The fracture microstructures of the various YN specimens after CSP at 200 °C are presented in Fig. 5. In the 1YN specimen, with the smallest amount of yttrium nitrate, a microstructure consisting of Y2O3 particles is observed. However, as the amount of yttrium nitrate increased (Fig. 5b and c), the morphology develops into a dense form in which the voids between the Y2O3 particles are gradually filled. In addition, a smooth fractured surface is observed in the 20YN specimen (Fig. 5d), in which the largest amount of yttrium nitrate was used. It can be inferred that the voids between the Y2O3 particles are filled by the second phases such as Y4O(OH)9NO3 and Y(OH)3 which form during CSP.