Structural and chemical analyses of a thermally grown oxide scale in thermal barrier coatings containing a platinum-nickel-aluminide bondcoat

The structure and chemistry of the thermally grown oxide scale in a thermal barrier coating having an electron beam-physical vapor deposited yttria-pa rtially stabilized zirconia (YPSZ) topcoat and a platinum-nickel-aluminide (Pt-Ni-Al) bondcoat were studied using transmission electron microscopy. Th e scale consisted of hexagonal aluminum oxide (alpha-Al2O3) in both as-coat ed and thermally cycled specimens; no metastable Al2O3 polymorphs were obse rved In as-coated specimens, the scale's grains had a fine, columnar morpho logy. ZrO2-rich dispersoids were observed both intra- and intergranularly t hroughout the scale. Thermally cycled specimens had a duplex scale structur e one band of grains adjacent to the YPSZ had an equiaxed morphology and co ntained ZrO2-rich dispersoids; a second band of dispersoid-free grains adja cent to the bondcoat had a coarse, columnar morphology. Porosity and cracks were associated with the interface between the bands. The formation of the banded morphology and the cracking are proposed to be associated with the presence of the ZrO2-rich dispersoids.