Research Of The Fabrication Of High Performance Silicon Nitride Ceramic By Slip Casting

To develop a process of silicon nitride ceramic for ceramic mill pot with high performance is the fundamental goal of the present study. In view of technical design, slurry preparation, green body drying, burning out and sintering were studied. To seek a practical slip casting technical route applied for non-oxides ceramic forming, particularly for silicon nitride-based ceramics with gas press sintering, mechanism of the slip casting process was discussed.Firstly, the surface bond change of silicon nitride powder particales by calcinations were studied. Then, to improve the stability and the suspension of the slurry of silicon nitride powder, and to fabricate the green body with high flexural strength, a series of parameters of the slurry, solid content, PH value, Zeta potential, rheological property and the adsorption amount of the dispersant, were designed. Finally, through the drying, the burning out, the sintering, the silicon nitride ceramic sample stick with the high relative density by slip casting were fabricated.The decomposition of silicon nitride powder in aqueous media was analysed. The analysis show there are a lot of silanol[Si-O] groups on the particles surface of silicon nitride powder. The silanol groups can contribute to water wetting and improving the dispersion of silicon nitride powder. Studying the oxidation behavior of silicon nitride particles, and discussing the transformation of the crystalline of silicon oxide coating film and the coating state, we determine850℃was the optimum calcinations temperature. The surface layer composition of the as-received and calcined silicon nitride powders was determined by X-ray photoelectron spectroscopy(XPS) analysis. The silicon oxide content of the calcined powders was16%of the total surface oxygen content. Then, we determined2hour was the holding time of the calcinations. By using ball milling prepared the high stability slurry of67.5wt%of solids and lower than110mPa.s of viscosity with the addition of ammonium citrate as dispersant. We study the adsorption of ammonium citrate on the calcined silicon nitride powders, and the amount of ammonium citrate have the influence on the property of the slurry, and effected by PH value, the content of silanol.By an sintering aid surface coating, we fabricate the perfect suspension of slurry of75wt%slip and lower than110mPa.s of viscosity with4wt%of ammonium citrate as dispersant.To analyze the drying rate effected by the temperature and humidity of the surruding, we study the model of green body drying as an opportunity, and determine the drying stage is divided into two phases:the slow drying phase and the fast drying phase. The slow drying phase put in practice in the constant temperature and humidity chamber with40℃and70%of high humidity. The fast drying phase combine with burning out. We study DSC and TG of the green body of slip casting, and established the technology of burning out.By the basis of the sintering kinetics of silicon nitride and the technology of SEM, the microstructure and phase transformation of the sample stick by slip casting was analyzed, then established the sinter technology for slip casting. The results showed the amount of liquid in silicon nitride sintered body increased by calcination. The calcination decreased the sintering temperature. We analyzed the sample stick caused bubbling by the higher sintering temperature.By gas pressure and liquid phase sintering, the optimum sintering technology of silicon nitride ceramic by slip casting was1700℃×7MPa. The perfect property of the silicon nitride ceramic by slip casting was HRV91.5of hardness and99.5%of the relative density.