Plasma derived ceramic-metal hybrid PEO coatings with embedded bismuth phases for multifunctional magnesium surfaces

Engineering hybrid surface architectures that couple corrosion resistance with biological functionality is critical for extending the performance envelope of biodegradable magnesium-based systems. In this study, a plasma-derived ceramic-metal hybrid coating was engineered on a multicomponent magnesium alloy using plasma electrolytic oxidation (PEO) with controlled incorporation of bismuth species. Sodium bismuthate (1 and 5 g/L) was introduced into a phosphate-alkaline electrolyte, enabling plasma-assisted in situ formation of a multiphase composite coating comprising a MgO/Mg₃(PO₄)₂ ceramic matrix embedded with metallic Bi and Bi₂O₃ phases. X-ray diffraction confirmed the evolution of phase assemblage with increasing bismuth content, while SEM-EDS analyses revealed a refined hybrid microstructure characterized by uniformly distributed Bi-containing domains and partial sealing of discharge channels. This composite architecture resulted in enhanced surface hydrophilicity and modified electrolyte transport pathways. As a consequence, the Bi-rich hybrid coating exhibited a substantial reduction in corrosion rate from 1.52 mm/year for the Bi-free PEO coating to 0.56 mm/year, accompanied by moderated alkalization during 28 days of immersion in Dulbecco’s Modified Eagle Medium. Beyond electrochemical stabilization, the hybrid coating demonstrated multifunctional surface responses. Extract-based in vitro studies using MG63 osteoblast-like cells showed improved cytocompatibility, reduced apoptotic fraction, and enhanced osteogenic activity, as evidenced by increased alkaline phosphatase expression and extracellular matrix mineralization relative to the Bi-free coating. In parallel, the incorporation of Bi-containing phases imparted effective antibacterial performance, with the Bi-rich coating achieving approximately 80% inhibition against Escherichia coli and 85% inhibition against Staphylococcus aureus. Overall, this work demonstrates that plasma-enabled ceramic-metal hybridization via bismuth incorporation transforms conventional PEO layers into multifunctional composite coatings, where controlled degradation kinetics, electrochemical stability, and biological performance are intrinsically linked to the hybrid microstructural design.