Antibacterial activity and corrosion behavior of thermally evaporated copper coated AZ31 magnesium alloy with and without substrate homogenization

Biodegradable Mg alloys such as AZ31 are attractive for temporary implant applications, yet their rapid degradation in physiological media remains a major limitation. Here, AZ31 was evaluated in the as-received and homogenized conditions before and after deposition of a Cu thin film by thermal evaporation. The combined effects of substrate condition and Cu deposition on surface characteristics, antibacterial activity, and corrosion behavior were systematically investigated. XRD confirmed the presence of α-Mg and fcc Cu phases, with Scherrer-estimated crystallite sizes of 47.34 nm for the uncoated substrate and 72.43 nm for the Cu-containing layer. FE-SEM revealed a relatively continuous Cu-containing surface layer with dispersed nanoscale features, while EDS and elemental mapping confirmed the presence and distribution of Cu on the coated surfaces. AFM showed a marked reduction in the measured surface roughness after homogenization and standardized surface preparation, with Ra values of 29.63 nm for AZ31 and 6.88 nm for AZ31A; after Cu deposition, Ra values were 7.18 nm for AZ31–Cu and 9.21 nm for AZ31A–Cu. Electrochemical testing in simulated body fluid (SBF) showed that Cu deposition substantially reduced the corrosion resistance of AZ31, as evidenced by lower impedance and polarization resistance and higher corrosion current density. Antibacterial assessment against Escherichia coli by disk diffusion demonstrated enhanced antibacterial activity for the Cu-coated samples, with inhibition-zone diameters of 22 mm for AZ31, 24 mm for AZ31–Cu, and 26 mm for AZ31A–Cu. Ion-release measurements further supported accelerated substrate degradation, with Mg²⁺ concentration reaching 334.04 ppm after 7 days for AZ31A–Cu. Although Cu deposition enhanced antibacterial activity, it simultaneously accelerated Mg degradation in SBF, highlighting the need to balance antibacterial functionality with corrosion control in the development of Cu-functionalized biodegradable Mg surfaces.