EB-PVD processing of pyrochlore-structured La2Zr2O7-based TBCs

Surface and Coatings Technology 182 (2004) 175–183

EB-PVD processing of pyrochlore-structured La2Zr2O7-based TBCs B. Saruhan*, P. Francois, K. Fritscher, U. Schulz DLR German Aerospace Center, Institute of Materials Research, Cologne D-51170, Germany Received 14 March 2003; accepted in revised form 15 August 2003

Abstract New ceramic thermal barrier coating (TBC) compositions superior to state-of-the-art PYSZ material are considered to overcome the problems related to phase stability and service-induced sintering within the columnar feather-like structure of current EB-PVD TBCs. Among those candidates for gas turbine applications, the pyrochlore-structure based TBCs, e.g. undoped and RE-oxide doped La2Zr2O7 offer very attractive properties. This work describes the fabrication and refers to the relations regarding manufacture and microstructure of two La2 Zr2 O7 -coatings, deposited by single source EB-PVD processing. The compositions contain an undoped La2Zr2O7-coating and an Y2O3-doped La2Zr2O7-coating. The aspects leading to the successful manufacture of a homogeneously aligned microstructure in EB-PVD TBCs rely on the careful control of deposition parameters as well as ingot (evaporation source material) quality. Hence, the microstructure of the EB-PVD La2 Zr2 O7 –coating analyzed by SEMyEDX and XRD have been correlated with the processing conditions in order to understand the controlling parameters for achievement of these coatings. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Thermal barrier coatings; Electron-beam physical vapor deposition; Pyrochlore

1. Introduction Electron-Beam Physical Vapor Deposition (EB-PVD) of thermal barrier coatings (TBCs) is used on advanced turbine blades to increase engine efficiency and to improve blade performance. These TBCs offer the advantage of superior strain and thermal shock tolerance due to their unique columnar microstructure. Partially Yttria Stabilized Zirconia (PYSZ) is the standard material for current TBC applications, providing a low thermal conductivity (2 Wmy1 Ky1), a relatively high coefficient of thermal expansion (11=10y6 Ky1) and chemical inertness in combustion atmospheres. The thermal stability of PYSZ-based TBCs, however, is seriously affected at demanding service temperatures by aging mechanisms and by considerable phase transformation and sintering-induced volume changes. Sintering, in particular, will degrade the columnar structure of EBPVD coatings and raise the modulus of elasticity and, as a result, restrict the favorable strain tolerance of PYSZ TBCs w1,2x. *Corresponding author. Tel.: q49-2203-601-3228; fax: q49-220369-64-80. E-mail address: [email protected] (B. Saruhan).

In order to avoid the problem of service-induced sintering within the columnar structure of EB-PVD coatings as observed with the state-of-the-art material (PYSZ), alternative oxide chemistries are suggested. These oxides have various crystal structures such as perovskite, spinel, magnetoplumbite, pyrochlore, etc. The cubic pyrochlore-based TBCs are of particular interest, owing to their distinctive arrangements of ions and vacancies within the AxByOz compositional structure. They offer some favorable properties such as low thermal conductivity (1.56 Wmy1 Ky1), relatively high thermal expansion coefficient (9.1=10y6 Ky1), and phase stability up to 23008C for TBC applications w3– 5 x. In binary compounds of AxByOz, the first metal cation A is a rare-earth element, typically a Lanthanide such as La, Gd, Nd, etc., and the second metal cation B is Zr, Hf or Ti. In the case of La2Zr2O7, the cubic crystal structure consists of six ZrO6 octahedra which are connected by La3q-ions. There are vacancies randomly distributed at La3q, Zr4q and O2y sites, which do not affect phase stability. The ionic conductivity of this group of material depends on the concentration of mobile vacancies and their mobility. The pyrochlores,

0257-8972/04/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.08.068

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depending on their composition, show different levels of ionic conductivity. So the maximum oxygen ion conductivity is with the defect fluorite solid solution GdxZr1yxO2y1y2x at xf0.5 while the minimum conductivity on same ordering is with NdxZr1yxO2y1y2x w6x. It was demonstrated that La2Zr2O7TBCs can be successfully manufactured by plasma spraying (PS) w7x Up to date no EB-PVD production line of this compound has been reported although the viability is proven w3,5x. This paper presents the EB-PVD processing of two pyrochlore variations; Comp A being pure La2Zr2O7 and Comp B 3 wt.% Y2O3-doped La2Zr2O7. Literature data on vapor pressures predict a difference of three orders of magnitude between ZrO2 and La2O3 at 3000 8C w8x. If vapor pressures of oxide components fall too far apart from each other as suspected in the present case, selective evaporation and selective deposition may result. In order to overcome this, Y2O3 is used as a dopant as well as a process regulator. The microstructure of coatings is analyzed by SEM and EDX, the phase changes are determined by XRD. Both are correlated with processing conditions. The role of Y2O3-doping of La2Zr2O7 is discussed in terms of coating quality and processing conditions during EB-PVD. 2. Experimental methods 2.1. Evaporation source preparation Two ingot compositions were produced (HTM Reetz, Germany) via a powder route by using fused homogeneous mono-phase powders (Treibacher Auermet, Austria). After cold-isostatical pressing, the ingots were heat-treated at approximately 1600 8C for obtainment of single phase formation of pyrochlore. The ingots were fabricated in dimensions of 62.5-mm diameter and 150mm length. 2.2. Physical vapor deposition For the EB-PVD process of the coatings, pilot plant equipment (Espri von Ardenne, Germany) was used, having a maximum EB-power of 150 kW. Dense stabilized zirconia rods and alumina plates were taken as substrates. During deposition the average substrate temperature was adjusted to 975"25 8C. The ingot source material was bottom fed in a water cooled copper crucible for evaporation. The electron beam power on the source was equilibrated to 60 kW during deposition at constant focus and beam pattern conditions, thereby keeping a constant evaporation rate. The substrates were rotated during coating at 12 miny1. Deposition was carried out under controlled oxygen flow into the deposition chamber. Vacuum pumping was set constant, however, no gas pressure adjustment was used.

Fig. 1. XRD pattern of the ingot prepared with the undoped La2Zr2O7-composition (Comp A), showing formation of crystalline pyrochlore phase.

2.3. Methods of characterization XRD-measurements were carried out on a Siemens D 5000 diffractometer over scattering angles of 2us108 to 808 using nickel-filtered CuKa radiation with a step size of 0.0208 2u and a counting time of 10 s per step. The samples were characterized microstructurally and compositionally by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) (LEITZ LEO 982, Germany). Other compositional determinations were carried out by an X-ray fluorescence analyzer (XRF) (Oxford MESA 5000, Germany). 3. Results 3.1. Evaporation source Fig. 1 shows the XRD-data of the undoped La2Zr2O7 ceramic ingots (Comp A) and demonstrates the single phase crystalline structure. XRD of the Y2O3doped La2Zr2O7 composition (Comp B) yields also the typical pyrochlore reflections with no apparent difference to that of the pure La2Zr2O7 composition (not shown here). Composition of ingots and coatings was analyzed by X-ray fluorescence (XRF). These analyses show that the chemical composition of ingot A is near to the calculated stoichiometric composition, although slightly higher ZrO2 and lower La2O3-contents are measured. At ingot B, 2.7 wt.% Y2O3 was detected which partially replaces La2O3 in the calculated stoichiometric composition (see also Table 1). 3.2. Microstructural observations SEM observations given in Fig. 2 on the pure La2Zr2O7 EB-PVD coatings (Comp A) reveal that their morphology differs from that of the state-of-the-art

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Table 1 XRF analysis of ingots and coatings (composition A and B) compared with the calculated values for La2Zr2 O7 and 3 wt.% Y2O3-doped La2Zr2O7 Calculated value

La2O3 (wt.%) ZrO2 (wt.%) Y 2O 3 (wt.%)

Comp A

Comp B

Ingot powder A

57.0

54.0

55.1

59.0

52.4

55.6

43.0

43.0

44.9

41.0

44.9

41.8

–

–

2.7

2.6

–

3.0

Coating composition A surface

Ingot powder B

Coating composition B surface

Fig. 2. SEM micrograph of the La2 Zr2 O7 -EB-PVD-coating (Comp A) showing the morphology at the cross section (a) and at the coating tip (b), high-magnification picture of column tip.

Fig. 3. SEM micrograph of the La2yxYxZr2 O7 EB-PVD coating (Comp B) showing the morphology at the cross section (a) and at the coating tip (b), high-magnification picture of column tip.

material EB-PVD PYSZ coatings, showing more branching of the columnar structure (Fig. 2a) This indicates a repetitive re-nucleation of columns during deposition. The column tips show a cauliflower-like appearance (Fig. 2b). SEM observations of the 3 wt.% Y2O3 doped La2Zr2O7 (La2-xYxZr2O7) EB-PVD layer (Comp B) reveals a similar branched columnar structure as that of pure La2Zr2O7 layer (Fig. 3a). The column tips have a somewhat pyramidal shape (see Fig. 3b), which refers to the cubic lattice of the pyrochlore compound. This agrees more with that of the state-of-the-art material

PYSZ, however, the pyramidal diameters are finer (approx. 1–2 mm), columns are more irregularly distributed and contain some intercolumnar gaps. The high magnification observations of the column tip reveal that the fine pyramids appear to be connected with subset prisms, which may have displayed a different orientation than the main pyramid (Fig. 3b). 3.3. Compositional determination Fig. 4 shows the effect of chamber gas pressure on composition and morphology of the La2Zr2O7-coating

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Fig. 4. Effect of gas pressure (top) on composition (center) and morphology (bottom) of EB-PVD La2Zr2O7 coating (Comp A).

(Comp A). Close examination of Fig. 4 reveals that there exists a correlation between the deviations of gas pressure and compositional changes during deposition. In general, a decrease in gas pressure results in a decrease in Zr and an increase in La. Chamber gas pressure shows a decrease (3=10y3 mbar) at the fourth minute of deposition and recovers back towards the sixth minute to an almost constant level of 9=10y3 mbar. Accordingly, EDX analyses of the coating show abrupt decrease in zirconium and increase in lanthanum contents at approximately 35–40 mm above the substrate–coating interface. At the same location a change in coating morphology is also observed. The morphology between approximately 40 and 90 mm displays almost no feather arm formation, instead,

only very narrow and isolated columns as the highmagnification SEM picture on the left side of Fig. 4 shows. At this stage, i.e. between the seventh and eleventh minutes of the deposition, the gas pressure remains fairly constant. However, achievement of near stoichiometrical La2O3 yZrO2-ratio in the composition proceeds very slowly taking all these four minutes. After 100 mm thickness and later than the eleventh minute of deposition, the morphology becomes more branched as shown in Fig. 2 and the coating composition more uniform (Fig. 4). Fig. 5 shows the correlation between the chamber gas pressure deviation and the coating composition and morphology of the 3 wt.% Y2O3 doped La2Zr2O7coating (Comp B) during EB-PVD coating process.

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Fig. 5. Effect of gas pressure (top) on composition (center) and morphology (bottom) of EB-PVD 3 wt.% Y2 O3 -doped La2 Zr2 O7 coating (Comp B).

EDX line scan analysis along the cross section of the Comp B coating yields a more homogeneous distribution of lanthanum and zirconium, apart from the changes at approximately 60 mm thickness. Yttrium is detected in trace amounts and displays a high scattering throughout the layer, which can be partly attributed to the limited peak separation caused by the use of L-lines. The

chamber gas pressure remained almost constant throughout the deposition at a level between 8=10y3 and 1=10y2 mbar other than a larger deviation occurring at approximately the 8th minute of the coating. Compositions of coatings at the surface of Comp A and Comp B were measured by XRF. These results indicate that the coating becomes near to stoichiometri-

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cal composition and, thus, zirconium–poorer and lanthanum–richer (see Table 1). The chemistry and microstructure of the EB-PVD coated pure La2Zr2O7 (Comp A) exhibits significant fluctuations along the thickness of the coating that are far more exaggerated than those for 3 wt.% Y2O3 doped La2Zr2O7 (Comp B). 3.4. Phase determination In order to follow the abrupt change observed by cross sectional EDX line scan analysis of composition A, the coatings were ground off layer by layer at approximately 50 mm distance. Each new exposed layer was analyzed by XRD. The results obtained along the thickness of the coating (Comp A) and (Comp B) are given in Fig. 6a and b. Both compositions show exclusive formation of a crystalline La2Zr2O7 phase. For Comp B, unlike Comp A, only a small shift was observed in the d-values compared to the reference data

Table 2 Diffraction pattern of Lanthanum Zirconium Oxide (La2Zr2O7) JCPDS file 73-444 ˚ d (A)

Relative Intensity I

Hkl

6.24 3.25 3.12 2.70 2.47 1.91 1.62 1.56 1.35

3 1 100 30 5 38 32 8 4

111 311 222 400 331 440 622 444 800

for La2Zr2O7 given in Table 2 (see also Fig. 6a and b). The main feature of these XRD investigations was that the peaks belonging to the N222M N400M N440M and N622M reflections show peak-splitting as well as peakshifting towards the higher d-values, compared to the

Fig. 6. XRD of the successive layers after grinding of undoped La2 Zr2 O7 -coating. The multiple peak–splitting can be an indication of the formation of various solid-solutions in the solubility range. The solubility range is given in the literature to vary from 0.87(La2O3).2(ZrO2 ) to 1.15(La2O3).2(ZrO2).XRD of the progressively ground 3 wt.% Y2 O3 -doped La2 Zr2 O7 -coating, displaying more sharper peaks at the second half of the coating. The layers deposited at the beginning of EB-PVD-process appear to bear some shoulder which may indicate the formation of various solid-solutions.

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Fig. 7. Changes in d–values of the primary and secondary peaks on the N400M and on the N222M planes of the LaO1.5 (1yx) ZrO1.5-phases in the cross-section of the undoped La2Zr2 O7 EB-PVD-coating. Dotted thick line is according to JCPDS. Changes in d-values of the primary and secondary peaks on the N400M and on the N222M planes of the LaO1.5 (1yx) ZrO1.5-phases in the cross-section of the 3 wt.% Y2O3-doped La2Zr2O7 EB-PVD-coating. Dotted thick line is according to JCPDS.

reference data (see Table 2, JCPDS file 73-444). No strong signs for a preferred growth of the TBCs have been observed since all peaks are present with intensity ratios close to the ideal values given in Table 2. The intensities of these split peaks were analyzed by a peak fitting program. Since the reflection N222M shows the highest intensity and N400M allows analyzing a possible texturing, the peak fitting has been carried out on these two reflections. After the subtraction of background and the separation of Ka1- and Ka2-peaks, the actual peaks, which are hidden beneath a peak having two summits were identified and their intensities were measured individually. Then d-values and intensities of the fitted peaks were correlated with those of the reflections given in the reference data (see Table 2). These XRD analyses show that at least two solid solution ranges of La2Zr2O7 phase are likely to form along the thickness of both EB-PVD-coated layers. In order to observe how these two compositions vary along the coating, d-values of two peaks on N400M and N222M reflections which were designated according to their

intensities as primary and secondary peaks (i.e. peaks from solid-solutions) were taken and plotted against the coating thickness in Fig. 7a and b. Fig. 7a shows an abrupt change in the d-values of the primary and secondary peaks of Comp A between 40 and 60 mm thickness, indicating a compositional switchover. At a thickness above 60 mm, the primary solid˚ solution range displays d-values approximately 0.06 A smaller than the stoichiometric one, while the secondary solid-solution range appears to remain very near to the ˚ from stoichiometry with d-values deviating only 0.01 A that of the stoichiometric composition. Otherwise, the d-values above 60 mm remain more or less constant. As for Comp B, Fig. 7b shows occurrence of a similar abrupt compositional switch-over at approximately 60 mm thickness. Above this thickness to the middle of the coating (i.e. between 60 and 150 mm), the d-values of the primary solid solution range remain almost at the stoichiometry for La2Zr2O7. Above 150-mm thickness, there occurs another d-value switch-over leading to the ˚ smaller than that of the stoichiomd-values up to 0.04 A

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etrical composition (Fig. 7b, right). Despite this switchover, the layer by layer XRD investigation as well as the peak-fitting analyses shows that Comp B (La2yxYxZr2O7) displays less peak-splitting than Comp A. This may indicate deposition of a more homogeneous layer throughout the EB-PVD-process with Comp B (see also SEM-picture in Fig. 5). While the N222M and N400M intensity ratio remains nearly constant over the entire thickness of Comp A, an intensity ratio change occurs for Comp B, exceptionally at higher coating thickness. At the lower part of the coating (i.e. below approx. 50 mm) N222M peaks have higher intensities compared to N400M peaks in accordance with the reference values given in Table 2. At higher coating thickness, however, intensity of N400M peaks firstly equals and then becomes higher than that of N222M (see Fig. 6b). This behavior may indicate development of a favored orientation on the N100M direction with increasing deposition time as the coating becomes homogeneous and sufficiently saturated with Y2O3. 4. Discussion In contrast to the strong texture observed in the stateof-the-art material PYSZ TBCs w2x, no development of a distinct texture was found for both lanthanum zirconate compositions (Comp A and Comp B). This was also supported by total absence (Fig. 2 for undoped La2Zr2O7) vs. rare occurrence of pyramids (Fig. 3 for 3 wt.% Y2O3-doped La2Zr2O7) that are otherwise representative for textured column tips in PYSZ TBCs. The intensity ratios of all peaks in both compositions (Comp A and Comp B) were close to the reference values given in Table 2. The ‘change’ in intensity ratios of N222M and N400M reflections in Comp B with progression of thickness indicates starting of a weak preferred orientation which correlates with the rare appearance of pyramids on the tip of the coating. The lack of strong texturing is perhaps due to the existence of many equally important crystallographic planes in the pyrochlore lattice which allow the survival of a number of crystallographic directions during column growth (such as N111M N100M and N311M). Due to the fact that La2O3 has a higher vapor pressure than ZrO2 and Y2O3, La2O3 evaporates predominantly. It seems that any irregularity, e.g. a compositionally or in density non-optimized evaporation source or other possible changes in ingot evaporation behavior or an excessive selective evaporation of the high vapor pressure component, causes a large change in composition of both vapor and coating. The change in vapor pressure during deposition turns out to affect heavily the chamber gas pressure leading to severe fluctuations. In the initial deposition phase the pressure condition in the chamber was essentially stable and the composi-

tion of the coating was relatively homogeneous. After a short while, the vapor pressure balance of the chamber changed for both compositions. This was observed for Comp A after approximately 4 min where a pressure decrease caused a higher content of La2O3 and for Comp B after approximately 8 min where a pressure increase lowered the La2O3 content. On predominant evaporation of La2O3, the composition of the vapor cloud as well as that of the melt formed on the ingot top changed and this favored the conditions for deposition of ZrO2-richer compositions. It is likely that such changes occur somewhat slower than the more abrupt changes in the chamber gas pressure since the melt depletes in La2O3 and the recovery back to the equilibrium composition may take some time. The observation of large fluctuations in composition through the thickness of a coating is very common for EB-PVD of multi-component systems having large vapor pressure differences w10–13x. Moreover, as detailed XRD-investigations along the whole coating thickness revealed, these fluctuations lead to formation of two diverging solid solution ranges (Figs. 6 and 7). As no distinctive localized morphological difference has been observed, one can postulate that these solid-solution ranges are not strictly separated or at least not visibly detectable by scanning electron microscopy. In particular, the multiple crystallographic growth patterns observed at the tip of Comp A may probably be a result of the alternating solid-solution ranges in the coating and of the resultant frequent renucleation of crystals. The variation of d-values throughout the thickness of the pyrochlore TBC correlates well with the changes in chamber gas pressure and composition of the coating. These may be due to the compositional changes within the solid-solution range of the La2Zr2O7 phase. This is plausible, since the pyrochlore structures can deviate from the stoichiometric A2O3 yBO2-ratio. The La2O3 – ZrO2 phase diagram shows a considerable solubility range for La2Zr2O7 from 0.87La2O3x2ZrO2 (i.e. 53.6 wt.% La2O3 and 46.4 wt.% ZrO2) to 1.15La2O3x2ZrO2 (i.e. 60.4 wt.% La2O3 and 39.6 wt.% ZrO2) whereby the crystal structure and habit remain unaffected w14,15x. One explanation for such fluctuations within an EBPVD deposited pyrochlore layer is that the rotation of the substrate in a mixed vapor cloud during deposition may disturb the homogeneity in terms of La2O3 and ZrO2 ratios based on differences in their gas mobility and diffusion. Regarding the observed compositional deviations, despite congruent melting behavior of La2Zr2O7 ceramic ingot, one can postulate that no direct evaporation of La2Zr2O7 from this melt does occur. Instead, La2O3 and ZrO2 evaporate individually. It is not clearly understood which mechanism is responsible for enhanced evaporation behavior yielding

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less fluctuation in Comp B after doping La2Zr2O7 with Y2O3. Pyrochlore structure contains unoccupied 8a sites in the lattice. The cation radius of La3q, Y3q and ˚ respectively w9x. The Zr2q are 1.16, 1.01 and 0.72 A, 3q relatively smaller Y -cations can occupy either vacancy or substitute lattice places in the pyrochlore. Furthermore, no pyrochlore formation is known between Y2O3 and ZrO2. As suggested in literature, Y2 O3 is known as fluorite former w9x. In either case, a higher packing density of the pyrochlore lattice and a stronger coupling of ZrO6 octahedra through Y3q-doping will be expected. As the experimental observations show, Y2O3-doping of pyrochlore suppresses some irregularities on growth and biases the growth direction especially onto N100M. Similarly, enrichment of the somewhat lower vapor pressure Y2O3 compared for more fugitive La2O3 may regulate the evaporation conditions, leading to a more controllable process as well as to achievement of a more homogeneous EB-PVD layer deposition. Moreover, with respect to the phase relations in the ternary phase diagrams La2O3 –Y2O3 –ZrO2 or Nd2O3 – Y2O3 –ZrO2, the presence of Y2O3 enlarges the pyrochlore phase field towards the Y2O3-corner and thus makes it possible to accommodate up to 20 mol.% Y2O3 in the pyrochlore lattice which possibly explains the more defined texture growth of EB-PVD-pyrochlore layers w15x. Other rare-earth oxides, for instance Ga2O3, Yb2O3, Dy2O3 and the trivalent Lanthanide oxides, are expected to improve properties in a similar manner. 5. Conclusions New pyrochlore-based ceramic coatings with compositions of undoped and 3 wt.% Y2O3-doped La2Zr2O7 have been manufactured successfully by single source EB-PVD-processing. – Compositional variations during EB-PVD-processing of La2Zr2O7 coatings appear to depend on gas pressure changes in the deposition chamber and the differences in vapor pressure of La2O3 and ZrO2. The respective partial vapor pressure of oxide mixtures turns out to be a critical parameter for EB-PVD processing to control the composition and morphology of coatings. – EB-PVD La2Zr2O7 coatings display a branched columnar microstructure and consist of some pyramids if doped with Y2O3. Undoped La2Zr2O7 coatings show no preferred orientation, whereas only a slight development of preferred orientation is observed in Y2O3-doped La2Zr2O7 coatings. XRD peak splitting

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and shifting correlate with compositional fluctuations and indicate coexistence of two solid solution ranges. – Y2O3-addition in La2Zr2O7 improves the chemical homogeneity of the coating. The presence of a lower vapor pressure oxide such as Y2O3 helps to stabilize the pressure conditions. Moreover, the smaller ionic radius of Y3q-ions appears to promote the preferred growth of pyrochlore EB-PVD-layers in the N100M crystal direction. References w1x K. Fritscher, F. Szucs, ¨ U. Schulz, B. Saruhan, W.A. Kaysser, Impact on thermal exposure of EB-PVD-TBCs on young’s modulus and sintering, CESP 23 4 (Part B) (2002) 341–352. w2x U. Schulz, K. Fritscher, C. Leyens, M. Peters, High temperature aging of EB-PVD thermal barrier coating, CESP 12 4 (Part B) (2001) 347–356. w3x R. Subramanian, S. Sabol, J.G. Goedzen, K.M. Sloan, S.J. Vance, Improved thermal barrier coatings for turbine components, WO01y23642 A2-US00y23201. w4x M.J. Maloney, Thermal Barrier Coating Systems, U.S. Pat. No. 6 177 200 B1, 2001. w5x R. Vassen, X. Cao, F. Tietz, G. Kerkhoff, D. Basu, D. Stover, ¨ Zirconates as new materials for thermal barrier coatings, J. Am. Ceram. Soc. 83 (8) (2000) 2023–2028. w6x L. Minervini, R.W. Grimes, Disorder in pyrochlore oxides, J. Am. Ceram. Soc. 83 (8) (2000) 1873–1878. w7x X.Q. Cao, R. Vassen, W. Jungen, S. Schwartz, F. Tietz, D. ¨ Stover, Thermal stability of lanthanum zirconate plasmasprayed coating, J. Am. Ceram. Soc. 84 (9) (2001) 2086–2090. w8x V.L. Stolyarova, G.A. Semenova, J.H. Beynon, Mass Spectrometric Study of the Vaporization of Oxide Systems, John Wiley and Sons, Chichester, 1990. w9x S.G. Terry, Evolution of microstructure during the growth of thermal barrier coatings by electron-beam physical vapor deposition, Ph.D. thesis in Materials. 2001, University of California, Santa Barbara: Santa Barbara, CA. w10x B.A. Movchan, Functionally graded EB PVD coatings, Surf. Coat. Technol. 149 (2–3) (2002) 252–262. w11x U. Schulz, K. Fritscher, M. Peters, EB-PVD Y2O3 and CeO2 y Y2O3 stabilized zirconia thermal barrier coatings-crystal habit and phase compositions, Surf. Coat. Technol. 82 (1996) 259–269. w12x U. Schulz, K. Fritscher, C. Leyens, M. Peters, W.A. Kaysser, Thermocyclic behavior of differently stabilized and structured EB-PVD thermal barrier coatings, Materialwissenschaft und Werkstofftechnik 28 (1997) 370–376. w13x U. Schulz, K. Fritscher, W.A. Kaysser, Cyclic lifetime of PYSZ and CESZ EB-PVD TBC systems on various Ni-superalloy substrates. In: J. Lecombe-Beckers, M. Carton, F. Schubert, P.J. Ennis (Eds.), COST 2002, Vol. 21–2 (2002) 483–492. w14x C.R. Stanek, L. Minervini, R. Grimes, Nonstoichiometry in A2B2O7 pyrochlores, J. Am. Ceram. Soc. 85 (11) (2002) 2792–2798. w15x R.S. Roth, T. Negs, L.P. Cook, in: G. Smith (Ed.), Figure 5232 Phase Diagrams for Ceramists, 4, Am. Ceram. Soc, Columbus OH, 1981.