Evaluating the stability of bsas-based EBCs in high water-vapor pressure environments

Barium Strontium Aluminosilicate (BSAS)-based environmental barrier coatings (EBCs) have been used successfully to protect the surface of Si-based fiber-reinforced ceramic-matrix composite (CMC) combustor liners in Solar Turbines Centaur 50S gas turbine engines. Two such EBC/CMC combustor liner sets were engine-exposed for times >14,000h. However, extensive microstructural characterization of the EBC/CMC liners after the long-term engine exposures showed a significant loss/recession of the BSAS-based EBC. The mechanisms by which BSAS recession occurs and the rate at which the BSAS recesses as a function of pressure, gas velocity, temperature, etc. are not fully understood but such information will be necessary for improving the life to ≫15,000h in order for the materials to be acceptable for use in an engine application. To this end, the thermochemical stabilities of candidate EBCs are being evaluated analytically and experimentally at very high H₂O pressures in a high-temperature, high-pressure furnace (Oak Ridge National Laboratory's "Keiser Rig"). Calculations based on generalized volatility reactions and mass flux of volatilized species have shown that high H₂O pressures can be used to compensate for the low gas-flow velocities in the Keiser Rig. An examination of the phase stability/volatilization of state-of-the-art EBC compositions (Ba- and Sr-based aluminosilicates) has been conducted through equilibrium thermodynamic calculations and initial experiments to validate assumptions via Keiser Rig exposures of the same EBC formulations conducted at 1250°C and 20 atm H₂O.