## 3 Results
### Reticulated Porous ZTA Specim Prepared by Coarse Zirconia Particles
#### 3.1.3 The Effect of the Sintering Temperature on the Reticulated Porous ZTA Specim  
Figure 4a shows the compressive strength outcomes of reticulated porous ZTA specimens at 25, 45, and 60 PPI according to the sintering temperature. It can be seen that the compressive strength increased as the sintering temperature and PPI were increased. The reticulated porous ZTA specimens at a pore density of 60 sintered at 1600 \({}^{\circ}\)C had the highest compressive strength (approximately 1.63 MPa). The outcomes with pore densities of 25, 45, and 60 of the reticulated porous ZTA specimens according to the sintering temperature are shown in Figure 4b. Overall, the trend for the densities was similar to that of the compressive strength. As the sintering temperature and pore density of the reticulated porous ZTA specimens were increased, the density increased. Figure 4c shows XRD patterns of the phases of reticulated porous ZTA with different sintering temperatures. For zirconia sintered at 1400, 1500, and 1600 \({}^{\circ}\)C, the main phase was the tetragonal phase, while the monoclinic phase of zirconia was not observed. In addition, as the sintering was temperature increased, the peak intensity of the tetragonal phase increased. Figure 4d shows the linear shrinkage outcomes as the sintering temperature was increased. As the sintering temperature was increased, the shrinkage rate of the reticulated porous ZTA increased. This is considered to stem from the number of open pores, which decreased due to the greater densification between the alumina and zirconia particles as the sintering temperature was increased. This can also be explained by the microstructure (Figure 5a-c). Typical backscattered electron images of the reticulated porous ZTA specimens when sintered at 1400, 1500, and 1600 \({}^{\circ}\)C, prepared from coarse zirconia particles, are shown in Figure 5a-c, respectively. The lighter color denotes the zirconia particles. It can be confirmed that the average particle size of alumina and zirconia increased as the sintering temperature was increased. When the reticulated porous ZTA was sintered at 1400 \({}^{\circ}\)C, there were many open pores between the alumina and zirconia particles, and densification was not achieved owing to the presence of many pores. Open pores between the particles and densification affect the compressive strength. The process of sintering can be explained by dividing it into the initial stage, intermediate stage, and final stage. In the initial stage of sintering, the interface of the particles to each other adheres to form a neck. In the intermediate stage of sintering, the pores gradually become round and grain growth occurs, resulting in larger particles than the raw material powder. After that, at the final stage of sintering, the pores become spherical and close, resulting in closed pores and additional grain growth. In addition, the higher the sintering temperature, the faster the sintering reaction occurs [27]. The microstructures when the reticulated porous ZTA is sintered at 1400, 1500, and 1600 \({}^{\circ}\)C correspond to the initial stage, intermediate stage, and final stage of the sintering process, respectively. Figure 4a shows that the compressive strength of the reticulated porous ZTA specimens was lowest when sintering was conducted at 1400 \({}^{\circ}\)C. When the reticulated porous ZTA specimens were sintered at 1500 \({}^{\circ}\)C, the number of open pores decreased and the grain boundaries between the alumina and zirconia particles became connected. When the reticulated porous ZTA specimens were sintered at 1600 \({}^{\circ}\)C, the grain size grew because there were no pores interfering with the grain growth process. These specimens showed the highest compressive strength due to the highest densification between the alumina and zirconia particles.