Ultralow Friction Study on Graphene Based on Multi-Size Microsphere Probe

Micro/nano devices have extremely high specific surface area due to size reduction, and surface adhesion and friction caused by the scale effect are prominent. Ultralow friction is an attractive method which could reduce friction and is the reliability of mechanical parts around various applications. Ultralow friction could be obtained when a rigid single-crystalline surface slide over another single-crystalline surface in dry incommensurate contact. Graphite is known as an excellent layered solid lubricant, which could be employed as an ultrathin lubricating layer on micro/nano devices surface. Here, the graphene samples were prepared by mechanical exfoliation method using Scotch adhesive tape and then deposited onto the clean Si substrate with about 300 nm SiO₂ layer in a super-clean workbench. The thickness and quality of the graphene were determined by atomic force microscope (AFM) and Raman spectra. Multi-size microsphere probes were prepared based on a homemade micromanipulator system to manipulate the microsphere and tip-less cantilever. A scanning electron microscope (SEM) was employed to image the multi-size microsphere probes. The friction measurements and adhesion measurements were carried out by atomic force microscope (AFM) in an ambient atmosphere. The normal spring constants and lateral detector sensitivities of the probes were calibrated by the noncontact method and anti-magnetic levitation method, respectively. Thus, the adhesion and friction could be studied quantitatively. The influences of scale effect on the adhesion and friction on graphene surface were investigated in this paper. The frictional tests were carried out to investigate the behavior of ultra-thin solid lubricant graphene at micro- and nanoscale. In this paper, the adhesion and friction on the graphene surface were acquired by using multi-size microsphere probes based on atomic force microscopy (AFM). The adhesion and friction increased with the increase of the tip size when the materials of the contact pair were the same according to the result of the test. The large adhesion and friction force between the tip and graphene surface would contribute to the transfer of graphene flake to the tip of the microsphere probes. The reduction of friction under high load, strengthening effect in atomic scale stick-slip motion and the PT model demonstrated the local interlayer sliding between graphene layers and the change of stiffness of the contact system during the friction measurement. These abnormal phenomena could be observed during the process of graphene transferring to the tip of microsphere probes. The adhesion and friction were strongly influenced by the materials of the contact pair. Once the graphene flake was attached to the surface of the tip successfully, the materials of contact pair were changed from SiO₂/graphene to graphene/graphene. Amazingly, the adhesion, friction and the friction coefficient of graphene/graphene contact pair were far lower than normal levels, and the friction coefficient was as low as 0.001, achieving ultra-low friction according to the classification of friction coefficient. Graphene/graphene contact pair would offer theoretical and experimental support for obtaining ultra-low friction in practical applications. The tribological system used in this study also offers an experimental platform to study the interlayer friction and adhesion between various two-dimensional materials, which could be used for searching for suitable contact pair materials at multiple scales under different conditions.