The field of nanotribology has long suffered from the inability to directly observe what takes place at a sliding interface. Although techniques based on atomic force microscopy have identified many friction phenomena at the nanoscale, many interpretative pitfalls still result from the indirect or ex situ characterization of contacting surfaces. Here we combined in situ high-resolution transmission electron microscopy and atomic force microscopy measurements to provide direct real-time observations of atomic-scale interfacial structure during frictional processes and discovered the formation of a loosely packed interfacial layer between two metallic asperities that enabled a low friction under tensile stress. This finding is corroborated by molecular dynamic simulations. The loosely packed interfacial layer became an ordered layer at equilibrium distances under compressive stress, which led to a transition from a low-friction to a dissipative high-friction motion. This work directly unveils a unique role of atomic diffusion in the friction of metallic contacts.

长期以来，纳米摩擦学领域一直无法直接观察滑动界面发生的情况。虽然基于原子力显微镜的技术已经在纳米尺度上识别出许多摩擦现象，但许多解释上的缺陷仍然是由接触表面的间接或非原位表征造成的。在这里，我们将原位高分辨率透射电子显微镜和原子力显微镜测量相结合，以在摩擦过程中提供对原子级界面结构的直接实时观察，并发现两个金属粗糙体之间松散堆积的界面层的形成，使得低拉应力下的摩擦。分子动力学模拟证实了这一发现。在压缩应力下，松散堆积的界面层在平衡距离处变成有序层，这导致从低摩擦运动到耗散高摩擦运动的转变。这项工作直接揭示了原子扩散在金属接触摩擦中的独特作用。