Rheological Properties and Consolidation of the Suspension in the Alumina Powder-Silicon Nitride Whisker System

Abstract

Dispersion, rheology and consolidation of the colloidal suspension in the alumina powder-silicon nitride whisker system were studied to control the microstructure and density of the green compact. Al₂O₃ particles with an average diameter of 0.15μm and Si₃N₄ whiskers with an average size of 0.4μm×3.7μm were electrostatically dispersed in water in the pH range 3 to 10 and consolidated by filtration. Well-dispersed stable suspensions were obtained at low pH. Decreasing the viscosity and at the same time increasing the solid content of the suspension is the key step in making the green compact with high density and a narrow pore size distribution. Application of isostatic pressing to compacts consolidated by filtration increased green density and shifted the pore size distribution to smaller size. These green compacts by colloidal processing were densified to relative densities of 98.4-99.4% by hot-pressing at 1500°C at a pressure of 39MPa in N₂ atmosphere.