The nature of edge pinning and vanishing friction in microscale structural superlubric graphite contact

Abstract

Abstract 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, especially for graphite, one of the most widely used materials for SSL. Here, for microscale graphite mesa, with detailed characterizations using atomic force microscopy, friction force microscopy, focused ion beam, high-resolution transmission electron microscope, and X-ray photoelectron spectroscopy, 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 104:103:1. The mechanism is revealed by full-atomic molecular dynamic simulations, which reproduce the measured friction quantitatively. Inspired by such understanding, through fabricating SixNy 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. Such a vanishing friction is 1-2 orders lower than all the values ever reported and approaches the theoretical limit of friction for structural superlubric contact.