Reaction mechanism of preparing porous silicon nitride/silicon carbide composite ceramics

The reaction mechanism of preparing porous silicon nitride/silicon carbide (Si₃N₄/SiC) composite ceramics was investigated by using solidified phenolic resin as carbon source. Argon was used as protective atmosphere. The polymer can pyrolyze to provide free carbon, and farther react with Si₃N₄ powder coated at different temperatures ranged from 1300°C to 1780°C. XRD, TEM, the microstructure of the samples, the thermodynamics and kinetics analysis show that the reaction mechanism mainly associates with liquid-solid reaction between C and Si(l) decomposed from Si₃N₄, and liquid-gas reaction between Si(l) and CO(g) which is the intermediate product during the reactions. Other reaction types involved are solid-solid reaction between C and Si₃N₄ gas-solid reaction between C and SiO₂ coated on the Si₃N₄ powder, and gas-solid reaction between C and SiO(g) or Si(g). The reactions between pyrolyzed carbon and Si₃N₄ begin at 1400°C, and form SiC layer. Keeping in higher temperatures can improve the formation of SiC layer and the prolongation of soaking time can increase the thickness of SiC layer significantly.