Low Cost CCVD Mullite and Alumina-Titania Interface Coatings for Ceramic Matrix Composites

Nicalon fiber-reinforced SiC matrix composites owe their good mechanical properties at room temperature to either C or BN interfaces, which provide a weak interfacial bond. However, C and BN interfaces encounter oxidation problems at elevated temperatures in oxidizing environments resulting in degradation of mechanical properties. Recent efforts to replace C and BN interfaces have identified alumina-titania and mullite as potential interface materials. However, the sol-gel deposition of alumina-titania and mullite coatings directly on Nicalon fibers degrade the fibers and result in Nicalon/SiC composites with low strength and toughness in the as-processed condition. Hence, there is a need to deposit these oxide coatings by alternative low cost approaches without degrading Nicalon fibers to obtain Nicalon/SiC composites with desired strength and toughness. The Combustion Chemical Vapor Deposition (CCVDsm) technique can quickly apply uniform, high quality, dense thin films of metals and oxides in a continuous manner at atmospheric pressure. The high quality CCVD process is low cost and versatile. It does not require a reaction chamber or furnace, yet the thin films produced are comparable in quality to CVD films, and CCVD technology allows substrates to be continuously passed through a deposition zone for production line or large area coatings. In this Phase I effort, alumina-titania and mullite films will be deposited by CCVD initially on flat substrates, such as LaAlO3, YSZ, and SiC. Subsequently, the deposition of alumina-titania and mullite coatings ( if necessary thin fugitive C coatings) will be carried out on Nicalon fibers and the morphology, microstructure, and mechanical properties of the coated fibers will be determined as a function of deposition conditions. A series of run studies will be performed to optimize the process, then a final run series will be performed to demonstrate reproducibility. This research will demonstrate the capability of CCVD to inexpensively deposit high quality, dense, smooth thin films of alumina-titania and mullite on to Nicalon without degradation of fiber strength for composite a