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Understanding Graphene’s Friction Properties

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A comparison between the energy corrugation of graphene (above) and fluorinated graphene (below). (Image Credit: University of Pennsylvania)

An interdisciplinary team of engineers from the University of Pennsylvania has made a discovery regarding the surface properties of graphene, the Nobel-prize winning material that consists of an atomically thin sheet of carbon atoms. On the macroscale, adding fluorine atoms to carbon-based materials makes for water-repellant, non-stick surfaces, such as Teflon. However, on the nanoscale, adding fluorine to graphene had been reported to vastly increase the friction experienced when sliding against the material. Through a combination of physical experiments and atomistic simulations, the Penn team has discovered the mechanism behind this surprising finding, which could help researchers better design and control the surface properties of new materials. Besides its applications in circuitry and sensors, graphene is of interest as a super-strong coating. As components of mechanical and electrical systems get smaller, they are increasingly susceptible to wear and tear. Made up of fewer atoms than their macroscale counterparts, each atom is that much more important to the component’s overall structure and function.

To test the friction properties of this material, the Penn researchers collaborated with Paul Sheehan and Jeremy Robinson of the Naval Research Laboratory. Sheehan and Robinson were the first to discover fluorinated graphene and are experts in producing samples of the material to specification. The researchers were surprised to find that adding fluorine to graphene increased the material’s friction but could not immediately explain the mechanism responsible.  The study determined that by adding fluorine, the energy corrugation landscape of the graphene changed. This essentially introduces electronic roughness, which at the nanoscale, can act like physical roughness in increasing friction.