Empirical Evaluation of Process Operations Affecting Sintered Density

The effect of fabrication process operations on the sintered density was empirically discussed in order to optimize operation conditions. High purity alumina powders were used as raw powders: One η/θ-phase and three a-phase alumina powders. Their crystal phase, specific surface area, particle size, degree of aggregation and shape of primary particle are characterized. Alumina with the η/θ-phase had a large specific surface area. Three a-phase alumina powders were different each other in the primary particle shape or the degree of aggregation. The alumina powders were formed by uni-axial pressing and sintered. The following process operations were changed: milling, additive, binder, spray drying, CIPing. Each green compact was sintered under the same condition. An empirical equation is established for correlating the sintered density with the compact green density under any process conditions. A diagram is proposed from the empirical equation, and the effect of process operations on the green and sintered densities was discussed on the diagram. The effects of the process operations on the green and sintered bodies depended on the primary particle properties, especially particle shape. The CIPing and addition of additives always increased the green and/or sintered densities. However, spray drying and milling were efficient in powders consisting of dendritic primary particles, but no or negative effect of spray drying and milling on the green and sintered densities was observed in powders consisting of round primary particles and weak aggregates.