Effect of water vapor on thermally grown alumina scales on bond coatings

The role of water vapor on thermal barrier coating (TBC) performance has been investigated for both Pt-modified aluminide and Pt-diffusion bond coatings by characterizing the alumina scale after exposures at 1150 °C. For both types of bond coatings, the average oxide scale thickness formed in dry O2 at 1150 °C was lower than that formed in wet air, especially for air + 10%H2O. In all cases, the average thickness was higher beneath the yttria-stabilized zirconia (YSZ) top coat. Characterization of the alumina scale formed without a YSZ top coat showed differences in morphology and Hf- and Ta-rich oxide precipitates at the gas interface depending on the environment. With and without water vapor, the β-(Ni,Pt)Al coatings showed a martensitic structure with 10–50 nm α-Cr(Re) precipitates. Alumina grain boundary segregation of Y and Hf from the superalloy substrate was easier to detect for the Pt-diffusion coatings.