Effect of MgO addition on the sinterability and mechanical properties of mullite ceramics

Abstract

Mullite is a refractory material with singular properties, although high temperatures and long sintering times are required to obtain this material with good densification. In this study, aluminum hydroxide and colloidal silica were used to produce mullite through reactive sintering and MgO was employed as a sintering additive. The compositions were prepared with different amounts of MgO and sintered at 1350, 1450, and 1550 °C, and then analyzed using X-ray diffraction, scanning electron microscopy, and measurements of apparent porosity (AP) and flexural strength. The results showed that the raw materials used allowed the mullite formation at relatively low temperatures (1350 °C), regardless of the amount of MgO added but with the increase in MgO content, a spinel phase appeared, resulting in a fraction of residual α-alumina. The MgO addition lowered the densification temperature at around 50 °C. Furthermore, the higher the sintering temperature and the MgO content, the larger and more anisotropic the mullite grains were. At sintering temperatures above 1450 °C, AP was reduced to approximately 10%. The MgO addition and increase in sintering temperature improved the flexural strength of mullite materials.