SYNTHESIS AND SINTERING OF MGO DOPED MULLITE ZRO2 COMPOSITE POWDERS USING THE SOL-GEL METHOD

Mullite/15 vol% 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 0.3 wt% MgO resulted in the formation of ZrSiO4 when calcined at 1400-degrees-C. This reaction is ascribed to the liberation of SiO2 from mullite grains due to the ex-solution 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-degrees-C was observed in the composition containing 1.0 wt% 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-degrees to 1450-degrees-C by the 1.5 wt% 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 560 MPa and K(IC) value of 5.8 MPa . m1/2 for the 1.5 wt% MgO doped composite fired at 1475-degrees-C.