Study On Nanofriction Of Graphene Surface Based On Atomic Force Microscopy

Graphene,with excellent mechanical,physical and chemical properties,is an ideal nano lubricating film for the gap surface of moving components in micro and nano electromechanical systems(MEMS/NEMS)to decrease interfacial friction and wear and reduce energy consumption.The outstanding nanofriction properties of graphene depend on the atomically-smooth inert surface and perfect honeycomb crystal structure,but the quality of surface and structure tends to decrease in the process of growth,preparation and post-treatment.Meanwhile,the out-of-plane corrugation is an inherent feature of graphene due to the ultra-thin and extremely-high flexible characteristics.Surface property,structure and out-of-plane deformation are the decisive factors to determine the nanofriction properties of graphene.In this paper,the surface properties and structure are regulated by plasma treatment,the out-of-plane deformation is tuned by introducing wrinkles,changing the surface properties of substrate and enhancing the substrate deformation.Then the effects and mechanisms of these factors on nanofriction of graphene are studied by atomic force microscopy(AFM).The research contents are mainly as follows:1.Regulating the nanofriction of graphene based on surface properties and structure.Plasma treatment was introduced to control the surface properties and structural defects of graphene,and then the effects of surface properties and structure defects on friction were studied.The results show that the surface adhesion and friction increase monotonously with the enhancement of surface hydrophilicity.The layer dependence of friction is gradually suppressed as the surface hydrophilicity enhances.The surface adhesion is insensitive to structural defects,but the surface friction increases with the enlarging structural defects.The stronger surface properties result in a larger interfacial sliding barrier by enlarging the adhesive interaction of tip-graphene interface and lead to a more enhanced asymmetric out-of-plane deformation under the tip,thus to cause greater interface friction.2.Comparatively studying the nanofriction behaviors of three kinds of graphene with different surface properties.An adhesion enhancement effect determined by dynamic sliding and surface properties is found in the friction process of tip-graphene interface.The degree of adhesion enhancement on pristine graphene increases with the enhancement of surface properties.A greater degree of adhesion enhancement effect occurs on graphene oxide due to the strong surface hydrophilicity,and fluorinated graphene has little adhesion enhancement because of its strong hydrophobic properties.The tip dynamic sliding increases the asymmetric out-of-plane deformation of graphene,and enhancing the surface properties further causes the local separation of upper layer,which leads to a stronger adhesion enhancement effect.3.Studying the effect of out-of-plane deformation on the nanofriction of graphene.The effect of wrinkle on nanofriction of graphene and the nanofriction behavior of wrinkle were investigated.The wrinkle deformation significantly increases the friction of graphene nearby by enhancing the out-of-plane corrugation,and the friction reversibly decreases when the wrinkles are removed.Flexible wrinkles can dynamically adjust the shape and position,resulting in a synchronous increase in friction.Both the friction and friction strengthening effect have an anisotropy with 180° periodicity due to the different deformation resistances of ripple along different directions.Enhancing the interaction of graphene-substrate interface effectively reduces the friction anisotropy because of the suppression of out-of-plane deformation.By studying the dependence of friction strengthening effect on tip velocity and load,it is found that the puckering effect plays a major role under higher load,whereas the contact quality evolving effect plays a main role under lower load.4.Studying the effect of substrate on the nanofriction of graphene.The bonding strength between graphene and substrate was increased by enhancing the surface properties of substrate through plasma treatment.It is found that the friction and the layer dependence of friction decrease with the enhancement of bonding strength due to the suppression of out-of-plane deformation.The nanofriction behaviors of graphene on soft elastic substrate were investigated.The deformation of soft substrate results in a synchronous deformation in graphene,and the friction of graphene increases faster and faster with the enhancement of deformation.Meanwhile,the friction of graphene shows a stronger thickness dependence.As the velocity increases,the friction exhibits a continues increase,and the dependence of friction on load changes from linear to nonlinear.The out-of-plane deformation of graphene enhanced by the deformation of soft substrate as well as the energy dissipation of soft substrate deformation increases the friction of graphene.The friction of graphene is mainly controlled by surface properties,structure and out-of-plane deformation.The friction increases with the enhancement of surface properties or structural defects.Also,the friction increases with the enhancement of out-of-plane deformation by introducing wrinkles or increasing the deformation of soft substrates.However,enhancing the surface properties of substrate decreases the friction of graphene due to the inhibition of out-of-plane deformation.Moreover,the surface properties are able to induce further out-of-plane deformation of graphene when the surface properties are enhanced to a certain extent.The surface properties and structural defects mainly determine the friction of graphene by changing the interfacial potential barrier,whereas the out-of-plane deformation mainly affects the friction by changing the interfacial contact area and quality.