Observation of High-Speed Microscale Superlubricity in Graphite

A sheared microscopic graphite mesa retracts spontaneously to minimize interfacial energy. Using an optical knife-edge technique, we report first measurements of the speeds of such self-retracting motion (SRM) from the $\mathrm{mm}/\mathrm{s}$ range at room temperature to $25\mathrm{m}/\mathrm{s}$ at $235°\mathrm{C}$. This remarkably high speed is comparable with the upper theoretical limit found for sliding interfaces exhibiting structural superlubricity. We observe a strong temperature dependence of SRM speed which is consistent with a thermally activated mechanism of translational motion that involves successive pinning and depinning events at interfacial defects. The activation energy for depinning is estimated to be 0.1–1 eV.

Figure 1

(a) Structure of the graphite mesa and the scheme of the manipulation process; (b) shearing and retraction under optical microscope; and (c) experimental setup used for optical knife-edge method.

Figure 2

(a) Two typical displacement vs time curves showing SRM of a graphite mesa at $26°\mathrm{C}$ and $40°\mathrm{C}$, respectively. The curves are shifted slightly for comparison. (b) The displacement vs time curves for a mesa showing a maximum SRM speed of $25\mathrm{m}/\mathrm{s}$. Several SRMs for this mesa are shown, to demonstrate the reproducibility of the process. The speeds obtained from fitting the linearly rising section of the curve are $21\mathrm{m}/\mathrm{s}$, $25\mathrm{m}/\mathrm{s}$, $17\mathrm{m}/\mathrm{s}$, and $20\mathrm{m}/\mathrm{s}$ from left to right.

Figure 3

(a) Temperature dependence of SRM speed, $V_m$, for mesas from two batches. The filled triangles and squares present measurements performed for one mesa from batch 1 and 2, correspondingly. The open triangles show the speeds obtained for 20 other mesas from batch 1. Similarly, the open squares display the speeds for 12 mesas from batch 2. (b) Results obtained for 8 mesas from batch 2 after oxidation at high temperature are indicated by green diamonds. Data for batch 2 before oxidation are shown for comparison. (c) Linear fits of $\ln (V_m)$ ($\mathrm{m}/\mathrm{s}$) to $1/k_{B}T$ for the data corresponding to the filled triangles and squares in (a).