Simulation And Experiment Study On Tribological Properties Of Graphene

Graphene is a typical two-dimensional material,made of a single layer of carbon atoms.With its exotic mechanical,electrical,thermal and chemical properties,graphene has attracted a lot of attention from researched and has broad application prospects in the fields of flexible electronic components,energy storage,and heat dissipation materials.Friction as the normal physical phenomena,needs to be considered in the design and manufacture process.Therefore,observing friction behavior,predicting the influence law of different factors,digging out the key element,and exploring the origin of the friction are the prerequisites for future mechanical systems.As the main research method,molecular dynamics simulation is applied to observe the friction and energy dissipation during relative sliding process,and explore the effects of sliding speed,temperature,the stiffness of the applied spring,and contact state on the friction behavior of the graphene.Combining with the atomic force microscopy experiments,the friction anisotropy properties of graphene is also studies in this paper.Firstly,a three-layer graphene model was established in the software package LAMMPS,based on the Prandtl-Tomlinson model and the experimental principles of the atomic force microscopic.After observing the friction phenomena of graphene,the influence of sliding speed and temperature on the energy dissipation is studied.The research results exhibit that when sliding speed is small,the graphene tip completely feels the periodic interaction with the graphene substrate.At the same time,the instantaneous friction shows a strong periodicity,under the influence of the stick-slip phenomena.There is a local peak of the friction force appearing at 50 m/s,while the whole trend of friction is downward with the increase of the sliding speed.After artificially adjusting the set temperature of the system,the friction decreases with the rise of the set temperature at low sliding speed.With the understanding the friction phenomenon of the graphene,an attempt is made to achieve superlubricity by adjusting the stiffness of the applied spring and the commensurability of the interface.The results of this research show that for small sliding velocities,the condition of elastic instability exists when the applied spring stiffness is smaller than the interfacial stiffness,resulting in the stick-slip movement of the tip.When the applied spring stiffness is larger than the interfacial stiffness,the condition of elastic instability does not exist,and the movement of the tip translates from stick-slip motion to continuous sliding,with vanishing of the friction and energy dissipation.According to the hexagonal lattice structure of graphene,the contact mode between the tip and the substrate turns to incommensurate contact with the relative rotation of the substrate.It is found that the friction and energy dissipation almost vanish in incommensurate contact.It is observed that the topography of the interlayer potential energy and the interfacial stiffness have seen an evident change with commensurate-incommensurate transition.A novel mechanism is proposed which explains the difference of the graphene friction properties under different contact modes.Finally,the friction anisotropy properties of graphene were explored with atomic force microscopy.After determining the surface lattice structure of the graphene sample,a silicon tip slides across the crystal surface in different directions by rotating the graphene sample.The research indicates that friction of graphene has significant anisotropy.When the tip slides in the armchair direction,the friction force is larger than the results of the zigzag direction.In the range of 4 μm/s～500 μm/s,the friction signal increases with the rise of the sliding speed.Through the simulation research,it is confirmed that factors such as sliding speed,temperature,spring stiffness,and interfacial contact mode are able to affect the energy dissipation and friction during the relative sliding.Through experimental studies,it is confirmed that the nanotribological properties of graphene are anisotropic.All these works provide data and design basis for the future applications of graphene and the design of micro-nano electronic components.