Novel mechanism revealed by a research group from Tsinghua for tuning friction of two-dimensional materials to a nearly superlubric state
Controlling, and in many cases minimizing, friction is a goal that has long been pursued in history. Prof. Qunyang Li and Prof. Xi-Qiao Feng from the Department of Engineering Mechanics at Tsinghua University, together with their collaborators, recently discovered a novel way of tuning friction via simple mechanical stretching. This work was published online as a research article titled “Tuning friction to a superlubric state via in-plane straining” in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) on October 28th. The research team experimentally demonstrated that friction of a graphene sheet can be actively and reversibly modulated by in-plane straining. In particular, by applying a tensile strain (up to ~0.6%), a nearly frictionless (or known as “superlubric”) state can be achieved on the surface of the stretched graphene.
It has been widely speculated in modern tribological studies that friction would scale with the true contact area for clean, elastic and adhesion-dominated sliding interfaces. The pre-factor of the scaling relation, often referred to as the friction shear stress, is determined by the atomic-scale features of the sliding interface, which typically cannot be altered on demand. Tuning friction in a dynamic and controllable way has been a great challenge in the community.
Figure 1. (A) A schematic showing the friction measurement on suspended graphene with different strains. (B) Variation of the coefficients of friction with the strain of graphene. (C)-(D) Schematic illustrations of the contact interfaces between the tip and (C) a relaxed graphene sheet and (D) a stretched graphene sheet. The relaxed graphene shows a better flexibility and could readjust its configuration to offer better pinning capability.
Profs. Qunyang Li and Xi-Qiao Feng, together with their collaborators, used a bulged micro-bubble to impose different magnitudes of strain on graphene samples. The research team found that the friction of the suspended graphene significantly decreased as the sample was stretched. More dramatically, when the in-plane tensile strain reached ~0.6%, a superlubric state (with an effective coefficient of friction nearly 0.001) could be observed. This unusual friction modulation effect by strain was confirmed to be completely reversible regardless of choice of specific probe materials. Using atomistic simulation and lattice-resolved friction measurements, the research team revealed that the in-plane strain could effectively modulate the flexibility of graphene, resulting in changes in local atomic pinning capability along the contact interface thereby indicating different frictional resistance.
Figure 2. A schematic of a nanoscale cluster sliding on a nearly frictionless stretched graphene sheet.
This work demonstrates, for the first time, that interfacial atomic-scale contact quality and surface friction can be rationally controlled by applying strains on a macroscale, offering a dynamic and reversible means for modulating the frictional behavior of 2D materials. The experimental results directly verify the hypothesis that contact quality can act as a dominant factor for regulating friction, which was proposed in the previous work by Prof. Qunyang Li’s group (The evolving quality of frictional contact with graphene) (Nature, 539, 2016). The present work is another breakthrough by Prof. Li's research group in the field of the micro-mechanisms of friction in addition to their pioneering work on thickness-dependent frictional behaviors of ultra-thin two-dimensional material (Science, 328, 2010), the mechanism of bonding effect on interfacial friction evolution (Nature, 480, 2011), the negative friction coefficient on modified graphite surface (Nat. Mat., 11, 2012) and the robust ultra-low friction state of graphene via moiré superlattice confinement (Nat. Comm., 7, 2016).
This work was done by Prof. Qunyang Li and Prof. Xi-Qiao Feng from the Department of Engineering Mechanics at Tsinghua University through collaboration with Dr. Luqi Liu, Dr. Zhong Zhang from the National Center for Nanoscience and Technology, and Prof. Suzhi Li from Xi’an Jiaotong University. Mr. Shuai Zhang, a doctoral student at Tsinghua University, Mr. Yuan Hou, a doctoral student at the National Center for Nanoscience and Technology, and Dr. Suzhi Li, a professor at Xi’an Jiaotong University, are the co-first authors. Prof. Qunyang Li is the corresponding author of this work. The work was supported by NSFC, 973 Program, and the State Key Laboratory of Tribology of Tsinghua University.
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