Conductive edge-warping graphite mesas for robust structural superlubricity

Abstract

Structural superlubricity (SSL) refers to a state of ultralow friction and zero wear when two solid surfaces slide against each other. Recent investigations have identified amorphous carbon at the edge of the graphite mesa as the primary source of friction in such SSL systems. Here, the tensile stress of metal thin film is exploited to engineer vertically conductive edge-warping graphite mesas (EWGM). Through this approach, robust SSL performance is realized, demonstrated by sliding an 8 µm side length square EWGM on an atomically smooth Au substrate for 10000 cycles at a constant voltage of 1 mV. In this SSL system, differential friction coefficients lower than 1.5 × 10⁻⁴ are achieved, with static contact resistance between EWGM and Au substrate as low as 28 Ω and sliding contact resistance as low as 32 Ω. Moreover, the EWGM exhibits SSL behavior on polished Si wafer substrates. Furthermore, because of the no-edge contact with the substrate during sliding, friction is independent of the sliding speed of the EWGM. This study presents the first successful fabrication of conductive EWGM. Remarkably, in both EWGM-Au and EWGM-Si SSL systems, the measured frictions are more than one order of magnitude lower than those of ordinary self-retracting graphite mesas with no-edge warping, and no wear is observed during extended current-carrying sliding. Overall, these findings establish a solid groundwork for the future realization of macroscale conductive SSL systems.