Sintering of mullite-alumina composites from diphasic precursors

The densification characteristics of diphasic mullite-alumina composite precursors (with 10 vol% excess alumina over the stoichiometric mullite composition) were studied by both pressureless and pressure-assisted sintering. The precursors were prepared by either a coprecipitation method or a colloidal method using different starting materials. However, independent of the preparation method used, high temperatures up to 1600 degrees C were required to densify the precursors either by pressureless or pressure-assisted sinter ng. This sintering behaviour was mainly due to the presence of the excess alumina after the formation of mullite at around 1200 degrees C, eliminating the viscous phase and retarding the densification. For the coprecipitated precursor, the excess alumina precipitated out in the form of platelets in the pressureless sintering; the morphology of the alumina grains is less typical in the pressure-assisted sintered microstructure due to a change of the relation between densification and grain growth caused by the applied pressure. (C) 1999 Elsevier Science Limited and Techna S.r.l. All rights reserved.