## 4 Discussion  
Generally, a dental subtractive zirconia block is subjected to high pressures prior to semi-sintering, which increases porosity and degrades mechanical properties. However, during the additive manufacturing of zirconia, in general, there are no processes involving the application of pressure to the zirconia object [24]. To enhance the mechanical properties of zirconia, we explored a novel top-down SLA system for fabricating dental zirconia laminates. The resulting additively manufactured zirconia had fewer pores and exhibited an almost homogeneous structure without any layer-by-layer distinction. However, the sintered zirconia had a lower mechanical strength, relatively close to that of subtractive zirconia fabricated via subtractive manufacturing. For additive manufacturing using SLA-based systems, the resin composition used for preparing the slurries is also important for obtaining highly dense ceramics [5, 25]. We prepared a UV-curable acrylate based on a low-viscosity resin containing several acrylate monomers. The TG-DTA data indicated a weight loss at 230 \({}^{\circ}\)C in the range of 230-500 \({}^{\circ}\)C. Therefore, de-binding was carried out for up to 600 \({}^{\circ}\)C. [PERSON] et al. examined the use of 48-56 vol% zirconia-added slurries. They demonstrated that the higher the zirconia concentration in a slurry, the higher the strength. However, a slurry with a high concentration of zirconia has a high viscosity, making printing difficult [26]. The zirconia concentration increased further; however, small scratches were made when spreading the zirconia slurry by the blade; therefore, we adjusted it to 46 vol% to improve spreading behavior. Cross-sectional SEM analysis of the green body of additively manufactured zirconia revealed the presence of white lines, separated by approximately 50 \(\upmu\)m, which is consistent with the paste layer's thickness. This can be attributed to the different compositions of the zirconia particles and resin, possibly because heavy zirconia particles can sink into the slurry. The high-magnification SEM image of the green body revealed that the zirconia particles were well distributed. After sintering, the low-magnification SEM image did not have lines in the green body. The sintered additively manufactured zirconia did not contain alumina particles, similar to the subtractive zirconia. Both the additive and subtractive manufacturing of zirconia involved zirconia with a low alumina content. SEM observations revealed that the additively manufactured zirconia obtained after sintering exhibited small holes; however, its density was almost the same as that of the subtractive zirconia fabricated via subtractive manufacturing. Although multiple SEM images were taken for each specimen, SEM is a localized surface analysis technique, and its primary purpose in this study was to qualitatively compare representative microstructural features. Therefore, the statistical evaluation of features such as porosity or grain size distribution was not applicable in this context. The biaxial bending strengths of additive and subtractive manufactured zirconia were almost identical. This result is consistent with the SEM observations. Improving the density of zirconia is important for achieving desirable mechanical properties. In general, subtractive zirconia is prepared via compression molding. Additive manufacturing involves using a layer-by-layer zirconia-resin slurry without additional pressure. For this reason, the additive manufacturing of zirconia often results in several holes and peeling between layers. However, in this study, the additively manufactured zirconia did not exhibit detachment between layers. Based on the SEM analysis, compared to the subtractive zirconia, the additively manufactured zirconia had more holes, which has the potential for improvement to reduce the number of holes. There are three commercially available systems for 3D-printing zirconia for dental applications.