JTTEE5 DOI: 10.1007/s11666-015-0254-y 1059-9630/$19.00  ASM International

N. Schlegel, D. Sebold, Y.J. Sohn, G. Mauer, and R. Vaßen (Submitted January 30, 2015; in revised form March 11, 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.

Keywords

columnar microstructure, perovskite ceramic, suspension plasma spraying (SPS), thermal barrier coating, thermal cycling test

1. Introduction Current research in fields of thermal barrier coatings (TBCs) for gas turbines in the aircraft and power generation industry deals with both, the improvement of the TBCsÕ microstructure, and the development of new materials. Conventionally, yttria partially stabilized zirconia (YSZ) is used for these application. However, YSZ has some shortcomings in long-term operation at temperatures higher than 1200 C. Under these conditions, phase transformations and sintering effects can lead to the failure of the entire system (Ref 1). Therefore a lot of investigations are performed to develop new materials (Ref 2-5), which provide high melting points and which do not undergo detrimental phase transformations at temperatures higher than 1200 C. They have to be thermomechanically and thermochemically compatible with the substrate material. Furthermore, no detrimental phase transformations during the plasma spraying process must occur. Often

N. Schlegel, D. Sebold, Y.J. Sohn, G. Mauer, and R. Vaßen, Institut fu¨r Energie- und Klimaforschung - Werkstoffsynthese und Herstellungsverfahren (IEK-1), Forschungszentrum Ju¨lich GmbH, 52425 Ju¨lich, Germany. Contact e-mail: n.schlegel@ fz-juelich.de.

Journal of Thermal Spray Technology

evaporation of volatile oxides is found for these new materials during plasma spraying (Ref 5, 6), because of different vapor pressures of their constituents (Ref 7-10). In general, three material classes show major advantages at high temperatures: aluminates, pyrochlores, and perovskites (Ref 2-5). In this work, the rare-earth perovskite La(Al1/4Mg1/2Ta1/4)O3 (LAMT) was investigated. The bulk material has a thermal expansion coefficient of ~10 9 10 6 1/K (at 1000 C, Ref 11), almost similar to that of YSZ. It provides a thermal conductivity of ~2 W/ (m K) (at 1000 C, Ref 11), which is lower than the thermal conductivity of the 3YSZ bulk material (~2.4 W/ (m K) at 1000 C, Ref 11) and that of some other perovskite-type bulk materials, namely SrHfO3 (~2.4 W/ (m K) at 1000 C, Ref 12), BaHfO3 (~4.7 W/(m K) at 1000 C, Ref 12) and Ba(Mg1/3Ta2/3)O3 (~2.8 W/(m K) at 1000 C, Ref 11). In contrast to the thermal conductivity of bulk pyrochlores (~1.2-2 W/(m K) at 1000 C, Ref 13, 14), the thermal conductivity of LAMT is higher, but as the microstructure of the coatings has a significant influence on the thermal conductivity, LAMT seems to be a suitable material for the application as TBC. In addition to these features, it shows good resistance to CMAS (calcium-magnesium-aluminum-silicate) attack (Ref 5). However, its fracture toughness is low: 0.6 MPa m1/2 (Ref 11). Therefore, it is usually applied as top layer in double-layer systems with YSZ (Ref 11, 15). For its application the suspension plasma spraying (SPS) process was used in order to produce a columnarstructured coating, which provides strain tolerance. In thermal cycling rigs, their potential for TBC application was tested. Microstructures and phase composition of the as-sprayed and of the thermally cycled samples were investigated.

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Cycling Performance of a ColumnarStructured Complex Perovskite in a Temperature Gradient Test

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2. Experimental Procedures 2.1 Powder and Suspension Preparation La(Al1/4Mg1/2Ta1/4)O3 powder was synthesized from stoichiometric amounts of La2O3 (99.9%, Treibacher Industrie AG, Althofen, Austria), Al2O3 (99.8%, Martinswerk GmbH, Bergheim, Germany), MgO (>99%, Sigma Aldrich, St. Louis, MO, USA), and Ta2O5 (98.95%, Treibacher Industrie AG, Althofen, Austria). Ethanol was added, and the suspension was ground mechanically and mixed homogeneously by ball milling (150 min 1, 12 h). Subsequently, the precursors were dried and calcinated (1250 C, 3 h). One more time the powder was activated mechanically by ball milling in an ethanol suspension (150 min 1, 48 h) and dried and calcinated (1600 C, 4 h). The powder was spray-dried, heat treated (1000 C, 2 h) and milled afterward. For phase analysis, x-ray diffraction (XRD) of the obtained powder was performed with a D4 Endeavor diffractometer (Bruker, Karlsruhe, Germany) in Bragg-Brentano geometry using Cu Ka radiation. For SPS experiments, a suspension with 20 wt.% solid load was produced. Therefore, ethanol, YSZ milling balls (d = 3 mm, Sigmund Lindner GmbH, Warmensteinach, Germany) and 4.5 g of the dispersant polyethylenimine (Polysciences, Warrington, PA, USA) were added to the powder and milled on a roller cylinder (120 min 1, 24 h) in order to produce a 30 wt.% suspension. This suspension was diluted to 20 wt.% and homogenized on a roller cylinder. The final suspension had a dynamic viscosity of 1.55 mPa s at a shear rate of 10 s 1 (measured with the viscosimeter Physica MCR 301, Anton Paar Germany GmbH, Ostfildern, Germany) and a d50 of approx. 700 nm (determined by the particle sizer Horbia LA-950, Retsch Technology GmbH, Haan, Germany) evaluated with Fraunhofer theory. Specimens for thermal cycling burner rig tests have an additional YSZ interlayer between bond coat (BC) and LAMT layer, which was applied by atmospheric plasma spraying (APS). The YSZ powder used is a Metco 204 NS (Oerlikon Metco, Wohlen, Switzerland).

2.2 Plasma Spraying Conditions The nickel-based superalloy IN738 was used as substrate (30 mm in diameter, 3 mm in thickness). It was coated with an Amdry 9954 (Oerlikon Metco, Wohlen, Switzerland) BC using a F4 plasma torch (Oerlikon Metco, Wohlen, Switzerland) in a vacuum plasma spray facility (Oerlikon Metco, Wohlen, Switzerland). Afterward, the YSZ interlayer was deposited by APS and the LAMT layer by SPS. For SPS and APS, a TriplexPro 210 plasma torch (Oerlikon Metco, Wohlen, Switzerland), with a nozzle diameter of 9 mm and mounted on a six axis robot in a Multicoat APS facility (Oerlikon Metco, Wohlen, Switzerland), was used. For the SPS, a pressure-based delivery system developed at Forschungszentrum Ju¨lich GmbH (Ju¨lich, Germany) was used. The suspension is pre-fragmented by a two-phase atomizer before entering into the plasma plume. The injection of the suspension into the plasma plume is carried out radially (Ref 16, 17).

For the deposition of the BC and the YSZ interlayer, internal well-established spraying parameters were used. The main spraying parameters for the deposition of the LAMT layer by SPS are given in Table 1.

2.3 Characterization of Coatings Metallographic cross sections have been prepared from samples to investigate the microstructure using scanning electron microscopy (Zeiss Ultra 55, Carl Zeiss NTS GmbH, Oberkochen, Germany or Hitachi TM3000, Hitachi High-Technologies Europe GmbH, Krefeld, Germany) with an EDS unit (INCA, Oxford Instruments, Abingdon, United Kingdom). X-ray diffraction was carried out on a D4 Endeavor (Bruker, Karlsruhe, Germany) using Cu Ka radiation for phase and crystallographic analysis.

2.4 Thermal Cycling Tests Gas burner test facilities operating with a natural gas/ oxygen mixture were used to evaluate the thermal cycling behavior of the coatings. More information on this test facility and the used specimens are given in Ref 18. The back of the substrate was cooled by compressed air to achieve the desired temperature gradient through the TBC. During the test, the surface temperature was measured with an infrared pyrometer using an emissivity of 1 for all coatings. Additionally, substrate temperature was measured by a thermocouple which was located in the center of the substrate (see Ref 18). The surface temperature was fixed at 1390 ± 50 C, while the substrate temperature was adjusted to 1050 ± 30 C during the high temperature phase, which lasted for 5 min. In the cooling period of 2 minutes, the whole TBC system (including substrate and bond coat) was cooled below 100 C with compressed air. Cycling was stopped when a clearly visible spallation or delamination of the coating occurred, as described in Ref 11. BC temperatures were calculated using the thermal conductivities of the coatings and the substrate, Table 2. For SPS LAMT layers, the thermal conductivity was determined for a free-standing

Table 1 Main spraying parameters used for the deposition of the LAMT layers by SPS Input power, kW Current, A Spraying distance, mm Plasma gas composition, slpm

~35/~50 400/500 65 40 Ar, 10 He

Table 2 Thermal conductivities used for the calculation of the temperature at the interface between BC and TBC

Material Substrate BC YSZ LAMT

Thermal conductivity, W/(m K)

Literature

29 29 1 2 (for APS), 0.9 (for SPS)

Assumed Ref 19-21 Assumed Ref 20, 21 Ref 22 Assumed Ref 11, measured data

Journal of Thermal Spray Technology

Peer Reviewed Fig. 1 SEM-BSE images showing the columnar microstructure of the coatings in the as-sprayed state. (left) ~35 kW and (right) ~50 kW gun input power

coating, which was coated by the use of a gun power of ~35 kW. The sample was heat treated for 12 h at 1400 C before the thermal conductivity measurement. Hence, the calculated temperatures at the BC/TBC interface shifted to lower values, in contrast to earlier presentations.

3. Results and Discussion 3.1 Microstructure and Phase Composition of As-sprayed LAMT Coatings

Journal of Thermal Spray Technology

Fig. 2 SEM-BSE image of the LAMT layer deposited with ~50 kW showing the slight decomposition of the as-sprayed coating

35 kW

Intensity [a.u.]

In Fig. 1, overviews of the as-sprayed coatings deposited with ~35 kW and with ~50 kW input power are shown. During the SPS process, columns are developed in both cases, which are expected to have a higher strain tolerance under thermal load. Besides, high porosity in the columns is observed. By scanning electron microscopy (SEM), different gray colors were detected, using back scattered electron (BSE) images. These differences in the average atomic number indicate a slight decomposition of the material, Fig. 2. Energy-dispersive x-ray spectroscopy (EDS) revealed a variation of the magnesium content in the LAMT coating. XRD analysis confirmed the decomposition of the material during spraying, Fig. 3. In addition to the orthorhombic LAMT phase, the La3TaO7 phase was found for the coatings sprayed with ~35 and ~50 kW. A Rietveld refinement confirmed that the amount of La3TaO7 is lower in the coating sprayed with ~35 kW. La3TaO7 is a stable phase in the La2O3-Ta2O5 system and has a melting point of 2020 ± 20 C (Ref 23). It has an orthorhombic lattice with the space group Cmcm (Ref 24-26). Obviously, for both coatings, slight phase decompositions occurred. A possible reason for the decomposition of the material can be the high temperatures in the plasma plume during the spraying process. In the first approach, the vapor pressures of the oxides (Ref 7-10) can be taken into account to explain the decomposition tendency of LAMT, Fig. 4. Even though the literature data show large scatter, vapor pressures of MgO and Al2O3 tend to be higher than the vapor pressures of La2O3 and Ta2O5. Hence, MgO and Al2O3 are prone to evaporate during the spraying process compared to the other two oxides. As a

50 kW

LAMT-powder

La3TaO7 10

20

30

40

50

60

70

80

2 Theta (Cu) [°]

Fig. 3 X-ray diffractograms of the surface of the as-sprayed LAMT layers sprayed with ~35 and ~50 kW along with their reference patterns (measured peak-positions of LAMT-powder; La3TaO7: Ref 26)

consequence, La- and Ta-containing secondary phases, such as the observed La3TaO7 are formed. The formation of secondary phases in the as-sprayed state of LAMT coatings is also known from the APS LAMT coatings (Ref 11, 27).

improved compared to that of the standard APS YSZ coatings as well as compared to that of the APS LAMT coatings (especially as the latter are tested at 1250 C TBC surface temperature). Thermal cycling lifetime behavior is in the same range for all tested SPS LAMT samples, taking into account the different temperatures at the BC/TBC interface. An important fact is that the lifetime of the singlelayer SPS LAMT coating was improved significantly compared to the single-layer APS LAMT coating. This is a first indication that the YSZ interlayer could even be omitted for those SPS LAMT coatings. Normally, the YSZ interlayer is TemperatureBC/TBC [°C] 1100

1070

1040

512

Time at high temperature [h]

The results of the thermal cycling tests are given in Table 3. All six tested SPS coatings demonstrate very good thermal cycling results. For a comparison of thermal cycling lifetime between these SPS samples and also in order to compare them with the standard APS YSZ coatings (Ref 28) and previously tested APS LAMT coatings (Ref 11), it is necessary to consider the temperature at the interface between BC and TBC. This temperature determines the growth rate of the thermally grown oxide (TGO) layer and, hence, also the failure of the system (Ref 22, 29, 30). For such BC-induced failure, lifetime data follow an Arrhenius-type, exponential dependence on the inverse BC/TBC temperature (Ref 22, 30, 31). Thus, the lifetime of TBC systems, tested under different conditions, consisting of different materials or different coating thicknesses, can be compared (Ref 22), by plotting the logarithm of the time to failure as a function of reciprocal temperature at the interface between BC and TBC (Arrhenius plot). Often, it is found that thermal cycling lifetime approx. halves, if the BC/ TBC temperature is increased by 30 C (this is indicated by the dotted line in Fig. 5). For the calculation of the BC/ TBC temperature, the thermal conductivities listed in Table 2 were used. The Arrhenius plot of the TBC systems presented in this paper is given in Fig. 5. In general, the thermal cycling performance of the SPS LAMT coatings is significantly

256

128

64

32

105

Vapor Pressure [Pa]

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3.2 Thermal Cycling Performance of the Coatings

16 0,72 La2O3 (a) Al2O3 (b-I) Al2O3 (b-II) Al2O3 (b-III) Al2O3 (c) MgO (b-IV) MgO (b-V) MgO (a) Ta2O5 (d)

100

10-5 3

4

5

6

104/Temperature [1/K]

Fig. 4 Vapor pressures of oxides as a function of inverse temperature (data taken from (a) Ref 9, (b) Ref 8, (c) Ref 7, (d) Ref 10) [in (b) experimental data from different test series are given]

0,73

0,74

0,75

0,76

0,77

1000/TemperatureBC/TBC [1/K] APS YSZ + SPS LAMT, ~50 kW, tested at 1390°C APS YSZ + SPS LAMT, ~35 kW, tested at 1390°C SPS LAMT, ~35 kW, tested at 1390°C APS YSZ + APS LAMT, tested at 1250°C APS LAMT, tested at 1250°C APS YSZ, tested at 1400°C

Fig. 5 Arrhenius plot of the thermal cycling behavior of samples with SPS LAMT top layer and with or without APS YSZ interlayer. In addition, thermal cycling results from a single- and a double-layer APS LAMT coating (Ref 11) and from standard APS YSZ coatings (Ref 28) are displayed. The temperatures indicated in the legend refer to the mean surface temperatures in the high temperature phase during thermal cycling tests

Table 3 Overview on the samples tested in the thermal cycling test, their thermal cycling lifetime, and testing conditions

No. 1 2 3 4 5 6

Description

LAMT thickness, lm

Thermal cycling lifetime, cycles

Double layer, ~50 kW Double layer, ~50 kW Double layer, ~35 kW Double layer, ~35 kW Single layer, ~35 kW Single layer, ~35 kW

~520 ~520 ~710 ~710 ~560 ~560

3091 2587 4392 4824 1440 2097

Calculated temperature at BC/TBC interface, °C ~1060 ~1050 ~1040 ~1040 ~1070 ~1060

Temperature at TBC surface, °C ~1390 ~1390 ~1390 ~1390 ~1390 ~1390

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3.3 Microstructure, Phase Composition and Failure Mechanism of Thermally Cycled LAMT Coatings The photographs and micrographs of the LAMT double- and single-layer coating after thermal cycling tests indicate that the failure of the TBC system actually took place at the BC/YSZ (in double-layer systems) or the BC/LAMT interface (in single-layer systems), Fig. 6.

SEM images of the top of the columns reveal the entire decomposition of the LAMT coating at the surface (Fig. 7, left) and also some decomposition in the middle of the columns (Fig. 7, right) after thermal cycling test. By EDS, it was found that bright areas are lanthanum- and tantalum-rich oxidic phases, whereas dark-gray areas are lanthanum- and aluminum-rich oxidic phases. Light-gray areas correspond to a complex perovskite with a composition close to that of LAMT. Less decomposition of the LAMT coating was found at the bottom of the columns, Fig. 8. The absence of magnesium on top of the coating is an obvious indication of Mg evaporation and needs to be further investigated in future work. In the double-layer systems, diffusion of lanthanum and tantalum into the YSZ layer takes place at the interface between LAMT and YSZ during thermal cycling tests,

Fig. 6 (top) Photographs and (bottom) SEM-BSE images of (right) sample no. 4 and (left) sample no. 6 sprayed with ~35 kW after failure in thermal cycling test

Fig. 7 SEM-BSE image of (left) the top of a column and (right) the middle of a column after failure in thermal cycling test, sample no. 1

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applied to overcome the drawback of the low fracture toughness of the new materials, which favors crack propagation during thermal cycling tests. The columnar microstructure of the SPS LAMT coatings probably reduces thermal stresses, so that they are able to withstand the harsh thermal cycling conditions, even without the interlayer.

locations, a full depletion from b-phase in the BC is observed. As mentioned above, this leads to the formation of fast growing oxides (e.g., like spinels). SEM analysis revealed that these undesired oxides developed during thermal cycling tests. Crack formation is found initiated by these oxide scales, Fig. 9. The XRD analyses of the surface of the thermally cycled samples confirm that LAMT is decomposed for the most part at the surface, Fig. 10. Instead of LAMT, La3TaO7, and LaAlO3 are found. The melting point of LaAlO3 is at 2110 C (Ref 36). The formation of those two oxides was detected in the APS coatings as well (Ref 11). Often, it is claimed that the formation of secondary phases is detrimental for TBC systems in general. However, this is not the case for the here presented SPS LAMT coatings. The coatings presented in this paper did not show any failure or cracks in the LAMT coating, after thermal cycling. It is very likely that the columnar microstructure and the high porosity inside the columns are beneficial. The high porosity inside the columns permits the coating to tolerate the sintering of the LAMT and the phase decomposition, whereas the columnar

~35 kW, sample no. 4, Z=4824

Intensity [a.u.]

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Fig. 8. It is assumed that this diffusion leads to an improved adhesion between the LAMT and the YSZ layer, because the coating did not fail at the interface between the YSZ interlayer and the LAMT layer after thermal cycling tests. As mentioned before, the failure of these samples took place at the BC/TBC interface, Fig. 6 and 9. The BC consists of c- and b-phase and forms an alumina-based oxide scale at high temperatures, the so called TGO (Ref 22, 29). Only after longer times the aluminum reservoir in the BC is depleted below a critical level. Usually fast growing mixed oxides will only be stable when b-phase is not sufficiently present. The formed alumina scale and even more fast growing oxides lead to additional stresses in the coating system which promote crack growth and failure in typical TBC systems (Ref 1, 32-34). Especially in our internal thermal cyclic rigs failure with short cycle length and hence high fatigue loading, failure is typically observed before the complete depletion of the aluminum reservoir. This is found for standard APS YSZ samples (Ref 22). However, in case of SPS LAMT/APS YSZ doublelayer systems, the lifetime is considerably higher. At some

~50 kW, sample no. 1, Z=3091

LAMT-powder La3TaO7 LaAlO3 10

20

30

40

50

60

70

80

2 Theta (Cu) [°]

Fig. 8 SEM-BSE image of SPS LAMT/APS YSZ sample after thermal cycling (sample no. 4). Bright lines at the LAMT/YSZ interface indicate the diffusion of lanthanum and tantalum into the YSZ layer in the double-layer systems

Fig. 10 X-ray diffractograms of the surface of the thermally cycled double-layer systems along with their reference patterns (measured peak-positions of LAMT-powder; La3TaO7: Ref 26, LaAlO3: Ref 35)

Fig. 9 SEM-BSE images showing (left) the b-phase depletion in the BC and (right) the formation of transient oxides at the interface between BC and TBC, sample no. 1

Journal of Thermal Spray Technology

7. 8.

4. Conclusion In the present study, it was shown that LAMT is a suitable new material with very good thermal cycling lifetime results. SPS was used to deposit the LAMT coating with a columnar highly porous microstructure. Lifetimes of the coatings were determined in the temperature gradient test, where a thermal gradient is induced in the coating through thickness direction. This reflects realistic circumstances, because in reality, these coatings are applied on turbine vanes that are exposed to very hot temperatures on the surface, whereas they are cooled from the inner part of the vane. Due to the excellent microstructure of the LAMT coating, the coatings are able to withstand the high thermal stresses and also the stresses resulting from secondary phase formation. Even though extensive decomposition of the LAMT on top of the coatings was found, this was not detrimental for the coating performance. The TBC systems failed at the BC/ TBC interface because of b-phase depletion, which led to the formation of brittle, fast growing transient oxides. In comparison to standard APS YSZ coatings, the lifetime of the SPS LAMT coatings was significantly improved.

Acknowledgment The authors kindly thank Karl-Heinz Rauwald and Frank Vondahlen for their support in the manufacturing of the samples, Michaela Andreas for the preparation of the powder and particle size measurement, Hiltrud Moitroux for the photographs and Nicole Adels for the thermal cycling tests (all Forschungszentrum Ju¨lich GmbH, Institut fu¨r Energie- und Klimaforschung - Werkstoffsynthese und Herstellungsverfahren (IEK-1)).

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microstructure endures the high thermal stresses during thermal cycling.

Peer Reviewed

29. N.P. Padture, M. Gell, and E.H. Jordan, Thermal Barrier Coatings for Gas-Turbine Engine Applications, Science, 2002, 296, p 280-284 30. C. Nordhorn, R. Mu¨cke, K.A. Unocic, M.J. Lance, B.A. Pint, and R. Vaßen, Effects of Thermal Cycling Parameters on Residual Stresses in Alumina Scales of CoNiCrAlY and NiCoCrAlY Bond Coats, Surf. Coat. Technol., 2014, 258, p 608-614 31. F. Tra¨ger, M. Ahrens, R. Vaßen, and D. Sto¨ver, A Life Time Model for Ceramic Thermal Barrier Coatings, Mater. Sci. Eng. A., 2003, 358(1-2), p 255-265 32. A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, and F.S. Pettit, Mechanisms Controlling the Durability of Thermal Barrier Coatings, Prog. Mater. Sci., 2001, 46(5), p 505-553

33. E.P. Busso, H.E. Evans, Z.Q. Qian, and M.P. Taylor, Effects of Breakaway Oxidation on Local Stresses in Thermal Barrier Coatings, Acta Mater., 2010, 58(4), p 1242-1251 34. D. Kim, I. Shin, J. Koo, S. Kim, D. Seo, J. Kim, and C. Seok, Quantitative Analysis on the Depletion Rate of b-NiAl Phases in MCrAlY Coating, J. Mech. Sci. Technol., 2014, 28(2), p 513-519 35. A. Nakatsuka, O. Ohtaka, H. Arima, N.A. Noriaki, and T. Mizota, Cubic Phase of Single-Crystal LaAlO3 Perovskite Synthesized at 4.5 GPa and 1273 K, Acta Crystallogr. Sect. E: Struct. Rep. Online, 2005, 61, p I148-I150 36. P. Wu and A.D. Pelton, Coupled Thermodynamic-Phase Diagram Assessment of the Rare Earth Oxide-Aluminium Oxide Binary Systems, J. Alloys Compd., 1992, 179(1-2), p 259-287

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