Sintering behaviour of precursor mullite powders and resultant microstructures

Abstract

Crystallization and sintering behaviour of two structurally different diphasic precursor mullite powders were studied. Powder derived from boehmite and colloidal silica formed mullite at around 1250°C and could be sintered to high density at this temperature by viscous flow sintering. Calcination of the powder up to 1000°C did not affect its sinterability but the weight loss decreased from 20% for the uncalcined powder to 5% for the 1000°C calcined powder. In contrast, powder derived from aluminium sulfate and colloidal silica formed mullite at around 1200°C and did not sinter much by the viscous flow, but showed enhanced solid-state sintering behaviour compared with crystalline mullite powders. Calcination of the powder at temperatures below mullite formation temperature did not affect its sinterability. Microstructural examinations revealed that dense mullite obtained by sintering of powder derived from aluminium sulfate-colloidal silica had much finer grain size (~135 nm) than that obtained from the boehmite-colloidal silica mixture (~1500 nm) and the fine grain size of the mullite was stable up to 1300°C upon long heat treatments (120 h).