Processing and characterization of carbon fibre reinforced ZrB2/SiC/WC composites

The sintering behavior and mechanical properties of carbon fibre reinforced ZrB2/SiC/WC composites was investigated. Sintering was carried out between 1800 and 2000°C via hot pressing at 40 MPa. The sample sintered at 1800°C is characterized by higher porosity, while the sample sintered at 2000°C is fully dense but fibres are severely degraded. A temperature of 1900°C allows to obtain 92% dense composite without excessively damaging the fibres. The microstructure is characterized by the presence of ZrB2 and solid solutions of W-Si-B. Four-point flexural strength is in the range of 150-200 MPa, while fracture toughness is 4-6 MPam0.5. The sample sintered at 1800°C is characterized by higher KIc which was attributed to a higher degree of fibre pull- out due to weak fibre/matrix interface. The sample sintered at 2000°C is characterized by very strong fibre/matrix interface due to extensive reaction of the fibre with the surrounding ceramic matrix, leading to brittle fracture behaviour. A temperature of 1900°C was found as the best compromise to maximize strength and avoid brittle fracture. Comparing these results with an un-doped ZrB2/SiC composite, mechanical properties do not seem to be significantly influenced by the presence of WC, but the presence of WC leads to the formation of a more compact oxide scale, improving the oxidation resistance