IN-SITU FORMED ALPHA-ALUMINA PLATELETS IN A MULLITE-ALUMINA COMPOSITE

A diphasic aluminosilicate gel with 10 vol% excess alumina over the st oichiometric mullite composition was prepared by a coprecipitation met hod and used to prepare a mullite-alumina composite. The microstructur e development of this composite, including the precipitation of the ex cess alumina and its influence on mullite grain morphology was studied by sintering the green compacts at different temperatures and for dif ferent times. At low sintering temperature (1400 degrees C), mullite f ormation takes place at local regions of dozens nm and a poorly crysta llised mullite phase exists among well crystallised mullite grains. Th e excess alumina exists in the form of low crystallised and fine theta -alumina crystallites distributed among the small mullite grains. At h igher sintering temperature, the alumina grains develop in preferred o rientations forming alpha-alumina platelets which are [0 0 0 1] orient ed. The excess alumina present favours the formation of equiaxed mulli te grains. (C) 1998 Published by Elsevier Science Limited. All rights reserved.