EFFECTS OF PH AND PARTICLE-SIZE ON THE PROCESSING OF AND THE DEVELOPMENT OF MICROSTRUCTURE IN ALUMINA-ZIRCONIA COMPOSITES

The aqueous processing of binary suspensions that contain 0.50-mu-m alumina and either 0.20- or 0.71-mu-m zirconia was studied from pH 3 to 11, emphasizing colloidally stable systems. Differences in particle size and density can induce differential sedimentation and lead to a nonuniform oxide distribution in pellets prepared by pressure filtration. Uniformity improves when the zeta potentials of the unlike particles are of the same sign but differ greatly in magnitude, i.e., in the pH range 5 to 6, where the oxides experience a weak, mutual association. However, when they are more similar electrostatically, they do not mutually associate and uniformity suffers. This nonuniformity occurs with the adjustment of pH to << 5 (pos ive particles) and the adsorption of soluble species that strongly alter the zeta potential, e.g., after the preexposure of zirconia to aluminum chloride (positive particles) and the addition of polyacrylic acid (negative particles). In these cases, either high viscosity or rapid consolidation are required to ensure a uniform distribution. The disparity in the particle number fraction, derived from the size differences, and the thermodynamic interparticle energies control the prefired microstructure and are central to the prediction of the proper processing conditions.