Interfacial Microchemistry and Surface Stability of Monolithic and Layered Dental Restorative Systems: A Comparative SEM–EDS–XPS Study

Background/Objectives: Restorative system configuration may influence the surface chemistry and interfacial behavior of dental ceramic and metal–ceramic systems, but direct comparative evaluations integrating SEM, EDS, and XPS remain limited. Methods: This in vitro study compared three restorative configurations: Monolithic ZrO2, Feldspar/ZrO2, and Feldspar/Co-Cr. Thirty specimens were fabricated and allocated into three groups (n = 10/group). Surface and cross-sectional morphology were assessed by scanning electron microscopy, while elemental composition and interfacial transition patterns were evaluated using energy-dispersive X-ray spectroscopy, including point analysis, elemental mapping, and line-scan profiles. X-ray photoelectron spectroscopy was used to characterize material-specific surface chemical profiles. Results: Monolithic ZrO2 showed a compact and homogeneous ceramic morphology, with a dominant Zr–O–Y chemical profile. Feldspar/ZrO2 specimens exhibited a sharply defined ceramic–ceramic interface, with limited elemental overlap between the feldspathic veneer and zirconia core. In contrast, Feldspar/Co-Cr specimens showed a broader ceramic–metal transition zone, mainly supported by SEM–EDS evidence of Cr-, Co-, and O-related interfacial signals. XPS provided complementary surface chemical information, without definitive oxide phase identification. Conclusions: Within the limitations of this in vitro study, the analyzed restorative configurations exhibited distinct morphological, elemental, and surface chemical patterns. Feldspar/Co-Cr showed a broader ceramic–metal elemental transition than Feldspar/ZrO2; however, the present findings should not be interpreted as evidence of superior bond strength, mechanical stability, fatigue resistance, or clinical performance. Further mechanical and aging studies are required to determine the functional significance of these interfacial differences.