Microstructure evolution of ZrO2–YbTaO4 thermal barrier coatings

Abstract

The ZrO2–YbO1.5–TaO2.5 system offers significant promise in the development of next generation thermal barrier coatings (TBCs) but there is a paucity of information on its processability by established technology and its subsequent evolution upon aging. This investigation provides insights on the as-deposited microstructure of a ZrO2–20YbO1.5–20TaO2.5 (mol.%) TBC produced by electron-beam physical vapor deposition (EB-PVD), and its evolution during aging at 1250–1700°C. Complementary studies were performed on precursor-derived materials of the same composition, as well as on a chemically similar ZrO2–20YO1.5–20TaO2.5 EB-PVD TBC. The coatings are deposited as a tetragonal single phase supersaturated (and hence metastable) solid solution which decomposes upon aging at 1250°C into a uniform dispersion of coherent nano-scale tetragonal YbTa(Zr)O4 precipitates within the depleted tetragonal Zr(Yb, Ta)O2 matrix. Similar microstructures were produced from precursor-derived materials when aged above the tantalate solvus, identified as ∼1450°C. Aging of similar samples below the solvus leads to much coarser YbTa(Zr)O4 second phase, with monoclinic tantalate emerging below 1350°C and the tetragonal form appearing between 1350°C and 1450°C. The remarkable phase stability of the TBC after aging is attributed to the fine scale dispersion of the second phase.