Characterization of hot-pressed short carbon fiber reinforced ZrB₂-SiC ultra-high temperature ceramic composites

Densified ZrB₂-SiC-based ultra-high temperature ceramics reinforced with short carbon fibers were prepared by conventional hot-pressing. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The C_sf/ZrB₂-SiC composite had improved fracture toughness of 6.6 MPa m^1/2 compared to ZrB₂-SiC composite of 4.3 MPa m^1/2 due to fiber debonding, fiber pull-out and fiber bridging as well as crack deflection. It was found that the low modulus of carbon fiber and a graphitization transition layer between fiber and matrix led to the decreased flexural strength. The oxidation resistance tests were carried out on C_sf/ZrB₂-SiC using an oxyacetylene torch. The temperature of the oxidized specimen exceeded 1800 °C and the surface layer appeared dense and adherent. No macro-cracks or spallation were detected, suggesting that these composites possess a better oxidation resistance than ZrB₂-SiC. The improved oxidation resistance is attributed to the formation of a coherent SiO₂ rich scale, which acts as an effective barrier against the inward diffusion of oxygen. The results presented here point to a potential way for improving toughness of composite without sacrificing oxidation protection.