The evolving quality of frictional contact with graphene

Atomistic simulations reproduce experimental observations of transient frictional strengthening of graphene on an amorphous silicon substrate, an effect which diminishes as the number of graphene layers increases. Why monolayer graphene is no lubricant Graphite and other lamellar materials are used as dry lubricating agents for macroscale metallic sliding components and high-pressure contacts. But how does monolayer graphene compare as a lubricating agent? Ju Li and colleagues studied the atomic stick–slip motion of nanoscale tips sliding against both suspended and supported graphene systems. Their simulations reveal that the quantity of atomic-scale contacts (true contact area) increases with time, and that the commensurability of the graphene with the surface evolves. In other words, carbon atoms become more strongly pinned to the substrate, and the pinned atoms show greater synchrony in their stick–slip behaviour. This is a direct result of the flexibility of the graphene. As additional graphene layers are added, the flexibility of the contact graphene layer is reduced, leading to the reduced frictional strengthening. The effect can be tuned by pre-wrinkling the graphene contact layer. Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction 1 . The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction 2 , 3 , 4 , 5 , 6 , 7 . Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits 8 , 9 , 10 , 11 , 12 , 13 unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhanc