Study On Low Temperature Densification And Properties Of Si₃N₄-ZrB₂ Ceramics

Si₃N₄ ceramics were used widely in aerospace,mechanical industry,armor and substrate materials because of its high hardness,high strength,high thermal conductivity,good wear resistance,excellent thermal shock and oxidation resistance.Si₃N₄ ceramics had a low efficiency and precision of machining because of its high hardness and brittleness,so machining was difficult applyto Si₃N₄ ceramics.The Si₃N₄-ZrB₂ conductive ceramics could be fabricated by introducing high-performance ZrB₂ conductive phase into Si₃N₄ ceramics,which could be used in the process of EDM,for improving the machining process of Si₃N₄-based ceramics.But the properties of Si₃N₄-ZrB₂ ceramics was greatly restricted by the coarser particle size of available commercial raw materials of ZrB₂.Therefore,this paper firstly intended to synthesize submicron ultrafine ZrB₂ powder through the novel borothermal reduction method.Then,the ultrafine ZrB₂ powder was used as raw materials to fabricate high properties of Si₃N₄-ZrB₂ ceramics under hot-pressing sintering process with low temperature?1500??.Finally,the Si₃N₄-ZrB₂ ceramics doped low ZrB₂ content were studied under the condition of good electrical conductivity of Si₃N₄-based ceramics,and the large-size Si₃N₄-based particles prepared by the spray-drying method as well as the self-synthesized ultrafine ZrB₂ powder were used as raw materials in this method.Firstly,Zr O2 and B powder were used as raw materials to study the synthesis of ultrafine ZrB₂ powder by novel borothermal reduction method.The phase composition,microstructure and electrical resistivity of Si₃N₄-20vol% ZrB₂ ceramics prepared by commercial ZrB₂ coarse powder and self-synthesized ZrB₂ fine powder were compared.Results showed that the new borothermal reduction method?two-step reduction and intermediate water washing process?,which could significantly reduce the particle size,was used to prepare sub-micron ZrB₂ powders.When the commercial ZrB₂ coarse powder was used as the raw material,the Si₃N₄-ZrB₂ ceramics containedthe main phases of ?-Si₃N₄,?-Si₃N₄ and ZrB₂,as well as the weakYb4Si2N2O7 phase.At the same time,the electrical resistivity was very high because of the isolation of ZrB₂ grains.However,the Si₃N₄-ZrB₂ ceramics containedthe main phases of ?-Si₃N₄,?-Si₃N₄ and ZrB₂,as well as the weak phases of Yb4Si2N2O7 and ZrN,when the self-synthesized ZrB₂ fine powderwas used as the raw material.Simultaneously,ZrB₂ grains remained in the connected state which led to significantly reducedof the electrical resistivity.Then,Si₃N₄-ZrB₂ ceramics was prepared by hot-pressing sintering at 1500? in argon or nitrogen atmosphere with self-synthesized sub-micron ZrB₂ powder and commercial Si₃N₄ powder as raw materials.The phase composition,relative density,microstructure,flexural strength,fracture toughness,hardness and electrical resistivity of the samples were systematic analyzed through controlling the content of ZrB₂ phase,sintering atmosphere and sintering additives.The results showed that it could improve the densification ofSi₃N₄-ZrB₂ ceramics when nitrogen was used as sintering atmosphere.But it could lead to the formation of secondary Zr N phase.The hardness of the Si₃N₄-ZrB₂ samples which sintered under1500 ? hot-pressing in the argon atmosphere,the ZrB₂ contentincreased from 15 vol% to 45 vol%,were 20.5 GPa,18.5GPa and 18.3 GPa,respectively.The fracture toughness of samples were 9.4 MPam1/2,9.8 MPam1/2 and 7.8 MPam1/2,respectively.The bending strength of sampleswere536.8 MPa,702.0 MPa and 669.8 MPa,respectively.The electrical resistivity of sampleswere 9×105?·cm,5×101?·cm and 6?·cm,respectively.The hardness of Si₃N₄-20vol%ZrB₂ ceramics prepared under hot-pressing and added with Y2O3-MgO and Yb2O3-MgO were 19.8 GPa and 20.1 GPa,respectively.And fracture toughness of sampleswere 9.6 MPam1/2,and 10.9 MPam1/2,the flexural strength of samples were754.9MPa and 773.8 MPa,respectively.The electrical resistivity of samples were7×102?·cm and 4×102?·cm,respectively.Compared with the traditional Si₃N₄-based ceramics,Si₃N₄-ZrB₂ composite ceramics with high hardness,high toughness and low electrical resistivity.Finally,Si₃N₄-ZrB₂ ceramics with three-dimensional conductive network structure were prepared under hot-pressing sintering at 1500 ? through controlling the mixing medium and using Si₃N₄ microspheres prepared by spray drying andsubmicron ZrB₂ as raw materials.The results showed that the solidified three-dimensional conductive network structure could be formed when acetone and proper amount of ammonia were used as the mixing medium,because it could keep the integrality of large-size Si₃N₄ microspheres and prevent the agglomeration of the ultra-fine ZrB₂ powder.When the ZrB₂ content was 15vol%,the electrical resistivity of Si₃N₄-ZrB₂ ceramics were only 2×102 ?·cm.This study significantly reduced the ZrB₂ phase content under the condition of ensuring the electrical conductivity.