Study On The Effect Of Phase Transformation And Densification Of Silicon Nitride At Low Temperature On Sintering At High Temperature

Silicon nitride ceramic is widely used in the fields of aerospace,machinery and chemical engineering,because it possesses high strength and hardness and excellent wear resistance.However,so far,the studies on phase transformation and densification of silicon nitride prepared at low-temperature,and their effects on the sintering behavior in the process of high-temperature are little.So silicon nitride ceramic was prepared by liquid sintering using ?-Si₃N₄ as raw material and Al₂O₃-Y₂O₃ as sintering additives,and its microstructure and properties were studied in detail in this paper.The effects of sintering temperature?1410¹510??,sintering additive content?4%¹0%?and holding time?4¹0h?on the phase transformation and densification of silicon nitride during pressureless sintering at low temperature were investigated.It is found that densification behavior of silicon nitride during sintering is generated prior to the phase transition from ? to ? phase,and the aspect ratio of ?-Si₃N₄ prepared at low temperature is small.When the holding time is 6h and the sintering additive content is 10%,the content of ? phase increases from 15% to 95%,and the relative density increases from 55% to 78%,as the sintering temperature increases from 1410? to 1510?.When the holding time is 6h and the sintering temperature is 1490?,the content of ? phase changes little from 77% to 78%,but the relative density increases obviously from 48% to 85%,as the content of sintering additive increases from 4% to 10%.The effects of phase transformation and densification of silicon nitride sintered at low temperature on the properties of the final silicon nitride ceramic prepared at high temperature were investigated.It is found that the phase transition of silicon nitride sintered at 1470? for the holding time of 6h is relatively low where the content of ? phase is less than 50%,and the relative density is higher where the relative density is close to 80%.Then the pre-sintered Si₃N₄ ceramic was sintered at 1700? for 2h.The results show that the relative density and fracture toughness of as-prepared ?-Si₃N₄ ceramic are 96.94% and 6.15MPa·m1/2,respectively.In the process of gas pressure sintering in nitrogen atmosphere,the effect of gas pressure?0.2¹.6MPa?on the phase transformation of silicon nitride was investigated.It is found that the percentage of the phase transition from ? to ? phase of Si₃N₄ decreases by about 7%,as the nitrogen pressure increases from 0.2MPa to 1.2MPa.When the nitrogen pressure increases from 1.2MPa to 1.6MPa,the percentage of the phase transition from ? to ? phase of Si₃N₄ increases obviously,and the increased percentage is about 9%.