Microstructure evolution of ZrO2-YbTaO4 thermal barrier coatings

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.