## 3 Results and Discussions
### Effects of Multiple Slurry Coatings on Reticulated Porous Zirconia  
Thus far, this study has focused on the effects on the compressive strength when changing the properties of the ceramic slurry. In another method, the authors investigated whether multiple slurry coatings could effectively improve the compressive strength. There are several methods that can be used to improve the compressive strength of reticulated porous ceramics, such as vacuum infiltration (reticulated porous mullite) [22], centrifugation coating [23], slurry rheology modification (reticulated porous alumina) [24], and multiple slurry coatings (reticulated porous silicon carbide [9]). Among these methods, there has been little discussion of multiple slurry coatings. The authors admit that this method is inefficient economically, but it is worth studying at this point to expand the field of research. Three slurry coatings were used, and sintering was performed every time. Figure 9a shows the density and compressive strength of the reticulated porous zirconia specimens according to number of coatings, which ranged here from one to three. As the number of slurry coatings increases, the compressive strength of the reticulated porous zirconia is more than doubled from 4.37 to 9.43 MPa. One study concentrated on reticulated porous alumina with multiple slurry coatings applied. In that study, the compressive strength for a single coating was 0.07 MPa, and that after a triple coating it was 0.2 MPa [25]. This data showed a pore density of 10 PPI with sintering at 1400 \({}^{\circ}\)C. Although both the pore density and the sintering temperature were different in each case, it was noted that reticulated porous zirconia could more effectively enhance the compressive strength when using multiple slurry coatings as opposed to reticulated porous alumina. The pore size distributions of specimens with multiple slurry coatings are shown in Figure 9b. Regardless of the number of coatings, the results indicate the highest number of pores in the range of approximately 10 to 100 \(\upmu\)m.  
The log differential intrusion is also slightly decreased as the number of coatings increases. These results can be explained by examining the microstructure in Figure 10. The fractured strut microstructures of reticulated porous zirconia with (a) one coating, (b) two coatings, and (c) three coatings are shown in Figure 10. When coated once, fractured strut walls of reticulated porous zirconia formed thin triangular voids. These voids can significantly affect the compressive strength because the sample becomes brittle. When coated twice, the fractured strut walls of reticulated porous zirconia formed thick triangular voids, and when coated thrice, the internal voids were completely filled. It can be judged that these microstructures coincide with the results in Figure 9b, showing a decrease in the log differential intrusion outcome. Figure 10: SEM image of (**a**) a fractured strut wall of reticulated porous coated with zirconia once, (**b**) thick reticulated porous coated with zirconia twice, and (**c**) a filled strut wall of reticulated porous coated with zirconia three times. Figure 9: (**a**) Densities and compressive strength outcomes, and (**b**) pore size distributions when increasing the number of coatings of reticulated porous zirconia. In Figure 11a, at a pore density of 45 PPI, the three-dimensional microstructure of the reticulated porous zirconia reconstructed by \(\upmu\)-CT is shown. A typical cross-section of the \(\upmu\)-CT reconstruction is shown in Figure 11b. The \(\upmu\)-CT image indicates that the reticulated porous zirconia has highly interconnected pore channels.