Study On Low Temperature Sintering And Densification Behavior Of Si₃N₄ Ceramics At Ambient Pressure

Si₃N₄?silicon nitride?ceramics are widely used as critical materials in metallurgy,military,nuclear reactor,aerospace,automotive engines and so on.As a strong covalent bond compound,Si₃N₄ is sintered by hot-pressing,gas-pressure and other high-cost sintering methods.The facilities and equipment for preparation and fabrication are complicated and expensive.Therefore,pressuresless sintering seems the emerging focus of the present related research.Based on the low-temperature preparation of Si₃N₄ ceramic in air,a pressureless sintering technology of Si₃N₄ ceramics in electric furnace was presented.The low temperature preparation of Si₃N₄ ceramics in air were realized by using Li₂O-based sintering additives.The effects of sintering additives based on Li₂O-Al₂O₃ and LiAlO₂ system on the densification of Si₃N₄ ceramics at different temperatures and proportionings were studied.?1?The sintering method of Si₃N₄ ceramics in the traditional electric furnace was designed.Through the analysis of the gas phase composition,the possible reactions and products of different systems in the sintering process,it was considered that the condition of the sample was micro-positive pressure N₂ atmosphere,and only a small amount of Si₂N₂O was generated in the sintering process.the feasibility of sintering Si₃N₄ ceramics in traditional air furnace was proved.?2?The low temperature sintering of Si₃N₄ ceramics in air were realized by using Li₂O-Al₂O₃ sintering additives?Li₂O was introduced by Li₂CO₃?.With the certain composition of the sintering additive,the densification degree of the samples increased with the increase of sinering temperature.Meanwhile,at the same sintering temperature,with the increase of the addition of Li₂CO₃ in the sintering additive,the mass loss of the samples increased,while the linear shrinkage,relative density,and?-Si₃N₄ conversion of the samples increased at beginning and then decreased.At sintering temperature of 1650?and the mole ratio of 0.7:1 of Li₂CO₃ and Al₂O₃ in the sintering additive,the densification of the sample reached the highest,the relative density of the sample was 92.6%,and the?-Si₃N₄ conversion of the samples was close to 55%.The SEM diagram of the sample could already see obvious rod-like?-Si₃N₄ grains,and with the increase of sintering temperature,it could be seen that the size of?-Si₃N₄ increased.However,there were still the residual micro-pores in the sample.?3?The complete phase of LiAlO₂ was synthesized by pretreatment of Li₂CO₃ and Al₂O₃ with mole ratio of 1:1 at 1000?for 2 h,and the average crystal grain size was about 1?m.LiAl₅O₈ was formed with more Al₂O₃,but Li₂CO₃ was left with more Li₂CO₃.?4?Compared with the Li₂O-Al₂O₃ sintering additives,the densification tend of Si₃N₄sintered by the LiAlO₂ additive was almost same.With the increase of temperature,the relative density increased,while with the increase of residual Li₂CO₃,the relative density increased decreased.When the sintering temperature was 1650?,the mole ratio of Li₂CO₃and Al₂O₃ in the pre-treated sintering additive was 0.7:1,the densification degree reached the highest.The relative density of the sample was 95.2%,and the?-Si₃N₄ conversion of the samples was close to 63%,compared with the Li₂O-Al₂O₃ sintering additives,the densification behavior of the samples was improved.The grain size of the Si₃N₄ in microstructure was larger with the higher sintering temperature,but the growth of the Si₃N₄ grain was inhibited by the higher Li₂CO₃ content.The densification behavior of the samples was improved and the residual pores decreased markedly by adoption of the sintering additives pretreated.