The effects of disordered edge and vanishing friction in microscale structural superlubric graphite contact.

Structural superlubricity (SSL), a state of ultralow friction and no wear between two solid surfaces in contact, offers a fundamental solution for reducing friction and wear. Recent studies find that the edge pinning of SSL contact dominates the friction. However, its nature remains mysterious due to the lack of direct characterizations on atomic scale. Here, for microscale graphite mesa, we unambiguously reveal the atomic structure and chemical composition of the disordered edge. The friction stress for each contact condition, namely, edge/edge, edge/surface, and surface/surface contacts are quantified, with the ratio being 10:10:1. The mechanism is revealed by all-atom molecular dynamic simulations, which reproduce the measured friction qualitatively. Inspired by such understanding, through fabricating SiN caps with tensile stress, we further eliminate the friction caused by the edges through disengaging the edges from the substrate. As a result, an SSL contact with ultralow friction stress of 0.1 kPa or lower is achieved directly.