The Influence of In Situ Alumina Defect Filling on the CMAS Corrosion Resistance of High-Entropy Rare-Earth Zirconate Thermal Barrier Coatings and Its Anti-Penetration Mechanism

As turbine inlet temperatures increase, the long-term stability of conventional yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) becomes difficult to maintain. Molten calcium–magnesium–alumino-silicate (CMAS) rapidly infiltrates atmospheric plasma-sprayed (APS) coatings through interconnected pores and cracks. In this work, an AlCl3-derived precursor sol prepared from AlCl3·6H2O in a deionized water–ethanol mixture (1:1, v/v) was introduced into the defects of high-entropy rare-earth zirconate (HE-REZ) coatings by vacuum-assisted drop-casting and subsequently converted into Al2O3 in situ through heat treatment. The influence of this treatment on the microstructure, surface wettability, and CMAS corrosion resistance of the coatings was examined. The newly formed Al2O3 was present on the coating surface and within pores and microcracks, where it narrowed or sealed the infiltration paths. After modification, surface roughness decreased by 8.96%, and the mean apparent contact angle measured on cooled CMAS/coating cross-sections increased by approximately 28°. Defect filling reduced the effective size of the infiltration channels, while dissolution of Al2O3 into the melt increased its viscosity and slowed capillary infiltration. Filling retained the dominant corrosion mechanism but shifted CMAS attack from rapid, localized penetration to a more uniform advance of the reaction front, reducing the corrosion rate and structural damage. Alumina filling therefore improves the CMAS corrosion resistance of APS HE-REZ TBCs by restricting infiltration channels and increasing melt viscosity.