Small Distortion Of Silicon Nitride Ceramic During The Sintering

Silicon nitride（Si₃N₄）ceramic has been used in automobile,aerospace,electronic circuit and other fields,with excellent comprehensive properties.With the rapid development of science and technology,the requirements on the complexity and performance of Si₃N₄ ceramic structure are increasing.At present,there are various methods for preparing Si₃N₄ ceramic blanks with complex shapes,but still need to be sintered to obtain high-performance Si₃N₄ceramics.In the process of pressureless sintering,accompanied by huge volume shrinkage changes,there are problems such as large sintering shrinkage and distortion.In this thesis,ceramic powder pressing technology and pressureless sintering technology were used to form Si₃N₄ ceramic.Systematically study the effects of sintering aid types,content,distribution and green density on the relative density and shrinkage of Si₃N₄ ceramics.Simulate the temperature field of the vacuum atmosphere sintering furnace used in the experiment.Provide a method for preparing Si₃N₄ ceramics with high relative density,high performance and uniform shrinkage.The main research is as follows:In order to facilitate the expression of sintering deformation,a shrinkage anisotropy factor,A_XY=shrinkage in X direction-shrinkage in Y direction,is used to describe the results in the study.The effect of different sintering aids Y₂O₃-Al₂O₃-Si₃N₄,Y₂Si₂O₇and Y₂Si₂O₇-MUL on the sintering relative density and shrinkage of Si₃N₄ ceramics were studied.It is found that the relative density increased with the increase of temperature;the effect of the sample on the relative density of Si₃N₄ ceramics is different,but all of them are quickly densified near the eutectic point of the liquid phase.Finally,Y₂O₃-Al₂O₃-Si₃N₄、Y₂Si₂O₇-MUL-Si₃N₄ ceramics with the relative density of 97%and 99.25%were prepared at 1800℃.The linear shrinkage and volume shrinkage of the sample increase with the increase of temperature.The amount of shrinkage is different in different temperature sections,but both shrink rapidly at the temperature of liquid phase formation.The shrinkage of the sample in the length and width directions is basically the same,which is smaller than that in the height direction,showing shrinkage anisotropy.And the shrinkage anisotropy factors A_HL and A_HD increase with increasing temperature.At the same sintering temperature,the shrinkage anisotropy factors A_HL and A_HD of samples with different sintering aids were different.Finally,Y₂Si₂O₇-MUL-Si₃N₄ ceramics with shrinkage factors A_HL of 0.53%were prepared at 1700℃.In order to obtain Si₃N₄ ceramics with high density and low shrinkage anisotropy,sintering aids with low liquid phase eutectic temperature should be selected.The effect of sintering aid content on the sintering relative density and shrinkage of Y2SM-Si₃N₄ ceramics were studied.It is found that the sample showed no obvious change in relative density and shrinkage without the addition of sintering aid.Samples with the same content of additives have higher relative densities at higher temperatures;high levels of sintering aids can obtain higher relative densities at low temperatures,but sintering aids have less effect on relative density at high temperatures.The highest relative density is obtained when the content is 10wt%.The samples with sintering aids have anisotropy in sintering shrinkage;the higher the sintering temperature of the samples with the same content of the auxiliary agent,the higher the shrinkage and the anisotropic factors A_HL and A_HD;The greater the shrinkage anisotropy factors A_HL and A_HD.In order to obtain high relative density,low shrinkage anisotropic Si₃N₄ ceramics,the content optimum content of sintering aid is 10wt%.The effects of green density and sintering aid distribution on the sintering density and shrinkage of Y₂O₃-Al₂O₃-Si₃N₄ ceramics were studied.It is found that before 1600℃,the higher the sample body density,the higher the relative density;The density effect is small,and the final relative density of the samples reached 97%.For samples of the same body density,linear shrinkage and volumetric shrinkage increase with increasing temperature;at the same temperature,the higher the body density of the sample,the smaller the shrinkage.And the sample anisotropy factor A_HL is minimum 0.2%when the density of the green body is maximum.The relative density,shrinkage and anisotropy of Si₃N₄ ceramics introduced by sintering aids through co-precipitation and ball milling increase with increasing temperature.At 1800℃,the relative density is 97%.At the same temperature,the shrinkage of the ball-milled sample is smaller than that of the co-precipitated sample;but the co-precipitated powder distribution is more uniform,and the anisotropy factor A_HL of sintering shrinkage is 0.3%,which is smaller than the ball mill sample 1.38%.In order to obtain high-density,low-shrinkage anisotropic Si₃N₄ ceramics,a green compact with uniform components and higher volume density is suitable.The temperature field distribution of atmosphere sintering furnace was studied by using simulation method.It is found that there is a significant uneven temperature distribution during the sintering process.Quarter model has the lowest temperature at the intersection of the wall surface,and the higher the temperature near the heating element.Long-term heat preservation can improve the temperature field distribution and increase the area of the average temperature zone.At the wall surface,the temperature uniformity zone is distributed along the wall surface in an annular shape;at the center cross section,the temperature of the center cross section is the lowest during the heating process.The closer to the heating element,the higher the temperature,and the larger the average temperature area.