Studies On Densification And Preparation Of Silicon Nitride Ceramics With LiAlO₂ Based Sintering Additives At Low Temperature

Silicon nitride?Si₃N₄?ceramic material is a kind of good structural ceramic material and functional ceramic material with extensive applications as its excellent mechanical properties such as good wear resistance and corrosion resistance,high bending strength,good fracture toughness,great hardness and high thermal conductivity.However,the high sintering temperature and pressure dependent sintering are the main factors that restrict the industrial production and applications of Si₃N₄ ceramics.Sintering temperature of Si₃N₄ ceramics with acceptable properties could be realized by adding a suitable sintering additive.The influences of LiAlO₂and LiAlO₂-Y₂O₃ sintering additives on the sintering behavior of Si₃N₄ ceramics were investigated in present paper.Based on the densification research,large-size Si₃N₄ ceramic samples were prepared by the optimized technologies and their properties related application were investigated.Effects of LiAlO₂ content and holding time on the densification of Si₃N₄ceramics were studied at relatively lower sintering temperature of 1650?by the addition of LiAlO₂ sintering additive.Relative density of 97.7%,the?-Si₃N₄ phase transformation rate of 67%,could be obtained when the content of LiAlO₂ is approaching 12%,which has the best precipitation and growth of?-Si₃N₄ grains comparing to the content of 6,9,15 and 18%.Lower or higher contents of sintering additive show a negative effect on the precipitation and growth of?-Si₃N₄ grains.With the increase of the content of LiAlO₂ sintering additive,the conversion rate of?-Si₃N₄ to?-Si₃N₄ increases continuously.When the holding time is more than 4hours,little effects on densification but higher conversion rate of?-Si₃N₄ with larger grain size could be observed with the increase of holding time.Influences of sintering temperature and proportion of composite sintering additives on the densification of Si₃N₄ ceramics were studied by LiAlO₂-Y₂O₃ as composite sintering additive.The relative density of Si₃N₄ ceramics can be further increased to over 98.0%,the?-Si₃N₄ phase transformation rate is 72%.The phase transition of?-Si₃N₄ to?-Si₃N₄ will start at the temperature range from 1400?to1500?,but with much lower phase transition rate.When the sintering temperature is higher than 1600?,the phase transition rate increases accordingly.Otherwise,when Y₂O₃ content in the composite additives increased,the relative density will decrease slightly but phase transformation rate is promoted obviously.The?-Si₃N₄grains with good morphology can be also observed by the introduction of Y₂O₃.Large-sized Si₃N₄ ceramic products were finally prepared by optimized proportion of sintering aids and sintering process and the influences of molding process on products as well as their performances were investigated.Si₃N₄ powders with more uniform particle size distribution,better sphericity,better particle cohesion and improved fluidity can be obtained by isostatic pressure granulation rather than mechanical pressure granulation,as a result of better forming property of green bodies.Furthermore,the density,machinability and formability of green bodies can be improved with optimized PVA content of 3%comparing to that 1%and 2%.The ellipse section of the specimen placed horizontally and elephant foot deformation placed vertically can be improved significantly by V-groove support sintering.Si₃N₄ceramic samples prepared by the optimized technologies show good performances,such as the initial bending strength is 580MPa,the residual strength rate of 80.0%after three thermal shock tests at 820?,mass change rate of 0.20%after 40 hours oxidation at 1200?,and mass change rate of 0.06%,0.31%,0.46%and 0.67%respectively after being eroded with 10%,15%,20%and 25%NaOH solution at110?for one hour,which present a great potential of low temperature sintering method for the Si₃N₄ ceramic production.