Mullite-zirconia composites prepared from halloysite reaction sintered with boehmite and zirconia

In this study, Algerian halloysite, a naturally occurring clay mineral, was used as low-cost precursor for the production of mullite-zirconia composites. The halloysite was reaction sintered with boehmite and zirconia in the temperature range 1250–1650 °C for 2 h. Differential thermal analysis (DTA), thermogravimetry (TG), dilatometry, high temperature X-ray diffraction (XRD), and scanning electron microscopy (SEM) complementary techniques were used to characterize the prepared materials. The influence of ZrO₂ content on the microstructure, densification, hardness, and coefficient of linear thermal expansion of the composites was investigated. Algerian halloysite was found suitable material for the synthesis of low-cost mullite based composites. All prepared samples exhibited same phase transformations that ended at 1550 °C with the formation of monolithic mullite in halloysite-boehmite mixture and mullite-zirconia composites in halloysite-boehmite-zirconia mixture. The composite materials showed higher values of hardness and coefficient of linear thermal expansion compared with monolithic mullite. The composite containing 10% ZrO₂ possessed the highest hardness value of 13.5 GPa. The composite containing 30% ZrO₂ possessed the lowest value of linear coefficient of thermal expansion of 7.5725 × 10^{− 6} K^{− 1} between 200 and 1500 °C.