Thermal gradient chemical vapor infiltration of silicon carbide matrix surface composites

Chemical vapor infiltration (CVI) is a processing technique wherein a solid material is deposited within a porous preform from the vapor phase. CVI is ideally suited for the fabrication of ceramic matrix composites (CMCs) because refractory matrices can be deposited under relatively low temperatures. However, under isothermal conditions (ICVI) the matricies tend to be deposited preferentially at the exterior of the preform. By applying a thermal gradient across the preform and introducing the reactants to the cooler side, preferential deposition of the matrix is promoted at the interior of the preform.; Thermal gradients were achieved in preforms which consisted of a thin layer of fibers applied to the surface of an internally heated ceramic tubular monolith. The kinetics of the CVI process for this geometry were investigated by infiltration experiments and by simulations with a numerical model. The optimum conditions predicted by the CVI model were in agreement with those identified in the experiments. The density of the surface composites was maximized at high internal temperatures and intermediate concentrations of the reactant. Surface composites with densities as high as 90% of theoretical value were fabricated in 3 hours.; A thin composite layer was found to improve the mechanical properties of the monoliths. Because the composite acts as a load-bearing member, the effective strength of the surface composites was found to be higher than that of the bare monolith. In addition, the strength of the specimens was found to be insensitive to damage caused to the outer diameter by indentation. The mechanisms of damage tolerance were identified as pullout of the fibers and delamination of the composite. Damage tolerance was retained in oxidized specimens due to brittle fracture of the porous composite coating.; The kinetics of chemical vapor deposition (CVD) of SiC from methyltrichlorosilane (MTS)-H{dollar}sb2{dollar} mixtures were examined. The kinetics of the MTS-H{dollar}sb2{dollar} system are explained by a model which includes the mechanisms of homogeneous decomposition, parallel deposition paths, and generation of a by-product that inhibits deposition. A consistent set of rate parameters were extracted from the literature and used to predict the optimum conditions for fabrication of surface composites.