The specimens were prepared by molding the mixture of silica fume(w(SiO2)=94.5%;average particle size:0.08 μm) and silicon nitride(≤0.074 mm) with a mass ratio of 11,carbon embedded firing at 1 300 ℃,1 450 ℃,1 500 ℃,1 550 ℃ and 1 600 ℃ for 3 h in air,and then water-cooling,respectively.The microstructure and phase composition of the specimens were analyzed.The results show that:(1) silica fume reacts obviously with Si3N4 forming Si2N2O above 1 550 ℃.The edges and corners of Si3N4 grains become smooth and the Si3N4 grains distribute in the continuous cementation phase of Si2N2O forming the dense structure of Si2N2O packed Si3N4;(2) below 1 500 ℃,the edges and corners of Si3N4 grains are clear,Si2N2O doesn't form,and only SiO2 crystallizes from silica fume which happens obviously at 1 300 ℃.
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