Influence of yttria-alumina surface coating on the colloidal processing of silicon nitride slips

Silicon nitride (Si3N4) powders coated with 6 wt.% Y2O3 and 4 wt.% Al2O3 were prepared by coprecipitation. For comparison, the Si3N4 powder containing the same amount of additives was also prepared by mechanical mixing. The resulting powders were dispersed in water with ammonium polyacrylate (NH4PA) to produce 32 vol.% slips. The influence of pH on the dispersion properties of 32 vol.% coated Si3N4 slips with NH4PA were investigated and compared with those of the mixed Si3N4 slips. The microstructure and density of green cast samples were determined and related to the degree of slip dispersion. In addition, the influence of the surface coating of Si3N4 on the sintering behavior was investigated. Lower viscosity values of the coated Si3N4 slips were found in the pH range studied. The better slip dispersion of the coated Si3N4 slips resulted in higher green density values. A linear correlation between the sintered density and the green density was found. The densification and the α- to -β phase transformation of Si3N4 were not affected and enhanced, respectively, with the surface coating of Si3N4 particles with sintering aids.