Manipulation and Characterization of Submillimeter Shearing Contacts in Graphite by the Micro-Dome Technique.

Manipulation techniques are the key to measuring fundamental properties of layered materials and their monolayers (2D materials) on the micro- and nanoscale as well as a necessity to the solution of relevant existing challenges. An example is the challenge against upscaling structural superlubricity, a phenomenon of near-zero friction and wear in solid contacts. To date, the largest single structural superlubric contact only has a size of a few tens of micrometers, which is achieved on graphite mesa, a system that has shown microscale superlubricity. The first obstacle against extending the contact size is the lack of suitable manipulation techniques. Here, a micro-dome technique is demonstrated on graphite mesas by shearing contacts 2500 times larger in area than previously possible. With this technique, submillimeter graphite mesas are opened, characterized for the first time, and compared to their microscale counterparts. Interfacial structures, which are possibly related to the failure of superlubricity, are observed: commensurate grains, external steps, and carbon aggregates. Furthermore, a proof-of-concept mechanical model is developed to understand how the micro-dome technique works and to predict its limits. Finally, a dual-axis force measuring device is developed and integrated with the micro-dome technique to measure the normal and lateral forces when shearing submillimeter mesas. These results provide a platform technique for future research on structural superlubricity on different scales and manipulation of structures of layered materials in general.

Origin of Friction in Superlubric Graphite Contacts.

More than thirty years ago, it was theoretically predicted that friction for incommensurate contacts between atomically smooth, infinite, crystalline materials (e.g., graphite, MoS_{2}) is vanishing in the low speed limit, and this corresponding state was called structural superlubricity (SSL). However, experimental validation of this prediction has met challenges, since real contacts always have a finite size, and the overall friction arises not only from the atoms located within the contact area, but also from those at the contact edges which can contribute a finite amount of friction even when the incommensurate area does not. Here, we report, using a novel method, the decoupling of these contributions for the first time. The results obtained from nanoscale to microscale incommensurate contacts of graphite under ambient conditions verify that the average frictional contribution of an inner atom is no more than 10^{-4} that of an atom at the edge. Correspondingly, the total friction force is dominated by friction between the contact edges for contacts up to 10 µm in lateral size. We discuss the physical mechanisms of friction observed in SSL contacts, and provide guidelines for the rational design of large-scale SSL contacts.