**2.3 Summary**

This study examined the characteristics of hot-pressed pure yttria ceramic that was annealed in an oxidation atmosphere. As the sintering temperature was increased, the color of the sintered body changed to black. The weight of the specimens after the oxidation of yttria ceramics showed a tendency to increase with increasing temperature. The hot-pressed yttria ceramics could be applied to marking samples with a crystalline structure and full density. Grain growth tended to show a continuous increase with increasing temperature, which has a direct effect on the porosity, density, and crystal phase. The relationship between the grain size and crystal phase increased with increasing indentation crack length and was attributed to the presence of local cracks acting as grain spalling. KIC was calculated to be 1.2–1.9 MPa, and the Vickers hardness of hot-pressed yttria ceramics was 28 GPa. The specimens before heat treatment showed high strength in the hightemperature region, and those after heat treatment showed relatively high strength in the low-temperature region. The bending strength of the hot-pressed yttria ceramics was approximately 140 MPa. The yttria crystalline phase varied according to the synthesis conditions and sintering methods, but the Y2O3 crystal phase identified in hot-pressed yttria ceramics means that the sintering temperature and oxidation reaction had no effect.
