Use of Very High Water-Vapor Pressures to Evaluate Candidate Compositions for Environmental Barrier Coatings

Environmental barrier coatings (EBCs) are used to protect Si-based ceramics from accelerated oxidation and volatilization in the presence of elevated water-vapor pressures at high temperatures and high gas velocities. Previously, an analysis based on mass flux of volatilized species has shown that operating at very high H2O pressures can be used to compensate for the low gas velocities found in many laboratory exposure facilities so as to conduct first-stage screening of EBC compositions for volatility resistance. To test this prediction experimentally, a high-temperature furnace was modified to accommodate gas pressures of up to 20 atm and bulk specimens of barium-strontium aluminosilicate (BSAS), barium aluminosilicate (BAS), and strontium aluminosilicate (SAS), silica (SiO2), and silicon carbide (SiC) were exposed at 1250°C in 90% H2O-10% air. This set of materials provided an adequate spread in susceptibility to volatilization to evaluate the proposed approach because BSAS, BAS, and SAS are significantly more volatilization resistant in water-bearing environments than SiO2 and Si-bearing ceramics (such as SiC) that form silica under these conditions. The gravimetric results provided clear trends in volatilization resistance. The three aluminosilicates exhibited similar kinetic behavior and had significantly lower rates of mass losses than the SiO2 and SiC. These findings provided the experimental proof-of-principle for using high-pressure, low-gas-velocity exposures for qualitative differentiation of degrees of volatilization resistance among different candidate materials being developed for EBC applications.