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 thermalbarrier coating having an electron beam-physical vapor depositedyttria-partially stabilized zirconia (YPSZ) topcoat and aplatinum–nickel–aluminide (Pt–Ni–Al) bondcoat werestudied using transmission electron microscopy. The scale consisted ofhexagonal aluminum oxide (α-Al2O3) in both as-coated and thermallycycled specimens; no metastable Al2O3 polymorphs were observed. In as-coatedspecimens, the scale's grains had a fine, columnarmorphology. ZrO2-rich dispersoids were observed both intra- andintergranularly throughout the scale. Thermally cycled specimens had aduplex scale structure: one band of grains adjacent to the YPSZ had anequiaxed morphology and contained ZrO2-rich dispersoids; a second band ofdispersoid-free grains adjacent to the bondcoat had a coarse, columnarmorphology. Porosity and cracks were associated with the interface betweenthe bands. The formation of the banded morphology and the cracking areproposed to be associated with the presence of the ZrO2-rich dispersoids.