Some factors affecting the reactive hot-pressing behaviour of alumina

Abstract

The densification behaviour of metastable cubic aluminas during reactive hot-pressing exhibits a region of enhanced densification followed by a region of reduced rate of densification. It has been established that the transition to the region of reduced rate of densification occurs after the phase transformations to α-Al₂O₃ are complete. Results are presented of further studies on the influence of applied pressure and minor additions of impurities on the nature of the region of reduced densification for three different cubic Al₂O₃ powders. It was observed that those parameters that assisted the phase transformations to α-Al₂O₃ also reduced the transition temperature between the two regions by a differing amount for each powder. It is believed that the differences in hotpressing behaviour of the three cubic Al₂O₃ powders arise from their initial powder characteristics, and that these differences may lead to a better understanding of the region of reduced rate of densification.

A further aspect of the present work has been a brief study of the influence of uniaxial pressure upon the high temperature phase transformation of cubic θ-Al₂O₃ to hexagonal α-Al₂O₃. Over a limited pressure range up to 1 kbar the influence of pressure on the irreversible phase transformations was observed to be dT/dp − 0.1 K bar⁻¹.