High-Entropy Ceramics for Thermal Barrier Coatings Produced from ZrO2 Doped with Rare-Earth Metal Oxides

Current studies of ceramic topcoat materials for thermal barrier coatings (TBCs) of next generation focus mainly on ternary, quaternary, and more complex oxide systems. The use of high-entropy ZrO2-based ceramics doped with a mixture of rare-earth metal (REM) oxides for TBCs was studied, and the properties of thermal barrier ceramic layers deposited by EB-PVD in one process cycle were examined. For research, a CeO2-based REM concentrate (light concentrate (LC)) with the following composition (wt.%) was chosen: 62.4 CeO2, 13.5 La2O3, 10.9 Nd2O3, 3.9 Pr6O11, 0.92 Sm2O3, 1.2 Gd2O3, 0.24 Eu2O3, 2.66 ZrO2, 1.2 Al2O3, and 1.7SiO2; the total content of other oxides was 1.38. Targets for depositing the thermal barrier ceramic layer were produced by the ceramic synthesis method from the 85 wt.% M-ZrO2–15 wt.% LC mixture. Two-layer metal/ceramic TBCs were deposited employing the UE-174 industrial electron-beam installation operated at ELTECHMACH (Vinnytsia) onto model blades manufactured by directional crystallization from the ZhS-26VI alloy. A rough dense glossy coating was produced. The coating on the blade suction side and leading edge was 85 μm thick and on the blade pressure side was 70 μm thick. The coating phase composition represented a mixture of F-ZrO2 and M-ZrO2. Dense columnar crystallites collected into feather-like structures were observed in the ceramic layer. Vertical pores that formed between the crystallites were located perpendicularly or at an angle to the surface. A complex Al2O3-based spinel layer 2–2.5 μm thick grew between the ceramic topcoat and metallic bond coat layers. The ceramic layer acquired a laminar structure under the synergistic effect of components in the ZrO2–REM concentrate mixture in the vapor deposition process. The microstructural features of the coating determined the microhardness gradient in height. Thermal cycling experiments showed that this coating withstood 161 thermal cycles, which was higher than the standard YSZ coating did (138 thermal cycles). Previous studies showed that ZrO2 stabilization with REM oxide concentrates was promising for the microstructural design of the thermal barrier ceramic layer.