Observation of normal-force-independent superlubricity in mesoscopic graphite contacts

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Observation of normal-force-independent superlubricity in mesoscopic graphite contacts

Cuong Cao Vu, Shoumo Zhang, Michael Urbakh, Qunyang Li, Q.-C. He, and Quanshui Zheng

Phys. Rev. B 94, 081405(R) – Published 5 August 2016

Abstract

We investigate the dependence of friction forces on normal load in incommensurate micrometer-size contacts between atomically smooth single-crystal graphite surfaces under ambient conditions. Our experimental results show that these contacts exhibit superlubricity (superlow friction), which is robust against the application of normal load. The measured friction coefficients are essentially zero and independent of the external normal load up to the maximum pressure of our experiment, 1.67 MPa. The observation of load-independent superlubricity in microscale contacts is a promising result for numerous practical applications.

Received 10 February 2016

DOI:https://doi.org/10.1103/PhysRevB.94.081405

Physics Subject Headings (PhySH)

Friction Lubrication

Graphite

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Cuong Cao Vu^1,2, Shoumo Zhang^2,3, Michael Urbakh⁴, Qunyang Li^2,3,5, Q.-C. He^1,6,*, and Quanshui Zheng^2,3,5,†

¹School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China
²Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China
³Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China
⁴School of Chemistry, Tel Aviv University, 69978 Tel Aviv, Israel
⁵Applied Mechanics Laboratory and State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
⁶Université Paris-Est, Laboratoire de Modélisation et Simulation Multi Echelle, UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Vallée, France

^*Corresponding author: qi-chang.he@u-pem.fr
^†Corresponding author: zhengqs@tsinghua.edu.cn

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Vol. 94, Iss. 8 — 15 August 2016

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Images

Figure 1
Schematic diagram of the experimental setup. (a) An objective lens is coupled to the AFM head to observe the relative movement of the sheared flake with respect to the substrate. Experiments are performed in an ambient condition with 1.5 $μ m / s$ of sliding velocity ( $v$ ) and 0.8 $μ m$ of sliding distance ( $x$ ). The normal ( $N$ ) and lateral ( $F$ ) forces are applied to the top flake through its $Si O_{2}$ cap using the same AFM probe that contacts the central area of the cap [the cross in (d)]. The two force components are measured simultaneously by the AFM. (b), (c) Two optical images obtained for the top flake sheared in the forward and backward directions, respectively. (d), (e) AFM topographic images of the selected sample. In order to generate sufficiently large lateral forces for very low normal loads, we made an indentation in the central area of the $Si O_{2}$ cover. The inset to panel (d) shows the AFM scanning profile (the blue curve) measured along the dashed blue line across the indentation [see (d)]. The profile shows that the width and depth of the indentation are about $1 μ m$ and 10 nm, respectively.
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Figure 2
The lateral force ( $F$ ) and applied normal force ( $N$ ) for a typical mesa sample of $3 μ m \times 3 μ m$ . (a) Lateral force's loop corresponds to forward [ $F^{+}$ (red)] and backward [ $F^{-}$ (blue)] directions. The corresponding normal forces are shown in the left inset. The right inset illustrates the repeatability of the force loops for five cycles. (b) The lateral force—displacement loops measured for various normal forces $N$ up to 15 μN. (c) The dependencies of the friction forces on the normal force measured for five self-retractable mesas of the sizes $3 μ m \times 3 μ m$ . The friction force ( $F_{fr}$ ) is estimated as the mean value of the $\frac{1}{2} (F^{+} - F^{-})$ over the sliding displacement range $300 nm < x < 800 nm$ . (d) Variations of friction forces with normal load measured for two randomly chosen graphite mesas of the same size $3 μ m \times 3 μ m$ but not showing SRM. Compared with the $N$ -independent friction observed for the superlubric contacts, as shown in (c), here we found the usual linear increase of friction with $N$ , and the friction forces are one order of magnitude higher than for the superlubric state.
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Figure 3
Effect of heating on friction forces measured with the five selected mesas. (a) Friction forces measured at room condition according to four different protocols: (1) before heating (navy squares), (2) after heating at $150^{\circ} C$ for 1 h and then exposing to room temperature for $10 - 30$ min (pink up triangle), (3) exposed to air for 14 days (purple diamonds), and (4) after reheating at $200^{\circ} C$ for 1 h and then exposing to room temperature for 30 min (violet right triangle). (b) Variation of friction forces with gradually increasing normal force observed for the samples, which have been treated according to the protocols (1)–(4), respectively. The error bars show the standard deviation of measurements for each plot.
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