The higher operating temperatures in gas-turbine engines enabled by thermal barrier coatings (TBCs) engender new materials issues, viz silicate particles (sand, volcanic ash, fly ash) ingested by the engine melt on the hot TBC surfaces and form calcium-magnesium-alumino-silicate (CMAS) glass deposits. The molten CMAS glass degrades TBCs, leading to their premature failure. In this context, we have used the concept of optical basicity (OB) to provide a quantitative chemical basis for the screening of CMAS-resistant TBC compositions, which could also be extended to environmental barrier coatings (EBCs). By applying OB difference considerations to various major TBC compositions and two types of important CMASs -desert sand and fly ash-the 2ZrO 2 ÁY 2 O 3 solid solution (ss) TBC composition, with the potential for high CMAS-resistance, is chosen for this study. Here, we also demonstrate the feasibility of processing of 2ZrO 2 ÁY 2 O 3 (ss) air-plasma sprayed (APS) TBC using commercially developed powders. The resulting TBCs with typical APS microstructures are found to be single-phase cubic fluorite, having a thermal conductivity <0.9 WÁ(mÁK) -1 at elevated temperatures. The accompanying Part II paper presents results from experiments and analyses of high-temperature interactions between 2ZrO 2 ÁY 2 O 3 (ss) APS TBC and the two types of CMASs.

Based on the application of OB considerations (Part I) to various major thermal barrier coating (TBC) compositions and two types of important calcium-magnesium-alumino-silicates (CMAS)-desert sand and fly ash-the 2ZrO 2 ÁY 2 O 3 solid solution (ss) TBC composition, with high CMAS-resistance potential, is chosen for studying molten-CMAS/TBC interactions. It is demonstrated that 2ZrO 2 ÁY 2 O 3 (ss) air plasma sprayed (APS) TBCs are highly resistant to high-temperature attack by both sand-CMAS and fly-ash-CMAS. Despite the differences in the compositions of the two CMASs, the overall CMAS-attack mitigation mechanisms in both cases appear to be similar, viz reaction between 2ZrO 2 ÁY 2 O 3 (ss) APS TBC and the CMAS, and the formation of main reaction products of Y-depleted c-ZrO 2 and nonstoichiometric Ca-Y apatite. Large differences in the OBs (DΛ) between the 2ZrO 2 Á Y 2 O 3 (ss) and the CMASs are good predictors of ready reaction between this TBC and these CMASs. While the details of the CMAS-mitigation mechanisms can depend critically on various other aspects, the OB difference (DΛ) calculations could be used for the initial screening of CMAS-resistant TBC compositions.

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