Tribological characteristics of few-layer graphene over Ni grain and interface boundaries

The tribological properties of metal supported few layered graphene (FLG) depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the wetting and frictional characteristics of the FLG system. We individually measured friction characteristics of FLG covering grains and grain boundaries of polycrystalline Ni metal substrate via an Atomic Force Microscopy (AFM) probe. The friction coefficient of FLG covered at grain boundaries is found to be lower than that on grains in vacuum (at 10-5 Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull-off forces were addressed in the course of friction measurements to explain the role of the out-of-plane deformation of graphene layer/s. Finite element simulations show good agreement with experiments and led to rationalize the observations. Thus, with grain boundaries the FLG tribology can be effectively tuned.