Synthesis and Sintering of MgO Doped Mullite/ZrO₂ Composite Powders Using the Sol-Gel Method

Abstract

Mullite/15vol% ZrO₂ composite powders doped with MgO were synthesized using sol-gel method. The effects of MgO addition on the powder characteristics, sinterability, microstructure and mechanical properties have been investigated. The addition of more than 0.3wt% MgO resulted in the formation of ZrSiO₄ when calcined at 1400°C. This reaction is ascribed to the liberation of SiO₂ from mullite grains due to the exsolution of MgO. ZrSiO₄ decomposed to t-ZrO₂ and SiO₂ liquid phases during sintering process. The decomposition temperature decreased as the MgO content increased, and the lowest decomposition temperature of 1440°C was observed in the composition containing 1.0wt% MgO. The sinterability of composite powders was greatly improved by the addition of MgO. The densification temperature necessary to achieve a relative density of higher than 99% was lowered from 1630° to 1450°C by the 1.5wt% MgO addition. Although the remarkable grain growth was observed with an increased addition of MgO, the amount of t-ZrO₂ in the specimens were also increased. Due to the transformation toughening, the mechanical properties were improved, such as flexural strength of 560MPa and K_IC value of 5.8MPa⋅m^1/2 for the 1.5wt% MgO doped composite fired at 1475°C.