Effect of particle size distribution on sintering of agglomerate-free submicron alumina powder compacts

Experiments were performed with colloidally processed submicron alumina powders to investigate the effect of particle size distribution on their sintering characteristics. The results showed that in the absence of agglomerates and macroscopic size segregation, a broader particle size distribution leads to two opposing phenomena during sintering—enhanced overall sintering characteristics and a higher degree of local differential densification. The former is a result of both the higher initial green density and smaller isolated pores in the final stage of sintering brought about by enhanced grain growth during the intermediate stage. The latter is promoted by a higher degree of variation in local particle packing and may negate the enhanced sintering effect at sufficiently broad particle size distribution. There therefore exists an optimum range of particle size distribution for best sinterability. Since the optimum particle size distribution may vary considerably even for a given powder system, depending on the compaction technique and conditions used, narrow size distribution powder is preferred to monosized or broad size distribution powders for high sinterability and microstructure control of powder compacts, provided that agglomerates in the starting powder are removed by appropriate means. For the agglomerate-free, submicron alumina powder system studied, the optimum particle size distribution was found to have a geometric standard deviation value lying in between 1.6 and 1.9.