## 5 Discussion  
The present study aims to evaluate the influence of ceramic veneering and of its firing parameters on the microstructure of SLM Co-Cr-W alloy, which was used as a substructure. The results of the current study reveal that the microstructure of the SLM substructure has been affected by the ceramic layer firing procedure; much more significantly, the behavior of the alloy is strongly dependent on the firing temperature. The ceramics themselves exhibited noticeably different behavior, by means of these structural changes, as a result of differences in the firing process. The XRD analysis provided detailed information about the crystallographic structure, chemical composition, and phase identification, also yielding information on how the samples' microstructures deviated from the initial structure when subjected to repeated firings. The as-3-D-printed samples, 1_P1 and 1_P2, consisting of only Co-Cr-W dental alloy that was not layered and not fired, displayed a low, crystalline, ordered structure. The structure transforms into a high crystalline cubic structure after the first firing of the opaquer layer, regardless of the ceramic used for veneering. The modification of crystallinity quality is clearly reflected in the intensity of the diffraction peaks (Figures 2 and 3). The subsequent firing steps caused a different evolution of the alloy's crystalline structure, depending on the ceramic used. In the case of P1, the structural changes were moderate, while in the case of P2, the crystalline structure of the dental alloy continued to change (Figure 4a-c). It is likely that the variations in the firing temperature (Table 1) between the two deposition processes were responsible for these differences. The Cr2O3 crystalline phase, which developed during the repeated firings (Figures 3b and 4b), indicated the different percentages of the (110) relative intensity: 5.3%, in the case of P1, and 15.65%, in the case of P2 (Table 5), once more emphasizing the behavioral differences. Chromium oxidation was proven to be a surface process (Figure 5), one that is quite normal and to be expected, as it plays an important role in the bonding of the ceramic to the metallic substructure. In the case of the metal-ceramic bond, chemical bonding is the predominant form; this refers to the reaction between the ceramic and the metal oxide layer, with the latter playing a fundamental role. Particularly in the case of base metal alloys, a certain saturation of both the metal and the ceramic with metal oxides occurs, ideally resulting in the formation of an oxide layer [37]. The dental Co-Cr alloys present oxidation surface morphologies and thicknesses that are known to be dependent upon the fabrication technique [34]. Much more significantly, differences in the alloy microstructures that are caused by the different manufacturing methods might also simultaneously change the nature of the surface oxidation, resulting in significantly modified adhesion behaviors [48]. The two veneering ceramics also showed different behaviors, starting with the first firing of the opaquer layer. It is common knowledge that the opaquer acts as an interface between the alloy and the ceramic and is also involved in the bonding between the two materials [49]. In the case of P1, the diffraction peaks were indexed as KAlSi2O6 (leucite), a typical crystalline phase related to the ceramic as a result of surface crystallization, along with ZrO2, a component of the opaquer. The second firing determines the complete formation of the KAlSi2O6 phase, with no formation of the impurity phases. The third and fourth firings only decreased the crystallinity quality of the KAlSi2O6 phase (Figure 1a). P2 showed a different behavior during the first firing, additionally highlighting the Si crystalline phase (Figure 2). After the second firing, KAlSi2O6 became a unique crystalline phase.