The influence of grain size and secondary phase on the fracture toughness (KIC) of sintered MgO bodies was investigated. Sintered MgO bodies doped with 1-10wt% Fe2O3 were made by hot-pressing. The grain size was changed by annealing. The fracture toughness (KIC) was measured by a chevron-notched beam method. The fracture toughness of sintered MgO bodies doped with Fe2O3 was independent of the grain size. However, it increased with the increase of the amount of Fe2O3, and reached 2.5MPa√m at the sintered MgO body doped with 10wt% Fe2O3, which is 50% larger than that of pure MgO. The fractography revealed that the toughening of Fe2O3-doped MgO bodies corresponded to the increase in the fraction of stepwise transgranular fracture on the fracture surfaces. It was considered that the change in fracture mode resulted from residual thermal stresses generated around MgFe2O4 precipitates.
