The role of surface chemistry in wetting dynamics and interfacial reactivity of molten Zn, and Zn−Ag alloys on biodegradable biomedical fe-based steels

Zn alloys constitute a major focus of current scientific research in the field of temporary medical implants, including orthopedic and vascular applications. In particular, Zn–Ag alloys are rapidly emerging as promising candidates due to their mechanical properties, controlled degradation rates, and intrinsic antimicrobial activity. Despite growing interest, little is known about their wetting and interfacial behaviour on advanced steels, which is critical for high-temperature applications such as coating, composite fabrication, and brazing. This study presents, for the first time, a structured analysis of the wetting behaviour of molten Zn, Zn2.5Ag, and Zn7Ag alloys on Hadfield (Fe-Mn-C) steel under two distinct surface conditions: 1) a mechanically polished (MP) surface with native oxides (FeO, MnO, FeOOH), and 2) a plasma-treated (PT) surface enriched in Fe₂O₃ and Mn₂O₃. Remarkably, ideal wetting (θ < 20◦) was observed for all Zn-based alloys on MP-surfaces, while wetting failure occurred on PT-surfaces after 300 s of contact, thus confirming a strong sensitivity to surface oxidation state. Although Fe–Zn intermetallic layers formed in both cases, their growth was significantly hindered on PTsurfaces, where droplet spreading stopped at the triple line within 300 s. Extended testing revealed that molten Zn could eventually overcome the oxide barrier (~ 2000 s), though accompanied by partial oxidation of the liquid. Notably, in Zn7Ag, formation of ε-Zn3Ag phase occurred within the drop and at the interface without forming a continuous reaction layer. For comparison, pure Ag exhibited consistent wetting (θ ~ 65◦) on both surface types under a non-reactive condition. The absence of interfacial compounds and identical wetting behaviour suggests the unique ability of Ag to dissolve surface oxides without forming new phases. These findings provide novel insights into the oxide–metal interactions that govern wetting and spreading kinetics, offering valuable guidelines for processing Zn–Ag-based materials in biomedical applications.