More than thirty years ago, it was theoretically predicted that friction for incommensurate contacts between atomically smooth, infinite, crystalline materials (e.g., graphite, ${\mathrm{MoS}}_2$) 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 $10\mu \mathrm{m}$ 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.

中文翻译：

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超润滑石墨触头的摩擦起因

三十多年前，从理论上预测，原子光滑，无限的结晶材料（例如，石墨， ${钼}_2$）在低速极限下消失，这种相应的状态称为结构超润滑性（SSL）。但是，对这种预测的实验验证遇到了挑战，因为实际接触始终具有有限的大小，并且总摩擦不仅由位于接触区域内的原子产生，而且还由在接触边缘处的原子产生，这可以贡献有限的量即使不相称的区域没有摩擦力 在这里，我们首次使用新颖的方法报告了这些贡献的解耦。在环境条件下从石墨的纳米级到微米级不等接触获得的结果证明，内部原子的平均摩擦贡献不大于${10}^{-4}$在边缘的原子 相应地，总摩擦力由接触边缘之间的摩擦力决定，直至达到$10\mu \mathrm{米}$在横向尺寸。我们讨论了SSL接触中观察到的摩擦的物理机制，并为合理设计大型SSL接触提供了指导。