Multilayer Fiber Interface Coatings for Improved Environmental Resistance and Slip in Ceramic Matrix Composites, Phase II

Application of fiber-reinforced ceramic matrix composites (CMC) can enhance the efficiency and performance, reduce the weight, improve the durability, and lower the cost of rocket engine combustion devices and turbomachinery components used in high temperature, high-stress environments. Meeting these objectives requires improvements in fiber-reinforced CMC materials and fabrication processes, particularly improved fiber/matrix interfaces, interface deposition processes, and oxidation protection. In previous work, Ultramet developed an ultraviolet-enhanced chemical vapor deposition (UVCVD) process that allows deposition of dense, strain-tolerant ceramics at room temperature, thus avoiding heat-induced material degradation and providing excellent material performance, including enhanced oxidation protection. Although these coatings have improved performance, identifying a single phase that best performs the two key functions of the interface coating, oxidation protection and interface slip, has proven elusive. Phase I focused on development of both conventional CVD and UVCVD deposition techniques that resulted in several novel multilayer interface coating systems utilizing oxide and carbide phases. Fiber tows coated with multilayer systems exhibited dramatic improvement in tensile strength compared to both uncoated tows and fiber coated with a single oxide layer. One multilayer system was employed in the fabrication of a carbon fiber-reinforced silicon carbide (C/SiC) CMC that demonstrated the highest mechanical strength yet achieved for C/SiC using Ultramet's melt infiltration densification process, verifying the beneficial effect of the multilayer system via a 33% strength increase. The Phase II project will build on this encouraging preliminary room temperature data via further optimization of multilayer interface deposition at Ultramet and extensive evaluation of both coated tows and CMCs utilizing the coatings at the elevated temperatures expected in actual use.