Improving the microstructural and mechanical properties of in-situ zirconia-mullite composites by optimizing the simultaneous effect of mechanical activation and additives

Abstract

The current study aims to investigate the microstructural properties, diametral tensile strength, Weibull modulus, and surface roughness (Ra) of the zirconia-mullite composites. The simultaneous effect of duration of the mechanical activation (MA) process of starting materials, as well as TiO2 and ZnO additions on the mentioned properties, were studied. X-ray diffraction analysis showed that mullite was the main crystalline phase in 6–24 h mechanically activated-TiO2 containing samples which were subsequently heat treated at 1450 °C. Prolonging the duration of the MA process to 72 h led to the crystallization of the tetragonal zirconia as one of the main crystalline phase in the mentioned samples. As the heat treatment temperature increased to 1550 °C, the dependence of the type of the crystalline phases on the milling time eliminated and mullite became the main crystalline phase in 6–72 h MA processed samples. The transformation of starting materials to mullite and t-zirconia was not progressed effectively in ZnO containing samples which were heat treated at 1450 °C. However, by increasing the temperature of the heat treatment process to 1550 °C, both mullite and zirconia were efficiently crystallized in 6–72 h MA processed ZnO containing ones. Scanning electron microscopy results revealed that prolonging the duration of the milling process of the additive containing mixtures from 6 to 72 h could eliminate the acicular morphology of the mullite phase. 72 h mechanically activated ZnO containing sample which was later heat-treated at 1550 °C had the highest noticeable diametral tensile strength of 270 ± 15 MPa. The Weibull modulus of composite samples increased from 10.07 to 12.37 and 20.95 by the addition of TiO2 and ZnO to the starting mixture, respectively. Finally, it was found that prolonging the milling time increased the Weibull modulus remarkably and decreased the Ra values as a result of the homogeneity development in the composite samples.