Study On Tribological Mechanisms Of Graphene At Nanoscale

About 1/3 of industrial energy consumption in the world is caused by friction,and about 80%of the failure of mechanical parts is caused by wear.Lubrication is an effective means to reduce friction and wear.As a two-dimensional material,graphene shows the unique lubricating properties that traditional solid lubricating materials do not possess.Graphene is a potential candidates solid lubricant.The lubrication effect of graphene varies greatly due to the difference of friction pairs,lubrication environment and its own surface morphology,etc.The mechanism behind these phenomena is not clear.The study on the tribological mechanism of graphene at the atomic scale is helpful to reveal the influence mechanism of lubrication conditions on the lubrication effect of graphene,so as to provide a theoretical basis for revealing the existing tribological phenomena and expanding new application of graphene in lubrication.In this paper,the nanoscale tribological mechanisms of graphene were studied by molecular dynamics（MD）simulation,including the mechanism of interfacial adsorption and moirésuperlattice structure affecting the tribological properties of graphene,and the mechanism of graphene improving the lubrication effect of nanoparticles.（1）Effect of the adsorption of monolayer graphene（MLG）on metal surfaces(nickel（Ni（111））,platinum（Pt（111））and copper（Cu（111））on the nanoscale adhesive friction and wear of MLG was investigated by MD simulation of a diamond tip sliding over metal-supported MLG.The chemisorption of MLG on the metal surface will seriously weaken the strength of C–C in MLG,enhance the chemical reactivity of carbon atoms and reduce the carrying capacity of MLG.The adsorption for MLG coating freely onNi（111）surface is a chemisorption and it is a physisorption for MLG coating freely on Pt（111）and Cu（111）surfaces.However,beneath the tip,the Pt（111）substrate exhibits a certain chemical weakening effect on MLG due to the contact pressure,while the weakening effect of Pt（111）substrate is weaker than that of the Ni（111）substrate;the Cu（111）substrate has no chemical weakening effect on MLG.This leads to the load carrying capacities of Ni（111）-and Pt（111）-supported MLG（rupturing under 7 and 11 nN,respectively,during sliding）much lower than that of Cu（111）-supported MLG（rupturing under 21 nN）.During wear,only Ni（111）substrate shows chemical weakening effect on MLG at crack tip,which results in lower tearing resistance（providing the main frictional resistance）of Ni（111）-supported MLG compared with Pt（111）-and Cu（111）-supported MLG.The decrease of the load carrying capacity and tearing resistance of MLG is due to the weakening of C–C bonds via charge transfer from metal to carbon atoms.（2）Effect of oxidation of metals on the nanoscale adhesive friction and wear of MLG was investigated by MD simulation of a diamond tip sliding over MLG supported by nickel/iron-based subsrates（Fe（111）,FeO,Fe₂O₃,Ni（111）,NiO and Ni₂O₃）.The MLG bearing capacity increases with the increase of oxidation degree of nickel/iron surface;at the same oxidation degree,the MLG bearing capacity supported by nickel-based substrate（NiO_x）is higher than that supported by iron-based substrate（FeO_x）.At300 K,oxygen atoms do not bond to MLG,but will reduce the chemical activity of metal atoms and the chemical adsorption of MLG on the nickel/iron substrate,and then reduce the chemical weakening of the nickel/iron substrate to MLG.Therefore,the oxidation of nickel/iron metal surface improves the wear resistance of MLG.Without the influence of load,the tear strength of MLG is not reduced after the oxidation of nickel/iron substrate,which leads to the friction resistance provided by MLG on the oxidized surface is greater than that on pure nickel/iron surface.The decrease of chemical weakening of metal atoms is due to the oxygen atoms with higher electronegativity reducing the charge transfer between metal and carbon atoms.The electronegativity of Feis lower than that of Ni,so electrons in Featoms are easier to transfer towards carbon atoms,which leads to a larger chemical weakening effect on C–C bonds.（3）Effect of the moirésuperlattice structure on the nanoscale friction of graphene was investigated by MD simulation of a tip sliding over Pt（111）-supported graphene（including pristine and hydrogenated graphene）with/without moirépattern under week/strong interfacial adhesion.Whether pristine or hydrogenated graphene,the peak value of friction force with moirépattern is larger than without moirépattern,which is due to the uphill effect caused by the geometric undulation of moirésuperlattice structure.The uphill effect increases the commensurability of the friction interface,causes lager asymmetry of the atomic lateral force on the tip surface.However,the moirésuperlattice structure will promote the movement of the tip due to the downhill effect,so the asymmetry of the atomic lateral force for the whole sliding process is reduced on moirésuperlattice structure,which leads to:the average friction force on hydrogenated graphene with moirépattern is less than that without moirépattern;the average friction force on pristine graphene with moirésuperlattice structure is similar to that without moirépattern.Under strong interfacial adhesion,hydrogenated graphene forms wrinkle around the tip,which causes larger strain.By comparing and analyzing the instantaneous friction force and the change rate of strain energy of hydrogenated graphene,it is found that the change rate of strain energy caused by the evolution of wrinkle is a direct factor affecting the friction force.The moirédistribution of hydrogen atoms（hydrogen atoms bond to the protruding areas of moirépattern）can enhance the adsorption between hydrogenated graphene and platinum substrate,inhibit the formation of wrinkle,reduce the change rate of strain energy,and then reduce the energy dissipated through the strain of hydrogen graphene.Therefore,for the same hydrogen coverage,the average friction force of hydrogenated graphene with moirépattern is significantly less than that without moirépattern ubder strong tip/hydrogenated graphene interfacial adhesion.（4）Friction properties of carbon nanodiamond（CND）and nanoscroll（CNS formed from graphene patch wrapping around nanodiamond）were studied by MD simulation of sliding diamond-like carbon（DLC）over the nanoparticles supported by amorphous silica（α-SiO₂）slabs.CNS reduces friction coefficient（COF）by 72%relative to CND and superlubricity（COF≤0.01）is enabled by CNS,which agrees well with experimental observations.Due to the wrapping graphene patch,the DLC/CNS contact area is smaller than the DLC/CND contact area and the CNS motion is repressed.Contrary to the rolling motion of ball bearings at the macroscale,the repressed motion of nanoparticles reduces the system friction dissipation.Therefore,graphene further improves the lubrication effect of CND by inhibiting the motion of CND.