Structural evolution and failure mechanisms of APS tantalate high-entropy ceramic coatings in response to thermal cycle up to 1500 °C

Thermal barrier coatings (TBCs) with high working temperatures and long service life are indispensable for the hot-end components of gas turbines and aircraft engines. This work designs and verifies that tantalate high-entropy ceramic (HEC) coatings are excellent TBCs with working temperatures reaching 1500 °C. We reveal the structural evolution and failure mechanisms of tantalate HEC coatings synthesized via air plasma spraying (APS). After they are subjected to thermal shock at 1500 °C for 614 cycles, thermal fatigue at 1150 °C for 12,830 cycles, and annealing at 1100 °C for 384 h. The thermal stress caused by the temperature gradient, differences in thermal expansion coefficients (TECs), and mechanical properties between ceramic coatings and bond coat (BC) lead to the spalling of coatings during thermal shock, while the effects of BC oxidation are limited. In thermal fatigue, the accumulative thermal stress between BC and thermally grown oxides (TGO) is higher than the fracture resistance when the h/R ratio is higher than 0.32 (h and R are the TGO thickness and undulation radius, respectively), which mainly leads to the spalling of coatings. Additionally, the effects of coating sintering and stiffness are also considered, which lead to surficial spalling during the high-temperature service. Two different failure mechanisms are proposed based on their microstructural evolution, and synthesized tantalate HEC coatings can be used at temperatures up to 1500 °C, which further promotes the design and application of high-performance TBCs.