Aging behavior of Mg-PSZ with 8.5 m/o MgO and 10.5 m/o MgO was studied. ZrO and MgO powder were mixed and sintered with solid solution reaction at 1800℃ for 4 hours. The sintered specimens were aged isothermally at 1420℃ 1100℃ and 800℃ for various time. Toughness and hardness were measured by indentation techniques. Water absorption was measured for the indirect observation of microcrack in the grain boundary region. Microstructure was studied by optical and scanning electron microscopes, X-ray diffraction and EDS analysis. In the specimens aged at 1420℃ and 1100℃ peak aging in the fracture toughness was observed. Peak aging occurred at 2 hours in 8.5 m/o MgO specimens and 6 hours in 10.5 m/o MgO specimens aged at 1100℃. Decomposition of cubic phase to monoclinic and MgO was not observed.
