Low-Cost, Low-Temperature Deposition of Fiber Interface Coatings

Conventional fiber-matrix interface coatings applied by chemical vapor infiltration (CVI) are expensive because they require costly, high-temperature equipment and must be applied at a slow rate in order to achieve uniform infiltration across large thermal gradients. In previous work for DoD and NASA, Ultramet developed a rapid, low-cost method of applying oxide and carbide interface coatings to carbon and silicon carbide fibers at low temperature (as low as 100°C) through ultraviolet-enhanced chemical vapor deposition (UVCVD). UV energy is used to decompose coating precursor gases at much lower temperatures than when thermal energy is used alone. The low temperature process virtually eliminates the fiber degradation that inherently occurs during conventional high-temperature CVI. Interface coatings have been applied to fiber tows as well as woven and braided structures, and were subsequently infiltrated with various carbide matrix materials through a novel melt-infiltration process. Ceramic matrix materials formed through melt processing do not exhibit the extensive microcracking that is common in most matrix materials. The very low microcracking improves interlaminar shear strength and substantially reduces the transfer of moisture and combustion gases into the composite, thereby further protecting the fiber reinforcement. Composite components in the form of thrust chambers and flat panels have demonstrated good mechanical and chemical survivability during high temperature hot-fire testing. In Phase I, Ultramet will develop UVCVD processing for rapid and low-cost application of a silicon-doped boron nitride coating, similar to that applied at high temperature, to silicon carbide fiber tows and panel preforms. Mechanical testing of composite panels will be performed before and after high-temperature oxidation testing in accordance with requirements defined for the Joint Strike Fighter aircraft. A cost comparison will be established between UVCVD coatings and those produced by conventional CVI. In Phase II, Ultramet will team with Lockheed Martin to establish a melt-infiltrated CMC property database, produce and test subscale components in conjunction with the Navy and Lockheed Martin, and demonstrate scaleup potential.