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Edited by Steve Granick, Institute for Basic Science, Ulju-gun, Ulsan, South Korea, and approved October 10, 2019 (received for review May 7, 2019)
Significance
It has long been conjectured that the steady-state friction of an elastic contact is an inherent property of the sliding interface, which depends on the contacting materials and typically cannot be altered on demand. In this work, we demonstrate that the friction on a graphene sheet can be actively modulated by in-plane straining. In particular, by applying a tensile strain (up to 0.60%), we are able to further reduce the surface friction of monolayer graphene to a superlubricating state. This unusual effect is attributed to the changes in the atomic contact quality of the sliding interface. Our work demonstrates the very example, where the atomic-scale interfacial interactions can be directly regulated via macroscopic operations.
Abstract
Controlling, and in many cases minimizing, friction is a goal that has long been pursued in history. From the classic Amontons–Coulomb law to the recent nanoscale experiments, the steady-state friction is found to be an inherent property of a sliding interface, which typically cannot be altered on demand. In this work, we show that the friction on a graphene sheet can be tuned reversibly by simple mechanical straining. In particular, by applying a tensile strain (up to 0.60%), we are able to achieve a superlubric state (coefficient of friction nearly 0.001) on a suspended graphene. Our atomistic simulations together with atomically resolved friction images reveal that the in-plane strain effectively modulates the flexibility of graphene. Consequently, the local pinning capability of the contact interface is changed, resulting in the unusual strain-dependent frictional behavior. This work demonstrates that the deformability of atomic-scale structures can provide an additional channel of regulating the friction of contact interfaces involving configurationally flexible materials.
Footnotes
↵1S.Z., Y.H., and S.L. contributed equally to this work.
- ↵2To whom correspondence may be addressed. Email: qunyang{at}tsinghua.edu.cn.
Author contributions: Q.L. designed research; S.Z., Y.H., S.L., and L.L. performed research; S.Z., Y.H., S.L., L.L., Z.Z., X.-Q.F., and Q.L. analyzed data; and S.Z., Y.H., S.L., L.L., and Q.L. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
Data deposition: All data discussed in the paper will be made available to readers.
See Commentary on page 24386.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1907947116/-/DCSupplemental.
Published under the PNAS license.
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Tuning friction to a superlubric state via in-plane straining
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