CHEMICAL-STABILITY OF THE FIBER COATING MATRIX INTERFACE IN SILICON-BASED CERAMIC-MATRIX COMPOSITES

Carbon and boron nitride are used as fiber coatings in silicon-based c omposites. In order to assess the long-term stability of these materia ls, reactions of carbon/Si3N4 and BN/SiC were studied at high temperat ures with Knudsen effusion, coupon tests, and by microstructural exami nation. In the carbon/Si3N4 system, carbon reacted with Si3N4 to form gaseous N-2 and SiC. The formation of SiC limited further reaction by physically separating the carbon and Si3N4. Consequently, the developm ent of high p(N-2) at the interface, predicted from thermochemical cal culations, did not occur, thus limiting the potential deleterious effe cts of the reaction on the composite. Strong indications of a reaction between BN and SiC were shown by TEM and SIMS analysis of the BN/SiC interface. In long-term exposures, this reaction can lead to a depleti on of a BN coating and/or an unfavorable change of the interfacial pro perties, limiting the beneficial effects of the coating.