O2 and Ar–H2 –H2 O atmospheres. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met9060692
  • 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
  • Mechanical characterisation of the protective Al2O3 scale in Cr2AlC MAX phases
    Gibson, J.S.K.-L. and Gonzalez-Julian, J. and Krishnan, S. and Vaßen, R. and Korte-Kerzel, S.
    Journal of the European Ceramic Society (2019)
    MAX phases have great potential under demands of both high-temperature and high-stress performance, with their mixed atomic bonding producing the temperature and oxidation resistance of ceramics with the mechanical resilience of metals. Here, we measure the mechanical properties up to 980C by nanoindentation on highly dense and pure Cr2AlC, as well as after oxidation with a burner rig at 1200 °C for more than 29 h. Only modest reductions in both hardness and modulus up to 980 °C were observed, implying no change in deformation mechanism. Furthermore, micro-cantilever fracture tests were carried out at the Cr2AlC/Cr7C3 and Cr7C3/Al2O3 interfaces after the oxidation of the Cr2AlC substrates with said burner rig. The values are typical of ceramic-ceramic interfaces, below 4 MPam, leading to the hypothesis that the excellent macroscopic behaviour is due to a combination of low internal strain due to the match in thermal expansion coefficient as well as the convoluted interface. © 2019 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2019.07.045
  • Microstructure and phase evolution of atmospheric plasma sprayed Mn-Co-Fe oxide protection layers for solid oxide fuel cells
    Grünwald, N. and Sohn, Y.J. and Yin, X. and Menzler, N.H. and Guillon, O. and Vaßen, R.
    Journal of the European Ceramic Society 39 (2019)
    Dense protective layers are needed to reduce chromium-related degradation in SOFC stacks. In particular, atmospheric plasma sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) coatings demonstrated low degradation rates in stack tests. We show that short-term annealing in air induces crack healing within these coatings. Parallel to this effect, a phase transformation is observed originating from oxidation that proceeds by solid state-diffusion. The present contribution reveals the basic mechanisms of the microstructural and phase changes of coatings in long-term annealing tests of up to 10,000 h at 700 °C. The layer develops differently at the air-facing surface and in the bulk. Due to cation deficiency, oxidation is dominated by cation outward diffusion, leading to a Co-enriched surface layer. The bulk displays a fine distribution of the initial (rock salt) and the final (spinel) phases. Understanding the mechanisms leading to these irreversible changes enables predictions to be made concerning durable protectivecoatings in SOFCs. © 2018 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2018.08.027
  • Miniaturization of low cycle fatigue-testing of single crystal superalloys at high temperature for uncoated and coated specimens [Miniaturisierung der Versuchstechnik für Niedrig-Lastwechsel-Ermüdung bei Hochtemperatur an Proben aus einkristallinen Superlegierungen mit und ohne Schutzschichten]
    Meid, C. and Waedt, U. and Subramaniam, A. and Wischek, J. and Bartsch, M. and Terberger, P. and Vaßen, R.
    Materialwissenschaft und Werkstofftechnik 50 (2019)
    A newly developed miniature specimen and respective fixture for high temperature low cycle fatigue testing of nickel based single crystal superalloys is presented. Miniaturization allows the preparation of test specimens in all main crystallographic orientations of the cubic nickel crystal using laboratory sized material samples and enables excellent utilization of the costly material. The specimen geometry is optimized by means of parameter studies employing numerical calculations such that for the main crystallographic orientations the stress concentration at the fillet between gauge length and specimen head is minimized, and failure is likely to occur within the gauge length. The designed fixture allows easy specimen mounting and provides sufficient support for applying an extensometer for strain measurement. Protective metallic coatings against oxidation can be applied on the specimen by plasma spraying for studying the effect of coatings on the fatigue lifetime. The functionality of the specimen geometry and fixture design for low cycle fatigue testing is demonstrated for temperatures up to 950 °C. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201800135
  • Molten salt shielded synthesis of oxidation prone materials in air
    Dash, A. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Nature Materials 18 (2019)
    To prevent spontaneous oxidation during the high-temperature synthesis of non-oxide ceramics, an inert atmosphere is conventionally required 1,2 . This, however, results in high energy demand and high production costs. Here, we present a process for the synthesis and consolidation of oxidation-prone materials, the ‘molten salt shielded synthesis/sintering’ process (MS 3 ), which uses molten salts as a reaction medium and also to protect the ceramic powders from oxidation during high-temperature processing in air. Synthesis temperatures are also reduced, and the final product is a highly pure, fine and loose powder that does not require additional milling steps. MS 3 has been used for the synthesis of different ternary transition metal compounds (MAX phases, such as Ti 3 SiC 2 3 , Ti 2 AlN 4 , MoAlB 5 ), binary carbides (TiC) and for the sintering of titanium. The availability of high-quality powders, combined with energy and cost savings, may remove one of the bottlenecks for the industrial application of these materials. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstract10.1038/s41563-019-0328-1
  • Oxide Dispersion Strengthened Bond Coats with Higher Alumina Content: Oxidation Resistance and Influence on Thermal Barrier Coating Lifetime
    Vorkötter, C. and Hagen, S.P. and Pintsuk, G. and Mack, D.E. and Virtanen, S. and Guillon, O. and Vaßen, R.
    Oxidation of Metals 92 (2019)
    The oxidation resistance of the bond coat in thermal barrier coating systems has significant influence on thermal cycling performance of the protective coating. In this study, the influence of varying the alumina content of plasma-sprayed oxide dispersion strengthened bond coats with CoNiCrAlY matrix material on the oxidation resistance was analysed by thermogravimetric analysis, SEM and TEM. Yttrium ions at the alumina scale grain boundaries and the grain size in the scale appear as major factors influencing oxidation properties. The ODS material with 2, 10 and 30 wt% alumina content was applied in TBC systems as an additional thin bond coat. The thermal cycling performance of those advanced TBC systems, in burner rig tests, was evaluated with respect to the ODS material properties. Thermal cycling behaviour is in good correlation with the isothermal oxidation resistance. All results indicate that TBC systems with 10 wt% alumina content in the ODS bond coat have a superior thermal cycling performance, as compared to ODS bond coats with lower or higher alumina content. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s11085-019-09931-z
  • Preface
    Pawłowski, L. and Vaßen, R. and Lusvarghi, L.
    Surface and Coatings Technology 371 (2019)
    view abstract10.1016/j.surfcoat.2019.05.084
  • 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
  • Repair of Ni-based single-crystal superalloys using vacuum plasma spray
    Kalfhaus, T. and Schneider, M. and Ruttert, B. and Sebold, D. and Hammerschmidt, T. and Frenzel, J. and Drautz, R. and Theisen, W. and Eggeler, G. and Guillon, O. and Vassen, R.
    Materials and Design 168 (2019)
    Turbine blades in aviation engines and land based gas-turbines are exposed to extreme environments. They suffer damage accumulation associated with creep, oxidation and fatigue loading. Therefore, advanced repair methods are of special interest for the gas-turbine industry. In this study, CMSX-4 powder is sprayed by Vacuum Plasma Spray (VPS) on single-crystalline substrates with similar compositions. The influence of the substrate temperature is investigated altering the temperature of the heating stage between 850 °C to 1000 °C. Different spray parameters were explored to identify their influence on the microstructure. Hot isostatic pressing (HIP) featuring fast quenching rates was used to minimize porosity and to allow for well-defined heat-treatments of the coatings. The microstructure was analysed by orientation imaging scanning electron microscopy (SEM), using electron backscatter diffraction (EBSD). The effects of different processing parameters were analysed regarding their influence on porosity and grain size. The results show that optimized HIP heat-treatments can lead to dense coatings with optimum γ/γ′ microstructure. The interface between the coating and the substrate is oxide free and shows good mechanical integrity. The formation of fine crystalline regions as a result of fast cooling was observed at the single-crystal surface, which resulted in grain growth during heat-treatment in orientations determined by the crystallography of the substrate. © 2019
    view abstract10.1016/j.matdes.2019.107656
  • Sintering behavior of columnar thermal barrier coatings deposited by axial suspension plasma spraying (SPS)
    Zhou, D. and Malzbender, J. and Sohn, Y.J. and Guillon, O. and Vaßen, R.
    Journal of the European Ceramic Society 39 (2019)
    During the last decade, Suspension Plasma Spraying (SPS) attracted a lot of interest as an alternative process to produce columnar Thermal Barrier Coatings (TBCs). In this study, columnar TBCs were deposited with SPS. After spraying, samples were isothermally annealed at 1373 K for 1 h, 3 h, 10 h and 50 h, respectively. Microstructures and mechanical properties of the ceramic coatings were investigated as a function of annealing time. Annealing resulted in healing of micro-cracks, coarsening of pores, growth of domain size, companied with a decrease of porosity within columns. The change of coating microstructure led to change of mechanical properties. In addition, residual stress in SPS coatings was also investigated. Furthermore, as-sprayed coatings and pre-annealed coatings were subjected to burner rig tests. Short time pre-annealing allowed to enhance thermal cycling lifetime of such SPS coatings. The thermal cycling results were related to microstructure modifications of coatings. © 2018 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2018.09.020
  • Superior cyclic life of thermal barrier coatings with advanced bond coats on single-crystal superalloys
    Vorkötter, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Surface and Coatings Technology 361 (2019)
    Advanced thermal barrier coatings are essential to further increase the efficiency of gas turbine engines. One limiting factor of the TBC lifetime is the temperature dependent formation of the thermally grown oxide (TGO) during thermal exposure resulting in critical stress levels at the top coat-bond coat interface. Oxide dispersion strengthened (ODS) bond coats demonstrated slower oxygen scale growth during thermal exposure in comparison to standard bond coats. In this study for the first time TBC samples on single-crystal substrates (comparable to CMSX4) with thin ODS Co-based flash coats on the same Co-based bond coat (Amdry 995) and a porous atmospherically plasma sprayed (APS) yttria stabilized zirconia (YSZ) topcoat were manufactured by thermal spray techniques and evaluated with respect to their thermal cyclic behavior. As the major performance test cyclic burner rigs, which can establish thermal conditions similar to those in gas turbines, were applied. TBC samples with the new material combination show superior performance compared to previous samples. Cross sections of the samples were analyzed by scanning electron and laser scanning microscopy. Lifetime data and failure mode of the samples are discussed with respect to material properties such as thermal expansion coefficients, microstructural changes and TGO growth. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.01.001
  • Surface roughening of Al2O3/Al2O3-ceramic matrix composites by nanosecond laser ablation prior to thermal spraying
    Gatzen, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Journal of Laser Applications 31 (2019)
    Al2O3/Al2O3 ceramic matrix composites are candidate materials for high-temperature applications such as gas turbines. As water vapor corrosion of Al2O3/Al2O3-CMC (ceramic matrix composite) is a major issue, the application of suitable environmental barrier coatings is inevitable. An important factor for coating adhesion, especially in thermal spraying, is mechanical interlocking. Therefore, a rough substrate surface is needed. Although it has been proven that laser ablation is a suitable method for surface preparation of metallic substrates, no studies on Al2O3/Al2O3-CMCs are available. Therefore, the suitability of surface preparation of an Al2O3/Al2O3-CMC by laser ablation for use prior to atmospheric plasma spraying was examined. The laser ablation threshold fluence for Al2O3/Al2O3-CMC was determined. The effects of different processing parameters on the surface were studied. Various surface morphologies were obtained, such as cauliflower and honeycomb structures. The samples were characterized by white light interferometry, laser microscopy, and scanning electron microscopy. The obtained surface structures were coated with Gd2Zr2O7. It was found that the adhesion strength of coatings on laser treated samples was drastically increased. © 2019 Laser Institute of America.
    view abstract10.2351/1.5080546
  • Synthesis of Ti3SiC2 MAX phase powder by a molten salt shielded synthesis (MS3) method in air
    Dash, A. and Sohn, Y.J. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Journal of the European Ceramic Society 39 (2019)
    Titanium silicon carbide (Ti3SiC2) powder was synthesized by molten salt shielded synthesis route of elemental reactants. Potassium bromide (KBr) was used for gas-tight encapsulation of the consolidated reaction mixture for further high temperature processing. The synthesis of Ti3SiC2 powder was carried out in air, the salt cladding and molten salt pool provided for the protection of the material against oxidation both at low and high temperature. The process yielded free flowing Ti3SiC2 powders without the need of a milling step. Al addition to the reaction mixture resulted in a high purity (96 wt. %) of Ti3SiC2 at a synthesis temperature of 1250 °C. The synthesized micro-metric Ti3SiC2 can be milled to nano-metric powders. © 2019 Elsevier Ltd
    view abstract10.1016/j.jeurceramsoc.2019.05.011
  • Tailored microstructures of gadolinium zirconate/YSZ multi-layered thermal barrier coatings produced by suspension plasma spray: Durability and erosion testing
    Mahade, S. and Zhou, D. and Curry, N. and Markocsan, N. and Nylén, P. and Vaßen, R.
    Journal of Materials Processing Technology 264 (2019)
    This work employed an axial suspension plasma spray (SPS) process to deposit two different gadolinium zirconate (GZ) based triple layered thermal barrier coatings (TBCs). The first was a ‘layered’ TBC (GZ dense/GZ/YSZ) where the base layer was YSZ, intermediate layer was a relatively porous GZ and the top layer was a relatively dense GZ. The second triple layered TBC was a ‘composite’ TBC (GZ dense/GZ + YSZ/YSZ) comprising of an YSZ base layer, a GZ + YSZ intermediate layer and a dense GZ top layer. The as sprayed TBCs (layered and composite) were characterized using SEM/EDS and XRD. Two different methods (water intrusion and image analysis) were used to measure the porosity content of the as sprayed TBCs. Fracture toughness measurements were made on the intermediate layers (GZ + YSZ layer of the composite TBC and porous GZ layer of the layered TBC respectively) using micro indentation tests. The GZ + YSZ layer in the composite TBC was shown to have a slightly higher fracture toughness than the relatively porous GZ layer in the layered TBC. Erosion performance of the as sprayed TBCs was evaluated at room temperature where the composite TBC showed higher erosion resistance than the layered TBC. However, in the burner rig test conducted at 1400 °C, the layered TBC showed higher thermal cyclic lifetime than the composite TBC. Failure analysis of the thermally cycled and eroded TBCs was performed using SEM and XRD. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.09.016
  • Thermo-mechanical response of FG tungsten/EUROFER multilayer under high thermal loads
    Qu, D.D. and Wirtz, M. and Linke, J. and Vaßen, R. and Aktaa, J.
    Journal of Nuclear Materials 519 (2019)
    With the aim to evaluate the performance of functionally graded (FG) tungsten/EUROFER multilayer under fusion relevant transient heat loads, this study summarizes the experimental results on type-I edge localized mode (ELM)-like thermal shock exposures of a FG tungsten/EUROFER multilayer. Absorbed power densities of 0.19 and 0.38 GW/m 2 with a pulse duration of 1 ms, and base temperatures of room temperature and 550 °C were chosen. We assessed the effect of variable FG-layers that consisted of three designed thicknesses and two kinds of layer numbers. The conclusion is that the introduction of FG-layers improves the thermal shock resistance. The thermal shock crack thresholds at RT and 550 °C of five multilayers lie between 0.19 and 0.38 GW/m 2 . © 2019
    view abstract10.1016/j.jnucmat.2019.03.019
  • Water vapor corrosion test using supersonic gas velocities
    Gatzen, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Journal of the American Ceramic Society 102 (2019)
    Testing of the corrosion resistance of environmental barrier coating (EBC) systems is necessary for developing reliable coatings. Unfortunately tests under realistic gas turbine conditions are difficult and expensive. The materials under investigation as well as parts of the test setup have to withstand high temperatures (≥1200°C), high pressure (up to 30 bar) as well as the corrosive atmosphere (H2O, O2, NOx). Therefore most lab scale test-rigs focus on simplified test conditions. In this work water vapor corrosion testing of EBCs with a high velocity oxy fuel (HVOF) facility is introduced which combines high temperatures and high gas velocities. It leads to quite high recession rates in short periods of time, which are comparable to results from literature. It was found that high flow velocities can easily compensate low gas pressures. HVOF-testing is a simple and fast way to measure the recession rate of an EBC-system. As proof of concept the recession rates of an oxide/oxide CMC with and without EBC were measured. © 2019 The American Ceramic Society
    view abstract10.1111/jace.16595
  • YAlO3-a novel environmental barrier coating for Al2O3/Al2O3-ceramic matrix composites
    Gatzen, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Coatings 9 (2019)
    Ceramic matrix composites (CMCs) are promising materials for high-temperature applications. Environmental barrier coatings (EBCs) are needed to protect the components against water vapor attack. A new potential EBC material, YAlO3, was studied in this paper. Different plasma-spraying techniques were used for the production of coatings on an alumina-based CMC, such as atmospheric plasma spraying (APS) and very low pressure plasma spraying (VLPPS). No bond coats or surface treatments were applied. The performance was tested by pull-adhesion tests, burner rig tests, and calcium-magnesium-aluminum-silicate (CMAS) corrosion tests. The samples were subsequently analyzed by means of X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Special attention was paid to the interaction at the interface between coating and substrate. The results show that fully crystalline and good adherent YAlO3 coatings can be produced without further substrate preparation such as surface pretreatment or bond coat application. The formation of a thin reaction layer between coating and substrate seems to promote adhesion. © 2019 by the authors.
    view abstract10.3390/coatings9100609
  • 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
  • Development of W-coating with functionally graded W/EUROFER-layers for protection of First-Wall materials
    Emmerich, T. and Qu, D. and Vaßen, R. and Aktaa, J.
    Fusion Engineering and Design 128 (2018)
    To protect First-Wall components, made of reduced activation ferritic martensitic steel, against the plasma of future fusion reactors, tungsten coatings are a feasible option. The difference in coefficient of thermal expansion between the coating and the steel substrate can be compensated using functionally graded material layers. Such layers were successfully produced by vacuum plasma spraying. This technique reduces, however, the hardness of the substrate surface near zone. Modified spraying parameters moderate the hardness loss. The parameters may, though, affect also the layer bonding toughness which is evaluated in this work by four point bending tests. Furthermore, the layers behavior on First-Wall Mock‐ups and under different thermal loads is investigated by finite element simulations. The measurement of the layer adhesion indicates that the layer adhesion decreases only for modified spraying parameters that do not reduce the substrate hardness. It follows also from the toughness calculation that without layer residual stresses the toughness values depend on coating thickness. In regard to the Mock‐up behavior the simulations show that intermediate steps are necessary during heating and cooling to prevent artificial stresses and inelastic deformation. It is, however, not possible to avoid stresses and inelastic deformation completely as they originate from the residual stresses. © 2018
    view abstract10.1016/j.fusengdes.2018.01.047
  • Environmental resistance of Cr2AlC MAX phase under thermal gradient loading using a burner rig
    Gonzalez-Julian, J. and Go, T. and Mack, D.E. and Vaßen, R.
    Journal of the American Ceramic Society (2018)
    Dense Cr2AlC materials were tested under a gradient loading for the first time using a burner rig. The severe thermal cycling conditions consist of 500 short cycles at 1200°C, with an accumulative time at the maximal temperature of more than 29 hours. The samples showed no visible damage under these conditions due to the formation of an outer protective α-Al2O3 layer, which shows a strong adhesion with the Cr2AlC substrate. No cracks, delamination or damage were observed at the interface between the different layers. This excellent response under cyclic loading shows the excellent potential of Cr2AlC compounds for high-temperature applications. © 2018 American Ceramic Society.
    view abstract10.1111/jace.15425
  • High-temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity
    Gonzalez-Julian, J. and Onrubia, S. and Bram, M. and Broeckmann, C. and Vassen, R. and Guillon, O.
    Journal of the American Ceramic Society 101 (2018)
    Cr2AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90-180 μm, 180-250 μm, and 250-400 μm. Sacrificial template technique was used as the processing method, utilizing NH4HCO3 as a temporary pore former. Cr2AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in-situ formation of an outer thin continuous protective layer of α-Al2O3. The in-situ α-Al2O3 protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter- to transgranular fracture mode. According to the excellent high-temperature response, heat exchangers and catalyst supports are the potential application of these foams. © 2017 The American Ceramic Society
    view abstract10.1111/jace.15224
  • Influence of coating thickness on residual stress and adhesion-strength of cold-sprayed Inconel 718 coatings
    Singh, R. and Schruefer, S. and Wilson, S. and Gibmeier, J. and Vassen, R.
    Surface and Coatings Technology 350 (2018)
    In the cold spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at the temperatures well below their melting point. The high particle impact velocity causes high local stresses which lead to deforming the particles and the substrate plastically in the proximity of the particle–substrate interface. As a result, high residual stresses are introduced in cold spray coatings due to the peening effect of the particles collisions with the substrate. In this study, a powder based on the chemical composition of IN 718 was cold-sprayed on IN 718 substrates by using nitrogen gas for an application as a repair tool for aero engine components. The magnitude of the residual stress and its distribution through the thickness were measured by using the hole-drilling and the bending methods. Residual stress was also estimated by using an approach based on the physical process parameters. Mainly compressive residual stresses were observed in cold-sprayed IN 718 coatings. Accumulation of residual stresses in the coatings is highly affected by peening during deposition and it decreases with increase in thickness. It has been observed that the adhesion-strengths of cold-sprayed IN 718 coatings are highly influenced by coating thickness and residual stress states of the coating/substrate system. In the presence of residual stresses in the coatings, adhesion-strength decreases with increasing coating thickness. The energy-release-rate criterion has been used to predict adhesion-strength with increasing coating thickness. Predicted bond-strength values are close to the measured adhesion-strength values and decrease with increase in coating thickness. © 2018 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2018.06.080
  • Influence of Feedstock Powder Modification by Heat Treatments on the Properties of APS-Sprayed Al2O3-40% TiO2 Coatings
    Berger, L.-M. and Sempf, K. and Sohn, Y.J. and Vaßen, R.
    Journal of Thermal Spray Technology 27 (2018)
    The formation and decomposition of aluminum titanate (Al2TiO5, tialite) in feedstock powders and coatings of the binary Al2O3-TiO2 system are so far poorly understood. A commercial fused and crushed Al2O3-40%TiO2 powder was selected as the feedstock for the experimental series presented in this paper, as the composition is close to that of Al2TiO5. Part of that powder was heat-treated in air at 1150 and 1500 °C in order to modify the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including EDS of metallographically prepared cross sections. Only a maximum content of about 45 wt.% Al2TiO5 was possible to obtain with the heat treatment at 1500 °C due to inhomogeneous distribution of Al and Ti in the original powder. Coatings were prepared by plasma spraying using a TriplexPro-210 (Oerlikon Metco) with Ar-H2 and Ar-He plasma gas mixtures at plasma power levels of 41 and 48 kW. Coatings were studied by XRD, SEM including EDS linescans of metallographically prepared cross sections, and microhardness HV1. With the exception of the powder heat-treated at 1500 °C an Al2TiO5-Ti3O5 (tialite–anosovite) solid solution Al2−xTi1+xO5 instead of Al2TiO5 existed in the initial powder and the coatings. © 2018, ASM International.
    view abstract10.1007/s11666-018-0716-0
  • Mechanical properties of sprayed overlayers on superalloy substrates, obtained via indentation testing
    Campbell, J.E. and Kalfhaus, T. and Vassen, R. and Thompson, R.P. and Dean, J. and Clyne, T.W.
    Acta Materialia 154 (2018)
    This paper concerns the use of a recently-developed methodology for inferring stress-strain curves from indentation data, based on iterative FEM simulation of the procedure. A relatively large indenter (2 mm diameter) is used, with deep penetration (to about 25% of the indenter radius). This has been carried out on (polished) free surfaces of sprayed superalloy overlayers on single crystal superalloy substrates. Both load-displacement data and residual indent profiles were obtained, with the overlayers being in two different conditions (as-sprayed and annealed). The overlayers were relatively thick (∼2.5 mm), so it was also possible to carry out uniaxial compression tests on them (in the through-thickness direction). The inferred stress-strain curves were similar in each case when derived from load-displacement data and indent profiles, and also close to the plots obtained by conventional uniaxial testing. The yield stress levels in both cases were around 1000 MPa, but the work hardening rate was significantly higher for the as-sprayed material. This kind of information is of considerable potential value when attempting to optimize the properties of such overlayers. The procedure can be employed, with some confidence, to cases for which uniaxial testing is difficult or impossible. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.05.043
  • 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
  • Tailoring columnar microstructure of axial suspension plasma sprayed TBCs for superior thermal shock performance
    Ganvir, A. and Joshi, S. and Markocsan, N. and Vassen, R.
    Materials and Design 144 (2018)
    This paper investigates the thermal shock behavior of thermal barrier coatings (TBCs) produced by axial suspension plasma spraying (ASPS). TBCs with different columnar microstructures were subjected to cyclic thermal shock testing in a burner rig. Failure analysis of these TBCs revealed a clear relationship between lifetime and porosity. However, tailoring the microstructure of these TBCs for enhanced durability is challenging due to their inherently wide pore size distribution (ranging from few nanometers up to few tens of micrometers). This study reveals that pores with different length scales play varying roles in influencing TBC durability. Fracture toughness shows a strong correlation with the lifetime of various ASPS TBCs and is found to be the prominent life determining factor. Based on the results, an understanding-based design philosophy for tailoring of the columnar microstructure of ASPS TBCs for enhanced durability under cyclic thermal shock loading is proposed. © 2018 The Authors
    view abstract10.1016/j.matdes.2018.02.011
  • Thermal cycling testing of TBCs on Cr2AlC MAX phase substrates
    Gonzalez-Julian, J. and Go, T. and Mack, D.E. and Vaßen, R.
    Surface and Coatings Technology 340 (2018)
    Thermal barrier coatings (TBCs) based on yttria-stabilized zirconia (YSZ) were deposited by Atmospheric Plasma Spray (APS) on highly dense and pure Cr2AlC substrates. The Cr2AlC/YSZ systems were tested under thermal cycling conditions at temperatures between 1100 and 1300 °C testing up to 500 h. The response of the system was excellent due to the strong adhesion between the substrate and the coating, and the formation of an outer and protective layer based on α-Al2O3. The oxide scale is formed due to the diffusion of Al atoms from the crystal structure of the Cr2AlC, followed by the reaction with oxygen in the air. The thickness of the oxide scale was 8.9, 17.6 and 39.7 μm at 1100, 1200 and 1300 °C, respectively, which is rather thick in comparison with the classical superalloy/TBC systems. Cr2AlC/YSZ systems survived without any damage under the severe cycling conditions at 1100 and 1200 °C due to the protective oxide scale layer and the sufficient thermal expansion match between the Cr2AlC, YSZ and α-Al2O3. At 1300 °C and after 268 h of cycling conditions, the system failed due to the formation of a porous carbide layer underneath of the oxide scale. The results are rather promising and confirm the potential of the MAX phases to operate under long term applications of high temperature and oxidizing environments. © 2018 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2018.02.035
  • Thermophysical and Mechanical Properties of Advanced Single Crystalline Co-base Superalloys
    Volz, N. and Zenk, C.H. and Cherukuri, R. and Kalfhaus, T. and Weiser, M. and Makineni, S.K. and Betzing, C. and Lenz, M. and Gault, B. and Fries, S.G. and Schreuer, J. and Vaßen, R. and Virtanen, S. and Raabe, D. and Spiecker, E. and Neumeier, S. and Göken, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 (2018)
    A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′ solvus temperature up to 1174 °C and γ′ volume fractions between 40 and 60 pct at 1050 °C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050 °C and 125 MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully. © 2018, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-018-4705-1
  • Vacuum plasma spraying of functionally graded tungsten/EUROFER97 coatings for fusion applications
    Vaßen, R. and Rauwald, K.-H. and Guillon, O. and Aktaa, J. and Weber, T. and Back, H.C. and Qu, D. and Gibmeier, J.
    Fusion Engineering and Design 133 (2018)
    As structural materials for future fusion power plants, reduced activation ferritic martensitic steels as EUROFER97 can be used. Unfortunately, the interaction of the plasma with the steel would result in a limited lifetime, so protective layers are investigated. An excellent protective material is tungsten, as it shows unique properties with respect to low sputtering, high melting points and low activation. However, the mismatch of thermo-physical properties between tungsten and EUROFER97 can lead to large stress levels and even failure. A possible way to overcome this problem is the use of functionally graded material (FGM). The paper will describe the manufacture of these FGMs by vacuum plasma spraying and their characterization. First of all, two different feeding lines have been used to produce the coatings. A major problem lies in different melting points of tungsten and steel. So the particle size distribution has to be adjusted to achieve sufficient melting of both materials during the spray process. In a second step, the feeding rates were optimized to obtain the wanted amount of tungsten and steel phases in the graded structures. In a thermal spray process, the gradient cannot be made continuously, however it has to be applied in a step-wise manner. In this investigation, samples with 3 and 5 different concentrations (excluding the pure steel and tungsten part) have been produced. The microstructures of these layers have been investigated. In addition, hardness was measured and the residual stress state was determined by the hole drilling method. © 2018
    view abstract10.1016/j.fusengdes.2018.06.006
  • Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials, Processes, and Properties
    Bakan, E. and Vaßen, R.
    Journal of Thermal Spray Technology 26 (2017)
    The ceramic top coat has a major influence on the performance of the thermal barrier coating systems (TBCs). Yttria-partially-stabilized zirconia (YSZ) is the top coat material frequently used, and the major deposition processes of the YSZ top coat are atmospheric plasma spraying and electron beam physical vapor deposition. Recently, also new thermal spray processes such as suspension plasma spraying or plasma spray-physical vapor deposition have been intensively investigated for TBC top coat deposition. These new processes and particularly the different coating microstructures that can be deposited with them will be reviewed in this article. Furthermore, the properties and the intrinsic–extrinsic degradation mechanisms of the YSZ will be discussed. Following the TBC deposition processes and standard YSZ material, alternative ceramic materials such as perovskites and hexaaluminates will be summarized, while properties of pyrochlores with regard to their crystal structure will be discussed more in detail. The merits of the pyrochlores such as good CMAS resistance as well as their weaknesses, e.g., low fracture toughness, processability issues, will be outlined. © 2017, ASM International.
    view abstract10.1007/s11666-017-0597-7
  • 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
  • Diffusion-related SOFC stack degradation
    Menzler, N.H. and Beez, A. and Grünwald, N. and Sebold, D. and Fang, Q. and Vaßen, R.
    ECS Transactions 78 (2017)
    As part of two different stack tests with four-plane short stacks and their intensive post-test characterization, two varying diffusionrelated degradation mechanisms were investigated. The first was a short-term test (∼1250h) with two different chromium evaporation protection layers on the air-side metallic interconnect and frame and the second was a long-term endurance test (∼ 35,000h). For the first stack, two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS), while the other two planes were coated with a manganese-cobalt-iron spinel layer by atmospheric plasma spraying (APS). The voltage loss in the planes with a WPS-coated interconnect was markedly higher than in those coated by means of APS. Finally, it was shown that the microstructure of the layers plays a key role in minimizing Cr evaporation. In this stack, gas-phase diffusion prevails over degradation. In the long-term stack, severe degradation due to solid-state manganese diffusion was observed. This paper draws an interaction hypothesis. © The Electrochemical Society.
    view abstract10.1149/07801.2223ecst
  • 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
  • Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray
    Mahade, S. and Curry, N. and Björklund, S. and Markocsan, N. and Nylén, P. and Vaßen, R.
    Journal of Thermal Spray Technology 26 (2017)
    7-8 wt.% Yttria-stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used by the gas turbines industry due to its excellent thermal and thermo-mechanical properties up to 1200 °C. The need for improvement in gas turbine efficiency has led to an increase in the turbine inlet gas temperature. However, above 1200 °C, YSZ has issues such as poor sintering resistance, poor phase stability and susceptibility to calcium magnesium alumino silicates (CMAS) degradation. Gadolinium zirconate (GZ) is considered as one of the promising top coat candidates for TBC applications at high temperatures (>1200 °C) due to its low thermal conductivity, good sintering resistance and CMAS attack resistance. Single-layer 8YSZ, double-layer GZ/YSZ and triple-layer GZdense/GZ/YSZ TBCs were deposited by suspension plasma spray (SPS) process. Microstructural analysis was carried out by scanning electron microscopy (SEM). A columnar microstructure was observed in the single-, double- and triple-layer TBCs. Phase analysis of the as-sprayed TBCs was carried out using XRD (x-ray diffraction) where a tetragonal prime phase of zirconia in the single-layer YSZ TBC and a cubic defect fluorite phase of GZ in the double and triple-layer TBCs was observed. Porosity measurements of the as-sprayed TBCs were made by water intrusion method and image analysis method. The as-sprayed GZ-based multi-layered TBCs were subjected to erosion test at room temperature, and their erosion resistance was compared with single-layer 8YSZ. It was shown that the erosion resistance of 8YSZ single-layer TBC was higher than GZ-based multi-layered TBCs. Among the multi-layered TBCs, triple-layer TBC was slightly better than double layer in terms of erosion resistance. The eroded TBCs were cold-mounted and analyzed by SEM. © 2016, ASM International.
    view abstract10.1007/s11666-016-0479-4
  • 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
  • Fabrication of Oxide Dispersion Strengthened Bond Coats with Low Al2O3 Content
    Bergholz, J. and Pint, B.A. and Unocic, K.A. and Vaßen, R.
    Journal of Thermal Spray Technology (2017)
    Nanoscale oxide dispersions have long been used to increase the oxidation and wear resistance of alloys used as bond coatings in thermal barrier coatings. Their manufacturing via mechanical alloying is often accompanied by difficulties regarding their particle size, homogeneous distribution of the oxide dispersions inside the powder, involving considerable costs, due to cold welding of the powder during milling. A significant improvement in this process can be achieved by the use of process control agent (PCA) to achieve the critical balance between cold welding and fracturing, thereby enhancing the process efficiency. In this investigation, the influence of the organic additive stearic acid on the manufacturing process of Al2O3-doped CoNiCrAlY powder was investigated. Powders were fabricated via mechanical alloying at different milling times and PCA concentrations. The results showed a decrease in particle size, without hindering the homogeneous incorporation of the oxide dispersions. Two powders manufactured with 0.5 and 1.0 wt.% PCA were deposited by high velocity oxygen fuel (HVOF) spraying. Results showed that a higher content of elongated particles in the powder with the higher PCA content led to increased surface roughness, porosity and decreased coating thickness, with areas without embedded oxide particles. © 2017 ASM International
    view abstract10.1007/s11666-017-0550-9
  • Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray
    Mahade, S. and Curry, N. and Björklund, S. and Markocsan, N. and Nylén, P. and Vaßen, R.
    Surface and Coatings Technology 318 (2017)
    7-8. wt.% yttria stabilized zirconia (YSZ) is the standard ceramic top coat material used in gas turbines to insulate the underlying metallic substrate. However, at higher temperatures (>. 1200. C), phase stability and sintering becomes an issue for YSZ. At these temperatures, YSZ is also susceptible to CMAS (calcium magnesium alumino silicates) infiltration. New ceramic materials such as pyrochlores have thus been proposed due to their excellent properties such as lower thermal conductivity and better CMAS attack resistance compared to YSZ. However, pyrochlores have inferior thermo mechanical properties compared to YSZ. Therefore, double-layered TBCs with YSZ as the intermediate layer and pyrochlore as the top ceramic layer have been proposed. In this study, double layer TBC comprising gadolinium zirconate (GZ)/YSZ and triple layer TBC (GZdense/GZ/YSZ) comprising relatively denser GZ top layer on GZ/YSZ were deposited by suspension plasma spray. Also, single layer 8YSZ TBC was suspension plasma sprayed to compare its functional performance with the multi-layered TBCs. Cross sections and top surface morphology of as sprayed TBCs were analyzed by scanning electron microscopy (SEM). XRD analysis was done to identify phases formed in the top surface of as sprayed TBCs. Porosity measurements were made using water intrusion and image analysis methods. Thermal diffusivity of the as sprayed TBCs was measured using laser flash analysis and thermal conductivity of the TBCs was calculated. The multi-layered GZ/YSZ TBCs were shown to have lower thermal conductivity than the single layer YSZ. The as sprayed TBCs were also subjected to thermal cyclic testing at 1300. C. The double and triple layer TBCs had a longer thermal cyclic life compared to YSZ. The thermo cycled samples were analyzed by SEM. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.12.062
  • 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
  • Lifetime and failure modes of plasma sprayed thermal barrier coatings in thermal gradient rig tests with simultaneous CMAS injection
    Mack, D.E. and Wobst, T. and Jarligo, M.O.D. and Sebold, D. and Vaßen, R.
    Surface and Coatings Technology 324 (2017)
    Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources such as sand, volcanic ash, fly ash, or variable quality fuels has been a tenacious issue during the recent years. This follows from the fact that engines are required to operate under increasingly harsh conditions in all kind of gas turbine applications following the demands for higher efficiency and operational flexibility. While the understanding of the mechanism of CMAS induced degradation of TBCs as well as approaches for mitigation of CMAS attack by means of advanced TBC compositions have grown remarkably, most of the reported results have been obtained from lab testing at isothermal conditions or from evaluation of ex-service components, either. The isothermal tests are not reproducing important thermomechanical effects from service conditions, and it may be hard to figure out the thermal history of the ex-service examples. In this study a burner rig facility has been used for the evaluation of TBC performance, where a thermal gradient is applied across the TBC at cyclic load and CMAS is continuously injected to the combustion environment to simulate the conditions closer to actual service in an engine while allowing control of important degradation limiting parameters. Tests have been performed on the state of the art material YSZ while loading parameters as high temperature dwell time and CMAS deposition rate have been varied. Tests have been evaluated in terms of chemical degradation, failure mode and TBC spallation lifetime by means of SEM, EDS and XRD. Test conditions and relevance to in-service operating conditions are discussed. Major impacts on thermal gradient cycling lifetime and similarities and discrepancies related to reported failure mechanism are reviewed. © 2017
    view abstract10.1016/j.surfcoat.2017.04.071
  • 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
  • Preface
    Pawłowski, L. and Vaßen, R. and Petrov, I. and Berndt, C.C.
    Surface and Coatings Technology 318 (2017)
    view abstract10.1016/j.surfcoat.2017.04.063
  • 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
  • Residual stress depth distributions for atmospheric plasma sprayed MnCo1.9Fe0.1O4 spinel layers on crofer steel substrate
    Back, H.C. and Mutter, M. and Gibmeier, J. and Mücke, R. and Vaßen, R.
    Materials Science Forum 905 MSF (2017)
    In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the use of ferritic stainless steel can lead to degradation in cell performance due to chromium migration into the cells at the cathode side [1]. Application of a coating on the ferritic stainless steel interconnect is one option to prevent Cr outward migration through the coating. MnCo1.9Fe0.1O4 (in the following designated as MCF) spinels act as a diffusion barrier and retain high conductivity during operation [2]. Knowledge about the residual stress depth distribution throughout the complete APS coating system is important and can help to optimize the coating process. This implicitly requires reliable residual stress analysis in the coating, the interface region and in the substrate. For residual stress analysis on these specific layered systems diffraction based analysis methods (XRD) using laboratory X-ray sources can only by applied at the very surface. For larger depths sublayer removal is necessary to gain reliable residual stress data. The established method for sublayer removal is electrochemical etching, which fails, since the spinel layer is inert. However, a mechanical layer removal will affect the local residual stress distribution. As an alternative, mechanical residual stress analyses techniques can be applied. Recently, we established an approach to analyse residual stress depth distributions in thick film systems by means of the incremental hole drilling method [5, 6]. In this project, we refined our approach for the application on MCF coatings with a layer thickness between 60 – 125 µm. © 2017 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.905.174
  • Self-healing atmospheric plasma sprayed Mn1.0Co1.9Fe0.1O4 protective interconnector coatings for solid oxide fuel cells
    Grünwald, N. and Sebold, D. and Sohn, Y.J. and Menzler, N.H. and Vaßen, R.
    Journal of Power Sources 363 (2017)
    Dense coatings on metallic interconnectors are necessary to suppress chromium poisoning of SOFC cathodes. Atmospherically plasma sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) protective layers demonstrated reduced chromium related degradation in laboratory and stack tests. Previous analyses revealed strong microstructural changes comparing the coating's as-sprayed and operated condition. This work concentrates on the layer-densification and crack-healing observed by annealing APS-MCF in air, which simulates the cathode operation conditions. The effect is described by a volume expansion induced by a phase transformation. Reducing conditions during the spray process lead to a deposition of the MCF in a metastable rock salt configuration. Annealing in air activates diffusion processes for a phase transformation to the low temperature stable spinel phase (T &lt; 1050 °C). This transformation is connected to an oxygen incorporation which occurs at regions facing high oxygen partial pressures, as there are the sample surface, cracks and pore surfaces. Calculations reveal a volume expansion induced by the oxygen uptake which seals the cracks and densifies the coating. The process decelerates when the cracks are closed, as the gas route is blocked and further oxidation continues over solid state diffusion. The self-healing abilities of metastable APS coatings could be interesting for other applications. © 2017 Elsevier B.V.
    view abstract10.1016/j.jpowsour.2017.07.072
  • Simulation of the effect of the porous support on flux through an asymmetric oxygen transport membrane
    Unije, U. and Mücke, R. and Niehoff, P. and Baumann, S. and Vaßen, R. and Guillon, O.
    Journal of Membrane Science 524 (2017)
    Asymmetric membranes provide a low ionic resistance of the functional separation layer together with a high mechanical stability. However, the microstructure of the porous support in the membrane assembly affects the overall flux significantly. This effect was studied by applying the binary friction model (BFM) for the support together with a modified Wagner equation for the dense membrane using transport relevant parameters obtained from micro computed tomography data of a tape cast Ba0.5Sr0.5Co0.8Fe0.2O3– δ support. The influence of different pore diameters and thicknesses of the support were compared for different feed gases (oxygen and air) and flow configurations (3-end, 4-end, assembly orientation). The effect of the support at large pore diameters (&gt;35 µm) for the 3-end mode transport process using oxygen as feed gas, was negligible. This was not the case for the 4-end mode irrespective of the feed gas, and for the 3-end mode using air as feed gas. This was attributed to the binary diffusion term in the BFM. Thin small-pored supports yield the same flux as thick large-pored supports considering a non-linear relationship between thickness and pore size. This can be used for the optimization of the support's microstructure with regards to mechanical strength and permeability. © 2016 Elsevier B.V.
    view abstract10.1016/j.memsci.2016.10.037
  • Special Focus on Next Generation Coatings for Gas Turbines
    Lau, Y.C. and Dorfman, M. and Li, L. and Vaßen, R.
    Journal of Thermal Spray Technology 26 (2017)
    view abstract10.1007/s11666-017-0594-x
  • Study of stability of microstructure and residual strain after thermal loading of plasma sprayed YSZ by through surface neutron scanning
    Gibmeier, J. and Back, H.C. and Mutter, M. and Vollert, F. and Rebelo-Kornmeier, J. and Mücke, R. and Vaßen, R.
    Physica B: Condensed Matter (2017)
    Yttria stabilized zirconia (YSZ) is often applied as thermal barrier coating on metal parts as e.g. turbine blades made of nickel base super alloys. The coating process in combination with the preconditioning of the substrate material induces characteristic residual stress distributions in the coating system consisting of topcoat, bondcoat and the substrate material. Knowledge about the residual stress depth distribution in the coating and at the interfaces down to the substrate material is essential for the assessment of the mechanical integrity and the reliability of the coating. In this regard the stability of the microstructure and the residual stresses is of particular interest; hence this forms the scope of our investigations. Yttria (8 wt.%) stabilized zirconia with a NiCoCrAlY bondcoat was deposited by atmospheric plasma spraying (APS) at different spray conditions on a nickel base super alloy substrate material. The coatings were subjected to different heat-treatment processes, i.e. static aging and cyclic thermal loadings. Through surface scanning using neutron diffraction was carried out for the as sprayed condition and for the thermally loaded samples. Based on the measured diffraction data the stability of the microstructure (phases) and the residual strain/stresses through the depths of the coating system were assessed. © 2017 Elsevier B.V.
    view abstract10.1016/j.physb.2017.12.014
  • 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
  • Aging of atmospherically plasma sprayed chromium evaporation barriers
    Vaßen, R. and Grünwald, N. and Marcano, D. and Menzler, N.H. and Mücke, R. and Sebold, D. and Sohn, Y.J. and Guillon, O.
    Surface and Coatings Technology 291 (2016)
    Chromium evaporation barriers are frequently used in solid oxide fuel cells to protect the porous cathode from chromium poisoning. Volatile chromium species are generated at the operation temperature of about 600-900 °C in a humid atmosphere for chromia scale forming steels as interconnect materials. In order to reduce this effect, barrier coatings are applied, often by atmospheric plasma spraying. However, also in these coatings microstructural changes as densification and in parallel formation of large pores have been observed. In order to clarify these mechanisms plasma sprayed Mn1.0 Co1.9Fe0.1O4 ("MCF") are deposited on ferritic steels and furthermore coated with a perovskite based contact layer as used in stack build-up. These coatings are annealed in air up to 1000 h and the microstructural changes and bending of the samples are studied. The results show increasing bending with increasing aging time. High temperature curvature measurements indicate that the amount of bending is not significantly dependent on temperature. As an explanation the creep deformation of the substrate/coating system at high temperatures due to compressive stress levels in the coating is given. The origin of the stress is related to phase changes in combination with the oxidation of the coatings. In addition, interdiffusion and densification processes are discussed. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.02.005
  • 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
  • Gadolinium zirconate/YSZ thermal barrier coatings: Mixed-mode interfacial fracture toughness and sintering behavior
    Frommherz, M. and Scholz, A. and Oechsner, M. and Bakan, E. and Vaßen, R.
    Surface and Coatings Technology 286 (2016)
    In this work, the delamination toughness and the sintering behavior of modern double-layer thermal barrier coatings of type gadolinium zirconate (GZO)/yttrium-stabilized zirconia (YSZ) are investigated in detail. These properties mainly determine the strain tolerance and thus the performance of thermal barrier coatings (TBCs). The delamination toughness was determined using a modified four-point bending setup. It is shown that the delamination behavior differs significantly from conventional monolayer coatings and is highly dependent on the specific microstructure of the GZO layer. The stiffness and sintering behavior of freestanding GZO layers were determined using impulse excitation, a test method sensitive to the global stiffness of the ceramic coating. The increase in stiffness is thereby correlated to microstructural changes, explicitly the healing of micro-cracks and the sintering of inter-lamellar cracks and unmolten particles. The results reveal that the specific spray structure of GZO has a great influence on the sinter stability which results in a characteristic temperature dependency. In this case the GZO coatings have an advantage in comparison to conventional YSZ coatings at temperatures above 1300 °C. © 2015 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2015.12.012
  • 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
  • Near Net Shaping of Monolithic and Composite MAX Phases by Injection Molding
    Gonzalez-Julian, J. and Classen, L. and Bram, M. and Vaßen, R. and Guillon, O.
    Journal of the American Ceramic Society 99 (2016)
    Injection molding of Ti3SiC2, Ti2AlC, and Ti2AlC containing 10 wt% of SiC fibers has been investigated to enable the production of reliable complex near net shaped components. Gear wheels with 9 cogs, 32 mm external diameter, and 4 mm thickness have been successfully produced by injection and sintering, obtaining homogeneous components with the characteristic layered structure of the MAX phases and good surface quality with average roughness below 2 μm. Injection molding of MAX phases is reported for the first time, offering the potential for large-scale production of functional parts for these compounds at affordable prices. © 2016 The American Ceramic Society
    view abstract10.1111/jace.14466
  • Probabilistic lifetime model for atmospherically plasma sprayed thermal barrier coating systems
    Nordhorn, C. and Mücke, R. and Mack, D.E. and Vaßen, R.
    Mechanics of Materials 93 (2016)
    Calculations of atmospherically plasma sprayed thermal barrier coating durability were facilitated by the development of a numerical lifetime model including probabilistic fracture mechanical analyses of thermally induced topcoat stress field evolutions. The stress distributions were determined in finite element analyses taking into account oxide scale growth and topcoat sintering as transient degradation effects. The influence of interface microstructure was investigated by implementing two different interface approximation functions. Subsequent fracture mechanical analyses of subcritical crack growth were performed at numerous different and permanently assigned abstract crack positions. A comparison of the transient energy release rate to its critical value, which depends on crack length and therefore position, results in statistical distributions of system lifetime as a function of simulated thermal cycling conditions. The model was calibrated by presetting an experimental lifetime distribution which was determined in thermal cycling experiments performed at a burner rig facility. The associated cycle-dependent calibration parameter reflects the effect of fracture toughness increase for increasing crack lengths. Experimental reference values for system lifetime were found to be reproduced by the lifetime model. The stress field inversion directly correlated to oxide scale growth rate was identified as the main failure mechanism. The expectation values and standard deviations of the calculated lifetime distributions were found to be in accordance to the experimentally obtained lifetime data and the data scattering typically observed in thermal cycling. © 2015 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.mechmat.2015.11.002
  • 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
  • Evolution of microstructure and mechanical properties of coated Co-base superalloys during heat treatment and thermal exposure
    Webler, R. and Ziener, M. and Neumeier, S. and Terberger, P.J. and Vaßen, R. and Göken, M.
    Materials Science and Engineering A 628 (2015)
    New γ'-strengthened Co-base superalloys show an interesting potential for high temperature applications. However, protective coatings are needed as for Ni-base superalloys to ensure sufficient oxidation and corrosion resistance. Therefore the properties of a commercial coating on a multinary new γ'-strengthened Co-base superalloy have been studied. Especially the influence of the coating process on the substrate also after long term annealing is discussed. It was found that the highly deformed areas at the coating-substrate interface indicated by a high local misorientation and caused by the sandblasting process led to a recrystallization of the interdiffusion zone during the age hardening heat treatment. A chemical gradient of γ and γ' promoting elements was found in the interdiffusion zone causing a change in hardness as measured by nanoindentation. Depending on the composition two separate recrystallized regions formed in the interdiffusion zone, one with single phase γ-(Co,Ni) and the other with a cellular two phase microstructure of discontinuously grown γ and γ'. © 2015 Elsevier B.V.
    view abstract10.1016/j.msea.2015.01.060
  • Influence of titanium nitride interlayer on the morphology, structure and electrochemical performance of magnetron-sputtered lithium iron phosphate thin films
    Bünting, A. and Uhlenbruck, S. and Dellen, C. and Finsterbusch, M. and Tsai, C.-L. and Sebold, D. and Buchkremer, H.P. and Vaßen, R.
    Journal of Power Sources 281 (2015)
    Pure LiFePO4 (LFP) thin films with different thicknesses are deposited at room temperature by a radio frequency (RF) magnetron-sputtering process. Ti foils with and without titanium nitride (TiN) coating as well as thermally oxidized Si wafers coated with Ti or TiN are used as substrates. In a subsequent annealing step, LiFePO4 thin films are crystallized at 500°C. The interaction between Ti and LiFePO4 as well as between TiN and LiFePO4 is characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX), secondary ion mass spectrometry (SIMS), cyclic voltammetry (CV) and galvanostatic measurements. A severe diffusion of Ti into LiFePO4 is found and leading to the formation of impurity phases which resulting in disturbing crystallization behaviour and rough surfaces. Moreover, 80 nm LiFePO4 thin films do not show the desired electrochemical characteristics when they are deposited on Ti foils directly. By using a TiN interlayer, the diffusion of Ti into LiFePO4 can be blocked resulting in smooth morphologies and improving crystallisation behaviour. Impurity phases do not develop and all samples exhibit the expected electrochemical characteristics. Therefore, TiN is a promising candidate for the use as a current collector in all-solid-state batteries with LiFePO4 electrodes. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2015.02.003
  • Isothermal aging of a γ'-strengthened Co-Al-W alloy coated with vacuum plasma-sprayed MCrAlY bond coats
    Terberger, P.J. and Sebold, D. and Webler, R. and Ziener, M. and Neumeier, S. and Klein, L. and Virtanen, S. and Göken, M. and Vaßen, R.
    Surface and Coatings Technology 276 (2015)
    Cobalt-based superalloys with a γ/γ' microstructure were discovered in 2006 and are currently being investigated as an alternative to nickel-based superalloys for high-temperature, high-load applications in gas turbine blades. They promise a better castability combined with a similar creep strength. Superalloy turbine blades are commonly coated with oxidation resistant bond coats. For this reason their compatibility needs to be studied. Co-9Al-9W specimens with a γ/γ' microstructure were coated with either a nickel-based or cobalt-based MCrAlY bond coat using vacuum plasma spraying. After aging at 900. °C in air for up to 500. h no decomposition of the γ' phase was found in the bulk superalloy. The interdiffusion zone shows several different W-rich topologically close-packed phases arising from the dissolution of the γ' phase in this region. The W-rich phases are identified to be μ phase for both bond coats and R phase for the nickel-based bond coat only. Their total volume is higher for the nickel-based bond coat. Therefore the cobalt-based bond coat is better suited for the Co-based superalloy substrate. Room temperature hardness and Young's modulus were measured using nanoindentation in the initial state and after heat treatment. A significantly higher Young's modulus was found for the cobalt-based bond coat. © 2015 Elsevier B.V..
    view abstract10.1016/j.surfcoat.2015.06.048
  • 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 evaluation of hydrogen desorption kinetics controlled by surface reaction and bulk diffusion for magnesium hydride
    Drozdov, I.V. and Vaßen, R. and Stöver, D.
    RSC Advances 5 (2015)
    The 'shrinking core' model has been applied for the evaluation of hydrogen desorption kinetics during decomposition of magnesium hydride. According to our estimation, the full desorption time is expected to have a quadratic dependence on the size of powder particles, if the bulk diffusion of hydrogen atoms in magnesium is a rate controlling step. However, for the actual diffusion rate for hydrogen in magnesium bulk the diffusion cannot significantly influence the overall desorption kinetics for microand nano-powders. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c4ra08089k
  • 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
  • Preparation and sintering behaviour of La5.4WO12-δ asymmetric membranes with optimised microstructure for hydrogen separation
    Deibert, W. and Ivanova, M. E. and Meulenberg, W. A. and Vassen, R. and Guillon, O.
    Journal of Membrane Science 492 (2015)
    La5.4WO12-delta(LaWO) is a promising membrane candidate for a variety of H-2-related applications due to its appreciable levels of mixed proton electron conduction and its stability in moist reducing atmospheres at elevated temperatures. Governed by Wagner theory, the H-2 permeation performance of a membrane can be enhanced by reducing its thickness. Therefore, the present work deals with preparing LaWO supported membranes with reduced thickness and optimised microstructure. Combining a dense membrane with a porous supporting layer is associated with mismatched sintering rates, which ultimately lead to bending effects. Therefore, the sintering behaviour of both the dense membrane and the porous substrate must be carefully adjusted to each other. For this purpose, single and co-fired membrane and substrate layers were produced by tape casting. Sintering experiments were carried out with an optical dilatometer. The shrinkage and microstructural evolution of the layers were evaluated in terms of the anisotropic shrinkage forces and the membrane rigidness counteracting the substrate shrinkage. The results were used to develop asymmetric LaWO membranes with optimal microstructure. High membrane density was combined with a substrate porosity of similar to 30% and minimised bending (40 pm). The LaWO membrane-substrate assembly displayed a He leakage of 10(-5) hPa dm(3), cm(-2) s(-1), which is a value that satisfies further practical demands. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.
    view abstract10.1016/j.memsci.2015.05.065
  • Redox-stable high-performance thin-film solid oxide fuel cell
    Keuter, T. and Roehrens, D. and Menzler, N.H. and Vaßen, R.
    ECS Transactions 68 (2015)
    In this work, a mechanically redox-stable SOFC with a 1 μm thin-film sol-gel electrolyte is presented. With this electrolyte a power output larger than 1.25 W/cm2 at 0.7 V and an operating temperature of 600°C could be demonstrated. Half cells were re-oxidized in excess air, in order to test the redox stability of these SOFCs. No cracks were found in the sol-gel electrolyte after re-oxidation for 4 hours at 600°C and 30 minutes at 800°C, respectively. Due to the fact, that the energy release rate is proportional to the thickness of the thin-film, a thinner film is more stable against cracking than a thicker film at constant tensile stresses. The SOFC with the thin-film sol-gel electrolyte can be considered as stable against re-oxidation, because the long re-oxidation time of 4 hours at an operating temperature of 600°C is unlikely to happen under real conditions. © The Electrochemical Society.
    view abstract10.1149/06801.2001ecst
  • Scale Formation of Alloy 602 CA During Isothermal Oxidation at 800-1100 A degrees C in Different Types of Water Vapor Containing Atmospheres
    Schiek, M. and Niewolak, L. and Nowak, W. and Meier, G. H. and Vassen, R. and Quadakkers, W. J.
    Oxidation of Metals 84 (2015)
    The oxidation behavior of the nickel base Alloy 602 CA in atmospheres, relevant to gas separation units in fossil fired power plants, was investigated in the temperature range 800 to 1100 A degrees C. Isothermal oxidation kinetics were determined in Ar-7 %H2O, Ar-4 %H-2-7 %H2O, Ar-20 %O-2-7 %H2O and, for comparative purposes, in Ar-20 %O-2. The alloy formed an external alumina scale during oxidation at 800 A degrees C, regardless of the atmosphere. Increasing the temperature results in gradual replacement of the alumina scale by an external chromia layer and internal alumina precipitates. The chromia growth rate is affected by the gas composition, due to titanium incorporation in the scale and outwardly protruding metallic nodules. The external alumina scale formation was strongly affected by surface cold work during specimen grinding. This effect, which promoted external alumina scale formation, was retained during exposure at 800 A degrees C but was rapidly lost at higher temperatures resulting in external chromia scale formation and internal oxidation of aluminum.
    view abstract10.1007/s11085-015-9595-7
  • Three-Dimensional, Fibrous Lithium Iron Phosphate Structures Deposited by Magnetron Sputtering
    Bünting, A. and Uhlenbruck, S. and Sebold, D. and Buchkremer, H.P. and Vaßen, R.
    ACS Applied Materials and Interfaces 7 (2015)
    Crystalline, three-dimensional (3D) structured lithium iron phosphate (LiFePO4) thin films with additional carbon are fabricated by a radio frequency (RF) magnetron-sputtering process in a single step. The 3D structured thin films are obtained at deposition temperatures of 600 °C and deposition times longer than 60 min by using a conventional sputtering setup. In contrast to glancing angle deposition (GLAD) techniques, no tilting of the substrate is required. Thin films are characterized by X-ray diffraction (XRD), Raman spectrospcopy, scanning electron microscopy (SEM), cyclic voltammetry (CV), and galvanostatic charging and discharging. The structured LiFePO4 + C thin films consist of fibers that grow perpendicular to the substrate surface. The fibers have diameters up to 500 nm and crystallize in the desired olivine structure. The 3D structured thin films have superior electrochemical properties compared with dense two-dimensional (2D) LiFePO4 thin films and are, hence, very promising for application in 3D microbatteries. © 2015 American Chemical Society.
    view abstract10.1021/acsami.5b07090
  • Ceramic materials for H2 transport membranes applicable for gas separation under coal-gasification-related conditions
    van Holt, D. and Forster, E. and Ivanova, M.E. and Meulenberg, W.A. and Müller, M. and Baumann, S. and Vaßen, R.
    Journal of the European Ceramic Society 34 (2014)
    This work focuses on the synthesis, characterization and testing of mixed protonic-electronic conducting membrane materials for H2 separation from gas mixtures capable of operating in a membrane reactor at temperatures higher than 600°C. La5.5WO12-δ and selected substituted barium zirconates with stoichiometries BaCe0.5Zr0.4Y0.1O3-δ and BaCe0.2Zr0.7Yb0.08Ni0.02O3-δ were therefore characterized and tested under coal-gasification-related conditions at 600-900°C. Sintered samples of the synthesized substituted barium zirconates were characterized by measuring the total conductivity and the thermal expansion coefficients. Also particle size distributions, BET surface-areas and elemental analysis of the starting powders, including commercial La5.5WO12-δ were specified. The compounds were exposed to syngas with steam, as well as to an atmosphere mainly consisting of CO2. The microstructure and phase composition of the membrane materials were studied by SEM, EDX and XRD before and after exposure. BaCe0.2Zr0.7Yb0.08Ni0.02O3-δ shows a very promising chemical stability from 600°C to 900°C and La5.5WO12-δ at 900°C. © 2014 Elsevier Ltd.
    view abstract10.1016/j.jeurceramsoc.2014.03.001
  • 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
  • Deposition and oxidation of oxide-dispersed CoNiCrAlY bondcoats
    Okada, M. and Vassen, R. and Karger, M. and Sebold, D. and Mack, D. and Jarligo, M.O. and Bozza, F.
    Journal of Thermal Spray Technology 23 (2014)
    CoNiCrAlY powder and nano-size alumina powder were milled by a high-energy-attrition ball-mill, and an oxide-dispersed powder was produced with a mixed structure of metal and alumina in each particle. The oxide-dispersed bond coat powder was deposited by HVOF. Pores, however, were observed in the coating since the alumina was deposited without sufficient melting. Isothermal oxidation tests were carried out for the bond coat specimens at a temperature of 1373 K up to 1000 h in air. As a result, oxidation proceeded inside the coating, since oxygen penetrated through pores formed in the dispersed alumina. However, the authors find that another deposition using higher power levels led to a bond coat without pores. A commercially available oxide-dispersed CoNiCrAlY powder was also deposited by HVOF and VPS, and isothermal oxidation tests were performed. The analysis clarifies that the HVOF bond coat exhibited the thinnest thermally grown oxide than those of the VPS bond coat and conventional metallic bond coat. Furnace cycling tests were conducted using the specimens with an additional ceramic thermal-barrier coating. The specimen with the bond coat sprayed by VPS using commercial oxide-dispersed powder showed almost same number of cycles to delamination compared with the specimen with the conventional metal bond coat. © 2013 ASM International.
    view abstract10.1007/s11666-013-0037-2
  • Effect of processing parameters on MCrAlY bondcoat roughness and lifetime of APS-TBC systems
    Nowak, W. and Naumenko, D. and Mor, G. and Mor, F. and Mack, D.E. and Vassen, R. and Singheiser, L. and Quadakkers, W.J.
    Surface and Coatings Technology 260 (2014)
    In the present work it is shown that the lifetime of air plasma sprayed thermal barrier coatings systems with high velocity oxyfuel bondcoats can be substantially extended by application of a thin air plasma sprayed "flashcoat" layer onto the base, high velocity oxyfuel bondcoat. This approach allows improvement of the bondcoat roughness profile to the extent typically obtained by optimized vacuum plasma spraying. The oxidation resistance of the flashcoat strongly depends on the spray parameters used. Deviation from the set of the optimized spray parameters was found to result in early catastrophic oxidation of the flashcoat and corresponding very rapid thermal barrier coating-failure. In order to explain the thermal barrier coating lifetime variations between the various bondcoat morphologies, the roughness profiles of high velocity oxyfuel, vacuum plasma sprayed and air plasma sprayed bondcoats were evaluated using fractal analysis. It is suggested that such an approach provides better correlation between the bondcoat morphology and thermal barrier coating-lifetime than the calculation of the arithmetic mean roughness (Ra) of the bondcoat frequently used in industrial practice. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.06.075
  • Effects of thermal cycling parameters on residual stresses in alumina scales of CoNiCrAlY and NiCoCrAlY bond coats
    Nordhorn, C. and Mücke, R. and Unocic, K.A. and Lance, M.J. and Pint, B.A. and Vaßen, R.
    Surface and Coatings Technology 258 (2014)
    Furnace cycling experiments were performed on free-standing high-velocity oxygen-fuel bond coat samples to investigate the effect of material composition, surface texture, and cycling conditions on the average stresses in the formed oxide scales after cooling. The oxide scale thicknesses were determined by SEM image analyses and information about the stresses were acquired by photo-stimulated luminescence-spectroscopy. Additionally, the scale thickness dependent stress fields were calculated in finite-element analyses including approximation functions for the surface roughness derived on the basis of profilometry data. The evolution of the average residual stress as a function of oxide scale thickness was subject to stochastic fluctuations predominantly caused by local scale spallations. In comparison to the supplemental modeling results, thermal stresses due to mismatch of thermal expansion coefficients are identified as the main contribution to the residual stresses. The theoretical results emphasize that analyses of spectroscopic data acquired for average stress investigations of alumina scales rely on detailed information about microstructural features. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.08.028
  • 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
  • High-precision green densities of thick films and their correlation with powder, ink, and film properties
    Mücke, R. and Büchler, O. and Menzler, N.H. and Lindl, B. and Vaßen, R. and Buchkremer, H.P.
    Journal of the European Ceramic Society 34 (2014)
    A precise geometrical method employing optical profilometry for green density measurements of thick films is presented that provides a typical reproducibility of 0.1-0.2% theoretical density (TD) and a measurement uncertainty of 0.2-0.4% TD for layer thicknesses of around 50. μm. The procedure can be applied for all thick films with a dried thickness of 10. μm or greater. In a case study, the green densities of screen-printed zirconia layers were investigated as a function of the starting powders (grain sizes from 0.1 to 0.4. μm), the solid content, the chain length of ethyl cellulose as binder and its concentration, and two different dispersants and their concentration. Rheological ink properties, surface roughness, drying stresses from deflection measurements, the mechanical properties of green films, and the equivalent compaction pressure were measured and correlated with the green density data. Compressive binder forces and lubrication effects dominated the packing of the particles. © 2014 Elsevier Ltd.
    view abstract10.1016/j.jeurceramsoc.2014.04.012
  • 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
  • Multi-layer thin-film electrolytes for metal supported solid oxide fuel cells
    Haydn, M. and Ortner, K. and Franco, T. and Uhlenbruck, S. and Menzler, N.H. and Stöver, D. and Bräuer, G. and Venskutonis, A. and Sigl, L.S. and Buchkremer, H.-P. and Vaßen, R.
    Journal of Power Sources 256 (2014)
    A key to the development of metal-supported solid oxide fuel cells (MSCs) is the manufacturing of gas-tight thin-film electrolytes, which separate the cathode from the anode. This paper focuses the electrolyte manufacturing on the basis of 8YSZ (8 mol.-% Y2O3 stabilized ZrO2). The electrolyte layers are applied by a physical vapor deposition (PVD) gas flow sputtering (GFS) process. The gas-tightness of the electrolyte is significantly improved when sequential oxidic and metallic thin-film multi-layers are deposited, which interrupt the columnar grain structure of single-layer electrolytes. Such electrolytes with two or eight oxide/metal layers and a total thickness of about 4 μm obtain leakage rates of less than 3 × 10 -4 hPa dm3 s-1 cm-2 (Δp: 100 hPa) at room temperature and therefore fulfill the gas tightness requirements. They are also highly tolerant with respect to surface flaws and particulate impurities which can be present on the graded anode underground. MSC cell tests with double-layer and multilayer electrolytes feature high power densities more than 1.4 W cm-2 at 850 C and underline the high potential of MSC cells. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2014.01.043
  • Oxygen transport through supported Ba0.5Sr0.5Co 0.8Fe0.2O3-δ membranes
    Niehoff, P. and Baumann, S. and Schulze-Küppers, F. and Bradley, R.S. and Shapiro, I. and Meulenberg, W.A. and Withers, P.J. and Vaßen, R.
    Separation and Purification Technology 121 (2014)
    The oxygen transport through supported membranes made of Ba 0.5Sr0.5Co0.8Fe0.2O 3-δ is investigated. For this, disc shaped membranes were manufactured by means of tape casting, consisting of a gastight layer with varying thickness (0.9 mm-20 μm) and a porous support with varying porosity (34%/41%). The sample's microstructure was analyzed using SEM and X-ray computer tomography and by this means characteristic values (i.e., porosity, tortuosity, and specific surface area) were determined. A modeling concept was developed based on literature approaches, extending the Wagner equation for bulk transfer with a geometrical factor β for the characteristic thickness accounting active supports and different surface microstructures. The results were compared with permeation measurements of samples under varying operation conditions (i.e., sweep flow rate, and feed gas). As a result, good agreement between model and measurement in case of a constant porosity is found for characteristic thicknesses Lc as reported in literature. However, calculations with varying porosity show indistinguishable results, indicating an underestimate of the geometric factor versus the influence of the characteristic thickness L c. Also, significant limitations of the oxygen permeation due to surface exchange and concentration polarization in the support is shown. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.seppur.2013.07.002
  • 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
  • Simulation of the effect of realistic surface textures on thermally induced topcoat stress fields by two-dimensional interface functions
    Nordhorn, C. and Mücke, R. and Vaßen, R.
    Surface and Coatings Technology 258 (2014)
    The simulation of thermally induced three-dimensional stress fields in multilayer systems with rough interfaces according to the distribution of stress levels by two-dimensional modeling approaches is investigated here by performing a case study on atmospherically plasma-sprayed thermal barrier coating systems. In order to analyze the microstructure effect, the pronounced interface roughness of these systems is simulated with different approximation functions, whose parameters are derived on the basis of measured surface roughness parameters. Finite element analyses of realistic three-dimensional and multiple two-dimensional models were performed ensuring that consistent boundary conditions were established in both cases. These analyses yielded stress distributions as a function of the thickness of a thermally grown oxide layer. In comparison to the reference histogram for the stress distribution in the three-dimensional model, the analyses of the two-dimensional approximation models result in histograms which correctly reflect essential oxide-growth-related features such as stress field inversion and reduction of maximum stress levels. However, these simplifying two-dimensional models do not reflect all the details of the stress distributions. The three-dimensional reference is found to be too complex with respect to the geometric interface features to be replaced by a single two-dimensional approximating function. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.09.032
  • Dense membranes for oxygen and hydrogen separation (DEMOYS): Project overview and first results
    Pinacci, P. and Louradour, E. and Wimbert, L. and Gindrat, M. and Jarligo, M.O. and Vassen, R. and Comite, A. and Serra, J.M. and Jewulski, J. and Mancuso, L. and Chiesa, P. and Prestat, M. and Ivers-Tiffée, E.
    Energy Procedia 37 (2013)
    This paper provides an overview of objectives, structure and first results of the DEMOYS project, financially supported by the European Commission in the frame of the 7th FP - Energy. The project started on May 1, 2010 and brings together fifteen Partners, including three Universities, five Research Organizations and seven Industries. The objective of DEMOYS is the development of thin mixed conducting membranes for O2 and H2 separation by using a new deposition technique "Plasma Spraying - Thin Film" (PS-TF) in combination with nano-porous, highly catalytic layers. © 2013 The Author.
    view abstract10.1016/j.egypro.2013.05.199
  • Design of next generation thermal barrier coatings - Experiments and modelling
    Gupta, M. and Curry, N. and Nylén, P. and Markocsan, N. and Vaßen, R.
    Surface and Coatings Technology 220 (2013)
    Thermal barrier coating (TBC) systems have been used in the gas turbine industry since the 1980s. The future needs both the air and land based turbine industry involve higher operating temperatures with longer lifetime on the component so as to increase power and efficiency of gas turbines. The aim of this study was to meet these future needs by further development of zirconia coatings. The intention was to design a coating system which could be implemented in industry within the next 3. years. Different morphologies of ceramic topcoat were evaluated; using dual layer systems and polymers to generate porosity. Dysprosia stabilised zirconia was also included in this study as a topcoat material along with the state-of-the-art yttria stabilised zirconia (YSZ). High purity powders were selected in this work. Microstructure was assessed with scanning electron microscope and an in-house developed image analysis routine was used to characterise porosity content. Evaluations were carried out using the laser flash technique to measure thermal conductivity. Lifetime was assessed using thermo-cyclic fatigue testing. Finite element analysis was utilised to evaluate thermal-mechanical material behaviour and to design the morphology of the coating with the help of an artificial coating morphology generator through establishment of relationships between microstructure, thermal conductivity and stiffness. It was shown that the combined empirical and numerical approach is an effective tool for developing high performance coatings. The results show that large globular pores and connected cracks inherited within the coating microstructure result in a coating with best performance. A low thermal conductivity coating with twice the lifetime compared to the industrial standard today was fabricated in this work. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.09.015
  • Effect of oxygen content in NiCoCrAlY bondcoat on the lifetimes of EB-PVD and APS thermal barrier coatings
    Song, P. and Naumenko, D. and Vassen, R. and Singheiser, L. and Quadakkers, W.J.
    Surface and Coatings Technology 221 (2013)
    The effect of oxygen content in NiCoCrAlY bondcoat on the cyclic oxidation lifetimes of EB-PVD and APS thermal barrier coatings (TBC) has been studied. The EB-PVD TBC system with an oxygen content of 0.05 wt. % in NiCoCrAlY bondcoat shows five times longer lifetimes compared to a TBC system with 0.2. wt% oxygen in the bondcoat. In the bondcoat with the high oxygen content the minor (0.3. wt.%) yttrium addition was found to be tied up by oxygen into fine precipitates of yttrium aluminates. Thereby the beneficial effect of yttrium onto the adherence of the alumina scale was significantly reduced. The critical scale thickness at failure was by about a factor of two lower for the high oxygen bondcoat than for the low oxygen bondcoat. In contrast to EB-PVD TBC systems, no detrimental effect of increasing oxygen content on the lifetime of APS-TBC systems was observed. This can be explained by a different failure mechanism of APS-TBC systems, whereby the lifetime is mainly determined by the rate of crack propagation through the ceramic topcoat. © 2013 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2013.01.054
  • Failure mechanisms of magnesia alumina spinel abradable coatings under thermal cyclic loading
    Ebert, S. and Mücke, R. and Mack, D. and Vaßen, R. and Stöver, D. and Wobst, T. and Gebhard, S.
    Journal of the European Ceramic Society 33 (2013)
    Abradable coatings have been used in low- and high-pressure sections of jet engine compressors for more than 40 years. Today, they are also used in the high-pressure turbine of jet engines and are gaining more interest for applications in industrial gas turbines. They minimise the clearance between the rotating blade tips and the stationary liners. Aside from being abradable, the coatings have to be mechanically stable and withstand high thermo-mechanical loadings. A typical material used in engines today is yttria-stabilised zirconia (YSZ). This material advantageously combines a suitable thermal conductivity with a high thermal expansion coefficient, but shows a temperature capability limited to 1200 °C in long-term applications. Typical abradable coating thicknesses are above 1. mm. With increasing coating thickness and limited cooling efficiency leading to high surface temperatures, there is a risk of premature failure. As a result, new ceramic materials have been developed with better high-temperature capability. The present work investigates an atmospheric plasma sprayed ceramic double-layer coating system composed of 7YSZ as an intermediate layer and magnesia alumina spinel as a top layer. This double-layer system was sprayed onto disc-shaped Inconel 738 superalloy substrates, which were coated with a vacuum plasma sprayed MCrAlY bondcoat. The lifetime of the coating system was assessed via thermal gradient cycling testing with surface temperatures above 1400 °C. During cycling, the samples showed a typical failure mechanism with exfoliation of thin coating lamellae starting from the coating surface. This failure mechanism was not observed in thermal barrier or abradable coatings in the past. The failure mechanism was analysed and mismatch stress calculations were carried out. © 2013 Elsevier Ltd.
    view abstract10.1016/j.jeurceramsoc.2013.06.021
  • Functionally graded vacuum plasma sprayed and magnetron sputtered tungsten/EUROFER97 interlayers for joints in helium-cooled divertor components
    Weber, T. and Stüber, M. and Ulrich, S. and Vaßen, R. and Basuki, W.W. and Lohmiller, J. and Sittel, W. and Aktaa, J.
    Journal of Nuclear Materials 436 (2013)
    Two coating technologies, magnetron sputtering and vacuum plasma spraying, have been investigated for their capability in producing functionally graded tungsten/EUROFER97 layers. In a first step, non-graded layers with different mixing ratios were deposited on tungsten substrates and characterized by nanoindentation, macroindentation, X-ray diffraction, transmission, Auger and scanning electron microscopy. The thermal stability of the sprayed layers against heat treatments at 800-1100 °C for 60 min was further analyzed. In a second step, the produced functionally graded layers deposited on tungsten substrates were joined to EUROFER97 bulk-material by diffusion bonding. The bonding and the graded joints were microscopically characterized and exposed to thermal cycles between 20 °C and 650 °C. Results from this study show that both coating technologies are ideal for the synthesis of functionally graded tungsten/EUROFER97 coatings. This is important in providing insights for fture development of joints with functionally graded interlayers. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jnucmat.2013.01.286
  • Influence of vacuum heat treatment parameters on the surface composition of MCrAlY coatings
    Keller, I. and Naumenko, D. and Quadakkers, W.J. and Vaßen, R. and Singheiser, L.
    Surface and Coatings Technology 215 (2013)
    In the present study the synergistic effect of vacuum heat treatment pressure and coating Y and O content on the MCrAlY surface oxide is investigated. For this purpose, free standing MCrAlY coatings with two different Y contents were exposed at 1100°C for times between 1 and 5h in vacuum at pressures of < 10-3Pa and 10-2Pa, and in Argon gas at 10-1Pa. The surface scale composition and morphology were analysed with a variety of analysis techniques. It is shown that the composition at the MCrAlY coating surface is mainly governed by two competing processes: Cr evaporation (β-NiAl formation) and Y oxidation (internal and external). The latter reaction has been observed to depend strongly on the Y reservoir in the coating. It can be observed that the chromium evaporation enhances for reducing pressure. More Y and less O in the coating result in enhanced Y rich oxide formation (Y2O3 and YAP; Yttrium Aluminium Perovskite) and decreased chromium evaporation. On rough surfaces β-NiAl enrichment in hills and yttria formation in valleys can be found. Additionally no alumina formation can be observed under all studied vacuum heat treatment conditions. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.09.066
  • 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
  • JTST special issue on "coatings for energy applications"
    Vardelle, A. and Vassen, R.
    Journal of Thermal Spray Technology 22 (2013)
    view abstract10.1007/s11666-013-9936-5
  • La-Sr-Fe-Co oxygen transport membranes on metal supports deposited by low pressure plasma spraying-physical vapour deposition
    Zotov, N. and Baumann, S. and Meulenberg, W.A. and Vaßen, R.
    Journal of Membrane Science 442 (2013)
    Dense La0.6Sr0.4Fe0.8Co0.2O3-δ (LSFC) membranes were successfully prepared on NiCoCrAlY metal supports by low pressure plasma spraying-physical vapour deposition. He leakage rate tests and microstructural analysis by SEM of the as-deposited films showed that a gas-tight membrane layer is achieved. Membrane thicknesses are between 40 and 65μm. The oxygen permeation flux was determined at atmospheric pressure in an air-Ar gradient at different temperatures between 700 and 950°C and compared with 1mm thick LSFC pellet. Although leakage was detected, noteworthy oxygen permeation rate of the supported membranes (~0.3mlcm-2min-1 at 885°C) is reported. © 2013 Elsevier B.V.
    view abstract10.1016/j.memsci.2013.04.016
  • Mechanical properties of zirconia composite ceramics
    Zhang, Y. and Malzbender, J. and Mack, D.E. and Jarligo, M.O. and Cao, X. and Li, Q. and Vaßen, R. and Stöver, D.
    Ceramics International 39 (2013)
    Composite materials based on 8 wt% yttria partially stabilized zirconia, with additions of gadolinium zirconate, lanthanum lithium hexaaluminate, yttrium aluminum garnet and strontium zirconate were characterized. Samples were fabricated by hot-press sintering at 1550° C. The effect of the secondary phase content on the mechanical properties of the composites was evaluated. Hardness, elastic modulus and fracture toughness of the fabricated composites were determined by means of depth-sensitive indentation testing. The fracture toughness of the samples as determined by the indentation method was found to increase with increasing YSZ content, reaching 3 MPa·m0.5 for samples with 80 wt% YSZ. The fracture toughness appeared to be affected by thermal expansion coefficient mismatch, crack bridging and crack deflection. © 2013 Elsevier Ltd and Techna Group S.r.l.
    view abstract10.1016/j.ceramint.2013.03.014
  • Microstructural characterization of porous thermal barrier coatings by IR gas porosimetry and sintering forecasts
    Cernuschi, F. and Golosnoy, I.O. and Bison, P. and Moscatelli, A. and Vassen, R. and Bossmann, H.-P. and Capelli, S.
    Acta Materialia 61 (2013)
    It is known that the thermal diffusivity of plasma sprayed coatings is quite sensitive to the operating conditions, namely the composition and pressure of the operating atmosphere. This makes it possible, in principle, to obtain information, in a non-destructive way, on the microstructure of thermal barrier coatings (TBCs) from thermal diffusivity measurements. An experimental assessment of this idea is presented in this paper. The microstructure of as-sprayed and sintered free-standing atmospheric plasma sprayed YPSZ TBC samples have been characterized by a new technique, named infrared (IR) gas porosimetry, as well as by mercury intrusion porosimetry and image analysis. The results from these different techniques have been compared. Furthermore, the microstructure and thermal diffusivity of sintered samples have been compared with the sintering forecasts produced by Cipitria's sintering code coupled with the Bruggeman model for thermal diffusivity estimation. Sample-to-sample variations have been discussed and uncertainties in experimental techniques have been analysed. Conditions for accurate microstructural estimations have been obtained and suggestions on the reliability of the statistical evaluation are provided. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.09.055
  • 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
  • Thermal Barrier Coatings
    Vaßen, R.
    Ceramics Science and Technology 4-4 (2013)
    Thermal barrier coatings (TBCs) are typically a duplex-type coating system which consists of a so-called metallic bond coat and a ceramic topcoat. The bond coat protects the substrates from oxidative and corrosive attack and improves the bonding between ceramic topcoat and substrate. This chapter focuses on the atmospheric plasma spraying (APS)-type TBCs that are frequently used in combustion chambers or on the vanes and blades of stationary engines. This more widespread use of APS TBCs mainly results from the much higher costs involved with the production of electron beam-physical vapor deposition (EB-PVD) coatings, which requires significantly more expensive equipment and higher investment costs. The chapter provides some details of the effort to seek ceramics that replace yttria-stabilized zirconia (YSZ) TBCs. It explains the two major manufacturing routes for TBCs namely EB-PVD and APS. The chapter discusses some basic properties of YSZ-based TBCS and the failure of YSZ-based TBC systems. © 2013 Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved.
    view abstract10.1002/9783527631940.ch52
  • 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
  • Component interactions after long-term operation of an SOFC stack with LSM cathode
    Malzbender, J. and Batfalsky, P. and Vaßen, R. and Shemet, V. and Tietz, F.
    Journal of Power Sources 201 (2012)
    The reliable long-term operation of stacks with a low degradation rate is a prerequisite for the commercialization of solid oxide fuel cell (SOFC) technology. A detailed post-test analysis of stacks is of major importance in understanding degradation mechanisms. Here the results are reported of a post-test analysis of an SOFC stack with anode supported cells with Ni/YSZ anode, 8YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode operated under steady-state conditions for 19,000 h. In particular, the microstructural and chemical analyses of the relevant metallic and ceramic components are reported. The interconnects were coated with a (Mn,Co,Fe) 3O 4 spinel by atmospheric plasma spraying, which prevented Cr evaporating into the cathode compartment. The diffusion of Mn from the (La,Sr)MnO 3 cathode into the 8YSZ electrolyte led to local enrichment at grain boundaries, which might have been responsible for the degradation via electronic pathways leading to partial short-circuiting across the electrolyte. However, the ultimate failure of the stack was the result of a weakening and fracture of the 8YSZ electrolyte along grain boundaries due to the local Mn enrichment. © 2011 Elsevier B.V.
    view abstract10.1016/j.jpowsour.2011.10.117
  • Damage characterization of thermal barrier coatings by acoustic emission and thermography
    Nies, D. and Rehmer, B. and Skrotzki, B. and Vaßen, R.
    Advanced Engineering Materials 14 (2012)
    Thermal barrier coatings allow increasing the operating temperature and efficiency of land-, sea-, or air-based turbines. As failure of the coating may result in serious damage of the turbine, reliable estimation of its lifetime is essential. To assess the lifetime, cyclic tests are conceived to combine thermal loading by heating the surface of the coating with laser irradiation and nondestructive methods for damage determination. Using laser irradiation allows a high reproducibility of the thermal load. The temperature of the sample surface during thermal loading is determined by an infrared-camera which also enables the possibility to detect damage in the coating via thermography. Additionally, four acoustic sensors, attached to the experimental setup, are used to detect damage in the sample and determine the source of acoustic events. Results of acoustic emission correlate well with thermographic images that visualize the formation and evolution of damage through delaminations in the samples. Thermal fatigue tests of thermal barrier coatings (TBCs) by heating the surface with laser irradiation were complemented by nondestructive methods. A comparison is made between results of acoustic emission and thermographic images in order to visualize the formation and evolution of damage through delamination in the samples. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200107
  • 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
  • HVOF spraying of Fe-based MMC coatings with in situ formation of hard particles by hot isostatic pressing
    Röttger, A. and Weber, S.L. and Theisen, W. and Rajasekaran, B. and Vaßen, R.
    Journal of Thermal Spray Technology 21 (2012)
    Thick (2-3 mm) Fe-base coatings with admixed ferrotitanium (Fe 30Ti 70) were applied to austenitic steel by a high-velocity oxy-fuel process (HVOF). Hot-isostatic pressing (HIP) was carried out to the decrease porosity and to increase the material strength, wear resistance, and adhesive bond strength of the deposited coating to the substrate material. SEM and XRD investigations confirmed the formation of hard titanium carbide (TiC) particles during HIP treatment as a result of strong carbon diffusion out of the metal matrix and into the Fe 30Ti 70 particles. The mechanical and wear properties of the densified coatings were investigated by means of shear tests, hardness measurements, and abrasive wear tests. A comparison of the coatings in the as-sprayed and the HIPed state showed a large increase in the wear resistance due to in situ TiC formation. © ASM International.
    view abstract10.1007/s11666-012-9736-3
  • 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
  • Processing of Ceramics by Plasma Spraying
    Vaßen, R.
    Ceramics and Composites Processing Methods (2012)
    view abstract10.1002/9781118176665.ch16
  • Status of solid oxide fuel cell development at forschungszentrum jülich
    Menzler, N.H. and Blum, L. and Buchkremer, H.P. and Groß, S.M. and De Haart, L.G.J. and Malzbender, J. and Mücke, R. and Quadakkers, W.J. and Peksen, M. and Peters, R. and Remmel, J. and Steinberger-Wilckens, R. and Tietz, F. and Uhlenbruck, S. and Vaßen, R.
    Procedia Engineering 44 (2012)
    view abstract10.1016/j.proeng.2012.08.432
  • 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
  • Suspension and air plasma-sprayed ceramic thermal barrier coatings with high infrared reflectance
    Stuke, A. and Kassner, H. and Marqués, J.-L. and Vassen, R. and Stöver, D. and Carius, R.
    International Journal of Applied Ceramic Technology 9 (2012)
    Yttria partially stabilized zirconia (YSZ) coatings are widely used for thermal barrier coatings (TBCs) to increase operating temperature of gas turbines. In the wavelength range where most of the radiation by walls and combustion gas is emitted within the gas turbine YSZ is semitransparent leading to increasing radiation heat flows into the components at increasing service temperatures. The objective of this work is to optimize the diffuse reflectance of plasma-sprayed TBCs by improving the coating microstructure such that the reflectance of radiation is increased. As a result, a more efficient thermal screening of the underlying metallic substrate is achieved. In this work, air plasma-sprayed and suspension plasma-sprayed (SPS) coatings of 7% YSZ using powder of different grain size distributions and different spray parameters were deposited. The reflectance and transmittance has been investigated in the wavelength range from 0.3 to 2.5 μm. The SPS-coatings showed the highest reflectance up to 94% at 1.5 μm wavelength. In addition, the scattering and absorption coefficients of the sprayed TBCs calculated with the Kubelka-Munk two flux model showed strong correlation with the measured porosity. By improving the microstructure, we were able to reduce thermal conductivity while increasing scattering of radiation, resulting in lower heat flow and lower temperature at the metallic substrate. These results are strengthened by numerical calculations. © 2012 American Ceramic Society and Wiley Periodicals, Inc.
    view abstract10.1111/j.1744-7402.2011.0202689.x
  • Testing and evaluation of thermal-barrier coatings
    Vaßen, R. and Kagawa, Y. and Subramanian, R. and Zombo, P. and Zhu, D.
    MRS Bulletin 37 (2012)
    Thermal-barrier coatings are complex systems with properties that largely depend on their specific microstructure. Their properties change during operation, typically leading to degradation. A further difficulty arises from the fact that this degradation also depends on specific loading conditions that can be rather complex. Different laboratory setups are described that simulate, at least partially, the actual loading conditions. In addition, sensing and nondestructive methods are described that are targeted toward reliable operation of a gas-turbine engine with thermal-barrier coated components. © 2012 Materials Research Society.
    view abstract10.1557/mrs.2012.235
  • Atmospheric plasma sprayed thermal barrier coatings with high segmentation crack densities: Spraying process, microstructure and thermal cycling behavior
    Karger, M. and Vaßen, R. and Stöver, D.
    Surface and Coatings Technology 206 (2011)
    Thermal barrier coatings (TBCs) with high strain tolerance are favorable for application in hot gas sections of aircraft turbines. To improve the strain tolerance of atmospheric plasma sprayed (APS) TBCs, 400μm-500μm thick coatings with very high segmentation crack densities produced with fused and crushed yttria stabilized zirconia (YSZ) were developed. Using a Triplex II plasma gun and an optimized spraying process, coatings with segmentation crack densities up to 8.9cracksmm-1, and porosity values lower than 6% were obtained. The density of branching cracks was quite low which is inevitable for a good inter-lamellar bonding. Thermal cycling tests yielded promising strain tolerance behavior for the manufactured coatings. Samples with high segmentation crack densities revealed promising lifetime in burner rig tests at rather high surface (1350. °C) and bondcoat temperatures (up to 1085. °C), while coatings with lower crack densities had a reduced performance. Microstructural investigations on cross-sections and fracture surfaces showed that the segmentation crack network was stable during thermal shock testing for different crack densities. The main failure mechanism was delamination and horizontal cracking within the TBC near the thermal grown oxide layer (TGOs) and the TBC. © 2011 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2011.06.032
  • 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
  • Diffusion and phase transformation at the interface between an austenitic substrate and a thermally sprayed coating of ledeburitic cold-work tool steel
    Röttger, A. and Weber, S. and Theisen, W. and Rajasekeran, B. and Vaßen, R.
    Steel Research International 82 (2011)
    An overlay coating against wear or corrosion on components is required for various technical applications. Thermal spraying is a well-established and near-net-shape deposition method. In this work, high-velocity oxy-fuel spraying of two different ledeburitic cold-work tool steels was employed to produce wear-resistant Fe-base coatings on a stainless steel substrate. This work focuses on the investigation of diffusion processes across the coating/substrate interface. Specimens were heat-treated for different dwell times and then analyzed by means of EBSD, XRD, OM, as well as SEM. Results of phase formation and diffusion profiles were compared with equilibrium and diffusion calculations obtained with ThermoCALC® and DICTRA®. The influence of diffusion processes across the coating/substrate interface on the mechanical properties, such as adhesive bond strength and hardness, was investigated by shear tests and microhardness profiles. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201000199
  • Emerging and innovative processes in thermal spraying
    Vardelle, A. and Vassen, R.
    Journal of Thermal Spray Technology 20 (2011)
    view abstract10.1007/s11666-011-9656-7
  • 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
  • Mechanical properties of thermally sprayed Fe based coatings
    Röttger, A. and Weber, S. and Theisen, W. and Rajasekeran, B. and Vassen, R.
    Materials Science and Technology 27 (2011)
    An additional coating against wear or corrosion on component parts is required for many applications. These coatings protect the substrate material against external influences, thus increasing the economic lifetime of the component. Coating processes such as build-up welding and thermal spraying are well established and commonly used. The thermal spray process, in particular, permits deposition of metals, ceramics, or cermets materials to produce near net shape coatings on complex surface geometries. However, commonly used coating materials suffer from high raw material costs, thus decreasing the cost effectiveness of the coating process. Fe based materials are low priced and possess noteworthy mechanical properties; they thus provide the possibility of substituting the expensive Ni and Co based materials commonly used for thermal spray processes. In this work, 2 mm thick high velocity oxyfuel sprayed Fe based coatings in the as sprayed and thermally sprayed and hot isostatic pressed condition were investigated with respect to their mechanical and wear properties. Additionally, the fracture surface was investigated by scanning electron microscopy to characterise the fracture behaviour. It could be demonstrated that the substrate and the heat treatment have the greatest impact on the shear strength of thermally sprayed cold work tool steel. It is shown that the substrate materials as well as the heat treatment are promoting diffusion processes across the interface between the coating and the substrate. Hence, a material integrated bond is formed. The microstructures of the thermally sprayed coatings become more important regarding the mechanisms of failure of the four point bending tests. The material strength is influenced by quenching and tempering and the specimen deflection is influenced by diffusion reactions induced by hot isostatic pressing treatment. The thermally sprayed coatings in the as sprayed condition feature the highest wear resistance due to their hardness. © 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.
    view abstract10.1179/1743284710Y.0000000002
  • 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
  • Solid particle erosion of thermal spray and physical vapour deposition thermal barrier coatings
    Cernuschi, F. and Lorenzoni, L. and Capelli, S. and Guardamagna, C. and Karger, M. and Vaßen, R. and von Niessen, K. and Markocsan, N. and Menuey, J. and Giolli, C.
    Wear 271 (2011)
    Thermal barrier coatings (TBC) are used to protect hot path components of gas turbines from hot combustion gases. For a number of decades, in the case of aero engines TBCs are usually deposited by electron beam physical vapour deposition (EB-PVD). EB-PVD coatings have a columnar microstructure that guarantees high strain compliance and better solid particle erosion than PS TBCs. The main drawback of EB-PVD coating is the deposition cost that is higher than that of air plasma sprayed (APS) TBC. The major scientific and technical objective of the UE project TOPPCOAT was the development of improved TBC systems using advanced bonding concepts in combination with additional protective functional coatings. The first specific objective was to use these developments to provide a significant improvement to state-of-the-art APS coatings and hence provide a cost-effective alternative to EB-PVD. In this perspective one standard porous APS, two segmented APS, one EB-PVD and one PS-PVD™ were tested at 700°C in a solid particle erosion jet tester, with EB-PVD and standard porous APS being the two reference systems.Tests were performed at impingement angles of 30° and 90°, representative for particle impingement on trailing and leading edges of gas turbine blades and vanes, respectively. Microquartz was chosen as the erodent being one of the main constituents of sand and fly volcanic ashes. After the end of the tests, the TBC microstructure was investigated using electron microscopy to characterise the failure mechanisms taking place in the TBC.It was found that PS-PVD™ and highly segmented TBCs showed erosion rates comparable or better than EB-PVD samples. © 2011 Elsevier B.V.
    view abstract10.1016/j.wear.2011.06.013
  • Stress distributions in plasma-sprayed thermal barrier coatings under thermal cycling in a temperature gradient
    Limarga, A.M. and Vaßen, R. and Clarke, D.R.
    Journal of Applied Mechanics, Transactions ASME 78 (2011)
    The residual stress distribution in plasma-sprayed zirconia thermal barrier coatings subjected to cyclic thermal gradient testing was evaluated using Raman piezospectroscopy and finite element computation. The thermal gradient testing (approximately 440°C/mm at temperature), consisted of repeated front-side heating with a flame and constant cooling of the back-side of the substrate either with front-side radiative cooling only or with additional forced air cooling between the heating cycles. The coatings exhibited characteristic "mud-cracking" with the average crack spacing dependent on the cooling treatment. This is consistent with finite element calculations and Raman spectroscopy measurements in which the sudden drop in coating surface temperature on initial cooling leads to a large biaxial tension at the surface. The key to proper interpretation of the Raman shifts is that the stress-free Raman peaks need to be corrected for shifts associated with the evolution of the metastable tetragonal phase with aging. © 2011 American Society of Mechanical Engineers.
    view abstract10.1115/1.4002209
  • A novel test approach for plasma-sprayed coatings tested simultaneously under CMAS and thermal gradient cycling conditions
    Steinke, T. and Sebold, D. and Mack, D.E. and Vaßen, R. and Stöver, D.
    Surface and Coatings Technology 205 (2010)
    Plasma-sprayed ceramic coatings, used as thermal barrier or abradable coatings in high-pressure stages of gas turbines, are exposed to high thermo-mechanical loading due to harsh operating conditions. Under certain conditions they also have to withstand attack by calcium-magnesium-alumino-silicate (CMAS) deposits resulting from the ingestion of siliceous minerals with the intake air. Resistance to this kind of attack becomes more important at higher temperatures, when the melting temperature of the CMAS deposits is exceeded and a penetration into the coating microstructural features (cracks and pores) can take place. During cooling the CMAS solidifies and the coating loses its strain tolerance, which can lead to coating failure. Although the basic principles of failure seem to be understood, a detailed analysis of the mechanisms and the possibilities of avoiding delamination are still lacking, especially because there are as yet no adequate test beds. This paper investigates the possibility of testing such coatings in a burner rig test facility under thermal gradient cycling conditions and at the same time CMAS deposition. This novel and unique test approach promises a coating screening and characterization test under service conditions. The CMAS rig was established because the test conditions simulated here are closer to actual engine conditions, as compared to previous tests with primarily CMAS deposition and subsequent thermal furnace testing. The experimental setup of this new test approach is described and the applicability is confirmed. Furthermore, a first evaluation for plasma-sprayed coatings is presented. A significantly reduced lifetime was found for samples tested with CMAS attack in comparison to tests with water deposition only. The lifetime was also reduced compared to older results without any corrosive media. A decrease of nearly two orders of magnitude was found. A microstructural evaluation of the coatings is presented and the mechanisms and reasons for the very early failure are discussed. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.09.008
  • 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
  • Corrigendum to "Effect of exposure conditions on the oxidation of MCrAlY-bondcoats and lifetime of thermal barrier coatings" [Surface & Coatings Technology. Volume 204 (2009) p. 820-823] (DOI:10.1016/j.surfcoat.2009.09.056)
    Subanovic, M. and Song, P. and Vassen, R. and Naumenko, D. and Singheiser, L. and Quadakkers, W.J.
    Surface and Coatings Technology 204 (2010)
    view abstract10.1016/j.surfcoat.2009.11.028
  • 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
  • Functionally graded thermal barrier coatings with improved reflectivity and high-temperature capability
    Vassen, R. and Kassner, H. and Stuke, A. and Mack, D.E. and Jarligo, M.O. and Stöver, D.
    Materials Science Forum 631-632 (2010)
    Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat isolative topcoat. In modern TBCs the ceramic topcoat is further divided into layers with different functions. One example is the double layer system in which conventional yttria stabilized zirconia (YSZ) is used as bottom and new materials as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. Examples of such systems will be shown. In modern gas turbines the increased temperatures and gas pressures lead to an increased fraction of radiative heat flow. Coatings with increased reflectivity can be used to avoid the direct heating of the metallic substrates by this radiation. An effective method to produce such coatings is suspension plasma spraying. These reflective coatings are deposited on top of the TBC system and will lead to a further grading and improved performance of the coating. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/MSF.631-632.73
  • Overview on advanced thermal barrier coatings
    Vaßen, R. and Jarligo, M.O. and Steinke, T. and Mack, D.E. and Stöver, D.
    Surface and Coatings Technology 205 (2010)
    During the last decade a number of ceramic materials, mostly oxides have been suggested as new thermal barrier coating (TBC) materials. These new compositions have to compete with the state-of-the-art TBC material yttria stabilized zirconia (YSZ) which turns out to be difficult due to its unique properties. On the other hand YSZ has certain shortcomings especially its limited temperature capability above 1200 °C which necessitates its substitution in advanced gas turbines.In the paper an overview is tried on different new materials covering especially doped zirconia, pyrochlores, perovskites, and aluminates. Literature results and also results from our own investigations will be presented and compared to the requirements. Finally, the double-layer concept, a method to overcome the limited toughness of new TBC materials, will be discussed. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.08.151
  • 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
  • Thermal-gradient testing of thermal barrier coatings under simultaneous attack by molten glassy deposits and its mitigation
    Drexler, J.M. and Aygun, A. and Li, D. and Vaßen, R. and Steinke, T. and Padture, N.P.
    Surface and Coatings Technology 204 (2010)
    Degradation of thermal barrier coatings (TBCs) in gas-turbine engines by molten calcium-magnesium-aluminosilicate (CMAS) glassy deposits is becoming a pressing issue, as engines are required to operate under increasingly harsh conditions. While new approaches for mitigating CMAS attack of TBCs are starting to emerge, there is a need for appropriate tests for evaluating the efficacy of those approaches. To that end, we present here a new thermal-cycling test for the evaluation of TBC performance, where a thermal gradient is applied across the TBC, with simultaneous injection of CMAS. The conditions simulated in this new test are closer to actual conditions in an engine, as compared to the conventional furnace test without thermal gradient. This new test is applied to a TBC with a new composition designed for mitigating CMAS attack, where the mitigation mechanisms are observed to be similar to those found in the conventional furnace test. However, based on a previously established mechanics model it is shown here that thermal-gradient cyclic testing is essential for evaluating thermomechanical performance of TBCs under attack by CMAS, and that the use of just the conventional furnace test can lead to the underestimation of the TBC performance. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.02.026
  • 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
  • ceramic materials

  • microstructure

  • physical vapor deposition

  • plasma jets

  • thermal barrier coatings

  • thermal spraying

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