In-situ Reactive Sintered Si₃N₄ Multiphase Ceramics And Its Environmental Performance

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.