UC Merced

Structural superlubricity is a physical state where friction between two surfaces in relative motion nearly vanishes, in the absence of any lubricants, purely as a result of structural incommensurability at the interface. Research on this intriguing phenomenon has accelerated rapidly in the past decade, with the discovery and investigation of new superlubric material systems by a few research groups around the world. These research advances have bolstered hopes for structural superlubricity to be applied in model mechanical systems, potentially revolutionizing their efficiency and longevity. However, many open questions remain regarding the physical limitations of structural superlubricity. In this thesis, we address the robustness of structural superlubricity with respect to rotations, environmental contamination, and sliding speed. Our fundamental studies employ atomic force microscopy (AFM) to manipulate gold nano-islands on graphite under various conditions. In particular, we describe a novel lock-in-amplifier-based manipulation technique that allows for precise displacement of nano-islands while significantly increasing the reliability of acquired data. Results show that friction forces remain ultra-low (under 1 nN) at gold-graphite interfaces even under contaminated conditions, at high speeds, and with significant rotations involved. In addition, we observe intriguing effects, such as spontaneous jumps in friction between friction “branches” as well as aging, which can be explained by the effect of environmental adsorbates depending on the coverage. Our findings contribute towards forming a fundamental understanding of structural superlubricity and its potential applicability in practical mechanical systems.