Mechanical behaviour of a 3-D braided, continuous SiC fibre-reinforced/CVI SiC matrix composite at ambient and elevated-temperatures

Three-dimensionally reinforced continuous-fibre ceramic matrix composites (CFCCs) were fabricated from preforms of braided SiC fibre (Nicalon^TM) tows that had been coated with 0.4-μm thick layer of pyrolytic graphite. A hybrid infiltration process of CVI and polymer precursor processing was used to promote the matrix and consolidate the composite. The as-fabricated CFCCs were composed of approximately 35 vol% fibre, approximately 40 vol% matrix and approximately 25 vol% residual porosity. Densities and elastic constants were determined prior to testing. Straight-sided and contoured coupons were tested in uniaxial tension and compression at 20 and 1000 °C. End tabs comprised of shaped potted-epoxy were used on all specimens to prevent crushing of the gripped specimen sections in the water-cooled, face-loaded hydraulic grips. Three- and four-point flexure tests were also conducted at 20 and 1000 °C. Linear stress-strain responses to well-defined proportional limits (approximately 75 MPa) were observed for tension and flexure at both temperatures. Nonlinear stress-strain behaviour was observed 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 fibre pullout for the 20 °C tests and evidence of brittle fracture due to environmental degradation for the 1000 °C tests.