High-temperature interactions between Yttria-stabilized zirconia thermal barrier coatings and Na-Rich calcia-magnesia-aluminosilicate deposits

We have evaluated the effectiveness of optical basicity, a chemical model, to predict and categorize the reaction behavior between calcia-magnesia-aluminosilicate (CMAS) deposits and ZrO₂-based thermal barrier coatings (TBCs), which are used to insulate and protect metallic components in gas-turbine engines. The attack behavior of two CMAS compositions (Na-lean and Na-rich) with 7 wt% Y₂O₃-partially-stabilized ZrO₂ (7YSZ) and 2ZrO₂·Y₂O₃(ss) free-standing TBCs at 1340 °C were evaluated and compared to previous studies. The behavior of Y³⁺ in the reaction is correlated with the optical basicity of the CMAS; more basic Na-rich CMAS melt resulted in lower Y-solubility and higher Y-content in the reprecipitated ZrO₂ grains than observed in the highly acidic CMAS attack for both tested TBCs. This behavior is consistent with previous studies of basic and acidic melts, and suggests that less acidic CMASs pose a unique danger to ZrO₂-based TBCs that rely on Y-rich secondary phases for CMAS mitigation.