Study On Preparation Of Si₃N₄-based Ceramic By Gel-casting And Toughening Mechanism

Si₃N₄ ceramic have excellent properties such as high temperature resistance,high hardness and high thermal stability,and are widely used in aerospace,mechanical processing and nuclear reactors.Aiming at the high hardness and brittleness of Si₃N₄ ceramic material itself,it is difficult to process in the later stage.In this paper,the near-net-size Si₃N₄-based ceramics are prepared by a advanced forming method gel-casting,which can achieve the purpose of less processing or no processing in the later stage.The effects of single addition and compound addition of ZrO₂ and NbC on Si₃N₄-based ceramics were studied to achieve the purpose of strengthening and toughening ceramics.The mechanical properties,SEM,XRD,EDS and other detection methods were used to study the density,flexural strength,fracture toughness,micro-morphology and strengthening and toughening mechanism of Si₃N₄-based ceramics.The main research contents and conclusions are as follows:?1?The effects of pretreatment of Si₃N₄ initial powder,pH value,dispersant content,milling time and solid phase content on the rheology of the slurry during the preparation of Si₃N₄ ceramic slurry were studied,and the solid phase content was as high as 52%.The Si₃N₄ slurry with variable performance meets the requirements of the gel-casting process.At the same time,the solidification mechanism of the slurry gel was studied,and the curing time was controlled to 8 minutes by controlling the content of the catalyst and the initiator.The final prepared Si₃N₄ceramic has a density of 90.24%and a flexural strength of 601MPa.?2?ZrO₂/Si₃N₄ ceramic were prepared by gel-casting process with ZrO₂ as the toughening phase.The effects of ZrO₂ content and sintering temperature on the mechanical properties and morphology of the materials were investigated.The results show that when the ZrO₂ content is 10wt%and the sintering temperature is 1650°C,the flexural strength and fracture toughness of ZrO₂/Si₃N₄ ceramics reach the maximum at the same time,which are 767MPa and 8.7MPa·m^1/2,respectively.XRD analysis shows that the ZrO₂-Si₃N₄ system generates a large amount of ZrN impurity phase which cannot be phase-changed after the sintering temperature is too high,which affects the ceramic properties.According to the fracture morphology,the toughening mechanism of ZrO₂ is stress-induced phase transition.?3?In the study of particle toughened Si₃N₄ based ceramics,the addition of hard particles NbC is beneficial to the formation and growth of long columnar?-Si₃N₄ grains.With the increase of NbC content,the microstructure of NbC/Si₃N₄ ceramics is gradually uniform.The density and flexural strength increase,the fracture toughness first rises and then decreases.When the NbC content is 10wt%and the sintering temperature is 1650°C,the Si₃N₄-based ceramics have achieved good comprehensive performance(the density is 95.56%,the flexural strength is 745MPa,and the fracture toughness is 9.1MPa·m^1/2).Backscattered electron imaging shows that NbC particles can be well dispersed in the Si₃N₄ ceramic matrix by gel-casting,and the ceramics are toughened by crack deflection and microcracking effects.At the same time,it is found that increasing the sintering temperature can make the?-Si₃N₄ grain distribution more uniform and reduce the porosity.?4?In the composite toughening series,ZrO₂ and NbC were used as the toughening phase,and the influence of the addition amount of the two toughening phases on the Si₃N₄-based ceramics was studied.The results show that the composite toughened particles with a small amount of 5%ZrO₂ and 5%NbC have higher performance than the single addition,and the density,flexural strength and fracture toughness are97.67%,887MPa and 9.3MPa·m^1/2,respectively.However,if the content of the second phase is too high,the?-Si₃N₄ grains will grow abnormally and the second phase particles will agglomerate.