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Graphene is known as a promising lamellar lubricant and coating material in the development of diamond-based micro/nanoelectromechanical systems and nanoheterostructures. As a rule, the friction performance on samples covered by few-layer graphene exhibits layer dependence; namely, the friction decreases with the increasing number of layers. However, in this paper, an inverse frictional response to graphene-covered diamond (111) was captured by a series of atomic force microscopy measurements in the low-load regime, which suggests that the routine layer dependence of friction on graphene remains a subject to debate. To explore the cause behind this, the friction process at the nanoscale was simulated by a molecular dynamic method. In the low-load regime, it is found that the friction forces on the indenter in the graphene-covered systems are exerted by the topmost graphene, and the layer-associated frictional response displays a trend of “climb-to-platform”. Such a layer-associated response of friction can be explained by the atomic force distribution on the indenter. It is corroborated that the distribution of atomic force in the 2-L system is changed significantly by the repeated squeezing and rubbing of graphene against the indenter and hence to the enhancement of friction induced by the pinning effect, which depends on the delicate competition between the adhesion of the topmost graphene to the nether substrate and that to the hemispherical indenter. This investigation is conducive to all-around insight into the friction characteristics of graphene on diamonds and provides a clue in designing graphene lubricants based on diamond nanodevices.
Chen et al. (Thu,) studied this question.