FORCES BETWEEN ALUMINUM-OXIDE GRAINS IN A SILICATE MELT AND THEIR EFFECT ON GRAIN-BOUNDARY WETTING

Experiments in which the disperson and flocculation behavior of aluminum oxide grains in a Ca-Al-Si-O silicate liquid are examined have been used to determine the nature of the forces between oxide grains in a silicate melt. From observations of the way in which the melt penetrates grain boundaries in a high-purity polycrystalline aluminum oxide, it is concluded that at short ranges the interaction between most grains is strongly repulsive. The formation of aggregated groups of grains in settling experiments, however, indicates that at larger separations the forces between grains become attractive and hence that there is an energetic minimum in the interaction between grains. The results are discussed in relation to models for the stability of intergranular films in liquid-phase-sintered ceramics. Microstructural observations made on the melt-penetrated aluminas are used to make a quantitative estimate of the fraction of grain boundaries that are wetted by the melt. The observations indicate that large clusters of grains connected by unwetted grain boundaries exist in liquid-phase-sintered aluminas.