Effect of the sintering additive content on the non-protective oxidation behaviour of pressureless liquid-phase-sintered α-SiC in air

The long-term oxidation resistance of pressureless liquid-phase-sintered (PLPS) α-SiC was investigated as a function of the content of sintering additive (in particular, YAG) at 1500 °C in air. It is shown that, regardless of the vol.% YAG, the oxidation is passive at that high temperature, with a kinetics given by the paralinear-rate law. This is because the oxide scales grow due to oxidation of the SiC grains, but recede due to the formation of a eutectic phase and to the carbothermal reduction of YAG. It is also shown that the oxidation resistance of PLPS SiC decreases markedly with increasing vol.% YAG, an effect that is especially marked above 7.3 vol.% YAG where a change in oxidation behaviour occurs. Thus, while up to 7.3 vol.% YAG the PLPS SiC ceramics gain mass during the entire oxidation process (500 h) because the oxide scales are at least semi-protective, from 11.1 vol.% YAG onwards the PLPS SiC ceramics first gain mass and then lose mass linearly over oxidizing time because the oxide scales are non-protective. Finally, implications for the design of PLPS SiC ceramics that can tolerate prolonged exposures at high temperatures in air are discussed.