## 5 Discussion
The subsequent firing steps determined the decrease in the crystalline structure until, in the case of the fourth firing, the ordered structure of the ceramic layer was destroyed, becoming amorphous; this indicates that the Si content may affect the behavior of the ceramic. In accordance with the XRD results, the AFM surface analysis indicates that the Co-Cr-W dental alloy is strongly affected by the repeated firings necessitated by the ceramic layering, especially by the first firing (opaquer layer). Both P1 and P2 samples display a large increase in the surface roughness, which is evident in both Sq and Sy. The height distribution abruptly increases from 6-52 nm, for the as-3-D-printed, Co-Cr-W dental alloy sample, to 6-130 nm for both the 2_P1_M and 2_P2_M samples, which is similar to the evolution of the crystalline structure (Figure 4a,b). Thus, from the perspective of the surface heights, in the case of the first firing, the temperature was primarily responsible for the structural modification. After the second firing, the roughness slightly decreased, followed by an increase during the subsequent firing steps. This behavior indicates that the firing process can cause inconsistent fluctuations in the surface roughness, depending on the firing step. It should be noted that this behavior was evident for both P1 and P2 samples. Based on the XRD results, the differences in the alloy surface can also be correlated with the development of the Cr2O3 layer, which occurs much more intensively in the 4_P2_M sample than in the 4_P1_M sample. SEM-EDS investigation confirmed that the different firing parameters affected both the C and M sides of the samples, their surface characteristics, and their chemical composition. In terms of the C sides, the SEM-EDS analysis revealed that the application of the opaque layer resulted in a rough surface with a relatively high volume fraction of pores and microcracks for both experimented firing temperatures of 930 \({}^{\circ}\)C (2_P1_C) and 940 \({}^{\circ}\)C (2_P2_C). However, the higher firing temperature (2_P2_C) resulted in a more homogeneous surface, with limited porosity. The further application of the dentin + enamel ceramic layers (4_P1_C and 4_P2_C), resulted in a surface texture that was characterized by few (4_P1_C) or even by no surface discontinuities (4_P2_C), the latter being fired at higher temperatures (Table 7). In the case of the M sides of the investigated samples, variable degrees of roughness may be observed, which is in accordance with the AFM results. The higher firing temperature of P2 led to a greater number of ceramic constituents on the samples' M sides, which can clearly be observed in Table 8. The higher Si content, observed in both the opaque (2_P2_C) and the multi-layered samples (4_P2_C) at higher firing temperatures, confirms the abovementioned finding. The present study clearly indicates that the repeated firings and the firing temperature have a certain influence on the microstructure of the investigated samples. Our findings are in accordance with those of [PERSON] et al. [41] and [PERSON] et al. [44], who reported that ceramic firing altered the microstructure of the SLM substructure. Much more significantly, [PERSON] et al. [44] also detected new phase formation by means of XRD. The study by [PERSON] et al., which used XRD, XPS, and metallographic microscopy for determining the microstructure and surface composition of an SLM Co-Cr dental alloy before and after ceramic firing, demonstrated an alteration of the alloy microstructure but, nevertheless, concluded that the samples retained a compact and homogeneous structure, and no significant differences in the surface composition after firing were detected [42]. The present study is in opposition to the findings of [PERSON] et al. [40], [PERSON] et al. [43], and [PERSON] et al. [34], which revealed no modification of the alloys' microstructure or the surface composition following ceramic firing.