CARBON-LAYER FORMATION AT SILICON-CARBIDE GLASS INTERFACES

A phenomenological model that describes the kinetics of carbon interph ase formation at SiC-glass interfaces is presented. It is based on an oxidation reaction at the interface, and so it considers mass transpor t of O2 and CO reaction product fluxes in and out of the interface reg ion. In the calculations presented here, it is intended to describe th e interface reactions that occur during processing of the composite in a hot-press; thus, the glass matrix itself is the only source of oxyg en. The model is tested against published data, and is then used to sh ow how variables associated with the glass and composite processing in fluence the carbon layer thickness and stability. The most important c onclusion is that the carbon interphase is transient in nature. Its ra te of formation and maximum thickness depend on the initial oxygen act ivity in the glass matrix; this activity is influenced by the melting history and the presence of variable-valence oxides. Once the oxygen a ctivity near the interface falls, the carbon interphase is consumed th rough CO out-diffusion. In principle, the model is general enough to b e applied to non-stoichiometric SiC phases, and to more complex oxidiz ing ambients. These cases will be considered in the future.