The anomalous effect of electric field on friction for microscale structural superlubric graphite/Au contact

Abstract

The current-carrying friction characteristics are crucial for the performance of a sliding electrical contact, which plays critical roles in numerous electrical machines and devices. However, these characteristics are influenced by multiple factors such as material surface quality, chemical reactions, and atmospheric environment, leading to a challenge for researchers to comprehensively consider these impacts. Structural superlubricity (SSL), a state of nearly zero friction and no wear between contact solid surfaces, provides an ideal experimental system for these studies. Here, with microscale graphite flakes on atomic-flattened Au surface under applied voltages, we observed two opposite friction phenomena, depending only on whether the edge of graphite flake was in contact with the Au substrate. When in contact the friction force would increase with an increasing voltage, otherwise, the friction force would decrease. Notably, when the voltage was turned off, the friction force quickly recovered to its original level, indicating the absence of wear. Through atmosphere control and molecular dynamics simulations, we revealed the mechanism to be the different roles played by the water molecules confined at the interface or adsorbed near the edges. Our experimental results demonstrate the remarkable tunable and robust frictional properties of SSL under an electrical field, providing an ideal system for the fundamental research of not only sliding electrical contacts, but also novel devices which demand tunable frictions.

Keywords: current-carrying friction; sliding electrical contact; structural superlubricity; water.

Figure 1.

Experimental set-up and samples for current-carrying friction measurements. (a) Experiment set-up. (b) Two contact states of graphite and Au: the ‘edge contact’ (top part) and ‘in-plane contact’ (sub part). (c) The SEM image of the two types of contacts, corresponding to the states illustrated in (b). (d) The topography of the Au surface by AFM scanning. The inset showing the profile of the red line.

Figure 2.

The opposite changing trends of the two types of contact states. (a) The frictional stress loop between graphite and Au, black for ‘edge contact’ and blue for ‘in-plane contact’. (b and c) The variation of friction caused by the applied voltages for ‘edge contact’ and ‘in-plane contact’, respectively. We increased the applied voltages from 0 V to 4 V each by 1 V and kept every voltage for ∼70 sliding cycles, and at last, returned the voltages to 0 V. (d) The changes of friction at each voltage for both contact states. The inset shows the relationship between and voltages, where , and is the friction force at each voltage, is the friction force without voltage.

Figure 3.

SSL characterization of both contacts. (a and b) The relationship between friction and normal load for ‘edge contact’ (a) and ‘in-plane contact’ (b), and obtaining the coefficients of friction (COFs) by linear fitting using least squares. (c and d) Raman spectra of the sliding interfaces of graphite flake for ‘edge contact’ (c) and ‘in-plane contact’ (d). The insets of (c and d) are the optical images of graphite flakes’ sliding interfaces, and the red points represented the characterized positions, corresponding to the lines in (c and d).

Figure 4.

The mechanism of the variation of friction in both contacts. All experiments in this section were conducted under nitrogen environment with different concentrations of water vapor. (a and b) The comparison of friction forces under low humidity (5%) and high humidity (40%) without electrics for ‘edge contact’ and ‘in-plane contact’, respectively. (c and d) The variation of friction forces with different applied voltages under low humidity (5%) and high humidity (40%) for the two types of contacts. (e) The illustration of MD model. (f) The structure factor of the oxygen atoms at the interface for E = 0 and 1 V/nm.