Oxidation Resistant CVI-C/SiC Composites via Molecular Level Matrix Inhibition

Carbon fiber reinforced silicon carbide (C/SiC) composites produced by chemical vapor infiltration (CVI) are actively being developed for a variety of high-temperature aerospace applications. Although CVI-based C/SiC composites are a promising class of materials for structural applications demanding high strength and toughness at temperatures to 1650 C, these materials are vulnerable to severe strength degradation when exposed to oxidizing environments above 425 C for prolonged periods. This limitation is due to the inherent process-induced matrix microcracks resulting from the large thermal expansion mismatch between the fiber and the matrix. These matrix cracks enable rapid intrusion of the environment which accelerates the degradation of the compliant fiber coating and/or reinforcing fiber, thus limiting the useful life of the component. The objective of this Phase I program is to investigate the feasibility of an oxidation- resistant C/SiC utilizing both fiber-level and molecularly-integrated matrix-level oxidation inhibition technology. Laminated C/SiC plates will be fabricated incorporating a duplex fiber coating system consisting of a 0.4 ¿m pyrolytic carbon (PyC) layer to promote strength and toughness, followed by a boron carbide (B4C) oxidation-barrier layer, and densified with a 10 v/o boron-enriched silicon carbide inhibited matrix produced by CVI co- deposition. Test samples will be prepared and experimentally evaluated in high- temperature static stressed oxidation environments. Results will be compared to baseline conventionally-processed C/SiC composites to establish the viability of the proposed approach.