Properties of slip-cast and pressureless sintered ZrB2-SiC composites

In view of the perspective to produce near-net shapes of ultra-refractory ceramics, ZrB2-SiC composite were produced by slip casting on plaster mould and pressureless sintered at 2150°C with holding time of 120 minutes, using silicon nitride as sintering aids. After sintering, slip cast samples reached a very high relative density over 98%, while, for comparison, the density of cold isostatic pressed samples was about 94%. Traces of the formation of secondary phases such as B4C, graphite, BN were found in all samples, while no grain boundary phases were revealed. RT flexural strength of as-sintered slip cast bars was affected by the presence of an external porous layer of coarse ZrB2 particles, while at 1500°C flexural strength increased, as a glassy layer sealed the superficial defects. Due to the absence of softening grain boundary phases, sub-critical crack growth did not occur during high temperature strength tests