2ZrO₂·Y₂O₃ thermal barrier coatings resistant to degradation by molten CMAS: Part I, Optical basicity considerations and processing

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 2ZrO₂·Y₂O₃ 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 2ZrO₂·Y₂O₃(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 W·(m·K)^-1 at elevated temperatures. The accompanying Part II paper presents results from experiments and analyses of high-temperature interactions between 2ZrO₂·Y₂O₃(ss) APS TBC and the two types of CMASs.