In situ formation of mullite strengthened SiC porous ceramics via gelcasting with high strength and good alkali resistance

Mullite-bonded porous SiC ceramics sintered in air by gelcasting are still challenges due to the high porosity induced severe oxidation of SiC, which results in the formation of large amount of detrimental cristobalite phase. Here in this work, small amounts of Y 2 O 3 and CaF 2 were added in SiC and Al(OH) 3 raw materials as sintering additives for the in situ growth of mullite reinforcement. This additive system promoted the reaction between oxidation-derived SiO 2 from SiC and Al 2 O 3 decomposed from Al(OH) 3 to mullite phase. Almost no cristobalite phase was detected when sintered at 1450℃/2 h with CaF 2 addition of more than 2.0 wt%. Mullite whisker reinforcement was in situ formed due to the gas reaction mechanism caused by CaF 2 addition. Thus obtained porous SiC ceramics exhibited a flexural strength of 67.6 MPa at porosity of 41.3%, which maintained exceeding 36 MPa after 8 h corrosion in 10 wt% NaOH 80℃ solution, being the best performance up to now. This high performance of porous SiC was attributed to the additive induces proper phase control and in situ formation of whisker-like mullite reinforcement.