Investigation On The Friction Behavior Of Graphene

Atomic-scale friction has been a hot topic in nanotribology in recent years. It is found that there exist clearly distinctions between the atomic-scale friction and the well-studied macroscopic friction. The complication of friction at the atomic scale severely limits our understanding of the underlying mechanism of friction. A clear understanding of the atomic scale friction is therefore very important for both scientific and industrial interests.The theme of this dissertation is to investigate the fundamental friction behavior of graphene. Graphene exhibits exceptional mechanical properties duo to its ultrastrong intralayer covalent bonds interactions. Especially, as a strictly two-dimensional material, graphene is remarkably stable with atomically flat and well-defined surface. Those extraordinary properties make the graphene as an ideal material for studying friction characteristics at the atomic-scale. Moreover, a clear understanding of the friction behavior of graphene will benefit the design of the graphene-based micro- and nano-electromechanical systems(MEMS/NEMS).The research contents are as follows:(1) We studied the influence of the supported stiffness of the substrate on the friction properties of graphene. Using molecular dynamics simulations, the friction behavior of a graphene slider sliding on a spring-supported graphene substrate has been investigated. The results show that the friction force decreases exponentially with the increasing stiffness. The stiffness is a dominant factor for the friction on a soft substrate, e.g., where superlubricity may be completely impeded. Furthermore, we relate the friction to the substrate deformation and find that the indentation depth can be an indicator for the friction of soft substrates.(2) We studied the effect of the slider stiffness on the friction properties of graphene. Based on molecular dynamics simulations, the sliding friction of a graphene slider and a bulk diamond slider on a spring-supported graphene are investigated. For the substrates with high stiffness, the friction between the substrate and the graphene or diamond slider, contributed mostly by the significant edge effects. For a softer substrate, the edge effect is still a main factor in the friction between the diamond slider and the substrate surface. In contrast, the edge effect is less significant for a graphene slider on a soft-spring-supported substrate. The main reason is that there exists a different interlayer pressure distribution along the contact interface between the susbtrate and the graphene or diamond slider. Moreover, we considered the influence of the stiffness and normal load on the interlayer superlubricity. Also, the relation between friction force and indentation depth at the incommensurate case has been obtained.(3) We investigated the effect of the contact edge on the interlayer friction of graphene. Using molecular dynamics simulations, we analyzed the friction characteristics of a rectangle graphene slider incommensurately placed onto a graphene substrate. The results show that the friction is significantly dependent on the length of the side that perpendicular to motion direction of the slider, but insensitive to the length of the side parallel to motion direction.(4) We studied the effect of the normal applied load on the friction on the surface of a suspend graphene. Using molecular dynamics simulations, we investigated the friction behavior of a diamond hemispherical probe scanning on a suspend graphene substrate to study the dependence of the friction on the normal load and real contact area. The results show a nonlinearly positive dependence of the friction on the normal load and real contact area, which make it apparently different from the friction characteristics on bulk materials.