High-temperature Performance and Failure Behavior of A6B2O17 (A = Hf, Zr; B = Ta, Nb) High-Entropy Thermal Barrier Coatings Prepared by Atmospheric Plasma Spraying

High-entropy A6B2O17 (A = Hf, Zr; B = Ta, Nb)/8YSZ double-layer ceramic coatings with three compositions are fabricated by atmospheric plasma spraying (APS). Thermal cycling tests are performed at 1150 °C with a 5 min heating hold followed by 5 min air cooling, while isothermal oxidation tests are performed at 1150 °C. Among the three coatings, H3Z1T2N1 coating exhibits the longest thermal cycling lifetime (1823 ± 153 cycles), followed by the H1Z1T2N1 coating (1123 ± 136 cycles) and H1Z3T2N1 coating (826 ± 114 cycles). After 180 h oxidation, the thermally grown oxide (TGO) thicknesses are 5 μm, 5.6 μm, and 6.6 μm, respectively. The H3Z1T2N1 coating possesses the lowest oxidation rate constant (kp = 0.1263 μm2·h−1). This superior performance is attributed to dense vertical cracks induced by higher thermal expansion, which relieve thermal stress, suppress oxygen ingress, and delay interfacial failure.