Observation of Microscale Superlubricity in Graphite

Abstract

Through experimental study, we reveal superlubricity as the mechanism of self-retracting motion of micrometer sized graphite flakes on graphite platforms by correlating respectively the lock-up or self-retraction states with the commensurate or incommensurate contacts. We show that the scale-dependent loss of self-retractability is caused by generation of contact interfacial defects.A HOPG structure is also proposed to understand our experimental observations, particularly in term of the polycrystal structure. The realisation of the superlubricity in micrometer scale in our experiments will have impact in the design and fabrication of micro/nanoelectromechanical systems based on graphitic materials. Nano-mechanical devices based on van de Waals forces in multi-walled carbon nanotubes (MWCNT) and HOPG (i.e., multilayered graphenes) have attracted intensive experimental and theoretical studies, owing to their superior properties, e.g., the nearly `freely' motion of inner shell inside the outer shell of a MWCNT [1,2,3], the MWCNT based oscillator with GHz resonance frequency [4], the extremely fast self-retraction motion of graphite flakes in HOPG islands [5] and so on. The role of the interlayer van de Waals interaction in driving the motion of such van de Waals devices has been well recognised and studied by various theoretical analysis and molecular dynamic simulations [3,4,6,7]. On the other hand, the interlayer van de Waals interactions also leads to potential corrugations due to the periodic atomic structures of the graphene layers, and in turn results in the interlayer friction/resistance force. The role of such friction force in the van de Waals micro/nano-mechanical devices, however, is largely overlooked and there is no experimental studies in micrometer scale up to now (except few scanning probe microscope (SPM) experiments with nanoscale sharp tip scanning on top of a graphene [8,9,10,11]). In this Letter, we will reveal the decisive role of such friction force in the van de Waals nano-mechanical devices. Our resultsshow that the superlubricity, as a result of the incommensurate contact of different graphene layers, is the necessary condition for the self-driven motion of CNT/graphene based micro/nanomechanical devices.Superlubricity is a phenomenon that friction force vanish or almost vanish when two solid surfaces are sliding over each other [12], and has attracted many attentions [13,14,15,16] since the introduction of the concept [17]. The structural incommensurate between two crystalline solid

Superlubricity of Graphite Sliding against Graphene Nanoflake under Ultrahigh Contact Pressure

“…Superlow friction would subsequently be achieved because of the extremely weak interaction and easily sliding between the transferred GNFs and graphite in the incommensurate contact 6, 7, 10, 19. According to the previous studies on the superlubricity of graphite, the superlow friction would disappear at two certain sliding orientations with an angle difference of 60° under small loads 6, 7. However, we did not observe this sliding orientation dependency of the superlubricity under the high contact pressure.…”

“…In this case, the GNFs may form the incommensurate sliding with the original graphite substrate because of their similar lattices. Superlow friction would subsequently be achieved because of the extremely weak interaction and easily sliding between the transferred GNFs and graphite in the incommensurate contact 6, 7, 10, 19. According to the previous studies on the superlubricity of graphite, the superlow friction would disappear at two certain sliding orientations with an angle difference of 60° under small loads 6, 7.…”

An atomistic investigation of the effect of strain on frictional properties of suspended graphene

“…Graphene, a two-dimensional material consisting of sp 2 -hybridized carbon atoms, has been attracting great interest for its outstanding electronic, 1 thermal 2 and mechanical 3 properties. Meanwhile, the superlubricity of graphite has been achieved at both nano-and microscales, [4][5][6] and it may be an promising solid lubricant for M/NEMS. 7,8 Therefore, an in-depth understand of the tribological properties of graphene is required for its engineering implementation in future M/NEMS.…”

Current trends in the physics of nanoscale friction

“…Structural superlubricity over micrometer length scale is reported on HOPG graphite [62,63]. In these experiments, the cap of micron-diameter graphite pillars is dragged laterally, producing a shear movement of the upper part relative to the base.…”