Abstract: Methods for making hermetic silicon carbide matrix composite structures by CVI. A fibrous preform is created by winding strands of yarn of refractory fiber material helically about a graphite mandrel. Impregnation of the preform is carried out in a subatmospheric pressure zone at a pressure of about 700 Pa or less by maintaining a relatively slow flow of a mixture of helium and an alkyl chlorosilane through a coating zone heated to a temperature of between about 1100.degree. to 1200.degree. C. at a rate between about 2 and about 10 times the diffusion rate of the gaseous atmosphere in the fibrous preform. Coating is continued until silicon carbide impregnates the interstices of the fibrous preform and preferably deposits an outer layer of dense pore-free silicon carbide thereupon. The resultant structures are hermetic and provide leak-free containment of gases such as helium at temperatures of as high as 1,000.degree. C. and a pressure differential as high as about 20 MPa.

Abstract: A method of making a fiber-reinforced ceramic matrix composite of complex shape by first rigidifying a fibrous arrangement made of woven or braided material by impregnating with a preceramic resin, and then curing and pyrolyzing the resin to form a rigid or stiff preform. The rigid preform is then densified by chemical vapor infiltration, e.g., using a forced-flow/thermal-gradient process. Using three-dimensional woven fabrics made of ceramic fibers, such as silicon carbide or silicon nitride, strong high-temperature resistant composites are produced having porosities from about 20 percent to less than about 5 percent.