A PERVASIVE MODE OF OXIDATIVE-DEGRADATION IN A SIC-SIC COMPOSITE

The oxidative durability of a SiC-SiC composite with Hi-Nicalon fiber and a BN interphase was investigated at 800 degrees C (where pesting i s known to occur in SiC-SiC composites) for exposure durations of up t o 500 h and in a variety of oxidant mixes and flow rates, ranging from quasi-stagnant room air, through slow-flowing oxygen that contained 3 0%-90% H2O, to the high-velocity flame of a burner rig. Degradation of the composite was determined from residual strength and fracture stai n in post-exposure mechanical tests and correlated with microstructura l evidence of damage to the fiber and interphase. The most-severe degr adation of composite behavior occurred in the burner rig and has been shown to be associated with the high oxidant velocity and substantial moisture content, as well as a thin sublayer of carbon that is indicat ed to form between the fiber and the interphase during composite proce ssing.