Textron SCS-6 silicon carbide fibers presently include an exterior PG (pyrolytic graphite) coating. A primary purpose of this coating is to form a titanium carbide layer in situ during the fabrication of titanium matrix composites. The TiC layer acts as a diffusion barrier, impeding movement of C, Ti, and Si atoms, thereby protecting the integrity of the load bearing SiC fibers. An improved diffusion barrier is needed to reduce the extent and complexity of the reaction zone created during processing, and to expand the operating regime for these composites to higher temperatures and/or longer times. There are several compounds in which C, Wi, and Ti have lower diffusion coefficients as compared to TiC, most notably the carbides of tantalum, zirconium, and hafnium. We will apply thin films of one or more of these compounds onto silicon carbide fibers and investigate the reaction zones created when these coated fibers are processed at elevated temperatures in the presence of titanium. Since diffusion occurs predominantly along grain boundaries in the temperature range of interest, we will also engineer the width, length, and content of the grain boundaries to partially block these "short circuit" paths. Anticipated gains include an order of magnitude decrease in the size of the reaction zone and a dramatic reduction in residual stress at the fiber-matrix interface.
