Unveiling the influence of interfacial bonding and dynamics on solid/liquid interfacial structures: An ab initio molecular dynamics study of (0001) sapphire-liquid Al interfaces

SD Ma and R Yan and NF Zong and RL Davidchack and T Jing and HB Dong, PHYSICAL REVIEW MATERIALS, 4, 023401 (2020).

DOI: 10.1103/PhysRevMaterials.4.023401

Nucleant substrates may induce formation of several quasilayers in the liquid metals in contact with the solid substrates. These liquid layers, called prenucleation layers (PNLs), normally form above the liquidus temperatures of the metal alloys and thus are believed to significantly affect the heterogeneous nucleation mechanism. In this work, a comprehensive study of the (0001) sapphire-liquid Al interfaces was conducted using the ab initio molecular dynamics method to unveil the key factors determining the substrate-liquid metal interfacial structural features, focusing primarily on revealing the underlying mechanisms of the PNL formation. Two types of (0001) sapphire surfaces, i.e., nonhydroxylated and hydroxylated surfaces, were investigated. Essentially the same equilibrated interfacial structure was observed for all the interfaces with the nonhydroxylated sapphire surfaces, featuring an Al-rich termination of the substrates and two distinguishable adjacent PNLs. These typical structural features are similar to the (root 31 x root 31) R +/- 9 degrees reconstructed surface, which supports the previous experimental observation that the presence of liquid Al promotes the reconstruction of the (0001) sapphire surface. A completely different structure was found at the interface with fully hydroxylated sapphire surface: the PNLs are absent, replaced by an Al- depleted region. To unveil the factors influencing the interfacial structure, we first analyzed the nature of interfacial bonding. Strong covalent bonds were observed between the nonhydroxylated sapphire surfaces and liquid Al, which constitute a relatively large proportion of the interfacial bonds at such systems. On the contrary, weak van der Waals force is the primary interaction between the fully hydroxylated sapphire surface and liquid Al. Then we characterized the dynamical behaviors of the substrate surfaces in the substrate-liquid Al interfaces. Better mobility and larger vibration amplitude were found for the fully hydroxylated sapphire surface compared to the equilibrated nonhydroxylated sapphire surface. We demonstrated that both the stronger bonding strength and the lower mobility of interfacial atoms contribute to the formation of the PNLs at the nonhydroxylated sapphire-liquid Al interfaces. Finally, the formation mechanism of the PNLs at the substrate-liquid metal interface is summarized.

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