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The tuning of flexible microscale friction is desirable for the reliability of wearable electronic devices, tactile sensors, and flexible gears. Here, the tuning of friction of atomically thin graphene on a flexible polydimethylsiloxane (PDMS) substrate was obtained with the elastic modulus using a 1H,1H,2H,2Hperfluorodecyltrichlorosilane (FDTS) self-assembly monolayers (SAMs)-modified microsphere probe with the diameter of 5 μm at the microscale. The friction can be tuned at a large scale with the difference in the elastic modulus of PDMS and thickness of graphene. The hydrophobic property of the FDTS SAMs-modified probe decreased friction by reducing interfacial adhesion and preventing the effect of capillary interaction; thus, the friction decreased with the increase in the elastic modulus of the PDMS substrate due to decreasing indentation depth and thus the interfacial contact area; and also, the enhanced out-of-plane stiffness effectively decreased the interfacial contact quality with the increase of the thickness of graphene. The flexible tuning of friction on graphene was further verified by the theoretical calculation from the aspects of the friction arising from the normal and lateral deformation around the contacting area. This work is meaningful for promoting the design and reliability of flexible micro-devices.

Robust superlubrication across nano-and microscales is highly desirable at the interface with asperities of different sizes in durable micro/nanoelectromechanical systems under a harsh environment. A novel method to fabricate superlubric interfaces across nano-and microscales is developed by combining a batch of surface modification with atomically thin graphene. The robust superlubric interface across nano-and microscales between hydrophobic 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) self-assembly monolayers (SAMs) and graphene was achieved under high relative humidity, sliding speed, and contact pressure. The superlubric mechanisms at the interface of FDTS/graphene could be attributed to the following at different scales: the hydrophobicity of FDTS SAMs and graphene preventing the capillary interaction of the interfacial friction under high relative humidity; the high elastic modulus of graphene leading to small interfacial contact area; the compressing and orientating of FDTS SAMs decreasing interfacial shear strength under high contact pressure; the surface modification of FDTS molecules reducing the interfacial potential barriers when sliding on the atomically thin graphene. The robust superlubric interface across nano-and microscales reducing the friction at the complicated interfaces with asperities at different scales and improving the performance and durability have great potentials in the field of micro/nano mechanical systems.

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.