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 ZrO2-20YbO(1.5)-20TaO(2.5) (mol.%) TBC produced by electron-beam physical vapor deposition (EB-PVD), and its evolution during aging at 1250-1700 degrees C. Complementary studies were performed on precursor-derived materials of the same composition, as well as on a chemically similar ZrO2-20YO(1.5)-20TaO(2.5) EB-PVD TBC. The coatings are deposited as a tetragonal single phase supersaturated (and hence metastable) solid solution which decomposes upon aging at 1250 degrees C into a uniform dispersion of coherent nano-scale tetragonal YbTa(Zr)O-4 precipitates within the depleted tetragonal Zr(Yb, Ta)O-2 matrix. Similar microstructures were produced from precursor-derived materials when aged above the tantalate solvus, identified as similar to 1450 degrees C. Aging of similar samples below the solvus leads to much coarser YbTa(Zr)O-4 second phase, with monoclinic tantalate emerging below 1350 degrees C and the tetragonal form appearing between 1350 degrees C and 1450 degrees C. The remarkable phase stability of the TBC after aging is attributed to the fine scale dispersion of the second phase. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.