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

Ceramic-matrix composites are being developed to operate at elevated temperatures and in oxidizing environments, Considerable improvements have been made in the creep resistance of SiC fibers and, hence, in the high-temperature properties of SiC fiber/SiC (SiCf/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen-enhanced track growth of SiCf/SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (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 glass layer on the fiber or within the fiber-matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical 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 from the oxidation of the interfacial layer and the resulting decrease of stress 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 of temperatures for d < d(c) and may occur at T > T-g for d > d(c). This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation.