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.
Fig. 1.High mass-contrast EPMA image of grains from Region 1 of 7YSZ TBC after attack by Na-rich CMAS for 24 h at 1340 ◦C. The ZrO2 grains and the CMAS appear gray and black, respectively, as labeled.
The reaction products from the Na-rich CMAS attack are similar in composition to those formed from Ca-rich CMAS (Ca/Si >1) as shown in Table 1. The formation of Y-rich ZrO2 grains has previously been attributed to CMAS with high Ca/Si ratios. However, these data suggesthat other basic (high OB) glass constituents in sufficient volume are more likely to cause enriched ZrO2 grains, regardless of the Ca/Si ratio (see Tables 1 and 3). Thus, the OB is a good descriptor of the final reaction behavior in 7YSZ coatings with CMAS: less acidic CMASs promote the formation of stabilizer-rich ZrO2 products, while more acidic CMASs have greater Y-solubility and induce the precipitation of Y-depleted ZrO2 grains. More tests are necessary to determine if Y-solubility is linearly correlated with OB. The Y-solubility change reported here is observed for only a small change in OB in the glass, which suggests that the OB is highly sensitive to changes in the glass composition. Previously, slight changes in OB (ΔΛ =0.01) have been correlated in glass refractive index, which demonstrates the quantitative power of the OB model. The results here indicate that OB-classified CMAS can be used to validate new TBCs with fewer tests because it reduces the large CMAS compositional space to only those with constituents of acidic or basic-nature. However, the true mitigation capability will depend on the constituents of the TBC themselves as exemplified by the difference in behavior between Zr4+and Y3+in the melt.
Table 1
Compositions (mol%) of synthetic CMASs used for 7YSZ/CMAS interaction studies and the reaction products reported from that interaction.
Table 2
CMAS compositions (mol%) used in the interaction studies. Their respective calculated optical basicity values (Λ) and Ca/Si ratios are also included.
Table 3
The CMAS compositions (cation basis, at%) after interaction with 7YSZ TBCs at 1340 ◦C for 1 min, 1 h and 24 h measured using EDS.
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