Corrosion Behavior and Failure Mechanism of (Sm0.2Gd0.2Dy0.2Er0.2Yb0.2)2(Zr0.7Hf0.3)2O7 Double-Ceramic Thermal Barrier Coatings in Na2SO4 + V2O5 Environment

To meet gas turbines’ growing demand for high-performance thermal barrier coatings (TBCs), this study addresses the limitations of traditional single-layer 8% Y2O3-stabilized ZrO2 (YSZ) coatings in high-temperature corrosive environments. Atmospheric plasma spraying (APS) was used to fabricate the double-ceramic TBCs with (Sm0.2Gd0.2Dy0.2Er0.2Yb0.2)2(Zr0.7Hf0.3)2O7 (RHZ) as the outer layer and YSZ as the inner layer; thermal cycling corrosion tests (1000 °C, Na2SO4 + V2O5 molten salt) were conducted to compare its performance with traditional single-layer YSZ. The results showed that the YSZ corrosion products were m-ZrO2 and YVO4, while RHZ/YSZ produced rare-earth vanadates, m-(Zr,Hf)O2, and t′-(Zr,Hf)O2, and corrosion degree was positively correlated with salt concentration (which was more impactful) and the number of cycles. Both coatings failed via molten salt penetration, thermochemical reaction, and crack-induced spallation. The corrosion mechanism between the RHZ/YSZ coating and the mixed salt can be explained based on the Lewis acid–base theory and the optical basicity. The RHZ layer on the surface of RHZ/YSZ coatings indeed hinders the penetration of corrosive molten salts into the underlying YSZ layer to some extent.