THE INFLUENCE OF GRAIN-SIZE AND SECONDARY PHASE ON THE FRACTURE-TOUGHNESS OF SINTERED MGO BODIES .3. ADDITION OF COO

The influence of grain size and secondary phase on the fracture toughness K(IC) for sintered MgO bodies was investigated. Sintered MgO bodies doped with 1-10 wt% CoO were made by hot-pressing. The grain size was changed by annealing. The fracture toughness K(IC) was measured by a chevron-notched beam method. The fracture toughness of sintered MgO bodies doped with CoO was nearly independent of the grain size and the amount of CoO, and indicated almost the same value as that of pure MgO. The fractography revealed that CoO-doped MgO bodies had the mixed mode of transgranular and intergranular fracture on the fracture surfaces, which is the same result as pure MgO. This result corresponded to little change in the fracture toughness by doping CoO to sintered MgO bodies. It was considered that fracture mode did not change in CoO-doped MgO because residual thermal stresses did not generate in MgO . CoO solid solutions. Furthermore, the relationship between the fracture toughness and the fraction of transgranular fracture was quantitatively estimated for MgO-based composites including the previous results. Toughening ceramics was discussed in connection with changing fracture mode.