Oxidation mechanisms and kinetics of SiC-matrix composites and their constituents

The oxidation kinetics and mechanisms of SiC-matrix composites fabricated by chemical vapor infiltration, and of their constituents (C or SiC-fibers, C or BN interphases and SiC matrix) are studied on the basis of an experimental approach and modelling. The oxidation of carbon fibers is rate-controlled by a combined diffusion/chemical reaction mechanism at low temperatures and its rate reduced by a 1600degreesC heat treatment. The oxidation rate of the pyrocarbon is similar to that of the fibers when they have been heat-treated. The oxidation kinetics of both the SiC-based fibers and matrix are parabolic and assumed to be rate-limited by the diffusion of gaseous species in the silica scale. A full kinetics law is given. The occurrence of water in the atmosphere increases the oxidation rate of the fibers and decreases the activation energy, water becoming the main oxidizing agent. The oxidation of the BN-interphase is complex and strongly anisotropic, its kinetics depending on composition, structure and texture. Finally, the oxidation of SiC-matrix composites, depicted for 1D-SiC/C/SiC and 2D-C/C/SiC composites, involves both diffusion of gaseous species in the composite porosity and heterogeneous oxidation reactions. Oxidation occurs through the thickness of the composites at low temperatures which consumes the carbon-based constituents. Conversely, it tends to be limited to near the composite surface at high temperatures, due to the formation of silica-based phases healing the material porosity and preventing the in-depth oxidation of the carbon-based constituents. (C) 2004 Kluwer Academic Publishers.