Anomalous layer-dependent lubrication on graphene-covered substrate: Competition between adhesion and plasticity

Friction on samples covered by few-layer graphene is well known to exhibit the unique layer dependence where friction measured by atomic force microscopy at the nanoscale is generally observed to decrease with the increasing number of layers. However, this trend is not always observed in experiments and simulations for tips comprised of different materials and for graphene on different substrates. Within this context, the exact role played by the interface, in particular graphene-tip/graphene-substrate adhesion and substrate plasticity, in the layer dependence of friction on graphene is not yet fully understood. We conduct molecular dynamics simulations to probe the evolution trend of the layer dependence with high interfacial adhesion under varied external normal loads. To better evaluate the frictional quality, we divide the contact interface into the adhesive and repulsive regions, and the total wrinkle variation is decomposed into the sum of dimple depth and pucker height. In the low load stage, contrary to the traditional perception, higher friction is observed in the bilayer system than that in the monolayer case. However, the bilayer system performs better in lubrication and wear protection when the load reaches high enough to induce plasticity in substrates. The anomalous layer-dependent frictional behavior originates from the interplay among interfacial adhesion, wrinkle of topmost graphene, contact roughness, and plastic deformation of substrates. We further corroborate our conclusion via the comparison with the case employing amorphous silicon substrates. In addition to surface roughness, out-of-plane deformation, and electron-phonon coupling, interfacial adhesion and substrate plasticity play pivotal roles in tribological performance and require extra considerations in designing lamellar lubricants such as two-dimensional materials.

Automated summary

Friction on few-layer graphene samples exhibits layer dependence, but the exact role of the interface and substrate plasticity in this layer dependence is not fully understood. Molecular dynamics simulations were conducted to study the evolution trend of layer dependence under different normal loads. It was found that high interfacial adhesion affects the layer dependence. The frictional quality was evaluated by dividing the contact interface into adhesive and repulsive regions, and it was observed that the bilayer system shows higher friction in the low load stage but better lubrication and wear protection when the load induces substrate plasticity.