Improvement in the colloidal processing of concentrated silicon nitride slips with ammonium polyacrylate by an yttria–alumina surface coating

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 adsorption behavior and interfacial charge properties of coated Si3N4 slips with NH4PA were investigated and compared with those of the mixed Si3N4 slips. Then, the influence of pH on the rheological properties of 32 vol.% coated and mixed Si3N4 slips was determined. The density of green cast samples was related to the degree of slip dispersion. In addition, the influence of the surface coating of Si3N4 on the sintering behavior was investigated. The surface coating improved the dispersion of 32 vol.% Si3N4 slips with NH4PA; lower viscosity values of the coated Si3N4 slips with respect to the mixed ones were found in the pH range studied. The better slip dispersion of the coated Si3N4 slips resulted in higher slip cast packing density compacts. The second stage of densification (solution precipitation) and the α- to β-phase transformation were promoted by the surface coating.