Microstructure and mechanical properties of three-dimensional carbon/silicon carbide composites fabricated by chemical vapor infiltration

Three-dimensional carbon/silicon carbide composites were fabricated by chemical vapor infiltration, and the microstructure and mechanical properties were investigated. For the composites (C/SiC) with no pyrolytic carbon interfacial layer, the mechanical properties (flexural strength, flexural elastic modulus, shear strength and fracture toughness) are increased with density of the composites. High density (ρ = 2.1 cm^-3) C/SiC composites exhibit high fracture toughness (16.5 MPa m^1/2) but brittle fracture behavior because of strong bonding between the fiber/matrix. Low density composites show non-catastrophic failure mode with bundle pull-out. The composites (C/PyC/SiC) with pyrolytic carbon interfacial layer exhibit good mechanical properties and a typical failure behavior involving fiber pull-out and brittle fracture of sub-bundle. Microstructural observations and theoretical analysis reveal that the tortuosity and bottleneck effect of the pores and large molar mass of reactant agent (methyltrichlorosilane) are three key issues to hinder the densification of composites. Cracks formed in the SiC matrix by thermal stress have two influences on the mechanical properties of the composites: to decrease mechanical properties and have some contribution on toughness and failure behavior by deflecting cracks.