  Molecular dynamics simulations have been used to study a collective atomic
transport phenomenon by repeated Ar$^+$ irradiations in the Ti/Pt interfacial
system. The ion-induced injection of surface atoms to the bulk, the ejection of
bulk atoms to the top layers together with surface erosion is strongly enhanced
by interfacial mixing. This process leads to a dense interfacial material, and
broadening of the interface region. The process scales with the relative
difference of the atomic masses. We find that surface roughening and
interfacial mixing is strongly coupled via an enhanced counterflow material
transport normal to the surface which might be a novel surface damage
mechanism. This cooperative phenomenon is active when the bilayer system is
subjected to a high dose ion irradiation (multiple ion irradiations) and leads
to surface cavity growth.
