Fabrication, Processing, and Characterization of Braided, Continuous SiC Fiber-Reinforced/CVI SiC Matrix Ceramic Composites

Three-dimensionally reinforced continuous-fiber ceramic matrix composites (CFCCs) were fabricated from preforms of braided SiC fiber (Nicalon(™)) tows that had been coated with 0.4 μm thick layer of pyrolytic graphite. A hybrid infiltration process of chemical vapor infiltration and polymer impregnation pyrolysis was used to form the matrix. The as-fabricated CFCCs were composed of ∼34 vol% fiber and ∼36 vol%matrix with ∼29 vol% residual porosity. Appropriate specimens were tested at 20° and 1000°C along the longitudinal braiding direction in uniaxial tension, compression and three and four-point flexure. Linear stress-strain responses to well-defined proportional limits (∼75 MPa) occurred for tension and flexure at both temperatures. Nonlinear stress-strain behaviour occurred beyond the proportional limit up to fracture at ultimate strengths in tension on the order of 175-200 MPa for 20°C and 100 MPa for 1000°C. SEM fractography revealed fiber pullout for the 20°C tests and evidence of brittle fracture due to environmental degradation for the 1000°C tests.