  The enhancement of surface diffusion (DS) over the bulk (DV) in metallic
glasses (MGs) is well documented and likely to strongly influence the
properties of glasses grown by vapor deposition. Here, we use classical
molecular dynamics simulations to identify different factors influencing the
enhancement of surface diffusion in MGs. MGs have a simple atomic structure and
belong to the category of moderately fragile glasses that undergo pronounced
slowdown of bulk dynamics with cooling close to the glass transition
temperature (Tg). We observe that DS exhibits a much more moderate slowdown
compared to DV when approaching Tg, and DS/DV at Tg varies by two orders of
magnitude among the MGs investigated. We demonstrate that both the surface
energy and the fraction of missing bonds for surface atoms show good
correlation to DS/DV, implying that the loss of nearest neighbors at the
surface directly translates into higher mobility, unlike the behavior of
network- and hydrogen-bonded organic glasses. Fragility, a measure of the
slowdown of bulk dynamics close to Tg, also correlates to DS/DV, with more
fragile systems having larger surface enhancement of mobility. The deviations
observed in the fragility and DS over DV relationship are shown to be
correlated to the extent of segregation or depletion of the mobile element at
the surface. Finally, we explore the relationship between the diffusion
pre-exponential factor (D0) and activation energy (Q) and compare to a ln(D0)-Q
correlation previously established for bulk glasses, demonstrating similar
correlations from MD as in the experiments and that the surface and bulk have
very similar ln(D0)-Q correlations.

Materials Science;Soft Condensed Matter