SECONDARY MINIMUM INTERACTIONS AND HETEROCOAGULATION ENCOUNTERED IN THE AQUEOUS PROCESSING OF ALUMINA ZIRCONIA CERAMIC COMPOSITES

Heterocoagulation calculations are used to explain the various colloidal stability phenomena encountered in aqueous binary suspensions that contain alumina and zirconia. Differential sedimentation of the particulates leads to a non-uniform oxide distribution in the pellets prepared by the pressure filtration of stable systems, yet consolidation under the conditions that promote a weak secondary minimum association of the dissimilar oxides makes the microstructures in the pellets uniform. This effect depends on the filtration pH. The mutual association is favored in stable systems if the zeta potentials of the unlike particles are of the same sign but differ significantly in magnitude. The best consolidation conditions for this binary system occur in the pH range 5-6, where each oxide is positively charged. The critical depths of the secondary minimum depend on the particle sizes and may exceed 10kT. Pre-exposure of zirconia to aluminum chloride and pre-exposure of each oxide to polyacrylic acid can alter the material distribution in pellets made from colloidally stable systems. Either additive's effect depends on its concentration and the filtration pH. Adsorption of either aluminum-containing species or polymer makes the heteroassociation of alumina and zirconia particles less favorable.