COLLOIDAL PROCESSING AND SINTERING OF NARROW-SIZE-DISTRIBUTION SUBMICRON ALUMINA

Abstract

Submicron alumina powders obtained from different sources were colloidally processed to obtain agglomerate-free and narrow-sized-distribution powder suspensions. They were then compacted by means of centrifuging and slip casting. The compacts exhibited uniform green microstructure and a high green density of 64 % th, which is comparable to that of a dense, random compact of equal-sized spheres, over the range of suspension concentrations, compacting conditions, and initial particle sizes studied. In contrast, samples compacted by dry pressing yielded relatively low green density of about 55% th as a result of the presence of agglomerates. The colloidally prepared compacts were found to sinter readily to full density at low temperatures regardless of the processing parameters used. A decrease in the initial size of the particles was also foun1 to be essential in reducing the sintering temperatures. Microstructural examinations of the colloidally prepared compacts sintered to various lengths of time at predetermined temperatures revealed that even though the compacts were agglomerate-free to start with, agglomeration took place during sintering as a result of local densification of the particles with different co-ordination numbers. Inter-agglomerate channel-like pores were formed before they were ovulated into small, isolated pores. The densification rate was kinetic controlled before the inter-agglomerate pores evolved into closed, isolated pores, after which it was thermodynamic controlled. From this stage on, grain growth was required to decrease the co-ordination numbers of the closed, inter-agglomerate pores to make them sinterable and to bring about further sintering. At low sintering temperatures, grain growth rate was slow which resulted in an abrupt drop in the densification rate and the phenomenon of end density. The abrupt drop in the densification rate corresponds to the transition from kinetic to thermodynamic controlled sintering.