Interlayer commensurability and superlubricity in rigid layered materials

Abstract

Superlubricity is a frictionless tribological state sometimes occurring in nanoscale material junctions. It is often associated with incommensurate surface lattice structures appearing at the interface. Here, by using the recently introduced registry index concept which quantifies the registry mismatch in layered materials, we prove the existence of a direct relation between interlayer commensurability and wearless friction in layered materials. We show that our simple and intuitive model is able to capture, down to fine details, the experimentally measured frictional behavior of a hexagonal graphene flake sliding on-top of the surface of graphite. We further predict that superlubricity is expected to occur in hexagonal boron nitride as well with tribological characteristics very similar to those observed for the graphitic system. The success of our method in predicting experimental results along with its exceptional computational efficiency opens the way for modeling large-scale material interfaces way beyond the reach of standard simulation techniques.

Stiffness-dependent interlayer friction of graphene

“…This is consistent with the previous studies on superlubricity in graphite or graphene [6,13,14,17,18,20,22] which showed that superlubricity may occur in incommensurate contacts. However, for a very soft substrate (e.g., with a support stiffness of k=0.05 nN/nm, nearly suspended),…”

“…This means that a soft substrate (e.g., with a support stiffness less than 0.05 nN/nm) may impede superlubricity, further indicating that, besides the commensurability [13,14,18,20,22,23], normal load [21], flake size [18,21], etc., the stiffness too is a dominant parameter for superlubricity. The physical mechanism of the friction in the commensurate case higher than that in the incommensurate case is correlated with the stick-slip behavior of the flake on the substrate.…”

“…The friction force generally increases with increasing the normal load because it reduces the interfacial spacing and thus increases the interfacial shear resistance [16,21]. When two graphene sheets incommensurately contact with each other, the interlayer friction is substantially lower than those in the commensurate contact [12][13][14][15][16][17][18], even achieving superlubricity, due to the fact that the lattice mismatch can lead to an ultrasmooth potential energy surface and cancellation of the shear forces [20,22]. The edge barriers induced by the contact edge may enhance friction energy dissipation, which consequently results in an edge effect on the nanoscale friction -it depends not only on the contact area [18,21] but also on the contact edge [21,[24][25][26][27].…”

On the lubricity of transition metal dichalcogenides: an ab initio study

“…In turn, the result is high static friction and stick-slip behaviour, resulting in high dynamical friction. 41 However, if the egg boxes are rotated relative to one another, yielding an incommensurate state, all unit cells now only have to cross a much smaller barrier at any point along the sliding path, i.e., we expect reduced resistance towards shear/sliding. Whereas we are overcoming obvious physical barriers when sliding egg boxes over one another, for (atomically smooth) lamellar materials the energetic barriers are caused by enhanced Pauli repulsions between the overlapping electron clouds as they encounter each other during the sliding process.…”

The Run-in Process for Stable Friction Fade-Out and Tribofilm Analyses by SEM and Nano-Indenter

“…Hirano further suggested in 2014 [15] that the superlubricity occurs only at the frictional contact between two crystals. However, since the publication of Superlubricity [16] by Martin and Erdemir in 2007, in which superlubricity was defined as having a friction coefficient smaller than 0.01, many DLC researchers, including those mentioned above, have adopted Martin's definition for developments in carbon films in which a friction coefficient smaller than 0.01 is frequently attained [8,[17][18][19].…”