The role of ZnO as an additive on the mechanical properties of zirconia-mullite composites

This study investigates the effects of ZnO addition on the physicochemical and mechanical properties of zirconia-mullite composites. Zirconia-mullite composites containing varying amounts of ZnO (0–15 wt.%) were fabricated by sintering the mechanically activated gibbsite-zircon-kaolinite mixtures at 1550 °C. The results demonstrated the positive effect of adding up to 5 wt.% ZnO on optimizing the phase composition, surface characteristics, and mechanical properties of the fabricated composites. The stabilization of the tetragonal zirconia phase and the formation of the acicular mullite phase were positively affected by the presence of ZnO, as determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The composites' flexural strength and diametral tensile strength (DTS) were increased from 65 to 155 MPa and 42 to 124 MPa, respectively, due to the stabilization of monoclinic zirconia and the formation of acicular mullite. Furthermore, a fracture toughness and Vickers microhardness of 4.67 ± 0.97 MPa.m^1/2 and 8 GPa were achieved in the composite containing 5 wt.% ZnO, respectively.