Study On Microstructure And Properties Of Pneumatic Sintered Silicon Nitride Ceramics

Silicon nitride ceramic is a structural ceramic with high strength,high hardness,high temperature resistance,abrasion resistance,corrosion resistance and good thermal shock resistance,as a result it is widely used in machinery,metallurgy,chemical,aerospace,electric power and other fields.However,the fracture toughness of silicon nitride has been the main factors restricting its further development,select the appropriate sintering temperature and sintering additives can effectively improve the strength and toughness of silicon nitride,which obtain good comprehensive propertiesIn this paper,gas pressure sintered silicon nitride were prepared by using a high purity a-Si3N4 powder as raw material and three kinds of oxides,MgO,Al2O3 and Y2O3,as composite sintering additives.Based on the analysis of microstructure and composition of silicon nitride ceramics,the effects of sintering temperature and the content of sintering aids on the grain size,phase composition and grain boundary structure of silicon nitride ceramics were studied.The effects of different sintering temperature and sintering additives on the mechanical properties of silicon nitride ceramics were studied.The main conclusions are as follows:In silicon nitride ceramics sintering with MgO,Al2O3 and Y2O3 sintering aids prepare by different sintering process,the a-Si3N4→β-Si3N4 phase change completely and the relativedensity reached more than 98%.With the increase of sintering temperature,the silicon nitride grains grow gradually,the grain diameter of P-Si3N4 grain also increased;with the increase of Y2O3 content,the grain size of silicon nitride has increased,the contents and the aspect ratio of long columnar grains are increased as well;when the Y2O3 content is 8%,the number of columnar grain and its size achieved the most.The grain boundary phases of silicon nitride ceramics are mainly SiO2,Y2O3 and Y2Si3N4O3,and the composition of grain boundary phase changing with the change of temperature and the content of sintering aids.The influence of sintering temperature and sintering additive content on the mechanical properties of silicon nitride was investigated by influencing the microstructure and grain boundary composition of silicon nitride ceramics.The density,hardness and beading strength of silicon nitride ceramics increased first and then decreased with the raise of sintering temperature,at 1760℃ reached the maximum value,while the fracture toughness increased with the increase of temperature.With the increase of the sintering aids content,the density,hardness and bending strength of the silicon nitride ceramics decreased first and then increased,while the fracture toughness increased first and then decreased.In self-pairing silicon nitride ceramics showed excellent wear resistance and flow friction coefficient,the wear mechanisms are mainly slight plastic deformation,surface crack generation and propagation,delamination wear and grain shedding.Silicon nitride ceramic friction under water lubrication,the formation of hydrogen silicon oxide film in the friction surface,can lower friction coefficient.With the increasing of friction speed,the friction coefficient reduced.