Corrosion Behavior and Failure Mechanism of (Sm_0.2Gd_0.2Dy_0.2Er_0.2Yb_0.2)₂(Zr_0.7Hf_0.3)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 (Sm_0.2Gd_0.2Dy_0.2Er_0.2Yb_0.2)₂(Zr_0.7Hf_0.3)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.