What is the real driving force of ion beam mixing?

TL;DR

This study uses molecular dynamics simulations to investigate the driving forces behind ion beam-induced interfacial mixing in metal bilayers, challenging the traditional thermal spike model and highlighting mass effects and vacancy supersaturation.

Contribution

It demonstrates that chemical forces are not the primary drivers of mixing, emphasizing the roles of mass differences and vacancy supersaturation over thermal spike effects.

Findings

Chemical forces like heat of mixing have no apparent effect up to 8 keV.

Mass differences cause backscattering, affecting atom ejection and injection.

Vacancy supersaturation induces thermally activated intermixing.

Abstract

Molecular dynamics simulations have been used to study the driving force of ion irradiation induced interfacial mixing in metal bilayers in which the relative mass of the constituents is considerable. We find no apparent effect of chemical forces, such as heat of mixing or cohesive energy up to 8 keV ion energy, although a considerable number of liquid and high energy particles (hot atoms) persist up to even 20 ps during the thermal spike. This result is in direct conflict with the widely accepted theory of thermal spike mixing (chemical interdiffusion model). The supersaturation of vacancies also occurs and which induces a thermally activated intermixing of the lighter constituent of the bilayer. The delay and the decoupling of the intermixing of the light constituent is explained as a backscattering effect at the interface: the interface acts as a diffusional barrier for high energy particles due to the large difference in atomic masses. The heavier atoms are predominantly ejected to the overlayer at the beginning of the thermal spike while the light atoms are injected to the bulk at the beginning of the cooling period (in Ti/Pt) or during the thermal spike with some time delay (Al/Pt).