Friction Scaling law of two dimensional heterojunctions

The weak interlayer interaction and strong intralayer bonding in two-dimensional (2D) layered materials give rise to distinguished ultra-low friction and wear properties when two incommensurate crystalline lattices are in contact, known as structural superlubricity.  However, due to experimental restrictions, the scaling law of friction in superlubricity remains ambiguous. Specifically, the correlations between friction and the coupling effects of contact area, edge length as well as normal load are not yet fully understood.  Here, using a novel geometry that decouples bulk from edge contributions to interlayer friction, we quantitatively demonstrate the generalised scaling law of friction in 2D hetero-junctions.  Consistent with theory, friction per internal atom is at least 4 orders of magnitude smaller than that of edge atom.  Moreover, friction force within a single Moiré unit cell scales in a power law of the Moiré unit cell area f~Am^γ with 1.2≤γ≤2.7, and the overal friction from the internal atoms scales linearly with the total number of Moiré unit cells.  Finally, depending on the Moiré unit cell size and edge length, both positive and negative coefficients of friction are observed as a result of the competition between the suppression of elevated Moiré ridges and mechanical deformation contributions.