【作者】 陈超；

【导师】 徐永东；

【作者基本信息】 西北工业大学 ， 材料学， 2006， 硕士

【摘要】 氮化硅是近年来广泛研究和应用的高温结构陶瓷材料,其具有强度高、抗热震性能好、抗氧化性能好等优点,尤其与铝合金熔体不润湿,在铝合金熔炼行业具有广泛的应用前景。冷等静压成型技术是近年来发展起来的一种粉体成型技术,具有坯体密度高且均匀、坯体强度高、易实现近净尺寸成型等优点。 本论文以硅粉为原料,通过冷等静压成型,采用反应烧结制备出纯Si₃N₄陶瓷和Si₃N₄基复相陶瓷。采用XRD、SEM和EDS对材料的相构成、微结构和元素组成进行了分析;针对铝合金熔炼的应用背景,研究了各种材料的环境性能;研究了Si₃N₄材料在铝合金熔炼中,对铝合金中硅元素含量的影响。主要研究内容和结果如下: （1）系统研究了硅溶胶—PVA、粘土—PVA和酚醛树脂等几种粘结剂对坯体强度的影响规律,确定了各粘结剂的最佳含量。冷等静压传统采用PVA粘结剂压制的坯体强度低（约2MPa）,不能直接机械加工,加工前需要预烧结。采用硅溶胶—PVA、粘土—PVA和酚醛树脂粘结剂,坯体抗弯曲强度分别达到3.1MPa、7.2MPa、21.0MPa。其中采用粘土—PVA和酚醛树脂作为粘结剂,实现坯体直接机械加工。在PVA含量确定的基础上,粘结剂最佳含量分别为:硅溶胶,17.2%;粘土,10.0%;酚醛树脂,9.7%。 （2）揭示了粘结剂构成与材料物相组成的关系。采用PVA、硅溶胶—PVA、粘土—PVA、酚醛树脂、酚醛树脂—粘土作为粘结剂,分别得到纯Si₃N₄陶瓷、Si₃N₄—Si₂N₂0、Si₃N₄—O′-Sialon、Si₃N₄—SiC、Si₃N₄—O′-Sialon—SiC复相陶瓷。Si₃N₄存在等轴颗粒、棒状晶粒、针状晶粒和晶须,Si₂N₂O为针状晶粒,SiC被Si₃N₄包裹形成不规则复合颗粒,O′-Sialon的晶粒形貌与Si₃N₄相似。 （3）确定了高压烧结工艺为:气氛压力1.3MPa,阶梯保温为1300℃,1h;1350℃,2h;1380℃,4h。与常压烧结比较,高压烧结时间由13h缩短为7h,实心、厚壁坯体氮化增重率由30%提高到55.6%。高压烧结制备的材料微观形貌与常压烧结材料一致,相对常压烧结的材料,高压烧结材料的晶粒更细小、晶粒结合更紧密;高压烧结材料的体积密度为2.38g/cm³,气孔率为16.51%。 （4）系统研究了纯Si₃N₄和复相陶瓷的相关环境性能,揭示了第二相对材料性能影响的规律。纯Si₃N₄和Si₃N₄—Si₂N₂0、Si₃N₄—O′-Sialon、Si₃N₄—SiC、Si₃N₄—O′-Sialon—SiC的气孔率分别为15.31%、17.38%、15.12%、17.15%和更多还原

【Abstract】 Silicon nitride is one kind of structural ceramics. Its properties of thermal shock resistance and oxidation resistance are very excellent. Especially, it doesn’t react with nonferrous metal. So silicon nitride is widely used in Al melting field. The advantages of green compacts formed by cold isostatic pressing （CIP） are high strength, high density, homogeneous density and so on. Especially, the CIP could achieve near net-shape or net-shape forming.In the paper, the silicon power was raw material, shaped in CIP. The pure Si₃N₄ and multiphase ceramics were prepared in reactive sintering. The components and microstructureof ceramics were studied by XRD, SEM, and EDS. The environmental performances of ceramics were studied for application in Al melting. The influence of ceramics to the content of Si in Al alloy was studied. The main content of study and conclusions are as follows:（1） The relation of silica sol-polyviyl alcohol （PVA）, clay-PVA, and phenolic resin with the bending strength of green compact are studied by the numbers. The best contents of each binder were confirmed. When PVA （the traditional binder） is used in CIP, the bending strength is only 2MPa. The compact can’t be machined directly, need be pre-sintered before machining. When the silica sol-polyviyl alcohol （PVA）, clay-PVA, and phenolic resin were used as binders, bending strength of green compact raises to 3.1MPa, 7.2MPa, 21.0MPa. When the binder is clay-PVA or phenolic resin, the as-prepared compact can be machined directly. The best content of each binder is: silica sol, 17.2%;clay, 10.0%;phenolic resin, 9.7%.（2） The relation of binder with components of ceramics was disclosured. When the PVA, silica sol-PVA, clay-PVA, phenolic resin, and phenolic resin-clay are binder separately, the pure ceramics and multiphase ceramics can be prepared, such as: Si₃N₄, Si₃N₄-Si₂N₂O, Si₃N₄-O’-Sialon, Si₃N₄-SiC and Si₃N₄-O’-Sialon-SiC. The Si₃N₄ forms equiaxed grains, rodlike crystals, acicular crystals, and whisker. Si₂N₂O forms acicular crystals. SiC is packed by Si₃N₄, forms erose grains. The micro-patterns of O’-Sialon are same as the Si₃N₄.（3） The best sintering in high pressure method was confirmed as follows: gaspressure, 1.3MPa. Step sintering is: 1300°C, lh;1350°C, 2h;1380°C, 4h. Compared with sintering in normal pressure, the time decreased from 13h to 7h, weight gain percent of thick-walled compact increased from 30% to 55.6%. The micro-pattern of Si3N4 is samed as the SJ3N4 sintered in normal pressure. Compared with the ceramics sintered in normal pressure, the crystals are thin, and the joint is compact. The density and porosity of ceramics sintered in high pressure are 2.38g/cm and 16.51%.（4） The environmental performance of each ceramics were studied by the numbers, the influence of the second-phase to performance of ceramics were discloused. Density of Si3N4, Si3N4 - Si2N2O, Si3N4-O’-Sialon, Si3N4 —SiC and Si3N4—O’-Sialon-SiC are: 2.53g/cm3, 2.47 g/cm3, 2.56 g/cm3, 2.49 g/cm3, and 2.53 g/cm3;the porosity are: 15.31%, 17.38%, 15.12%, 17.15%, and 16.75%;the bending strength are: 186.6MPa, 152.4MPa, 161.3MPa, 141.2MPa, and 151.2MPa. The bending strength of multiphase ceramics are lower than pure Si3N4. The strength of Si3N4—O’-Sialon is maxmium in multiphase ceramics. Thermal diffusivity of Si3N4 ceramics is depressed when the temperature is ascending, and the speed of depressing is slow. SiC can improve the thermal diffusivity;Si2N2O and O’-Sialon can reduce thermal diffusivity. The thermal shock resistance of ceramics are very excellent^ the retentive ratio of residual strength of multiphase ceramics are higher than pure Si3N4 （74%）, are more than 85%. Especially, thermal shock resistance of Si3N4—O’-Sialon is the best. The micro-cracks form in thermal shock can toughen the ceramics. In air atmosphere, weight gain of ceramics by oxidation follows parabola rule at 800 °C. The oxidation resistance of ceramics are: Si3N4 — Sii^O > Si3N4 — SiC > Si3N4 — O’-Sialon>Si3N4-O’-Sialon-SiC>Si3N4.（5） The influence of Si3N4 to content of Si in Al alloy in melting was studied. In the experiment, Si goes into Al alloy, content of Si will improves above 0.1%;in natural production, the content of Si will improve under 0.01%. We can certify that Si3N4 can be used in pure Al melting.更多还原