(Ho 0.2 Er 0.2 Tm 0.2 Yb 0.2 Lu 0.2 ) ₂ Ce ₂ O ₇ : A Promising Thermal Barrier Coating Material With Superior CMAS Corrosion Resistance

A single‐phase defect‐fluorite‐structured (Ho 0.2 Er 0.2 Tm 0.2 Yb 0.2 Lu 0.2 ) 2 Ce 2 O 7 high‐entropy ceramic is successfully prepared by high‐temperature solid‐state reaction method at 1600°C for 5 h. The phase evolution, microstructure, thermophysical properties, and molten silicate environment deposits (CaO‐MgO‐Al 2 O 3 ‐SiO 2 , CMAS) corrosion behavior of the ceramic were systematically investigated. The results show that the mixed rare‐earth oxide nanopowders will completely formed a solid solution into a single‐phase high‐entropy ceramic after calcination at 1600°C for 5 h. In addition, it has a low thermal conductivity of 1.19 W·m −1 ·K −1 at 1000°C, high thermal expansion coefficient of 11.85 × 10 −6 K −1 at 1200°C, excellent high‐temperature phase stability at 1500°C, and slow grain growth of 6.8 ± 0.55 nm/h during thermal exposure at 1500°C for 200 h. Furthermore, the high‐entropy ceramic exhibits excellent CMAS corrosion resistance at 1400°C, mainly due to the rapid formation of a continuous and dense single‐phase high‐entropy apatite‐type structure (Ca x RE (10 − x ) (SiO 4 ) 6 O 2 ) barrier layer at the corrosion front. Therefore, this study provides a feasible solution for the development of new thermal barrier coating materials for aircraft engines suitable for the next generation of aero‐engines operating in environments above 1400°C.