Properties of Slip‐Cast and Pressureless‐Sintered ZrB ₂ –SiC Composites

In view of the perspective to produce near‐net shapes of ultra‐refractory ceramics, ZrB ₂ –SiC composite were produced by slip casting on plaster mould and pressureless sintered at 2150°C with a holding time of 120 min, using silicon nitride as a sintering aid. 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 B ₄ C, graphite, and 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 ZrB ₂ particles, while at 1500°C the flexural strength increased, as a glassy layer sealed the superficial defects. Because of the absence of softening grain boundary phases, subcritical crack growth did not occur during high‐temperature strength tests.