In order to overcome the shortcoming of the LZA( LaZnAl11O19) coating,a LZA/YSZ double-ceramic-layer (DCL) TBC was produced. As shown in Fig.1a, the thickness of the LZA layer and YSZ layer is ～150 um and ～200 um, respectively. The adhesion of the LZA coating to the YSZ coating is perfect in the as-sprayed DCL TBC. Besides, no obvious deficiencies can be observed at the interface of the YSZ coating and bond coat. Fig.1b shows thesurface photo and cross-sectional SEM micrograph of the LZA/YsZ coating after thermal cycling failure. It affirms that the DCI coating spalled bit by bit from the LZA coating to the inner YSZ layer during thermal cycling, which is much different from the LZA single-layer coating. As shown in Fig.1c, the LZA/YSZ DCL TBC exhibits a thermal cycling life-time of ～669 cycles to a slight failure, which is almost 13times as long as that of the LZA single-layer TBC. The result indicates that the stress concentration caused by thermal expansion mismatch between the LZA coating and bond coat was significantly reduced by using the YSZ buffer layer.
As illustrated in Fig.1b, large cracks could be observed at the YsZ/bond coat interface while the interface between LZA and YSZ seems to be intact. The formation of vertical cracks in the LZA layer could relieve the thermal stresses and make it to be more strain tolerant, which can explain why few interface cracks formed at the LZA/YSZ interface. Due to the long-term heating, an obvious TGO layer with a thickness of ～3.2 um can be observed(Fig.1b). The TGO thickness was measured several times by the averaged value of 20 points. Therefore, the failure mode seems to be the same as the single YSZ coating, which is mainly attributed to TGO growth stresses and the thermal expansion mismatch between the YSZ coating and bond coat. Such a DCL coating also exhibits a thermal cycling lifetime that is much longer than that of the YSZ coating(392cycles). the average thickness of the TGO layer in the YsZ coating is ～6.8 um after 392 cycles.Therefore, the longer thermal cycling lifetime of the LZA/YSZ DCL coating may be due to the relatively lower TGO growth rate caused by the inferior oxygen conductivity of LZA.Compared with the LMA coating, the LZA coating has a higher TEC and less thermal shrinkage due to the relatively lower amorphous phase, and that is the reason why the LZA/YSZ coating has a longer thermal cycling lifetime than the LMA/YsZ coating(Fig.1c). Based on these results,a DCL coating based on LZA/YSZ might be a promising candidate for TBC applications at higher temperatures.
Fig.1Photos of the surface and SEM micrographs of the LaZnAl11O19/YSZ coating:(a) before and (b) after thermal cycling;(c) comparison of the thermal cycling lifetime of different TBCs.
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