In this work, we proposed a method for coating the whole surfaces of bearing balls uniformly by carbon film with a rotatable ball clamp. We studied the carbon/carbon friction with a self-designed current- carrying ball bearing friction test system. A notable and instant friction force drop of 28% and significant carbon film wear alleviation were found when currents were applied. By using TEM-, SEM-, and EDS-analysis, special carbon stacks with a mixture of large wear particles and oxide were found in the wear areas under current applied condition. We elucidated the current-carrying friction mechanisms as follows: (1) wear particles formation; (2) wear particles charged by tribomicroplasma; (3) formation of surface passivated carbon stacks under electric force; (4) sliding between passivated carbon surfaces. This work may facilitate the development of novel solid-lubricated ball bearings and lay some foundations for current-carrying rolling friction.

Carbon films with two different kinds of sp² nanocrystallited structure were investigated to study the stick–slip friction with the in-situ and ex-situ tests. In-situ transmission electron microscope (TEM) observation and nanofriction tests revealed that the origins of stick and slip varied with shear stress and film deformation. At the stick stage, shear stress gradually increased with the contact strengthened until reached the shear strength to break the interfacial adhesion; at the slip stage, the shear stress decreased and accompanied with film deformation. During the sliding process, adhesive deformation resulted in the large stick–slip step while ploughing deformation led to a smoother step. Ex-situ nanofriction tests on a series of sp² nanocrystallited carbon films with different irradiation energies showed the expected sliding behavior with the in-situ results. This study first clarified the mechanism of stick–slip friction with the in-situ TEM observation, which plays the important role for the micro and nano application of sp² nanocrystallited carbon films.

sp² nanocrystallited carbon films with large nanocrystallite sizes, smooth surfaces, and relative high hardness were prepared with different ion irradiation densities regulated with the substrate magnetic coil current in an electron cyclotron resonance plasma sputtering system. Their multiscale frictional behaviors were investigated with macro pin-on-disk tribo-tests and micro nanoscratch tests. The results revealed that, at an ion irradiation density of 16 mA/cm², sp² nanocrystallited carbon film exhibits the lowest friction coefficient and good wear resistant properties at both the macroscale and microscale. The film sliding against a Si₃N₄ ball under a contact pressure of 0.57 GPa exhibited a low friction coefficient of 0.09 and a long wear life at the macroscale. Furthermore, the film sliding against a diamond tip under a contact pressure of 4.9 GPa exhibited a stable low friction coefficient of 0.08 with a shallow scratch depth at the microscale. It is suggested that sp² nanocrystallites affect the frictional behaviors in the cases described differently. At the macroscale, the contact interface via the small real contact area and the sp² nanocrystallited transfer layer dominated the frictional behavior, while the sp² nanocrystallited structure in the film with low shear strength and high plastic resistivity, as well as the smooth surface morphology, decided the steady low nanoscratch properties at the microscale. These findings expand multiscale tribological applications of sp² nanocrystallited carbon films.