Densification Study and Mechanical Properties of Microwave-Sintered Mullite and Mullite-Zirconia Composites

The objective of this research is to evaluate microwave sintering as a viable technique to reliably process high-strength mullite and mullite-zirconia composites. In this report, the advantage of microwave processing over conventional heating is realized by comparing the densification behavior and mechanical properties of sintered mullite and mullite-zirconia composites. Commercially procured mullite powders were sintered using a 3 kW, 2.45 GHz microwave furnace in the range of 1400 degrees-1500 degrees C, for 1 h. For pure mullite, an increase in sintering temperature from 1400 degrees to 1500 degrees C decreased the porosity from 30% to 13%, which increased the compressive strength from 128 +/- 18 to 387 +/- 21 MPa with 1 wt% MgO as a sintering aid to mullite. Furthermore, yttria-stabilized tetragonal zirconia (3Y-TZP) was incorporated (up to 20 wt%) to improve the microwave absorption efficiency of mullite. A maximum density of 92% can be achieved for mullite with 1 wt% MgO and 10 wt% ZrO2 composites sintered at 1500 degrees C in microwave furnace and exhibited a maximum hardness of similar to 10.24 +/- 0.61 GPa, compressive strength of similar to 740 +/- 38 MPa and a moderately high indentation fracture toughness of similar to 3.65 +/- 0.43 MPa center dot m1/2. Microwave sintering data, when compared with those of conventional sintering, revealed that volumetric heating of microwaves led to considerable improvement in strength of mullite and mullite-zirconia composites while utilizing reduced time and energy.