Microstructure evolution and physical properties of ZrO2-(Y + Yb)O1.5-TaO2.5 thermal barrier coatings

Abstract

This investigation provides insight into the microstructure evolution and bulk thermomechanical properties of quaternary ZrO2-(Y/Yb)O1.5-TaO2.5 thermal barrier coatings (TBCs) fabricated by electron-beam physical vapor deposition and aged at 1250 °C and 1500 °C. A ZrO2-based coating containing all three dopant oxides with a ratio of (Y + Yb): Ta ≥ 1 exhibits a columnar microstructure and texture similar to the state-of-the-art tetragonal ZrO2–8 ± 1 mol% YO1.5 (8YSZ) TBCs. It was deposited as a single, supersaturated tetragonal phase that evolves into a two-phase mixture of cubic and non-transformable tetragonal phases upon aging. The mean coefficient of thermal expansion is marginally lower than that of 8YSZ in the temperature range of 200–1200 °C, differing by no more than 0.5 ppm·K−1. The toughness at room temperature is comparable to cubic YSZ, i.e. ~10 J/m2, but preliminary results indicate that modifying the composition to increase the tetragonal phase fraction can produce toughness values similar to 8YSZ, i.e. ~40 J/m2. With thermomechanical properties akin to 8YSZ but superior phase stability and thermal resistivity, quaternary compositions hold promising potential as next-generation TBCs capable of operating at temperatures above current limits, up to 1500 °C.