Friction on few-layer graphene is known to exhibit unique layer dependence where friction measured via atomic force microscopy (AFM) on the nanometer scale is generally observed to decrease with increasing number of layers. However, this trend is not always observed for AFM probe tips with different sizes and for graphene on different substrates. Within this context, the precise role played by the interface, in particular, the size of the contact and substrate roughness, in the layerdependence of friction on graphene is not yet completely understood. Here, we probe the origins of the roughness dependence of layer-dependent friction on graphene by a combination of AFM measurements and molecular dynamics (MD) simulations. In the experiments, friction is observed to monotonically decrease with increasing number of graphene layers for tips with various apex radii, while the roughness of the sample surface is observed to decrease. In the simulations, two opposite layer-dependence trends for friction are observed on few-layer graphene on substrates with different roughness values. The underlying mechanisms are investigated using atomistic details obtained from the simulations, where the different friction trends are found to originate from an interplay between surface roughness, the trajectory of the tip and the number of atoms in contact. Finally, the effect of topographical correlation length on the layer dependence of friction on graphene is discussed.
In this study, we investigated macro-and nano-scale tribological behaviours of single-layer graphene on steel parts. Single layer graphene was synthesized via Chemical Vapour Deposition (CVD) on copper foil and then transferred onto commercial journal bearing that has a considerable rough surface. Nanotribological tests were carried out by using Atomic Force Microscopy (AFM) under loadings differs from 5 to 30 nN, and macrotribological experiments were done using pin on disc type tribometer at three different loads of 10, 15 and 30 N within 90, 120 and 250 s sliding cycle durations. The results exhibited that graphene effectively diminish the wear rate of substrate material, whereas it has no significant improvement in coefficient of friction due to high asperity of surface. The worn surface analyses were characterized by scanning electron microscopy for the evaluation of wear mechanisms.
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