Molecular dynamics of single-particle impacts predicts phase diagrams for large scale pattern formation

Abstract

Energetic particle irradiation can cause surface ultra-smoothening, self-organized nanoscale pattern formation or degradation of the structural integrity of nuclear reactor components. A fundamental understanding of the mechanisms governing the selection among these outcomes has been elusive. Here we predict the mechanism governing the transition from pattern formation to flatness using only parameter-free molecular dynamics simulations of single-ion impacts as input into a multiscale analysis, obtaining good agreement with experiment. our results overturn the paradigm attributing these phenomena to the removal of target atoms via sputter erosion: the mechanism dominating both stability and instability is the impact-induced redistribution of target atoms that are not sputtered away, with erosive effects being essentially irrelevant. We discuss the potential implications for the formation of a mysterious nanoscale topography, leading to surface degradation, of tungsten plasma-facing fusion reactor walls. Consideration of impact-induced redistribution processes may lead to a new design criterion for stability under irradiation.

Surface instability and pattern formation by ion-induced erosion and mass redistribution

“…In the literature, it is often assumed that the ionirradiated layer has a constant thickness d (independent of the angle of incidence) and F S,rad Ád 3 is assumed to be constant [22,43]. However, the thickness d of an ionirradiated layer depends on ion energy and ion incidence angle and the correct dependence F S,rad (h, E ion )Ád(h) 3 should be used.…”

“…The time evolution of a perturbation h(u, v) of a flat surface due to ion irradiation can be obtained in linear approximation as a function of the surface curvature and ion angle of incidence by [1,6,22],…”

“…In the BH and CV theory, a gradient and curvaturedependent stochastic differential equation describes the stability (or instability) of a surface exposed to an oblique incident ion beam. According to recent publications by Madi et al [19], Davidovitch et al [20,21] and Norris et al [22], directed mass redistribution seems to be the dominating contribution to ripple pattern formation with ripple wave vectors parallel to the projected ion beam direction.…”

“…The equations for the height evolution dhðx; y; tÞ=dt of a surface, originating from atomic mass redistribution caused by ballistic displacements as well as ion beam erosion, are revisited. The calculations follow the description of Bradley and Harper [1], Carter and Vishnyakov [18] and Davidovitch et al [20] and descriptions found in papers on crater function analyses [22,[25][26][27][28][29]. However, I take into account additional contributions to the height evolution in the linear expansions, which were not considered so far.…”

“…Several theoretical approaches describe erosion as well as mass transport on the basis of crater functions (cf models) [22,[25][26][27][28][29] derived from molecular dynamics simulations. The crater function is the average height distribution h(x, y) following ion impact for a significant number of ions incident on an identical and initially flat surface with h 0 (x, y) = 0.…”