Structural lubricity of physisorbed gold clusters on graphite and its breakdown: Role of boundary conditions and contact lines

Abstract

The sliding motion of gold slabs adsorbed on a graphite substrate is simulated using molecular dynamics. The central quantity of interest is the mean lateral force, that is, the kinetic friction rather than the maximum lateral forces, which correlates with the static friction. For most setups, we find Stokesian damping to resist sliding. However, velocity-insensitive (Coulomb) friction is observed for finite-width slabs sliding parallel to the armchair direction if the bottom-most layer of the three graphite layers is kept at zero stress rather than at zero displacement. Although the resulting kinetic friction remains much below the noise produced by the erratic fluctuations of (conservative) forces typical for structurally lubric contacts, the nature of the instabilities leading to Coulomb friction could be characterized as quasi-discontinuous dynamics of the Moiré patterns formed by the normal displacements near a propagating contact line. It appears that the interaction of graphite with the second gold layer is responsible for the symmetry break occurring at the interface when a contact line moves parallel to the armchair rather than to the zigzag direction.

A Computational Study of Cluster Dynamics in Structural Lubricity: Role of Cluster Rotation

“…This is also in agreement with the nature of graphene as a 2D material, whereby the out-of-plane stiffness is much lower than the in-plane stiffness. Structurally, the second-to-the-bottom atomic layer of a gold (111) cluster breaks the mirror symmetry generically, which would intensify interfacial atom vibration at the propagating contact line, especially when it is normal to the sliding direction [14]. Compared to a semi-infinite slab, instability of a circular cluster is located primarily at its circumference, where the highest contrast in the moiré patterns are visualized as shown in Fig.…”

“…Although nominally rigidity holds the upper hand against elastic deformation and dislocations, sliding-induced vibration of individual atoms about their non-equilibrium steady-state positions annihilates the accumulation of instantaneous forces systematically, thereby a vanishing of mean lateral force (i.e., structural lubricity) arises. For a finite-sized geometry, instability can be introduced from a propagating contact line normal to the sliding direction as a result of the generically asymmetric structure of the slider [14], although such an effect will decay with cluster size in terms of the scaling argument [5,6].…”

“…Not surprisingly, lateral forces will rise at the contact lines where the stress gradients are the highest, particularly so when those contact lines are normal to the sliding direction. As a recent proof of this argument, Gao and Müser [14] demonstrated that one possible origin of instabilities can be from quasi-discontinuous dynamics of the moiré patterns (manifesting atom normal displacements) near a propagating contact line, during the sliding of a semi-infinite gold slab on a graphite substrate. Such sliding discontinuity can be attributed to the symmetry breaking nature of a multi-layer gold cluster with its (111) surface parallel to the graphite basal plane.…”

“…Motivated by the question above, we present here molecular dynamics (MD) simulations of nanoscale cluster manipulation performed on the representative goldgraphite system, which has been shown to exhibit structural lubricity under both ultra-high vacuum (UHV) and ambient conditions [6,9,14,20]. In particular, kinetic friction has been studied in the presence of spontaneous cluster rotations, whereby the tip-cluster interaction has been modeled in the form of a harmonic spring linked directly to the cluster's topmost-layer.…”