The Na-rich CMAS rapidly infiltrates the APS 7YSZ coating similarly to other previously tested CMASs, including the sand-CMAS tested here. However, the composition and crystal structure of the resulting ZrO2 grains depends on the Na-content of the CMAS. As a glass modifier, Na+reduces the viscosity and increases the ionic mobility of the CMAS-melt by breaking up the SiO2 network. Stott et al. studied the in-teractions between 7YSZ TBCs and Na2O–CaO–SiO2 glasses and concluded that reprecipitation would not occur because the increase in modifier (CaO and Na2O) content would increase the solubility of Zr4+. However, in the present study we did not observe a significant increase in the ZrO2 solubility with Na2O content in the CMAS . Furthermore, the shape of the grains and their composition suggests a dissolution-precipitation process. Fig. 1 shows a compositional ‘core-shell’ structure that suggests the grains form from an Ostwald ripening-type mechanism where small grains dissolve and precipitate on the neighboring larger grains. The formation of Y- and Ca -enriched ZrO2 grains instead of the Y-lean ZrO2 grains caused due to attack by Na-lean CMAS, indicates that the Na+changes the local thermodynamic equilibrium composition.
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