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.
It was observed in the research that the thermal reduction (heating at 750C for 30 min in presence of nitrogen and hydrogen) brings the adhesion force of graphene sheets almost to the level of untreated material. Interestingly, this was not the case with friction force. It was argued that the thermal reduction eliminates the functional groups created during plasma treatment, but it also creates structural defects (vacancy-like) by removing carbon atoms from the graphene lattice as shown in Fig. 2. This brings increased friction.
Results of the study can help in design of graphene base coatings in nano tribological applications. Further details can be found in the original article: Zeng, X. et al. Controllable Nanotribological Properties of Graphene Nanosheets. Sci. Rep. 7, 41891; doi: 10.1038/srep41891 (2017).
. Zeng, X. et al. Controllable Nanotribological Properties of Graphene Nanosheets. Sci. Rep. 7, 41891; doi: 10.1038/srep41891 (2017).