EFFECT OF RIGID INCLUSIONS ON THE SINTERING OF MULLITE SYNTHESIZED BYSOL-GEL PROCESSING

The effect of crystalline particulate inclusions of mullite or zirconi a on the sintering and crystallization of a mullite powder matrix was investigated as a function of the inclusion volume fraction and size. The mullite powder was synthesized by sol gel processing and, within t he limits of X-ray diffraction, was amorphous. Composites containing u p to 22.5 vol % zirconia reached almost full density after sintering a t 1500-degrees-C for 1 h. Under identical conditions, the sintered den sity of the composites containing crystalline mullite inclusions was c onsiderably lower. The zirconia inclusions were inert but the mullite inclusions enhanced the independent nucleation and growth rate of the mullite crystals in the matrix. The lower sintering rate of the matrix reinforced with crystalline mullite is attributed to the enhanced mat rix crystallization.