Graphene and functionalized graphene are promising candidates as ultrathin solid lubricants for dealing with the adhesion and friction in micro- and nanoelectromechanical systems (MEMS and NEMS). Here, the dynamic friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO), and fluorinated graphene (FG) were comparatively studied using atomic force microscopy (AFM). The friction as a function of load shows nonlinear characteristic on GO with strong adhesion and linear characteristic on PG and FG with relatively weak adhesions. An adhesion enhancement phenomenon that the slide-off force after dynamic friction sliding is larger than the pull-off force is observed. The degree of adhesion enhancement increases with the increasing surface energy, accompanied by a corresponding increase in transient friction strengthening effect. The dynamic adhesion and friction enhancements are attributed to the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip behaviors confirm that the interfacial interaction is enhanced during dynamic sliding, and the enhancing degree depends on the surface hydrophilic properties. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced in the dynamic friction process, which needs careful attention in the interface design of MEMS and NEMS.

Graphene oxide (GO) with properties of large-scale preparation and application is an attractive solid lubricant. Understanding the tribological properties of GO under an electric field is crucial for the application of GO as a lubricant in electromechanical devices. Here, nanofriction properties of GO was measured in real-time using atomic force microscopy (AFM) with a positively biased conductive tip. The nanofriction force between the AFM tip and GO decreased sharply in the initial few tens of scan cycles. Then, a gradual decrease in nanofriction force was observed. The nanofriction force leveled off as the scan cycles further increased. The significant decrease in nanofriction force resulted from the reduction of oxygen-containing functional groups during the scan of the positively biased tip, which also led to the drop of height in the scanned area. Also, biased voltages, relative humidity, scan velocities, and normal loads affected the reduction and thus the nanofriction force. The exhibited low nanofriction force of GO after reduction makes it become an economically viable coating with the potential to extend the lifetime of electromechanical systems.