Founder of TriboNet, Editor, PhD (Tribology), Tribology Scientist at ASML, The Netherlands. Expertise in lubrication, friction, wear and contact mechanics with emphasis on modeling. Creator of Tribology Simulator.
Controlling friction of graphene
Graphene, a wonder just one atom thick material, shows incredible wear-resistance and super low friction levels. This behavior is ascribed to its low inter-layer shear properties and high normal load carrying capabilities. However, in many cases controlling friction is preferable. Apparently, this is possible with graphene, which makes it even more attractive material in the field of tribology.
Researchers from China recently reported a way to control tribological behavior of graphene sheets using plasma treatment and thermal heat reduction. It was shown that plasma treatment introduces defects (by inducing nitrogen and oxygen containing functional groups) into the surface and their amount increases with treatment time. It was clearly shown, that the defects increase adhesion force (also hydrophilicity) and the time of the plasma treatment determines the degree of growth. According to Bowden and Tabor adhesion friction model, the increase of the adhesion force increases friction and this behavior was confirmed in the current study by the Atomic Force Microscopy measurements of friction on graphene sheets, as shown in the Fig. 1.
[1]. Zeng, X. et al. Controllable Nanotribological Properties of Graphene Nanosheets. Sci. Rep. 7, 41891; doi: 10.1038/srep41891 (2017).
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I am not sure that Zeng et al. have the control of friction with graphene that they think they have. Remember that graphene has been shown to be self-healing under compression and stress. Induced vacancies (holes) can self-heal over time, reducing the desired induced friction. As the lowest energy state for the carbon matrix is the hexagonal (graphene) configuration, it may be that any induced friction from vacancies is short-lived in a tribological contact setting.
You are right and they also considered the healing of graphene properties with time. They observed a clear trend of decreasing friction on treated graphene to the level corresponding to the untreated graphene, but the full recovery was not achieved (after 8 hours). Self-healing – a nice property of graphene!