Frictional aging in microscale structural superlubric graphite contacts

Devices based on structural superlubricity (SSL) technology are characterized by energy efficiency, low wear, longevity, and superior performance. However, the ubiquitous phenomenon of frictional aging, where static friction increases with contact time, can lead to issues such as startup failure and return-to-origin failure in SSL devices. To date, frictional aging in SSL systems has not been studied. This paper delves into the frictional aging behavior of SSL graphite systems. Our research reveals that microscopic graphite flakes in contact with various substrate materials, including silicon, exhibit significant frictional aging from 1 to 100 s of contact time. The static friction of graphite-silicon pairs logarithmically increases with contact time and remains constant or decreases with load. Furthermore, by comparing static friction values of graphite flakes in contact with small silicon mesas and silicon flat area, we decoupled the contributions to the total static friction from the edges and in-plane area of the contact and found that in-plane contributions could be close to or even larger than edge contributions, challenging the traditional belief that friction of a graphite flake is dominated by the edges. These results enrich the basic understandings of structural superlubricity, lay the foundation for developing techniques to effectively mitigate edge effects, and provide important references for the design of future SSL devices.