## 3 Results  
In this study, reticulated porous ZTA specimens with pore density of polyurethane foam of 45 PPI (pores per inch) were prepared using the replica template method. The pore density is expressed in PPI (pores per inch), and the higher the PPI, the greater the number of pores and the smaller the pore size. Reticulated porous ceramic with a pore density of more than 45 PPI is difficult to prepare. As the pore density of the reticulated porous ceramic approaches 80 PPI, it is difficult to remove the excessive slurry inside the polyurethane foam and form a thin ceramic coating on the strut walls due to the increase in the pore density. In addition, voids are formed inside the polyurethane foam, and the compressive strength is lowered. Figure 1 shows images of the polyurethane foam (left), and the as-prepared reticulated porous ZTA specimen (right) with a pore density of 45 PPI prepared using the replica method, respectively. Figure 2a shows a typical SEM image of a fractured strut wall of a polyurethane foam specimen. The reticulated porous ZTA specimens prepared via the replica template method were formed to coat the surface of the polyurethane foam with ZTA slurry, as shown in Figure 2a. Through a heat treatment and sintering, a polyurethane foam-like skeleton structure was formed, as shown in Figure 2b. A typical SEM image of the fractured strut wall of a reticulated porous ZTA specimen prepared via the replica template method is shown in Figure 2b. The triangular void obtained after the heat treatment and sintering of the polyurethane foam, shown in Figure 2b, affects the overall mechanical properties of reticulated porous ZTA. These inevitable triangular voids lower the compressive strength of reticulated porous ZTA. In order to solve this problem, various methods have been attempted, such as dividing the ceramic coating process on the polyurethane foam into two steps or using a ceramic precursor as a raw material [28; 29]. Figure 2b shows a reticulated porous ZTA specimen on which a uniform thickness of the strut wall is formed. The particle size distributions of the alumina and zirconia are shown in Figure 3a. The average particle size of the coarse alumina was 4.241 \(\upmu\)m, that of the fine alumina was 0.328 \(\upmu\)m, and that of the zirconia was 0.210 \(\upmu\)m. Figure 3b,c depict the microstructures of the coarse and fine alumina particles, respectively. The coarse alumina particles do not have enough sintering driving force to be densified fully due to the presence of coarse particles. Figure 1: Optical images of polyurethane foam with a pore density of 45 PPI (**left**) and a reticulated porous ZTA specimen (**right**). Figure 2: Typical SEM images of the fractured strut wall of (**a**) polyurethane foam, and (**b**) a reticulated porous ZTA specimen. Figure 4 shows part of the preliminary experiment before addition of zirconia; because it is impossible to conduct an experiment at the initial stage of this study, these values for the basis for the setting of the variables. As the amount of solid loading increases, the viscosity, and compressive strength values increase. The compressive strength was highest when the solid loading rate was 67 wt. %. Therefore, the solid loading content was fixed at 67 wt. %. Because slurry prepared from coarse alumina has very low viscosity compared to that with fine alumina, it is not possible to tightly coat the strut walls of polyurethane foam. To coat the strut walls of polyurethane foam tightly, it must have the proper viscosity level. Therefore, alumina particles must be indefinitely added until the coarse alumina approaches the adequate viscosity. However, this approach is not efficient. In addition, if the solid loading value is set based on the coarse alumina when the fine alumina also uses the same composition, the fine alumina cannot be uniformly coated to the inside of the strut walls of the polyurethane foam because the viscosity is too high, meaning that the compressive strength is decreased.