Wetting Behavior of Zn Droplets on Fe Surfaces: Insights from Molecular Dynamics Simulations

The liquid metal embrittlement (LME) induced by Zn melts in advanced high strength steels has seriously hindered their wide application in various industries. Microscopically, wetting is the precursor for LME; it is therefore crucial to understand the wetting of Zn melts on Fe surfaces. Molecular dynamics simulations were conducted to investigate the wetting behavior of Zn droplets on Fe(001), Fe(110), and Fe(111) surfaces from both thermodynamics and dynamics aspects. The simulation results reveal that the surface energy of solid Fe is significantly greater than the surface tension of liquid Zn and the interfacial energy of Fe-Zn solid-liquid interface at the pertinent temperatures. Consequently, Zn droplets tend to completely envelop the Fe substrates as they spread toward the equilibrium state. Specifically, Fe(111) surfaces possess the highest surface energy, whereas Fe(110) surfaces have the lowest surface energy. Meanwhile, the solid-liquid interfacial energy is minimal for Fe(111)/Zn and maximal for Fe(110)/Zn. These differences contribute to the strongest spreading driving force for Zn droplets on Fe(111) surfaces and the weakest on Fe(110). During the initial spreading stage, Zn droplets form precursor films on all Fe surfaces. Nonetheless, on Fe(111), the dissolution reaction between the substrates and the droplets destabilizes the precursor films, ultimately resulting in complete wetting. Conversely, no dissolution is observed between Zn droplets and the Fe(001) or Fe(110) surface. As a result, the equilibrium Zn droplet consists of a prefreezing precursor film that grows epitaxially on the substrate and a main body of the droplet exhibiting a convex hull shape corresponding to pseudopartial wetting. These findings provide new insights into the wetting behavior of metal droplets on metal surfaces, particularly for understanding the liquid metal embrittlement induced by Zn melts in steels.