To evaluate the high-temperature capability and phase stability, TGA-DSC curves of as-sprayed LC coating and LCT coating were mea-sured and illustrated in Fig.1. It is clear from TGA curves that there is no obvious mass change in the tested temperature range, only less than 1% of mass loss, which may be ascribed to the evaporation of physically absorbed water. And it can be seen from DSC curves that there is nei-ther endothermic peak nor exothermic peak up to 1523 K. These results reveal that no thermal decomposition or phase instability occurred in the coatings within the experimental temperatures. However, the test time is too short due to the rapid heating rate (20 K/min) and thus it is necessary to explore the thermal stability at rising temperatures and during long-term aging.
Fig.1.TGA-DSC curves of as-sprayed LC coating and LCT coating.
Based on the TGA-DSC results, some coating samples were sepa-rately quenched into water(about 298 K) from 623 K,973 K and 1273K for 1 h while others were respectively annealed from 40 h up to 1000h at 1573 K. All heat treated samples were then analyzed by XRD, and the results are shown in Fig.2. For LCT coatings, the fluorite structure is retained after water quenching but with a few weak peaks corresponding to La2O3 and La(OH)3. After long-term annealing at 1573K, the peaks of La2O3 and La(OH)3 are invisible due to the decomposition of La(OH)3 followed by the incorporation of La2O3 into the solid solution at high temperature, which could be confirmed by the slight shift of LCT peaks to the lower diffraction angles above 1273 K. With the aging time LCT peaks become sharper, meaning the increased crystal size with better crystallinity. Except for the above changes, no much difference among the XRD patterns was observed in LCT coatings. Accordingly, it can be concluded that LCT coating is thermally stable in the temperature range of interest for TBC applications at least 1573 K.Similarly, LC coating has the same case compared to LCT coating.
Fig.2. Change of XRD patterns of LCT coating (a) as well as LC coating(b):(1)as-deposited coating,(2)-(4) coatings quenched from indicated temperatures to298K,(5)-(8) coatings annealed at 1573 K for 40h,100h,300 h and 1000h, respectively.
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