Molecular Simulation Study Of Nanoscale Adhesive Wear Mechanism Of Few-Layer Graphene

Graphene has been found to possess ultrahigh mechanical strength,easy-shear capability and thickness thin to a few atomic layer.Thus it stands out as a potential solid lubricant for micro/nano electromechanical systems.However,the graphene sheet on rubbing surface is easy to be worn out and sensitive to environmental atmosphere,which has still limited the realistic application of graphene.In this paper,adhesive wear process and mechanism between the diamond tip and few-layer graphene(FLG)sheets were studied by large-scale molecular dynamics(MD)simulation.In addition,the chemical adsorption behavior of hydrogen atom in the adhesive wear process of graphene were also investigated.The main contents are as follows:(1)A diamond tip with different hydrogen coverages was prepared,and a simulation method was established for the sliding process of the diamond tip/few-layer graphene sheets model.By selecting suitable potential function and controlling the interaction between hydrogen atoms,then the simulation method for the sliding process of the diamond tip/few-layer graphene sheets model was determined.(2)The sliding process of the diamond tip with different hydrogen coverages on the few-layer graphene sheets was simulated,and the adhesive wear behavior and mechanism of few-layer graphene were investigated.Two typical adhesive wear processes caused by interfacial action were obtained in the simulation.By calculating variation of the number of interfacial bonds(between the tip and graphene),interlayer bonds(between graphene layers)and in-plane broken C-C bonds in the sliding process,the coupling effect of interfacial bonds and interlayer bonds on the adhesive wear behavior of graphene was revealed.Then we calculated the critical normal load for the adhesive wear of un-passivated tip/graphene model,and found that the critical normal load was significantly smaller than critical normal load of abrasive wear and the initial critical normal load of the graphene in the loading process.Through further investigating the evolution of the contact state between the tip and graphene during the sliding process,the reason why graphene has a lower critical normal load in adhesive wear was revealed.(3)The sliding process of the diamond tip on the few-layer graphene sheets in hydrogen environment was simulated.We studied the chemical adsorption behavior of hydrogen atoms in the adhesive wear process of the graphene,and hydrogen atoms were found to be mainly adsorbed on worn graphene surface where there was large strain or dangling bonds,namely,chemically enhanced carbon atoms caused by tensile strain,wear defects with dangling bonds and the convex puckering.By comparing with the corresponding wear process in the vacuum environment,influence mechanisms of different kinds of hydrogen adsorption on the wear process of graphene was studied,including asymmetric expansion of the wear cavity of graphene resulting from the hydrogen adsorption at the cavity tip,as well as suppressed adhesive wear caused hydrogen passivation.Besides,the actual effect of these influence mechanisms on wear degree of graphene was investigated by using different shape tips with various normal loads.The research content of this paper will help to understand the intrinsic microscopic mechanism of the wear failure process of graphene,as well as the influence mechanism of environmental hydrogen on the wear property of graphene.And it has a certain theoretical guiding significance for the practical application of graphene as a solid lubricant.