Evaluating steam oxidation kinetics of environmental barrier coatings

Environmental barrier coatings (EBCs) are a commercially proven means of protecting SIC-based materials in gas turbine environments. However, there are little specific data in the literature on the impact of coatings like Yb2Si2O7 on preventing accelerated SiO2 growth in the presence of H2O. Quantification of reduced rates are necessary for evaluating and comparing EBC effectiveness and incorporation of silica growth rates into future EBC lifetime models. In this study, baseline kinetics of silica formation on bare Si and chemically vapor deposited (CVD) SiC in the 1250-1425 degrees C range were obtained via 100 h isothermal exposures in dry air and steam environments utilizing a SiC reaction tube to mitigate specimen volatility. An Arrhenius plot of the resulting rates was constructed, representing baseline minimum and maximum rates for Si and SiC oxidation at ambient pressure. Various EBC systems on CVD SiC substrates including air plasma sprayed (APS) EBCs with and without a Si bond coating and with surface roughening to enhance Yb2Si2O7 adhesion were subjected to 1-h furnace cycle testing in air with 90vol%H 2 0 at 1250-1350 degrees C for up to 500 cycles. After exposure, silica formation rates were measured and compared to the baseline rates to assess EBC effectiveness, where EBC effectiveness is gauged as the propensity to reduce underlying rates of silica formation. With a Si bond coating, a similar to 180 mu m Yb2Si2O7 (YbDS) top coating reduced rates over the entire 1250 degrees-1350 degrees C range. Without a Si bond coating, similar to 60 mu m (YbDS) coatings deposited directly onto CVD SiC exhibited poor adhesion, and had to be deposited onto substrates with enhanced roughness at 1350 degrees C. While exhibiting good adhesion at 1350 degrees C, overall the single layer YbDS coating exhibited a decreasing effectiveness from 1250 degrees to 1350 degrees C.

Automated summary

The study shows that EBCs have a certain effect on reducing the rate of SiO2 formation, especially when paired with a Si bond coating. The adhesion of the coating and roughness of the substrate play a crucial role in the effectiveness of EBCs.