Microstructure and mechanical properties of slip cast sol-gel derived mullite ceramics

Abstract

Mullite ceramics were processed by pressureless sintering (1620°C, 2h) of slip cast mullite derived from single-phase gel calcined at 1070°C and attrition milled with ZrO2 balls. The four-point bending strength was determined from room temperature up to 1400°C. Creep behaviour in compression was determined at stresses of 20 and 100MPa and temperatures of 1200 to 1450°C. Microstructural and microchemical characterisation of mullite material was performed using scanning (SEM) and high resolving transmission electron microscopy (HRTEM) in conjuction with energy dispersive X-ray spectrometry (EDX). A remarkable abrasion of ZrO2, and the leaching of magnesium from ZrO2 grains occurred in milling process. The bending strength and the creep behaviour is determined by residual glassy phase, observed in triple points and at the grain boundaries of mullite/mullite and mullite/ ZrO2 grains. When ZrO2 grain participates in the formation of triple points, the composition of the glassy phase differs from that at the junction of three mullite grains. The EDX line microanalyses across the mullite/mullite grain boundaries revealed < 5 nm thick SiO2-rich glassy film. There are some grain boundaries at which only a compositional gradient of SiO2 occurred. The creep below 1300°C is most likely controlled by grain boundary sliding accommodated by solution-precipitation mechanism, and above by the grain boundary sliding accommodated by viscous flow of SiO2-rich phase.