Study On Electric-Carrying Friction Of Graphene Surface Based On Conductive Atomic Force Microscopy

Current-carrying friction is defined as the friction between two electrified surfaces.Current-carrying friction has important applications in industries such as aerospace,rail transit,communications,and electric power.Current-carrying friction and wear properties determine the lifetime and reliability of related devices.graphene,as a twodimensional material,has excellent tribological properties,as well as a series of other excellent properties such as electrical,mechanical and thermal properties.It is an ideal lubricant material for the current-carrying friction interfaces.The study on mechanisms and control of current-carrying nanofriction of graphene is helpful to develop its application in the current-carrying friction interfaces.In this thesis,the electrical properties of graphene during nanofriction were investigated using conductive atomic force microscopy(CAFM).Then,the effects of magnitude and direction of voltages and contact resistance on the nanofriction of graphene were also investigated.Finally,a novel method was proposed to precisely control the nanofriction of graphene.A cover of graphene on Au substrate could not only efficiently decrease the friction,but also ensure the electrical conductivity,because the low friction and high conductivity properties of graphene.In addition,the highly flexible graphene is easily deformed under external force to form a reliable surface contact,which greatly improves the stability of current transmission.Through analyzing the applied force and motion of tip,it was found that reducing the speed and increasing the load can improve the contact quality between the tip and the graphene,which further improved the stability of current transmission.The current-carrying friction of graphene was affected by the magnitude and direction of the tip voltage.The friction of graphene increased with voltages.When the value of voltage was the same,the friction under negative voltages was always larger than the friction under positive voltages,and the friction difference between positive and negative voltages increased with the value of voltages.On the one hand,the electrostatic force between the tip and the graphene increased quadratically with voltages,and the inherent work function difference between the tip and the graphene made the electrostatic force under the negative voltages larger than that under positive voltages.On the other hand,the influence of water molecules on friction under negative voltages was greater than that under positive voltages.Moreover,the friction increased rapidly with the under high negative voltages due to the electrochemical oxidation of graphene.Nevertheless,the friction under positive-bias voltages remained low and the structure of graphene was unchanged.The electron transfer between the tip and graphene determines the potential difference and friction of the interface.When the contact resistance was low,the electrons can transfer quickly from tip to graphene,which decreased the potential difference of the interface.Therefore,the applied voltage had little effect on the friction.When the contact resistance was high,the electron transfer was blocked at interface.The high potential difference between tip and graphene causes an increase in friction.The electron capacity of graphene increased with the size,and the surface potential of large size graphene changes slower than small size graphene.So,the friction of large size graphene was larger than small size graphene under same voltage.Increasing the load can decrease the contact resistance between the tip and the graphene and accelerate the electron transfer.So,the friction decreased with an increase in load.The local electrochemical functionalization of graphene is an effective method for controlling the friction of graphene.The controlled functionalization and friction of graphene were realized by introducing an electron releasing process to avoid electron saturation.Utilizing the polarization of the functionalized graphene under the electric field,the friction of graphene was further reversibly controlled by adjusting the applied voltage.