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

In previous work, Ultramet has developed and demonstrated a rapid, low-cost method of applying oxide, carbide, and nitride interface coatings to carbon, oxide, and silicon carbide (SiC) fibers at low temperature (as low as 100°C) via ultraviolet-enhanced chemical vapor deposition (UVCVD). The state-of-the-art interface coating for SiC fiber-reinforced SiC matrix (SiC/SiC) composites is boron nitride (BN). Unfortunately, in the presence of oxygen and water vapor at elevated temperatures, composites using BN interfaces have severe embrittlement problems due to the formation of a liquid and/or solid reaction product that attacks the fibers or strongly bonds the fibers to the matrix. The stability of the BN interface in moisture-containing environments such as turbine engines can be improved by depositing it at higher temperatures or by doping the BN with silicon. In Phase I, Ultramet demonstrated the feasibility of uniformly applying silicon-doped BN interface coatings onto carbon and SiC fibers at relatively low temperatures using the UVCVD process. After several process development trials, the silicon-doped BN interface was applied to a Sylramic fabric preform that was subsequently infiltrated with a SiC matrix to form a SiC/SiC composite. Uniform deposition of the silicon-doped BN interface was demonstrated throughout the fiber preform, and despite a very low fiber reinforcement volume fraction (<25%), room temperature flexural strength up to 40 ksi was measured. In Phase II, Ultramet will build upon the success demonstrated in Phase I and undertake process optimization focusing on compositional control, homogeneity, thickness, microstructure, and uniformity of the silicon-doped BN interface coating. Key mechanical and thermal properties of the coating, coated fibers, and SiC/SiC composites containing the interface will be assessed. The benefits of the silicon-doped BN coatings relative to conventional BN interfaces will be validated through simulated environmental testing involving direct exposure to elevated temperature oxidizing environments containing moisture. Scaleup efforts will apply the coating to net-shape fiber preforms and fabricate SiC/SiC composite components.