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

Graphene is considered to be an ideal solid lubricant due to its high mechanical strength,easy shear on the stacked surface and high chemical inertness.However,it was found that graphene has high mechanical strength and low friction coefficient at the nanoscale,but is easily worn out at the micro-and macroscale,which limits the practical application of graphene as a lubricating material.In this paper,the large-scale molecular dynamics simulation method was used to simulate the sliding process of a diamond tip on a few-layer graphene sheet.The effects of point defects,rough substrate and CH_x environment on the wear behavior of graphene were studied.Parts of the atomistic mechanisms that cause the weakening of the lubrication properties of graphene at the micro-and macroscale were revealed.The main contents are as follows:（1）The sliding process of the diamond tip on a few-layer graphene sheet with point defects was simulated,and the effect of point defects on the wear behavior of graphene was studied.It was found that the presence of point defects promotes the process of adhesive wear and abrasive wear of graphene,and reduces the critical loads for onset of initial wear.The presence of Stone-Wales,double vacancies and single vacancy defects reduces the critical normal load for onset of adhesive wear to 85%,15%and 11%of that of intact graphene,respectively,while reduces the critical load for onset of abrasive wear to 78-85%of that of intact graphene.By analyzing the potential energy surface and nanoindentation process of the defective graphene,we found that it was the maximum interaction energy that determines the critical normal load for the onset of adhesive wear,while the load carrying capacity of the defective graphene determines the critical load for the onset of abrasive wear.（2）The sliding process of a diamond tip on a single-layer graphene sheet supported by rough substrate was simulated,and the effect of rough substrate on wear behavior was studied.We found that the presence of rough peaks induces the occurrence of adhesive wear and abrasive wear of graphene,and the higher the height of the rough peak is,the earlier the wear occurs.By analyzing the change of the number of interfacial bonds and in-plane broken bonds and the maximum stress of the contact area between graphene and the tip,we found that the influence of the rough peak on the wear behavior of graphene is mainly due to the stress concentration of graphene caused by the rough peak,which enhances the chemical reactivity of graphene and weakens the mechanical strength of graphene.In addition,the tip further increases the stress and chemical reactivity of the graphene atoms during sliding through the rough peak.（3）The sliding process of a diamond tip on the few-layer graphene sheet in CH_x environment was simulated.The chemisorption behavior of CH_x ion on graphene during adhesive wear process and its effect on adhesive wear were studied.It was found that CH_x ions are mainly adsorbed on the edges of wear defects with dangling bonds,carbon atoms with large tensile stress and raised wrinkles.By comparing with the corresponding wear process in the vacuum environment,we found that the adsorption of CH ions changes the stress concentration region during the wear process,so that the graphene is earlier peeled off from the graphene sheet.Through the comparison of the effects of different ions,it was found that CH ions have the greatest promotion effect on the adhesive wear process,followed by CH₂ ions,and CH₃ ions have no obvious influence on the adhesion wear process of graphene.