Fabrication And Properties Of Porous Silicon Nitride And Silicon Nitride Based Composites Via Gel-casting

The rapid development of high-Mach-number missiles has posed severe challenges to radomes,urgently requiring the improvement of wave-transparent materials.On this background,porous silicon nitride and silicon nitride based composites were prepared by using Gel-casting forming method and pressureless sintering in order to cope with the relatively high dielectric constant and intrinsically poor fracture toughness of current silicon nitride ceramics.In this paper,the forming method was optimized,and the influence of material formulas and sintering parameters on the overall p erformance of ceramics was also studied.The optimization of gel-casting was developed.During the preparation of ceramic slurry,the content of p H regulator required increases with the increasing solid loading,and it is also necessary to add ceramic powders separately for many times before achieving high solid loading slurry.After pre-oxidized at high temperatures in air,silicon nitride powders show an increase in zeta values when the solution is alkaline,which is considered to be beneficial to the stability and the decrease of viscosity of silicon nitride slurry to some extent.The volume shrinkages of silicon nitride green bodies were studied.It is indicated that when the solid loading is lower than 35 vol.%,the volume shrinkage of ceramics before and after drying and sintering processes is higher than 20% and the porosity of materials cannot be improved efficiently by further decreasing the solid loading.Meanwhile,when the solid loading is higher than 40 vol.%,ceramic powders in gel preforms are closely stacking,which means that it is of less use to increase solid loadings in order to minimize the shrinkage.The effects of sintering additives' content,sintering temperatures and holding times on the mechanical and dielectric properties of porous silicon nitride were investigated.The introduction of sintering additives can enhance ceramics' mechanical properties but is detrimental to the decrease of volume shrinkage and d ielectric constant.Besides,relatively high sintering temperatures and long holding times are beneficial to the densification of ceramics and the growth of rod-like ? silicon nitride,which can contribute to the enhancement of mechanical properties.However,silicon nitride is prone to decompose when sintered at temperatures higher than 1750 °C for a long time,thus leading to a lower density and weaker strength.Specifically,porous silicon nitride sintered at 1650°C for 3h with the solid loading of 40 vol.% and 3wt.% sintering additives shows a relatively good performance.Its porosity reaches 43.1%,while the volume shrinkage is under 14.6%.The fractural strength and fracture toughness are 146 MPa and 1.44MPa·m1/2,respectively,and its dielectric constant and loss tangent are 3.4 and 0.008 at room temperature?14GHz?.The oxidizing behavior of raw silicon nitride powders was analyzed.The oxidation begins at around 1200°C,and the reaction rate grows with the increasing temperature.The oxidized powders are covered by a layer of amorphous SiO2,which can transform into crystal after being treated at over 1400°C.The effects of sintering temperatures on the phase composition and microstructure of silicon nitride based ceramics were studied.For the phase composition,when there is a low content of SiO2 in silicon nitride powders,the ceramics sintered at 1750°C by using such raw materials would be made up of silicon nitride-silicon oxynitride?Si₃N₄-Si₂N₂O?multiphase,but the Si₂N₂ O phase would decompose when the temperature increases to 1850°C.However,when the content of SiO2 is much higher,ceramics sintered at 1750°C and 1850°C would only include Si₂N₂ O phase.As to the microstructure,when the sintering temperature is lower than 1750 °C,the silicon nitride powders would react with and be bonded by SiO2 with high viscosity,which can promote the densification of ceramics.When it comes to 1850°C,the viscosity of SiO2 declines and the silicon nitride decompose s to produce gas,which would cause the oversintering of ceramics and serious foaming phenomenon.The effects of sintering temperatures on the mechanical and electric properties of Si₃N₄-Si₂N₂ O ceramics were investigated.By adopting the 1200°C pre-oxidized silicon nitride powders,the multiphase co mposites sintered at 1750°C for 2h display good properties,with the theoretical porosity and sintering volume shrinkage being at 42.0% and 18.6%.Their fractural strength,elastic modulus and fracture toughness can reach 87.9MPa,42.1GPa and 1.85MPa·m1/2 respectively,while dielectric constant and loss tangent are 3.6 and 0.0034 at room temperature?14GHz?.The introduction of Si₂N₂ O could help improve the fracture toughness and dielectric properties of silicon nitride ceramics,but the weak intergranular phase would also impair the fractural strength of ceramics.