Effect of electric current on the microstructural evolution and tribological behavior of highly oriented pyrolytic graphite

The sliding electrical contact behavior on the perpendicular surface of highly oriented pyrolytic graphite has been systematically studied under various electric current densities and environmental atmospheres. The results indicated that the friction coefficient decreased from 0.30 to 0.23 in wet air, and it increased from 0.32 to 0.39 in dry N₂ with increasing electric current density from 0 to 50 A/cm². The opposite tribological behavior induced by electric current between wet air and dry N₂ was attributed to the changes in the microstructure and carbon dangling bonds. Electric current caused the nanostructure defects and carbon dangling bonds to increase in both wet air and dry N₂. In wet air, these carbon dangling bonds were passivated by –H and –OH, which reduced the sliding force and caused the debris parallel to the sliding direction, thereby decreasing the friction coefficient. In dry N₂, however, the carbon dangling bonds could not be passivated for the lack of water molecules. As a result, electric current oriented the debris perpendicular to the sliding direction, and the friction coefficient increased. In addition, it provided a novel strategy for tuning the tribological behavior of carbon materials in situ by adjusting the environmental atmospheres and electric current density.