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

Abstract

Silicon nitride (Si 3N 4) powders coated with 6 wt.% Y 2O 3 and 4 wt.% Al 2O 3 were prepared by coprecipitation. For comparison, the Si 3N 4 powder containing the same amount of additives was also prepared by mechanical mixing. The resulting powders were dispersed in water with ammonium polyacrylate (NH 4PA) to produce 32 vol.% slips. The adsorption behavior and interfacial charge properties of coated Si 3N 4 slips with NH 4PA were investigated and compared with those of the mixed Si 3N 4 slips. Then, the influence of pH on the rheological properties of 32 vol.% coated and mixed Si 3N 4 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 Si 3N 4 on the sintering behavior was investigated. The surface coating improved the dispersion of 32 vol.% Si 3N 4 slips with NH 4PA; lower viscosity values of the coated Si 3N 4 slips with respect to the mixed ones were found in the pH range studied. The better slip dispersion of the coated Si 3N 4 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.