Effect of water vapor on thermally-grown alumina scales on Pt-modified and simple aluminide bond coatings

Photo-stimulated luminescence spectroscopy (PSLS), 3D microscopy and focused ion beam cross-sections were used to study the effect of water vapor on the cyclic (1 h cycle time) oxidation behavior of simple and Pt-modified aluminide bond coatings on several superalloy substrates at 1125° and 1150 °C. By tracking the same region over time, the change in surface roughness with thermal cycling could be attributed to large and small bond coating grains rising and sinking, respectively, with thermal cycling. Oxidation in air with 10% H2O showed a 0-37% increase in surface roughness compared to oxidation in dry air. Reducing the cycle temperature from 1150° to 1125 °C, increased the thermal barrier coating lifetime by > 4X but did not prevent bond coating rumpling. Compared to oxidation in dry air, PSLS identified more θ-Al2O3 formation on the Pt-modified aluminide bond coatings after 1 h at 1125° or 1150 °C in air with 10% H2O but only minor differences in the residual stress after longer cyclic oxidation exposures. Stress histograms produced by PSLS mapping help to elucidate Al2O3 scale damage accumulation in each condition but did not clearly identify a mechanism for the effect of water vapor on coating performance.