Effect of roughness on the layer-dependent friction of few-layer graphene

Abstract

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.

Velocity-dependent friction enhances tribomechanical differences between monolayer and multilayer graphene

“…The decreasing in friction with the increasing number of layers, as seen in Fig. 1, is in good agreement with results reported on graphene tribology literature 12,15,28 . The dependence of friction with the number of layers as observed in experiments involving the sliding of an AFM tip over the graphene surface is an intricate problem.…”

“…Many efforts have been reported on this issue, from both experimental 6,12–21 and theoretical 15,22–26 approaches. Such studies show that friction in graphene is influenced by parameters such as the number of graphene layers 12,15 , interaction with the substrate 14,15,19 , surface roughness 25,27,28 and crystallographic orientation 17,21,29,30 . The number of layers has an important role in the friction mechanism of graphene, as friction was observed to decrease with the increasing number of layers.…”

“…The interaction with substrate is of relevance as graphene deposited on atomically thin materials has shown much lower friction than graphene deposited on silicon substrates 15 . Such interaction influences the surface roughness, diminishing it and lowering the friction forces 28 . The crystallographic orientation of the graphene sample is also important, as the energy dissipation along the armchair direction can be higher than along the zigzag direction for a single-layer graphene 21 .…”

Solid Lubrication with MoS2: A Review

“…As the number of layers increases, the bending stiffness of the 2D material in the vertical direction also increases, leading to a decrease in puckering and consequently, in friction. While factors such as electronphonon coupling [87] as well as surface roughness [88] may also play a role in the layer-dependent friction of 2D materials, the puckering phenomenon is generally accepted to be the dominant mechanism.…”

Speed dependence of friction on single-layer and bulk MoS2 measured by atomic force microscopy

“…[7][8][9] Consequently, friction on 2D materials was evaluated in a number of studies conducted primarily via atomic force microscopy (AFM), focusing on the effects of applied load, the number of layers, structural defects, and humidity, among others. [10][11][12][13][14][15][16][17] Despite the extensive amount of work performed toward elucidating friction mechanisms on 2D materials, very few results were published on the dependence of friction forces on sliding speed. 18 This is potentially a critical concern, as components in various micro-and nano-scale mechanical systems designed to be lubricated by 2D materials are expected to move in a wide range of speeds during operation.…”