Hardness and fracture toughness of mullite–zirconia composites obtained by slip casting

Mullite–ZrO2 composites were obtained by reaction-sintering using three alumina to zircon weight ratios (45.5/54.5, 51.7/48.3 and 35.1/64.9 wt.%). Green compacts were made by slip casting of stabilized aqueous suspensions of alumina and zircon mixtures and then were reaction-sintered from 1450 to 1600 °C under different firing conditions. The phase evolution was followed by XRD, the densification degree was calculated from densities and the microstructures were studied by SEM. Hardness, H and fracture toughness, KIC were measured by Vickers indentation. Effects of both alumina/zircon ratios and sintering conditions on the H, KIC and the microstructure were examined. Inter- and intragranular mechanisms of fracture were observed. Mainly a phase transformation toughening was assumed although crack deflection and microcracking mechanisms cannot be discarded.