Influence of Chemical Composition on the Mechanical Properties of SiO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub> Ceramics

Abstract

Mechanical properties of SiO2-Al2O3 ceramics were studied in relation to the chemical composition. Raw powders of the composition ranging from 60 to 78wt% Al2O3 were synthesized by the spray pyrolysis method and sintered at 1650°C for 4h. The room temperature flexural strength and fracture toughness of the specimens showed no significant difference with the composition ranging from 66 to 78wt% Al2O3. But for the SiO2-rich specimens containing less than 66wt% Al2O3, those values decreased slightly. For the Al2O3-rich specimens which contain no glassy phase, the high-temperature strength and fracture toughness decreased gradually with increasing temperature. In contrast, the strength and the KIC of the specimens containing both mullite and a glassy phases increased with increasing silica content and increasing temperature up to 1300°C. While the flexural strength decreased drastically at 1400°C. The maximum fracture strength at high temperature depends on both chemical composition and the amount of grain boundary glassy phase. The increase in strength and fracture toughness for the SiO2-rich specimens at high temperatures is explained in terms of stress relaxation and/or crack healing in the grain boundary glassy phase, and pull out of acicular mullite grains due to decrease in viscosity of the glassy phase, respectively.