The high-temperature stability of an oxide/oxide composite at high water-vapor pressure

The microstructural and mechanical stabilities of a Nextel 720 (alumina+mullite) fiber-reinforced, alumina matrix composite were evaluated following exposure of the material to a high H₂O pressure (to simulate a gas turbine combustor environment) in Oak Ridge National Laboratory's (ORNL's) high temperature, high pressure facility (Keiser Rigs). Coupons were exposed at 1135°C and 1200°C in a high-pressure (10 atm) furnace containing 10% water vapor for 1000, 2000, and 3000 h. Extensive microstructural characterization was conducted after each exposure to evaluate fiber and matrix stabilities as a function of exposure time and temperature. Retained tensile strengths were also measured and compared to baseline properties following each exposure. It was found that there was no significant change in the composite microstructure or tensile strength after long-term exposure in the Keiser Rig at 1135°C, whereas significant Nextel 720 fiber degradation (grain growth and increased surface roughness) occurred during exposure for 3000 h at 1200°C. Statistical variations in the mean strength values between the as-processed and exposed composites indicated little change in strength, however. The composite degradation was primarily thermal and the addition of H₂O had little effect on changing or accelerating these effects.