Carbon oxidation in ceramic composites and the evaluation of interfacial sealing for oxidation resistant fiber-reinforced composite systems

Carbon offers desirable properties as a fiber-matrix interphase material in ceramic matrix composites (CMC's), but oxidation of carbon at temperatures above 500{dollar}spcirc{dollar}C has limited its utility. In an effort to better understand the kinetics associated with carbon oxidation pertaining to CMC applications, the origin of non-planar morphologies observed in the reaction front of carbon fibers and interphases receding into a ceramic matrix in the temperature range of 700{dollar}spcirc{dollar}C to 1000{dollar}spcirc{dollar}C was analyzed. A numerical simulation based on the finite difference method is utilized to evaluate the parameters which govern the morphology of the receding carbon reaction front. The study indicates that the morphology of the reaction front contains information regarding the interplay between oxidation behavior and microstructural features of the carbon. Carbon oxidation was found to obey "weak-link" behavior, that is, a sub-component which is more susceptible to oxidation governs the recession kinetics. The implications of weak link oxidation to preservation of a carbon interphase in a ceramic composite are discussed.; Interrupted interphases have demonstrated the ability to confine oxidation of a carbon interphase to a localized region adjacent to a matrix crack. Commercial SiC mono-filaments (SCS-6, Textron Specialty Materials) were modified with a laser to produce fibers with discontinuous carbon coatings that were used in experiments to study mechanical properties. The laser-scribed fibers were tested in isolation, used in single-fiber microcomposites, or incorporated into small bulk composite specimens using a powder processing route to produce the matrix. The mechanical performance of the various types of specimens prepared using the laser scribing technique is presented and these results are used in a simulation of ultimate composite properties.; The effect of fiber matrix fusion, by direct bonding or through a reaction product which seals the interface, was investigated with respect to composite mechanical performance. Stress concentration due to fiber-matrix sealing was evaluated using finite element modeling (FEM). Geometric and material parameters which govern the stress concentration were analyzed. FEM results are compared to tensile data obtained using SiC-SiC microcomposites.