Fabrication and oxidation behavior of sintered SiC fiber reinforced SiO₂-mullite composite with BN interphase

As a potential material for high temperature services, unidirectional sintered SiC fiber reinforced SiO₂-30 mol%Al₂O₂ composite with BN interphase was synthesized by liquid state hot pressing. The obtained composite had dense structure, and both the mullite and glassy SiO ₂ phases were found in the matrix. No chemical reaction among the constituents was observed. The composites were exposed to air at 1573 K up to 3600 ks and then oxidation behavior was investigated. Three-point flexural test was performed at room temperature after each exposure. Flexural strength decreased with an increase in the exposure time and then leveled off in 360 ks. This tendency corresponds to the crystallization of SiO₂ glass in the matrix. Change in the stress field which is caused by the crystallization and phase transformation of SiO₂ would lead to the reduction of the strength. It was confirmed that oxidation progressed along the fibers and the oxidized depth was limited within approximately 200 μm independent of exposure time (3.6∼3600 ks), The oxide layer with approximately 10 [im thickness including silicon, aluminum and oxygen was found on the surface of the composites. This layer will prevent the inner oxidation within 200 μm.