Influence of fibre content on the strength of carbon fibre reinforced HfC/SiC composites up to 2100 degrees C

The influence of carbon fibre content on the mechanical behaviour of HfC/SiC composites was investigated up to 2100 degrees C for specimens containing 40 or 55 vol% fibres. Silicon carbide was added as a sintering aid during hot pressing. Increasing the fibre content made infiltration more difficult, which resulted in higher porosity in the specimen with 55 vol% fibres. The room temperature flexural strength ranged from 340 to 380 MPa, and it increased to more than 400 MPa at 1800 degrees C due to stress relaxation. Increasing temperature was accompanied by a decrease in the slope of the load-displacement curve, indicating a decrease in elastic modulus, but plastic deformation was not observed below 2100 degrees C. At 2100 degrees C, the specimen containing a higher fibre content underwent significant deformation due to low interfacial strength between the fibre plies, retaining a strength at the proportional limit of 290 MPa and an ultimate strength of 520 MPa.