Surface Oxide Film and Antibacterial Activity of Zirconium-Based Bulk Metallic Glasses

Zr-based bulk metallic glasses (Zr-BMGs), Zr_58.4Cu_28.7Al_11.3Nb_1.6 (ZR01) and Zr_51.8Cu_18.2Ni_14.8Al_10.7Ti_4.5 (ZR02), show excellent mechanical properties and contain alloying elements with potential antibacterial activity, such as Cu. In this study, we investigated the surface oxide films and antibacterial activities of ZR01 and ZR02. These samples showed amorphous structures and comparable surface roughness values. Surface analysis revealed that thermodynamically stable metal oxides were detected from the sample surfaces, while Cu was detected in the metallic state. In particular, ZR01 had a higher surface content of Cu than ZR02, suggesting more pronounced Cu segregation, which is consistent with the difference in alloy composition. The in vitro evaluation against methicillin-resistant Staphylococcus aureus (MRSA) showed that ZR01 exhibited antibacterial activity, whereas ZR02 did not, despite a higher level of Cu ion release. Our results support the antibacterial activity of ZR01 caused by the direct contact between metallic Cu in the surface oxide film and bacteria, rather than from the Cu ion release. Furthermore, our results provide the importance of the segregation of Cu on the ZR01 surface for exhibiting its antibacterial activity. These findings suggest that the bulk composition of Zr-BMGs determines the composition of the surface oxide film, which, in turn, affects the surface properties, particularly the antibacterial activity.