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(TM)) tows that had been coated with 0.4 mu m 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 similar to 34 vol% fiber and similar to 36 vol% matrix with similar to 29 vol% residual porosity. Appropriate specimens were tested at 20 degrees and 1000 degrees 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 (similar to 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 degrees C and 100 MPa for 1000 degrees C. SEM fractography revealed fiber pullout for the 20 degrees C tests and evidence of brittle fracture due to environmental degradation for the 1000 degrees C tests.