Study On The Friction Properties Of Two-dimensional Materials Based On Atomic Force Microscopy

With the continuous miniaturization of modern micro/nano electromechanical systems（MEMS/NEMS）,the study and development of stable solid lubricants for enhancing efficiency,durability,and extending the lifetime of these devices is of great importance.As a two-dimensional material,the outstanding nanofriction properties of graphene depend on the atomically-smooth inert surface and perfect honeycomb crystal structure,making it very suitable as a lubricant for micro/nano electromechanical systems.However,the micro/nano electromechanical system is often accompanied by high temperature and high speed working environment,which will influence the nanofriction properties of graphene,therefore the micro/nano device will wear out and even fail.Therefore,studying the effect of annealing temperature and substrate on the tribological properties of graphene will promote the engineering application of graphene and other related 2D materials as lubricants in micro/nano electromechanical systems.In this paper,highly oriented pyrolytic graphite（HOPG）was used to prepare single-layer graphene（SLG）samples by mechanical exfoliation method.The tribological properties of graphene under different environmental and the tribological mechanism are studied by atomic force microscopy（AFM）.The research contents are mainly as follows:1.The potential degradation of the lubricative properties of graphene under high temperatures environment is particularly important to many micro-and nano-scale mechanical systems where graphene is used as solid lubricant.In this study,the tribological properties of single-layer graphene（SLG）after annealing in the environment were investigated by atomic force microscopy（AFM）.The variations of the friction performance before and after annealing at different temperatures were compared.The experimental result showed that friction force shows a significant enhancement after annealed in ambient air when annealing temperature reached 300℃.Raman spectroscopy was applied in this experiment to detect the induced defects of graphene and the defect density of graphene under different annealing temperature are evaluated.It was demonstrated that the detectable defects initially appeared between 200℃and 300℃and the defect density also increase with the annealing temperature.Finally,the relationship between friction force of SLG and defect density under different loads was established.The research is of great significance for evaluating the change of tribological properties of graphene as a solid lubricant in practical applications.2.The substrate has a significant influence on the control of the friction of the graphene surface.The friction of graphene on various substrates,such as Si O₂,bulk-like Mo S₂,graphite and PDMS,was investigated by atomic force microscopy（AFM）to establish the relationship between friction and interfacial interactions between graphene and substrate.It is found that the mechanically exfoliated graphene has low friction on an atomic flat substrate,such as the bulk graphite and Mo S₂.Moreover,the friction of graphene on a soft elastic substrate will show much higher friction force than that on hard Si O₂.Besides,the Si O₂substrate with the plasma treatment can efficiently reduce the friction force of graphene.The friction measurements revealed that adhesive attraction between graphene and the underlying surface is particularly important to the tribological properties of graphene.This result can be applied for the fabrication of 2D material device with reliable and high performance since the interaction between 2D materials and substrate can be modulated by controlling the morphologies of both surfaces.3.Studying the strong dependence of the surface friction of graphene and molybdenum disulfide on speed is helpful for the safe use of two-dimensional materials as solid lubricants under high-speed conditions.We recorded the friction force of graphene and Mo S₂as a function of sliding speed.The results of the experiments show that the atomic stick-slip friction on graphene and Mo S₂increase linearly with the logarithm of the sliding speed until speed reaches saturation.This phenomenon can be attributed to the hoping of the contact atoms between tip and sample induced by thermal activated.While the attempt frequency of hoping is related to the effective contact stiffness and tip-sample interaction corrugation in the thermally activated Prandtl–Tomlinson model.The difference in the speed dependence of friction of graphene and Mo S₂can be explained by the parameters fitted in PT model with this experiment results.The friction of graphene is mainly controlled by surface properties,substrate and velocity of cantilever.Defects in graphene will increase the friction on the graphene surface.This is mainly due to the formation of hydrophilic functional groups on the graphene surface,which enhances the hydrophilicity of the graphene surface.Enhancing the van der Waals interaction between the substrate and the graphene will reduce the friction on the graphene surface.The mechanism is to suppress the wrinkling of the graphene.The influence of the cantilever speed on the friction characteristics of graphene is well explained by the PT model,and it reveals the change in the friction characteristics of graphene is related to the effective contact stiffness and tip-sample interaction corrugation in the thermally activated Prandtl–Tomlinson model.