The optical basicity model was used to categorize the attack behavior of two different CMASs with 7YSZ and 2ZrO2⋅Y2O3(ss) APS TBCs. The Na-rich CMAS (higher OB) attack induced higher Y-content in the reprecipitated ZrO2 grains than those observed after attack from sand-CMAS (lower OB) in both 7YSZ and 2ZrO2⋅Y2O3(ss) TBCs. The Na-rich CMAS behaved similar to Ca-rich CMAS, suggesting that basic glass constituents (high OB) promote Y-rich ZrO2 grains and little Y-solubility. The low Y-solubility in the CMAS and Y-rich products suggest little apatite formation, which is used to mitigate further CMAS infiltration.
Therefore, the OB model can identify potentially ‘dangerous’ CMAS compositions that should be used to screen new TBC and EBC candidates for CMAS mitigation. The reaction evolution observed here further demonstrates the superior CMAS mitigation capabilities of apatite to other reaction products, like the Na-rich crystalline phase found here, and the importance of controlling its growth kinetics.
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