Structural superlubricity with a contaminant-rich interface

Structural superlubricity, which results from the cancellation of lateral force between contacted smooth solid surfaces with weak interaction, achieves ultralow friction and wear, inspiring disruptive innovations in applications on microscale. It is generally believed that this phenomenon requires the contact interface being ultra-clean, ensured by either a clean environment or mechanical cleaning of the contact. However, in this paper, the experimental observation of structural superlubricity with a contaminant-rich interface is unambiguously observed. After intentionally introducing airborne contaminants into microscale structural superlubric incommensurate graphite contacts, we find that the important features of structural superlubricity, the nearly-zero friction coefficient, ultralow friction, and symmetry in friction, are all well preserved. Moreover, contaminants are found to reduce the friction of a commensurate contact but increase the friction of an incommensurate contact, i.e., playing opposite roles. A theoretical model is proposed to fully grasp the contaminant effect. Our results bring challenges to the current understanding about the mechanism of structural superlubricity and provide solid support for its application under practical conditions.