Li Yutao, He Wenjie, He Qi-Chang, Wang Wen
School of Mechanical Engineering, <a href="https://ror.org/00hn7w693">Southwest Jiaotong University</a>, 610031 Chengdu, China.
<a href="https://ror.org/03x42jk29">Université Gustave Eiffel</a>, MSME, CNRS UMR 8208, F-77454 Marne-la-Vallée France.
Phys Rev Lett. 2024 Sep 20;133(12):126202. doi: 10.1103/PhysRevLett.133.126202.
The weak interlayer interaction and strong intralayer bonding in Van der Waals (vdW) layered materials give rise to ultralow friction at incommensurate contact interfaces, a phenomenon known as structural superlubricity. This phenomenon is complicated by the interplay between atomic degrees of freedom, twist angle, and normal force. In this Letter, we exploit naturally occurring cracks in vdW crystals and microfabrication techniques to qualitatively separate the contributions of edge, complete moiré tile, and incomplete moiré rim regions. It is observed that friction from the incomplete moiré rim region, scaling linearly with the rim area, plays a dominant role in determining the twist angle dependence of friction. Interestingly, despite lower friction contributions from complete moirés, friction from a complete moiré tile is independent of moiré size; consequently, friction from the moiré tile scales linearly with the total number of moirés. By integrating the contributions from edge, moiré tile, and incomplete moiré rim, a friction law for vdW heterojunctions is proposed. Finally, friction changes from positively to negatively correlating with normal force, contingent on the suppression of moiré ridge and the dissipation from edge atoms.
范德华（vdW）层状材料中较弱的层间相互作用和较强的层内键合在非共格接触界面处产生了超低摩擦，这一现象被称为结构超润滑。原子自由度、扭转角和法向力之间的相互作用使这一现象变得复杂。在本信函中，我们利用vdW晶体中自然存在的裂纹和微加工技术，定性地分离出边缘、完整莫尔条纹单元和不完整莫尔条纹边缘区域的贡献。据观察，来自不完整莫尔条纹边缘区域的摩擦与边缘面积呈线性比例关系，在决定摩擦的扭转角依赖性方面起主导作用。有趣的是，尽管完整莫尔条纹的摩擦贡献较低，但完整莫尔条纹单元的摩擦与莫尔条纹尺寸无关；因此，莫尔条纹单元的摩擦与莫尔条纹的总数呈线性比例关系。通过整合边缘、莫尔条纹单元和不完整莫尔条纹边缘的贡献，提出了vdW异质结的摩擦定律。最后，摩擦与法向力的相关性从正变为负，这取决于莫尔条纹脊的抑制和边缘原子的耗散。
