Observation of robust macroscale structural superlubricity

TL;DR

This paper demonstrates that macroscale structural superlubricity can be achieved in large graphite contacts, showing near-zero and even negative friction over broad load ranges, challenging previous size limitations.

Contribution

The study provides the first evidence of robust macroscale SSL in graphite, expanding superlubricity from micro- and nanoscale to macroscopic contacts, and shows its generality across layered materials.

Findings

Friction coefficients fluctuate around zero at macroscale graphite contacts.

Friction as low as 10^{-6} observed across a broad load range.

Similar superlubricity behavior found in graphite/MoS2 interfaces.

Abstract

Structural superlubricity (SSL) promises nearly frictionless and wearless sliding, but has until now been considered a special and extreme interfacial phenomenon limited to micro- and nanoscale contacts. Here, we demonstrate robust macroscale SSL within a single sub-millimeter graphite contact. Previously reported near-zero friction coefficients, where friction is nearly independent of normal load, have only been observed at microscale contacts under low loads. Our system expands both contact size and load into the macroscopic regime, exhibiting friction coefficients that fluctuate around zero and reach values as low as $10^{-6}$ across a broad load range from 1 mN to 0.5 N. Negative friction coefficients are also observed. Similar behavior is observed at graphite/MoS$_2$ interfaces, indicating that macroscale SSL is a generalizable phenomenon across flat layered materials. These findings overturn long-standing scaling limitations and establish macroscale SSL as a paradigm-shifting platform for next-generation mechanical and electromechanical systems.