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. 2019 Dec 3;116(49):24452-24456.
doi: 10.1073/pnas.1907947116. Epub 2019 Oct 28.

Tuning Friction to a Superlubric State via In-Plane Straining

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Free PMC article

Tuning Friction to a Superlubric State via In-Plane Straining

Shuai Zhang et al. Proc Natl Acad Sci U S A. .
Free PMC article

Erratum in

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.

Keywords: energy dissipation; friction; graphene; strain engineering; superlubricity.

Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Preparation and characterization of graphene with varying strains. (A) A schematic showing the topography and friction measurements on suspended graphene with different strains. (B) Typical 3D topographic images of pressurized monolayer graphene bubbles with different strains. The color represents the height. The heights of the bubbles are magnified (not to the ratio of the lateral size) to better illustrate the bulging. (C) Raman spectra of the suspended graphene with varying strains, measured at the center region of the graphene bubbles. (D) Friction versus normal force data measured on the supported graphene and the suspended graphene. The error bar represents the SD of the repeated measurements under the same normal load. (E) Variation of the coefficients of friction with the strain of graphene, acquired by fitting linearly the mean friction vs. normal force curves in D.
Fig. 2.
Fig. 2.
Atomically resolved friction curves and adhesion tests on graphene with varying strains. (A) Atomic-scale stick–slip curves on graphene with different strains, measured under a normal load of 1.4 nN and a sliding velocity of 30 nm/s. (B) Energy dissipation and the peak lateral force, calculated from the stick–slip curves in A. (C) Normalized adhesion obtained by normalizing the average adhesion force of the suspended graphene by the adhesion force on the supported graphene. Error bars represent the SD. (Insets) Adhesion maps of the suspended graphene with varying strains. (Scale bar, 2 µm.)
Fig. 3.
Fig. 3.
Atomistic simulations of friction on graphene with different in-plane strains. (A) Averaged peak friction force of the atomic stick–slip motion at different strain states. Minus and positive signs in strain correspond to the compressive and the tensile states, respectively. (Inset) Model setup and a typical stick–slip trace of the suspended graphene with −0.2% biaxial strain. (B) Variation of kurtosis with different applied strain. (Inset) Distribution of interfacial friction force when the lateral force reaches the local peak value. The color is coded according to the atomic-level friction force fi. The atoms with positive values (red) provide the pushing force to tip sliding, whereas those with negative values (blue) act as the pinning sites. (C and D) Schematic illustrations of the contact interfaces between the tip and (C) the relaxed graphene and (D) the stretched graphene. The relaxed graphene shows a better flexibility and could readjust its configuration to offer better pinning capability.
Fig. 4.
Fig. 4.
Friction modulation with cyclic strains. (A) Friction versus normal force data measured on the suspended graphene with cyclic strains. Error bar represents the SD of the repeated measurements under the same normal load. (B) COF and the strain values of graphene during cyclic loading. The coefficient of friction were calculated by performing a linear fit of the mean friction vs. normal force curves in A.

Comment in

  • Tunable superlubricity of 2-dimensional materials.
    Bonn D, Frenken J. Bonn D, et al. Proc Natl Acad Sci U S A. 2019 Dec 3;116(49):24386-24387. doi: 10.1073/pnas.1918084116. Epub 2019 Nov 13. Proc Natl Acad Sci U S A. 2019. PMID: 31723046 Free PMC article. No abstract available.

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  • Tunable superlubricity of 2-dimensional materials.
    Bonn D, Frenken J. Bonn D, et al. Proc Natl Acad Sci U S A. 2019 Dec 3;116(49):24386-24387. doi: 10.1073/pnas.1918084116. Epub 2019 Nov 13. Proc Natl Acad Sci U S A. 2019. PMID: 31723046 Free PMC article. No abstract available.

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