Synthesis, Microstructure And Properties Of Si₂N₂O Ceramics

Si₂N₂O is a low density, high hardness strong covalent compound, with finethermal shock resistance, oxidation resistance, and thermodynamic stability. It is a verypromising high temperature structural materials. Study on preparation and properties ofSi₂N₂O will expand its application in refractory and high-temperature heat-resistantcomponents, which will have very important significance.Si₂N₂O ceramic material was maded by Spark Plasma Sintering taking Si₃N₄andSiO2an raw materials and Lu₂O₃, Y₂O₃, Gd₂O₃as sintering additives. The densificationbehavior of Si₂N₂O material was studyed. And the effec of sintering temperature,content of additives and type of additives on the phase and microstucture were studyedcombinde with XRD and SEM. The law of mechanical properties and oxidationresistance changed with contents and type of sintering additives were also studyed. Theeffect of heat treatment on the phase and microstructure were discussed, and theoxidation resistance of annealling Si₂N₂O materials were researched.Displacement and displacement rate during SPS sintering process were discussed.When the conten of Y₂O₃was one percent molar, the beginning sintering temperaturewas between1150℃，the first maximum value of displacement rate appears between1200℃and1250℃. And the second maximum value of displacement rate was around1550℃. When the temperature rised1600and above, the displacement rate no longerchanged, the densification process completed. When the sintering temperature is1700℃,the relative density arrives more than98%. Increasing the content of Y₂O₃, thetemperatue which corresponds the second maximum displacement rate moves to hightemperature. When changing the type of additives, the temperature that corresponds thesecond maximum displacement rate move to low temperature according the order ofLu₂O₃, Y₂O₃, Gd₂O₃. In view of Lu3+<Y3+<Gd3+, material densification was easilyachieved with the iron radius of rare earth increased.XRD analysis and phase contents showed that when the sintering temperature is1700℃, holding time10min, a large number of silicon oxynitride were synthesised.Studys on the effect of Y₂O₃content on the synthesis of silicon oxynitride showed thatwhen the content of Y₂O₃were1mol%,1.5mol%,2mol%and3mol%, the content ofsilicon oxynitride increased first and then cut down. The yield maximum value appearsin the1.5mol%, the maximum content is86.25wt%. Different sintering aids candramatically affect the amount of silicon oxynitride, Gd₂O₃can promote more siliconoxynitride than Y₂O₃and Lu₂O₃, in addition of composite sintering aids can increase theyield of silicon oxynitride.The micro-morphology of silicon oxynitride is sheet structure and plate-like structure by SEM. Different sintering aids will affect the final size of silicon oxynitride.The greater of the ion radius of rare earth, the bigger of the plate.The bending strength of materials first reduced and then increased with increasingthe content of Y₂O₃, when the content of Y₂O₃is2mol%, the strength is the lowest.Withregard to different rare earth oxide, the bending strength of material with Lu₂O₃wasgreatest, secondly was with Gd₂O₃, the bending strength of material with Y₂O₃was thelowest.The vickers hardness of silicon oxynitride can reach16.21GPa with1.5mol%Y₂O₃.Combining the content of silicon oxynitride, it can conclude that silicon oxynitridepossess bigger hardness. The fracture toughness of Si₂N₂O ceramics calculated usingindentation method were relatively low, about3MPa·m^1/2. Through the study ofindentation crack, it can be seen that the path of crack is relative straight with lessobvious crack deflection and bridging toughening mechanism whicn can increasefracture toughness.With regard to the ceramics sintered by SPS, there was no obvious weight gain at1100℃and1200℃with4h’s oxidation. When the oxidation temperature reached1300℃, the maximum weight gain was2.0mg/cm². The maximum weight gain ofannealing SPS ceramics was1.1mg/cm². Considering the content of silicon oxynitridein the ceramics, it can conclude that silicon oxynitride can improve the oxidationresistance.