Oxidation Kinetics and Stress Effects for the Oxidation of Continuous Carbon Fibers within a Microcracked C/SiC Ceramic Matrix Composite

Carbon fiber-reinforced silicon carbide (C/SiC) composites have the potential to be utilized in many high-temperature structural applications, particularly in aerospace. However, the susceptibility of the carbon fibers to oxidation has hindered the composite's use in long-term reusable applications. In order to identify the composites limitations, fundamental oxidation studies were conducted to determine the effects of such variables as temperature, environment, and stress. The systematic studies first looked at the oxidation of the plain, uncoated carbon fiber, then when fiber was utilized within a C/SiC composite, and finally when a stress was applied to the C/SiC composite (stressed oxidation). The first study, oxidation of just the carbon fibers, showed that the fiber oxidation kinetics occurs in two primary regimes: chemical reaction control and diffusion control. The second study, oxidation of the C/SiC composite, showed the self-protecting effects from the SiC matrix at elevated temperatures when the composite was not stressed. The final study, stressed oxidation of the C/SiC composite, more closely simulated application conditions in which the material is expected to encounter thermal and mechanical stresses. The applied load and temperature will affect the openings of the as-fabricated cracks, which are an unavoidable characteristic of C/SiC composites. The main objective of the paper was to determine the oxidation kinetic regimes for the oxidation of carbon fibers in a cracked silicon carbide matrix under stressed and unstressed conditions. The studies help to provide insights in to the protective approaches, that could be used to prevent oxidation of the fibers within the composite.