Interlayer motion and ultra-low sliding friction in microscale graphite flakes

The interlayer friction force (F-f) of incommensurate graphite is a key material parameter to understand the mechanism of superlubricity and the unexpected properties of carbon-based materials; however, its determination by direct experiments under different ambient temperatures and sliding velocities is yet to be reported. In this letter, we report the direct, accurate experimental measurement of the F-f of microscale incommensurate graphite. The measured F-f per unit area, F-f = 0.015 +/- 0.008 MPa in ambient laboratory conditions, decreases with increasing temperature (22 degrees C <= T <= 185 degrees C), exhibits a logarithmic increase with respect to sliding velocities (40 nm/s <= v <= 12000 nm/s) and remains almost unchanged under various twist angles. At elevated temperatures, Ff can be lower than the resolution of the force sensor, which corresponds to F-f = 0.32 kPa per unit area. This is the lowest value reported for lamellar materials. These experimental measurements assist in understanding the mechanism of microscale superlubricity and such an ultra-low friction force will also introduce a wide range of applications in microelectromechanical systems (MEMS). Copyright (C) EPLA, 2019