Stress-corrosion cracking of silicon carbide fiber/silicon carbide composites

Ceramic-matrix composites are being developed to operate at elevated temper atures and in oxidizing environments, Considerable improvements have been m ade in the creep resistance of SiC fibers and, hence, in the high-temperatu re properties of SiC fiber/SiC (SiCf/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments bef ore they are widely accepted. Experimental weight change and crack growth d ata support the conclusion that the oxygen-enhanced track growth of SiCf/Si C occurs by more than one mechanism, depending on the experimental conditio ns. These data suggest an oxidation embrittlement mechanism (OEM) at temper atures <1373 K and high oxygen pressures and an interphase removal mechanis m (IRM) at temperatures of greater than or similar to 700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a g lass layer on the fiber or within the fiber-matrix interphase region. The f racture stress of the fiber is decreased if this layer is thicker than a cr itical value (d > d(c)) and the temperature below a critical value (T < T-g ), such that a sharp crack can be sustained in the layer, The IRM results f rom the oxidation of the interfacial layer and the resulting decrease of st ress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM occurs over a wide range o f temperatures for d < d(c) and may occur at T > T-g for d > d(c). This pap er summarizes the evidence for the existence of these two mechanisms and at tempts to define the conditions for their operation.