In Situ Formed α-Alumina Platelets in a Mullite–Alumina Composite

Abstract

A diphasic aluminosilicate gel with 10 vol% excess alumina over the stoichiometric mullite composition was prepared by a coprecipitation method and used to prepare a mullite–alumina composite. The microstructure development of this composite, including the precipitation of the excess alumina and its influence on mullite grain morphology was studied by sintering the green compacts at different temperatures and for different times. At low sintering temperature (1400°C), mullite formation takes place at local regions of dozens nm and a poorly crystallised mullite phase exists among well crystallised mullite grains. The excess alumina exists in the form of low crystallised and fine θ-alumina crystallites distributed among the small mullite grains. At higher sintering temperature, the alumina grains develop in preferred orientations forming α-alumina platelets which are [0 0 0 1] oriented. The excess alumina present favours the formation of equiaxed mullite grains.