YSZ thermal barrier coatings prepared by electron beam–physical vapor deposition

Accepted Manuscript Phase evolution, interdiffusion and failure of La2(Zr0.7Ce0.3)2O7 / YSZ thermal barrier coatings prepared by electron beam-physical vapor deposition Limin He, Xin Zhou, Bintao Zhong, Zhenhua Xu, Rende Mu, Guanghong Huang, Xueqiang Cao PII: DOI: Reference:

S0925-8388(14)02523-7 http://dx.doi.org/10.1016/j.jallcom.2014.10.094 JALCOM 32440

To appear in:

Journal of Alloys and Compounds

Received Date: Revised Date: Accepted Date:

24 August 2014 10 October 2014 21 October 2014

Please cite this article as: L. He, X. Zhou, B. Zhong, Z. Xu, R. Mu, G. Huang, X. Cao, Phase evolution, interdiffusion and failure of La2(Zr0.7Ce0.3)2O7 / YSZ thermal barrier coatings prepared by electron beam-physical vapor deposition, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.10.094

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Phase evolution, interdiffusion and failure of La2(Zr0.7Ce0.3)2O7 / YSZ thermal barrier coatings prepared by electron beam-physical vapor deposition c

c

Limin He*,a, , Xin Zhoua, c, d, , Bintao Zhongb, Zhenhua Xua, Rende Mua, Guanghong Huanga , Xueqiang Caoc a

Beijing Institute of Aeronautical Materials, Department 5, P.O. Box 81-5, Beijing

100095, China b

AVIC Aviation Power plant Research Institute, Zhuzhou 412002, China

c

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China d

University of Chinese Academy of Sciences, Beijing 100049, China

_________________________ *

Corresponding authors. Tel/Fax: +86-10-62496456, +86-431-85262285. E-mail

addresses: [email protected] (L.M. He),. c

These authors contributed equally to this work

Abstract La2 (Zr0.7Ce0.3)2 O7 (LZ7C3) has attracted great interest for thermal barrier coatings (TBCs) because it presents extremely low thermal conductivity, high thermal stability and is more resistant to sintering than yttria stabilized zirconia (YSZ). In the present study, an LZ7C3/YSZ double-ceramic-layer (DCL) TBC was deposited by electron beam-physical vapor deposition (EB-PVD) and the TBC system was

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investigated for its phase evolution, interdiffusion and failure pattern though thermal shock test at 1373 K. X-ray diffraction and Raman spectra results indicate that the as-deposited LZ7C3 coating transforms from fluorite to pyrochlore structure upon thermal shocking between 373 K and 1373 K. It seems that this phase change may have affected the durability of the DCL TBCs. The EDS mapping analysis indicates that some diffusion of Y from YSZ to LZ7C3 layer is occurred after thermal shock test. Additionally, an obvious outward diffusion of Cr from bond coat into LZ7C3 layer takes place due to the chemical reaction of LZ7C3 and Cr. The phase transformation of LZ7C3, the abnormal oxidation of bond coat, and the outward diffusion of Y and Cr alloying element into LZ7C3 coating would be the primary factors for the spallation of LZ7C3/YSZ thermal barrier coating. Keywords: Thermal barrier coatings; Double-ceramic-layer; La2 (Zr0.7Ce0.3)2 O7; Phase evolution; interdiffusion

1. Introduction Thermal barrier coatings (TBCs) have been extensively used in advanced gas turbine engines to protect hot components such as combustion chambers and turbine blades from high temperature gas aggression [1]. The application of TBCs enables engines to be operated at higher gas inlet temperature, giving rise to the improvement of the thrust-to-weight ratio and fuel efficiency of the aerospace engines [2]. State-of-the-art TBCs consist of a NiPtAl diffusion or NiCrAlY overlay bond coat (BC) as the oxidation resistant layer and a ceramic topcoat as the heat resistant layer.

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Up to now, the most studied and commercially used ceramic top coat is based on 6-8 wt% yttria stabilized zirconia (YSZ) which shows superior thermophysical and mechanical properties such as low thermal conductivity, high thermal expansion coefficient and high fracture toughness. However, the maximum operation temperature of YSZ is limited to 1473K for long-term application. At higher temperatures, YSZ coating suffers serious sintering and martensitic phase transformation accompanied by a 4–6% volume expansion, which tend to lead to early spallation failure of TBC [3-5]. In the next generation of advanced engines, further increases in thrust-to-weight ratio will require even higher gas temperature [6]. To meet the development of advanced gas turbine engines, great efforts have been made to identify new alternative TBC materials to YSZ for applications above 1473 K. Several ceramic materials such as multicomponent oxide-doped ZrO2 [7], garnet (Y3AlxFe5-xO12) [8], pyrochlores (Re2Zr2O7, Re=La, Nd, Sm and Gd) [9-13], perovskites (SrZrO3, BaZrO3) [3,9], fluorite-type Re2Ce2O7 (Re =La and Nd) [14], and magetoplumbite-type LaMgAl11O19 [15,16] have been evaluated as TBC materials. Among the numerous oxides that have been explored as alternate thermal barrier materials, the rare earth zirconates especially La2Zr2O7 (LZ) have received great attention. Compared with YSZ, LZ has lower thermal conductivity (1.56 Wm-1 K-1) and better sintering-resistance. However, the relatively low TEC of LZ tends to generate high thermal stress between the LZ coating and the metallic bond coat during thermal cycling.

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CeO2 was considered to dope in LZ because materials containing CeO2 usually have high TEC and good thermal shock resistance. In the series of La2(Zr1−xCex)2O7, La2 (Zr0.7Ce0.3)2 O7 (LZ7C3) has been proved to possess the lowest sintering ability and the lowest thermal conductivity [17]. As reported in ref [18], LZ7C3 is a mixture of pyrochlore and fluorite. The main phase in LZ7C3 is LZ with a small solubility of LC, and this phase keeps pyrochlore structure. The second phase is a solid solution of LC and LZ with fluorite structure. The thermal conductivities of bulk LZ7C3 have achieved 0.87 Wm-1 K-1 at 1273 K and smaller than half of that of YSZ (2.1–2.2 W m-1 K-1). Meanwhile, LZ7C3 has a high TEC (10.66×10-6 K-1), which can be compared with that of YSZ [19]. It is thermally stable from room temperature to its melting point (2413K), implying that LZ7C3 is very promising for TBC materials at high temperature. However, single LZ7C3 coating has a very short life because the chemical compatibility of LZ7C3 coating and thermally grown oxide (TGO) layer is unstable during thermal cycling test [6]. It seems that a double-layered ceramic design, which adopt a YSZ as bottom layer, is a practical way to improve the LZ7C3 lifetime [5,17]. Xu et al prepared a double-ceramic-layer (DCL) coating of LZ7C3/YSZ by electron beam-physical vapor deposition (EB-PVD) method [6]. The thermal cycling test result shows that its lifetime is not only much longer than that of single LZ7C3 coating, but also approximately 36% longer than that of YSZ coating at 1373 K. The DCL coating of LZ7C3/YSZ prepared by air plasma spray method also showed excellent performance in the burner-rig test as reported by Zhao et al [20]. It is generally accepted that the elements interdiffusion between different layers

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and phase changes during thermal cycling have significant influences on the lifetime of TBCs [21-23]. The inter-diffusion behavior has been extensively studied in conventional YSZ TBCs, however, the influence of the added LZ7C3 layer on the interface stability between the NiCrAlY bond coat and the ceramic coat in the DCL TBCs has never been investigated. In addition, the phase evolution of EB-PVD LZ7C3 coating during thermal exposure still remains unclear. More efforts should be devoted to these issues before the practical application of LZ7C3/YSZ DCL coating. In this work, LZ7C3/YSZ DCL coating was prepared by EB-PVD method and the durability of the coating was evaluated by thermal shock testing at 1373 K. The main focus of this work had been on the phase evolution and microstructure changes of LZ7C3/YSZ coating during thermal shock test and interdiffusion behavior of the DCL coating.

2. Experiment 2.1. Ingot of LZ7C3 coating Ingot powder with the desired composition was prepared by solid-state reaction at 1673 K for 12 h with La2O3 (99.99%, Shenghua Chemicals of Hunan), CeO2 (99.99%, Shenghua Chemicals of Hunan) and ZrO2 (99.9%, Dongfang Chemicals of Guangdong) as the starting materials. The as-synthesized ingot powder was ball milled together with Arabic gum, triammonium citrate and deionized water for 72 h. Then the obtained slurry was spray-dried (Jiangsu Yangguang Ganzao Co., Ltd.) to form free-flowing powders. After the cast-formation, the ingot was densified at 1773

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K for 12 h. The ingots were fabricated in dimensions of 68 mm diameter and 125 mm length. The commercial ingot of YSZ (GRINM, Beijing) was directly used for the deposition of YSZ layer. 2.2. Preparation of bond coat and top ceramic coat The directionally solidified Ni-based superalloy DZ125 (15 mm×10 mm×1.5 mm) was used as the substrate material. The substrates were sand-blasted before the bond coat (BC) of NiCrAlYSi was deposited by arc ion-plating (A-1000 Vacuum Arc Ion-Plating Unit). The BC used identically in this study had a nominal composition (wt%): 20–25Cr, 6–10Al, 0.08–0.4Y, 0.4–0.8Si, and Ni as balance. After the deposition of BC, the samples were heat-treated under high vacuum at 1143 K for 3 h in order to enhance the adhesion of bond coat to substrate. The ceramic coatings were deposited using commercial EB-PVD equipment with a maximum EB-power of 60 kW. The YSZ coating was firstly deposited onto BC followed by the deposition of LZ7C3 coating. Electron beam currents for depositions of YSZ and LZ7C3 were 1.6 and 1.2 A, respectively. The average substrate temperature for the deposition of YSZ coating was adjusted to 1223f50 K while the average temperature for LZ7C3 coating was 1123f50K. During deposition of ceramic coating, the rotation speed was 20 rpm and the deposition pressure of EB-PVD working chamber was measured to between 6×10−4 and 2×10−3 Torr. 2.3. Thermal shock test and sample characterization Thermal shock tests of the coating samples were performed using a vertical furnace which is equipped with an automation system allowing specimens moving in

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and out automatically. During testing, samples were heated in the air furnace at 1373 K for 5 min followed by removing out for cooling with airflow for 5 min. This process was stopped when 5% area of the ceramic coating was delaminated. The coating microstructure and composition evaluation was observed by scanning electron microscope (SEM, FEI-Quanta 600) equipped with energy dispersive spectroscopy (EDS, Oxford INCAx-sight 6427). All the coatings for cross-section analysis were firstly embedded in transparent epoxy resin and then polished with diamond pastes down to 1µm. X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation at a scan rate of 4◦ min−1 was used for the phase determination of powders and coatings. Raman spectrum was recorded at room temperature with a HR800 Ramanscope that uses an Ar-ion laser with a wavelength of 514 nm for incident radiation.

3. Results and discussion 3.1. Microstructure and composition of the as-deposited LZ7C3/YSZ DCL TBCs Fig. 1 shows the surface morphologies of as-fabricated LZ7C3/YSZ DCL TBCs. As shown in Fig. 1a, the coating exhibits columnar structure and the column tips show a cauliflower-like appearance, which are very common for EB-PVD coatings. The intercolumnar gaps introduced due to the “shadowing effect” are also observed in Fig. 1a. The gaps are beneficial to improve the lifetime of the coatings since they can partially release thermal stress during high temperature service. From Fig. 1b, it is clear to find that the column tips have a well-defined 4-sided pyramidal shape of

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fluorite derivative structures and are similar to those seen in 7YSZ and other zirconates [24-26]. The pyramidal diameters are measured to be approximately 0.5-0.8 µm, which is much smaller than those of EB-PVD YSZ coating. This phenomenon can be explained by the highly branched columns structure of LZ7C3 EB-PVD coating, which has been reported in our former work [27]. Because of repetitive re-nucleation of columns during deposition, each column of LZ7C3 coating consists of a number of subcolumns with different misorientations and the formation of these subcolumns prevents the establishment of a bigger final diameter in a later stage of columnar growth [28]. Thus, the frequent branching of columns is estimated to be responsible for the smaller average diameter of the columns. The cross-section micrograph of as-deposited LZ7C3/YSZ DCL TBCs is shown in Fig. 2. The individual LZ7C3 and YSZ layers are apparent. YSZ was deposited on the bond coat as the bottom layer and LZ7C3 was deposited on the YSZ layer as the top layer. Fig. 2a shows that the thickness of LZ7C3 layer and YSZ layer are 䍐65 µm and 䍐40 µm, respectively. The interface is clearly presented while these two layers are bond well and there is no notable interruption of column morphology from the YSZ layer to the LZ7C3 top layer was observed as shown in Fig. 2b. From the cross-section SEM image, it is also easy to find that the column diameter of LZ7C3 is much smaller than that of YSZ. Because vapor pressures of its compositional oxide are different (Vapor pressures of the above oxides at 2773 K are 2×10 −5 atm, 8×10 −5 atm and 2×10−2 atm for ZrO2, La2O3 and CeO2, respectively), partial decomposition of rare earth-zirconia composite oxide could occur during EB-PVD process, which

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inevitably leads to composition derivation of the deposited ceramic coating from the original ingot [6,25,26,27]. In such case, it is necessary to optimize the chemical composition of original ingot and processing parameters for deposition of coatings in order to obtain the optimal phase with nearly stoichiometric composition. In our former work [19], it was found that the content of La2O3 in the LZ7C3 coatings had increased as compared to the stoichiometric value while the excess of La2O3 is detrimental to coating durability since it would absorb moisture from the air with the formation of La(OH)3, leading to the swelling and then spalling of the coating. Therefore, ingots with a reduced La2O3 content and an optimized EB-PVD processing condition are adopted for deposition of DCL coating in this study. Chemical compositions of areasĀA” and “B” are listed in Table 1. As shown in Table 1, the chemical compositions of areasĀB” is similar to those of theoretical value of LZ7C3 and the chemical compositions of areasĀA” is similar to those of YSZ ingot, indicating that the deposition parameters used in this work are reasonable. Elemental distributions along the cross section of DCL coating were determined by EDS and their results are shown in Fig. 2. Apparently, neither La2O3 nor CeO2 is distributed at the beginning of coating formation by taking into account the testing error. Whereas they increase rapidly when the thickness of DCL coating is approximately 60-70 µm and then they keep nearly constant. It suggests that the DCL coating has been fabricated appropriately by controlling the deposition parameters accordingly.

3.2. Phase evolution of LZ7C3/YSZ DCL TBCs during thermal shock testing

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In order to study the phase evolution of LZ7C3/YSZ DCL TBCs, the samples were moved out from the cyclic furnace after some certain cycles and then examined by X-ray diffraction and Raman spectrum. The XRD patterns of LZ7C3 powder and surfaces of LZ7C3/YSZ coating with different thermal cycling time are compared in Fig. 3. It can be seen from Fig. 3b that the surface of as-deposited coating is mainly crystallized in cubic fluorite and pyrochlore structures. However, for LZ7C3 powder, the XRD peaks which belong to the pyrochlore structure are stronger than those of the fluorite structure (Fig. 3a), while the situation is reversed in the as-deposited LZ7C3 coating (Fig. 3b). It indicates that a solid solution of La2(ZrxCe1−x)2O7 with a fluorite structure is preferentially formed in LZ7C3 coating compared with that of LZ with a pyrochlore structure, which is also found in the APS process[20]. In addition, it is obvious that the peaks which belong to the fluorite structure in the coating slight shift to the larger 2θ-value (the smaller d-value) when compared with LZ7C3 powder. The phenomenon might indicate that the former contains a high content of LZ in the solid solution of La2(ZrxCe1−x)2O7 with fluorite structure. As Zr4+ (0.079 nm) ion has a smaller ionic radius than that of Ce4+ (0.092 nm), the more LZ in the solid solution should result in the larger 2θ-value shift of the coating. After 2286 cycles, the diffraction peaks get sharper, indicating that the high temperature thermal shock test will lead to the re-crystallization of all the phases presenting in the DCL coating. As shown in Fig. 3c, several new XRD peaks which are identified as LaCrO3, Ce2O3 and m-ZrO2 appear after 4741 cycles. The presence of these phases could be attributed to the reaction between LZ7C3 and Cr that diffused out from the bond coat during

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thermal cycling. As previously reported by Thornton et al.[29], partial reduction of the Ce4+ ions to Ce3+ may be possible by chromium in the bond coat and the reaction can be expressed as: 6CeO2+2Cr=3Ce2O3+Cr2O3

(1)

Then Cr2O3 reacts with LZ7C3 to form LaCrO3: La2(ZrxCe1−x)2O7+ Cr2O3=2LaCrO3+2(1-x)CeO2+2xZrO2

(2)

The detail mechanism of Cr diffusion will be discussed below. It is interesting to find that the intensities of peaks belong to pyrochlore are enhanced and the XRD pattern of the DCL coating after thermal shock test is nearly the same as that of LZ7C3 powder except for a few weak peaks. The phase evolution of the fluorite structure to the pyrochlore structure of LZ7C3 EB-PVD coating could be explained as following: Due to the fast cooling rate during EB-PVD deposition process, LZ is mainly formed with the metastable fluorite phase and it dissolves in LC, which is similar to that observed in Sm2Zr2O7 coating prepared by EB-DVD [25]. However, this metastable fluorite phase would transform to pyrochlore phase above 1273 K. As a result, LZ with a pyrochlore structure will partially recrystallized from the La2(ZrxCe1−x)2O7 solid solution during thermal shock test, which can be substantiated by the appearance of the characteristic peak of pyrochlore structure. Except for phases that have been detected on the surface of DCL coating after 4741 cycles, La2O3 is also observed after 6734 cycles and the intensity of m-ZrO2 become stronger, indicating the partial decomposition and failure of LZ7C3 after thermal shock test.

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In comparison with XRD patterns, the Raman spectra (Fig. 4) provide consistent but complementary structural details of the solid solutions [30]. For LZ7C3 powder, the Raman bands at䍐288,䍐383 and䍐490 cm−1 are visible. The most intense Raman band at about 288 cm−1 corresponds to the Eg Raman band from Fd-3m space group of a pyrochlore structure and two vibrational frequencies at around 383 and 490 cm−1 are due to the T2g modes [31-33]. These three Raman bands of pyrochlores slightly shift to low frequencies (red shifts) as compared with those of LZ, suggesting bond elongation and weakening due to a much bigger Ce4+ replacing its Zr4+ host. A weak Raman bands at 587 cm−1 is also detected in LZ7C3 powder, which is the characteristics of La2Ce2O7 [6,34].However, for the as-deposited coating, the Raman bands at䍐445 and䍐587 cm−1 which belong to fluorites are the two most intense peaks while peaks at 䍐383 and䍐490 cm−1 disappear. In addition, the Raman peak at䍐288 cm−1 is broadening. The phenomenon indicates that the as-deposited coating is mainly in a fluorite structure with a trace of the pyrochlore structure, which is consistent with the result of XRD. As shown in Fig. 4c, the intensity of peak at 288 cm−1 is enhanced and the Raman bands at䍐490 cm−1 belongs to the T2g modes of the pyrochlore structure appears again after 2286 cycles. On the other hand, after 2286 cycles, the characteristic peak of the fluorite structure at 䍐445 cm−1 is disappeared and the Raman peak at 䍐587 cm−1 becomes weaker as compared with the as-deposited coating, suggesting a phase transformation from the fluorite structure to the pyrochlore structure starts at or before 2286 cycles. From Fig. 4d, it can be seen that Raman peaks at 䍐288 and䍐490 cm−1 get sharper as compared with that of Fig. 4c and

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the peaks at 䍐383 cm−1 appears after 4741 cycles, indicating a continuous recrystallized of LZ with a pyrochlore structure from the La2(ZrxCe1−x)2O7 solid solution during thermal shock test. After 6734 cycles, the Raman peaks are nearly the same as those of LZ7C3 powder except for a weak peak at 䍐816 cm−1 which is assigned to La2O3 [6]. The Raman peaks of Ce2O3, m-ZrO2 and LaCrO3 are too weak to be detected due to their rather low content in coating. Combining with the results of XRD and Raman spectra, it can be concluded that a phase transformation from the fluorite structure to the pyrochlore structure took place during thermal shock test.

3.3. Microstructure evolution of LZ7C3/YSZ DCL TBCs during thermal shock testing Fig. 5 shows the SEM images of LZ7C3/YSZ DCL coatings surface before and after different thermal cycles. As shown in Fig.5e-f, although grain boundaries between columnar grains have almost disappeared after thermal shock test, column tips still present cauliflower-like appearance which is the characteristic microstructure of EB-PVD coating, indicating that LZ7C3 material has a low sintering rate and good thermal shock resistance. This phenomenon is consistent with the results reported in Refs.[6]. It is obvious that partial densification is also observed in Fig. 5b–d (as well as in Fig.5f-h). The densified coatings would reduce thermal stress/strain tolerance, which is detrimental to the durability of TBCs [6]. From Fig. 5a and Fig. 5e, the growth surface is composed of column tips bound by curved triangular facets, no microcrack occurs on the surface of as-deposited coating (Fig.1a). Microcracks

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originated between the clusters of columns or within the clusters are apparently after thermal shock test as shown in Fig. 5b–d. The width of the microcracks occurring on the coatings surfaces after different thermal cycles is measured from the high magnification SEM images (Fig. 5f-h). The microcracks are severally measured by the averaged value of 30 points, and the averaged width of microcracks on surfaces of samples after 2286, 4741 and 6734 cycles are about 2.41, 3.47 and 6.56 µm respectively. The possible reason for the surface cracks is that the ceramic coating is subjected to a tensile stress during thermal shock test. One more explanation for the occurrence of microcracks could be attributed to the reduction–oxidation of cerium oxide. The LZ7C3 ingot is heated in vacuum by electron beam source during deposition which is a reduced atmosphere and the cerium oxide is reduced to Ce3+ to a certain extent. When the reduced cerium oxide is heat treated in air, it would be oxidized and the coating would swell [35]. Therefore, the microcracks are easy to be formed due to the presence of cerium in both Ce3+ and Ce4+ oxidation states within the coating surface. In addition, it is expected that phase transformation from the fluorite structure to the pyrochlore structure as confirmed by XRD and Raman spectroscopy in this study would also generate tensile stress due to atomic rearrangement and volume change in these progresses. Fig. 6 shows the surface morphologies of the DCL TBCs after failure at 6734 cycles. Chemical compositions of the selected areas in Fig. 6c are presented in Table 2. Areas “A” and “B” are mainly composed of La, Ce, Zr and O, and the relative content of each element in area “A” is similar to that in area “B”, suggesting that

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delamination occurs in the interior of the LZ7C3 coating. The delamination of LZ7C3 layer can be ascribed to partial sintering of LZ7C3 coating surface after long-term thermal shock test which results in the generation of plane tensile stress in the outer region of LZ7C3 coating. Parallel cracks would initiate and propagate when the plane tensile stress is accumulated to some extent, leading to the spallation of the outer region of LZ7C3 coating. This process is repeated and LZ7C3 coating spalls gradually layer by layer [36,37]. AreaĀC” mainly contains Y, Zr and O, indicating entire spallation of the LZ7C3 coating. The EDS analysis of areaĀD” shows that Ni, Cr, Al, O are detected and the result indicates that the spallation location is occurred at the interface between the YSZ layer and the bond coat. The cross-sectional SEM images of the DCL coating with different thermal cycling time are presented in Fig. 7. After 2286 cycles, a thermally grown oxide (TGO) layer with a thickness of 䍐3.92 µm between the YSZ and bond coat is clearly observed in Fig. 7b. As shown in Fig. 7b, the TGO layer displays two different structures. The outer mixed zone of about 1 µm thickness is slightly porous and consists of mainly Al2O3 and some oxides of Ni and Cr as proved by EDS. While the inner part is dense and the composition is mostly α-Al2O3, only a very small quantity of NiO coexisted (the EDS results are not shown here). Although some vertical cracks can be observed, there is no evidence of localized interface separations between the YSZ and TGO layer or between the LZ7C3 and YSZ layer (Fig. 7a-c). After 2286 cycles, the feathery structure on the 7YSZ columnar surfaces has disappeared while the LZ7C3 feathery structure appears to have undergone a slower evolution (Fig.7c),

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similar to that observed in Sm2Zr2O7/YSZ and Gd2Zr2O7/YSZ DCL coating [23,38]. It can be seen that horizontal cracks begin to initiate at the LZ7C3/YSZ interface and within the TGO layer after 4741 cycles (Fig.7d and e). As shown in Fig. 7d, transverse cracks occur not only at the interface between the LZ7C3 and YSZ coatings, but also in the interior of the LZ7C3 layer. The YSZ/LZ7C3 interface is of particular interest in Fig. 7f. Although the top LZ7C3 coating is still attached to the YSZ, there is a dense region at the interface within the LZ7C3 coating. Upon further cycling, the microcraks apparently propagate and coalesce and the DCL TBCs eventually undergo delamination at interfaces. Fig. 7g exhibits a typical cleavage at the interface between LZ7C3 and YSZ layer. It is evident that the whole LZ7C3 layer spalls off in some areas. While in some locations, a thinner LZ7C3 remains is observed on the surface of the YSZ coating (Fig 7h). These phenomena are consistent with the results showed in Fig. 6. In Fig. 7i, some large vertical cracks pass through the two ceramic layers and further propagate down to BC surface. In this case, it is considered that the oxygen in the air could go through those cracks and induce an abnormal oxidation of BC. Although the cracking and delamination of the coating occurs after 6734 cycles, TBCs in some locations are still intact after long-term cycling as shown in Fig. 7j. It is worthwhile to note that both LZ7C3 and YSZ still remain a column structure. As shown in Fig.7, there is a continuous increase in the thickness of the TGO layer during thermal shock test. The growth of the TGO layer at the TBC–BC interface plays a very important role in determining the lifetime of TBC systems. Due

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to the TGO growth, stresses accumulate in the TBC system which ultimately results in the failure of the system. It is generally known that the location of delamination and cracks is mostly within the TGO layer or along the TGO/bond coat interface for EB-PVD coatings [39]. Therefore, although no critical TGO thickness for TBC failure exists, a slower TGO growth is appreciated in TBC systems [23]. In order to compare the response of the DCL coatings with that of standard YSZ coatings deposited on the same bond coat/substrate system under the same deposition conditions, the TGO thickness versus the thermal exposure time has been plotted for both coatings in Fig. 8. The data have been fitted to a parabolic TGO growth law: (x−x0)2=kpt, where x0 is the “as-deposited” TGO thickness, x is the thickness after accumulating a time t at 1373 K [38]. To reduce the scatter in the data, TGO thicknesses have been measured at a minimum of three locations for TBCs. As shown in Fig. 8, the DCL coating has a lower TGO growth rate (0.0667µm2/ h) than YSZ whose parabolic rate constant is 0.0958 µm2/ h, which can be explained by the lower oxygen diffusivity of LZ7C3. A lower TGO growth rate would be one of the reasons for the longer thermal shock life of the DCL TBCs.

3.4. Interdiffusion of LZ7C3/YSZ DCL TBCs during thermal shock testing EDS map scanning analysis of the DCL coating after 6734 cycles are shown in Fig.9. As expected, TGO is rich in Al and O as the content of Al and O are very high at the interface of the ceramic coats and bond coat, confirming that bond coat has been deeply oxidized during long-term thermal exposure. The distribution of the

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corresponding elements proves that there is no obvious diffusion of La, Ce and Zr from one layer into the other one by taking into account the testing error. However, a bit of Y element has diffused into the LZ7C3 top layer after 6734 cycles, which is evident in Fig. 9. This phenomenon suggests a faster diffusion coefficient of yttrium out of the tetragonal phase than that of La out of the mix phase of pyrochlore and fluorite. The outward diffusion of yttrium is not desirable in the DCL TBC systems as it may cause a phase transformation of YSZ layer from t′-ZrO2 to m-ZrO2 when the Y concentration is reduced beneath a critical level. In addition, the diffusion of Y may be responsible for the partial sintering at the bottom of the LZ7C3 columns as shown in Fig. 7f. Opposite to the current finding, in Sm2Zr2O7/YSZ DCL TBCs [38] or Gd 2Zr2O7/YSZ DCL TBCs [23] prepared by EB-PVD method, Sm (or Gd) has been found to diffuse into the 7YSZ in a limited thickness region while no yttrium outward diffusion into the pyrochlores. Identification of the reasons for this opposite behavior is the topic of ongoing research. As shown in Fig. 9, it is surprising to find that the alloying element of Cr in bond coat has partially diffused out and even extends to the LZ7C3 layer. The outward diffusion of Cr is also confirmed by the high magnification SEM image of the sample surface after 6734 cycles and corresponding EDS result as shown in Fig. 6d and f. It can be seen that small grains containing mainly in La, Cr, O elements are formed on the surface of sample after a long-term exposure. This phenomenon is also consistent with the XRD results in section 3.2 where LaCrO3 is detected on the coating surface after 6734 cycles, indicating that Cr has outward diffused to the sample surface.

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However, no obvious Cr diffusion can be observed in single YSZ coating after 6052 cycles from the EDS elements mapping analysis showed in Fig. 10. Since these two coatings were deposited on the same bond coat/substrate system and underwent the same thermal shocking conditions, the diffusion of Cr from bond coat to ceramic coat can be attributed to the presence of LZ7C3 top layer in the DCL TBCs. In the case of single YSZ coating, Cr should diffuse from bond coat to YSZ because of the present of a Cr concentration gradient. However, since the diffusion coefficient is extremely low, the content of Cr in YSZ coating is under the detection range even after a long-time thermal cycling. In the case of LZ7C3/YSZ DCL coating, a chemical reaction between CeO2 and Cr will take place as soon as Cr diffuses into LZ7C3 layer, which will break the local diffusion balance. As a result, Cr could migrate continuously from bond coat into LZ7C3 layer though the column grain boundaries. The migrating Cr atoms are eventually absorbed by the formation of LaCrO3 as having been substantiated by XRD (Fig. 3). Evidently these reactions remove Cr from YSZ columnar grain boundaries at a much faster rate than does the process of diffusion into the YSZ grains. The accumulation of Cr in LZ7C3 layer which is detected in the EDS map scanning analysis is therefore obtained in accessible time (̚561 h) and temperature (1373 K). The crystal structure of LaCrO3 below 513 K and above 553 K is orthorhombic- and rhombohedral-distorted perovskite structure, respectively. During the thermal exposure, it undergoes a phase transformation from orthorhombic to rhombohedral structure, which might introduce a discrete volume change. That could be one of the reasons for the initiation of microcracks within the

19

ceramic coatings as shown in Fig. 7. In addition, the abundant Cr diffusion is undesirable for the selective oxidation of the bond coat when it is reduced beneath a certain level, which may lead to a less stable TGO layer. What’s more, a strong outward diffusion of Cr from BC would be the main reason for the emergence and enlargement of the micropores in the DCL TBCs as shown in Fig. 7. It has been reported in our previous paper and by other studies that LZ7C3 sinters less than 7YSZ when parallel columns are considered. However, opposite findings are obtained in present work. After thermal cycling, LZ7C3 top layer is more compact than YSZ layer in the DCL TBCs as shown in Fig. 7j. This abnormal phenomenon would be attributed to the abundant Cr cation migrating in. In order to understand clearly the influence of Cr diffusion on the durability of the DCL TBCs and alleviate its adverse effects, more fundamental work is required in the future. Therefore, the phase transformation of LZ7C3, the abnormal oxidation of bond coat, and the outward diffusion of Y and Cr alloying element into LZ7C3 coating would be the primary factors for the spallation of LZ7C3/YSZ thermal barrier coating.

4. Conclusions LZ7C3/YSZ double-ceramic-layer TBCs were deposited by EB-PVD and their thermal shock behavior was systematically investigated. The findings can be summarized as follows: (1) A fluorite to pyrochlore ordering takes place for LZ7C3 coating during thermal shock test as detected by X-ray diffraction and Raman spectra. It seems that this 20

phase change may affect the durability of the DCL TBCs. (2) The LZ7C3 layer spalls from the YSZ surface after certain thermal cycles and the cracks occur not only at the interface between the LZ7C3 and YSZ layer, but also inside the LZ7C3 coating. Apparently, the occurrence of microcracks could be partially attributed to the phase transformation and the reduction–oxidation of cerium oxide of LZ7C3. (3) After cycling, some diffusion of Y from YSZ to LZ7C3 layer is observed. This Y diffusion is detrimental to the phase stability of YSZ. Additionally, an obvious outward diffusion of Cr from bond coat into LZ7C3 layer takes place due to the chemical reaction of LZ7C3 and Cr.

Acknowledgement The authors would like to thank Z.J. Xu for the bond coats preparation and Z.H. Deng for the careful manufacture of ceramic coats. Financial support from National Natural Science Foundation of China (21171160 and 21001017) and the projects of Lotus Scholars Program is also gratefully acknowledged.

References

[1] Q.L. Wei, H.B. Guo, S.K Gong, H.B Xu, Novel microstructure of EB-PVD double ceramic layered thermal barrier coatings, Thin Solid Films 516 (2008) 5736–5739. [2] X.L Chen, Y.F. Zhang, X.H. Zhong, Z.H. Xu, J.F. Zhang, Y.L. Cheng, Y. Zhao, Y.J

21

Liu, X.Z. Fan, Y. Wang, H.M. Ma, X.Q. Cao, Thermal cycling behaviors of the plasma sprayed thermal barrier coatings of hexaluminates with magnetoplumbite structure, J. Eur. Ceram. Soc. 30 (2010) 1649–1657. [3] W. Ma, D. E. Mack, R. Vaßen, D. Stöver, Perovskite-Type Strontium Zirconate as a New Material for Thermal Barrier Coatings, J. Am. Ceram. Soc. 91 (2008) 2630–2635. [4] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas-turbine engine applications, Science 296 (2002) 280–284. [5] X.Y. Xie, H.B. Guo, S.K. Gong, H.B. Xu, Lanthanum–titanium–aluminum oxide: A novel thermal barrier coating material for applications at 1300◦C, J. Eur. Ceram. Soc. 31 (2011) 1677–1683. [6] Z.H. Xu, S.M. He, L.M. He, R.D. Mu, G.H. Huang, X.Q. Cao, Novel thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition, J. Alloys Compd. 509 (2011) 4273–4283. [7] D.M. Zhu, R.A. Miller, Development of Advanced Low Conductivity Thermal Barrier Coatings, Int. J. Appl. Ceram. Technol. 1 (2004) 86-94. [8] N.P. Padture, P.G. Klemens, Low Thermal Conductivity in Garnets, J. Am. Ceram. Soc. 80 (1997)1018–20. [9] R. Vassen, X.Q. Cao, F. Tietz, D. Basu, Detlev Stöver, Zirconates as New Materials for Thermal Barrier Coatings, J. Am. Ceram. Soc. 83(2000) 2023–28. [10] J. Wu, X.Z. Wei, N.P. Padture, P.G. Klemens, M. Gell,

22

Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal Barrier Coating Applications, J. Am. Ceram. Soc. 85 (2002) 3031–3035. [11] Z.G. Liu, J.H. Ouyang, Y. Zhou, Structural evolution and thermophysical properties of (SmxGd1–x)2Zr2O7 (0 ≤ x ≤ 1.0) ceramic, J. Alloys Compd. 472(2009) 319–324. [12] Z.G. Liu, J.H. Ouyang, Y. Zhou, S. Li, High-temperature hot corrosion behavior of gadolinium zirconate by vanadium pentoxide and sodium sulfate in air, J. Eur. Ceram. Soc. 30(2010) 2707–2713.  ȏͳ͵ȐZ.G Liu, J.H. Ouyang, Y. Zhou, J. Li, X.L. Xia, Densification, structure and thermophysical properties of ytterbium–gadolinium zirconate ceramics, Int. J. Appl. Ceram. Technol. 6(2009) 485–491.  [14] X.Q. Cao, R. Vasen, W. Fischer, F. Tietz, W. Jungen, D. Stöver, Lanthanum–cerium oxide as a thermal barrier-coating material for high temperature applications, Adv. Mater. 17 (2003) 1438–1442. [15] X.Q. Cao, Y.F. Zhang, J.F. Zhang, X.H. Zhong, Y. Wang, H.M. Ma, Failure of the plasma-sprayed coating of lanthanum hexaluminate, J. Eur. Ceram. Soc. 28 (2008) 1979–1986. [16] R. Gadow, M. Lischka, Lanthanum hexaaluminate-novel thermal barrier coatings for gas turbine applications-materials and process development, Surf. Coat. Technol. 151-152 (2002) 392–399. [17] X.Q. Cao, R. Vassen, F. Tietz, D. Stoever, New double-ceramic-layer thermal barrier coatings based on zirconia–rare earth composite oxides, J. Eur. Ceram. Soc. 26

23

(2006) 247–251. [18] X.Q. Cao, J.Y. Li, X.H. Zhong, J.F. Zhang, Y.F. Zhang, R. Vassen, D. Stoever, La2(Zr0.7Ce0.3)2O7—A new oxide ceramic material with high sintering-resistance, Mater. Lett. 62 (2008) 2667–2669. [19] Z.H. Xu, L.M. He, X.H. Zhong, R.D. Mu, S.M. He, X.Q. Cao, Thermal barrier coating of lanthanum–zirconium–cerium composite oxide made by electron beam-physical vapor deposition, J. Alloys Compd. 478 (2009) 168–172. [20] S.M. Zhao, L.J. Gu, Y. Zhao, W.Z. Huang, L. Zhu, X.Z. Fan, B.L. Zou, X.Q. Cao, Thermal cycling behavior and failure mechanism of La2(Zr0.7Ce0.3)2O7/Eu3+-doped 8YSZ thermal barrier coating prepared by atmosphericplasma spraying, J. Alloys Compd. 580 (2013) 101–107. [21] A.G. Evans, D.R. Mumn, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Mechanisms controlling the durability of thermal barrier coatings, Progress in Materials Science, 46 (2001) 505-553. [22] A.G. Evans, D.R. Clarke, C.G. Levi, The influence of oxides on the performance of advanced gas turbines, J. Eur. Ceram. Soc. 28 (2008) 1405–1419. [23] A.U. Munawar, U. Schulz, G. Cerri, H. Lau, Microstructure and cyclic lifetime of Gd and Dy-containing EB-PVD TBCs deposited as single and double-layer on various bond coats, Surf. Coat. Technol. 245 (2014) 92–101. [24] H. Zhao, F. Yu, T.D. Bennett, H.N.G. Wadley, Acta Mater. 54 (2006) 5195. [25] H. Zhao, C.G. Levi, H.N.G. Wadley, Vapor deposited samarium zirconate thermal barrier coatings, Surf. Coat. Technol. 203 (2009) 3157–3167.

24

[26] B. Saruhan, P. Francois, K. Fritscher, U. Schulz, EB-PVD processing of pyrochlore-structured La2Zr2O7-based TBCs, Surf. Coat. Technol. 182 (2004) 175–183. [27] Z.H. Xu, L.M. He, R.D. Mu, S.M. He, X.H. Zhong, X.Q. Cao, Influence of the deposition energy on the composition and thermal cycling behavior of La2(Zr0.7Ce0.3)2O7coatings, J. Eur. Ceram. Soc. 29 (2009) 1771–1779. [28] U. Schulz, K. Fritscher, M. Peters, EB-PVD Y2O3- and CeO2/Y2O3-stabilized zirconia thermal barrier coatings- crystal habit and phase composition, Surf. Coat. Technol. 82 (1996) 259-269. [29] J. Thornton, A. Majumdar, G. McAdam, Enhanced cerium migration in ceria-stabilished zirconia, Surf. Coat. Technol. 94-95 (2009) 112-117. [30] Y.F. Wang, F. Yang, P. Xiao, Role and determining factor of substitutional defects on thermal conductivity: A study of La2(Zr1-xBx)2O7 (B = Hf, Ce, 0≤x≤0.5) pyrochlore solid solutions, Acta Mater. 68 (2014) 106–115. [31] Z.X. Qu, C.L. Wan, W. Pan, Thermal Expansion and Defect Chemistry of MgO-Doped Sm2Zr2O7, Chem. Mater. 19 (2007) 4913-4918. [32] C.J. Wang, Y. Wang, X.Z. Fan, W.Z. Huang, B.L. Zou, X.Q. Cao, Preparation and thermophysical properties of La2(Zr0.7Ce0.3)2O7ceramic via sol–gel process. Surf. Coat. Technol. 212 (2012) 88–93. [33] F.X Zhang, B. Manoun, S.K. Saxena, C.S. Zha, Structure change of pyrochlore Sm2Ti2O7 at high pressures, Appl. Phys. Lett. 86 (2005) 181906. [34] C.J. Wang, W.Z. Huang, Y. Wang, Y.L. Cheng, B.L. Zou, X.Z. Fan, J. L. Yang,

25

X.Q. Cao, Synthesis of monodispersed La2Ce2O7 nanocrystals via hydrothermal method:A study of crystal growth and sintering behavior, Int J Refract Met Hard Mater 2012(31):242-246. [35] Z.H. Xu, L.M. He, Y. Zhao, R.D. Mu, S.M. He, X.Q. Cao, Composition, structure evolution and cyclic oxidation behavior of La2(Zr0.7Ce0.3)2O7 EB-PVD TBCs, J. Alloys Compd. 491 (2010) 729–736. [36] L. Guo, H.B. Guo, H. Peng, S.K. Gong, Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings, J. Eur. Ceram. Soc. 34 (2014) 1255–1263. [37] W. Ma, S.K. Gong, H.F. Li, H.B. Xu, Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition, Surf. Coat. Technol. 202 (2008) 2704–2708. [38] H.B. Zhao, M.R. Begley, A. Heuer, R. Sharghi-Moshtaghin, H.N.G. Wadley, Reaction, transformation and delamination of samarium zirconate thermal barrier coatings, Surf. Coat. Technol. 205 (2011) 4355–4365. [39] Z.H. Xu, X. Zhou, K. Wang, J.W. Dai, L.M. He, Thermal barrier coatings of new rare-earth composite oxide by EB-PVD, J. Alloys Compd. 587 (2014) 126-132.

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Figure and Table Caption Fig. 1. SEM micrographs of the as-deposited LZ7C3/YSZ DCL coating top surface with different magnifications: (a) 300×and (b) 20,000×, the selected location A of (a). Fig. 2. Cross-sectional micrograph of the as-deposited LZ7C3/YSZ DCL coating and corresponding EDS line scanning. Fig. 3. XRD patterns of LZ7C3 powder (a), the as-deposited DCL coating (b), and coating during thermal shock test (c–d). Fig. 4. Raman spectra of LZ7C3 powder (a), the as-deposited DCL coating (b), and coating during thermal shock test (c–d). Fig. 5. SEM micrographs of LZ7C3/YSZ DCL coatings surface before (a) and after

27

thermal shock test: (b) 2286 cycles; (c) 4741cycles and 6734 cycles. Fig. 6. SEM surface morphologies of the DCL coating after spallation failure during thermal shock test. Fig. 7. Cross-sectional SEM images of the LZ7C3/YSZ DCL coating during thermal shock test. Fig. 8. TGO growth plotted against thermal exposure time for LZ7C3/YSZ DCL coating and YSZ coating Fig. 9 Cross-section SEM micrograph and corresponding element mapping of the DCL coating after 6734 cycles.

Fig. 10 Cross-section SEM micrograph and corresponding element mapping of YSZ coating after 6052 cycles.

Table 1. Chemical compositions of areas “A” and “B” as marked in Fig. 2. Table 2. Chemical compositions of selected areas in Fig. 6c (in wt.%).

28

Fig. 1. SEM micrographs of the as-deposited LZ7C3/YSZ DCL coating top surface with different magnifications: (a) 300×and (b) 20,000×, the selected location A of (a).

29

Y

Zr

Ce

La

Fig. 2. Cross-sectional micrograph of the as-deposited LZ7C3/YSZ DCL coating and corresponding EDS line scanning.

30

F F:Fluorite 1:LaCrO3

P

3:m-ZrO2 4:Ce2O3 P F 1 1 FP 4 P1 FPP

1 P FP 2 1 3 1 142 3 P 1 2 F

(e) P (d)

1P 4 3 1 1 3F

(c)

FP

P F FP

FP

FP

FP FP

(b)

FP

FP

(a)

P F

P 10

20

30

1

P

FP P

P

FP

FP

PP

F P

P F

P 40

P

P

FP F

F P

P:Pyrochlore 2:La2O3

50



FP 60

P F 70

80

90

2θ (deg.)

Fig. 3. XRD patterns of LZ7C3 powder (a), the as-deposited DCL coating (b), and coating during thermal shock test (c–d).

31

Fig. 4. Raman spectra of LZ7C3 powder (a), the as-deposited DCL coating (b), and coating during thermal shock test (c–d).

32

Fig. 5. SEM micrographs of LZ7C3/YSZ DCL coatings surface before (a) and after thermal shock test: (b) 2286 cycles; (c) 4741cycles and 6734 cycles.

33

(e)

Fig. 6. SEM surface morphologies of the DCL coating after spallation failure during thermal shock test.

34

Fig. 7. Cross-sectional SEM images of the LZ7C3/YSZ DCL coating during thermal shock test.

35

8 7

Single YSZ layer 2 kp ~ 0.0958 µm /h

TGO thickness (µm)

6 5

Bilayer LZ7C3/YSZ 2 kp ~ 0.0667 µm /h

4 3 2 1 0

100

200

300

400

500

600

thermal exposure time (h)

Fig. 8. TGO growth plotted against thermal exposure time for LZ7C3/YSZ DCL coating and YSZ coating

36

100 µm

Electron Image 1

Fig. 9. Cross-section SEM micrograph and corresponding element mapping of the DCL coating after 6734 cycles.

37

100 µm

Electron Image 1

Fig. 10. Cross-section SEM micrograph and corresponding element mapping of YSZ coating after 6052 cycles.

38

Table 1. Chemical compositions of areas “A” and “B” as marked in Fig. 2 La (wt.%)

Ce (wt.%)

Zr (wt.%)

Y (wt.%)

O (wt.%)

A

--

--

73.66

5.43

20.91

B

42.25

13.04

22.80

--

21.91

39

Table 2. Chemical compositions of selected areas in Fig. 6c (in wt.%). La

Ce

Zr

Y

O

A

48.92

12.26

18.51

--

17.60

--

2.71

--

B

49.53

13.78

17.21

--

16.08

--

3.40

--

C

--

--

72.60

5.20

24.20

--

--

--

D

--

--

--

--

12.33

52.83

16.97

17.86

40

Ni

Cr

Al

Highlights

1) No interruption of column morphology from YSZ to LZ7C3 layer in TBCs. 2) A fluorite to pyrochlore ordering occurs for LZ7C3 during thermal shocking. 3) Some diffusion of Y from YSZ to LZ7C3 layer is occurred after thermal shocking. 4) Outward diffusion of Cr takes place due to the chemical reaction of LZ7C3 and Cr. 5) The delaminations occur at interface of LZ7C3/YSZ and inside the LZ7C3 coating.

41