Nanomechanical devices can operate at much higher speeds than their macroscopic analogues, due to low inertia. For example, peak speeds >100m/s have been predicted for carbon nanotube devices. This stimulates our interest in the atomic-scale physics of friction at high velocity. Here we study a model nanosystem consisting of a graphene flake moving freely on a graphite substrate at >100m/s. Using molecular dynamics we discover that ultra-low friction, or superlubricity, is punctuated by high-friction transients as the flake rotates through successive crystallographic alignments with the substrate. We term this phenomenon frictional scattering and show that it is mathematically analogous to Bragg scattering. We also show that frictional scattering can be eliminated by using graphitic nanoribbons as frictional waveguides to constrain the flake rotation, thus achieving persistent superlubricity. Finally, we propose an experimental method to study nanoscale high-velocity friction. These results may guide the design of efficient high-frequency nanomechanical devices.
The Potential of Ti$_3$C$_2$T$_X$ Nano-sheets (MXenes) for Nanoscale Solid Lubrication Revealed by Friction Force Microscopy
Ti $_3$ C $_2$ T $_X$纳米片（MXene）是一类新兴的二维材料，具有在从纳米到宏观的长度尺度上用于能量存储、催化和摩擦电应用的突出潜力。尽管对这类新材料的兴趣迅速增加，但它们的纳米级摩擦特性，特别是它们在纳米级上的固体润滑潜力，尚未得到详细研究。在这个简短的通信中，我们提出了第一个结果MXene的纳米级摩擦特性，在一个孤立的Ti $_3$ C $_2$ T $_x$纳米片上通过摩擦力显微镜沉积在二氧化硅衬底上获得的摩擦地图的形式。我们的实验表明，少层Ti $_3$ C $_2$ T $_x$纳米片确实在纳米级上充当固体润滑剂，减少二氧化硅基底上的摩擦，尽管不如少层石墨烯有效。本文报道的结果为进一步研究Ti $_3$ C $_2$ T $_X$（MXene）纳米片实现的纳米级固体润滑铺平了道路。
