Effect of Prior Exposure at Elevated Temperatures on Tensile Properties and Stress-Strain Behavior of Four Non-Oxide Ceramic Matrix Composites

Thermal stability of four non-oxide ceramic matrix composites was studied. The materials studied were commercially available composites SiCSiNC CSiC CSiC-B4C CHYPR-SiCtrademark and SiCSiC-B4C SiCHYPR-SiCtrademarkr. COI Ceramics manufactured the SiCSiNC and CSiC composites using polymer infiltration and pyrolysis PIP. The CHYPR-SiCtrademark and SiCHYPR-SiCtrademark CMCs were manufactured by Hyper-Therm High-Temperature Composites using chemical vapor infiltration CVI. All four composites rely on a dense matrix for strength, stiffness, and oxidation protection. Fiber coating provides a fiber-matrix interface that allows fiber-matrix debonding and fiber pullout to occur, imparting fracture toughness to the CMC. The SiCSiNC, CSiC, and CHYPR-SiCtrademark composites were heat treated in laboratory air for 10 h, 20 h, 40 h, and 100 h at over-temp 1300 deg C and for 100 h at operating temperature 1200 deg C. The SiCHYPR-SiCtrademark composite was heat treated in laboratory air for 10 h, 20 h, 40 h, and 100 h at over-temp 1400 deg C and for 100 h at operating temperature 1300 deg C. Room-temperature tensile properties of heat treated and virgin material were measured, and effect of prior heat treatment on tensile properties was evaluated. Prior heat treatment caused a reduction of tensile strength of at least 10 for all materials. Both PIP-produced CMCs exhibited increased fiber-matrix bonding due to high temperature exposures, contributing to brittle fracture of clumped fiber bundles and thus reduced tensile strength. Both CVI-produced CMCs exhibited considerable matrix voids due to poor infiltration during fabrication. Void prevalence and the associated stress concentrations contributed to premature matrix cracking and composite failure. The CVI CMCs were susceptible to degraded tensile properties and brittle composite fracture due to strengthened fiber-matrix interfaces and fiber degradation caused by prior heat treatment.