Carbon-layer formation at silicon-carbide-glass interfaces

A phenomenological model that describes the kinetics of carbon interphase formation at SiC-glass interfaces is presented. It is based on an oxidation reaction at the interface, and so it considers mass transport of O₂ and CO reaction product fluxes in and out of the interface region. In the calculations presented here, it is intended to describe the interface reactions that occur during processing of the composite in a hot-press; thus, the glass matrix itself is the only source of oxygen. The model is tested against published data, and is then used to show how variables associated with the glass and composite processing influence the carbon layer thickness and stability. The most important conclusion is that the carbon interphase is transient in nature. Its rate of formation and maximum thickness depend on the initial oxygen activity in the glass matrix; this activity is influenced by the melting history and the presence of variable-valence oxides. Once the oxygen activity near the interface falls, the carbon interphase is consumed through CO out-diffusion. In principle, the model is general enough to be applied to non-stoichiometric SiC phases, and to more complex oxidizing ambients. These cases will be considered in the future.