The evolving quality of frictional contact with graphene
The evolving quality of frictional contact with graphene
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石墨烯摩擦接触质量的不断变化
DOI：
10.1038/nature20135
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发表时间：
2016-11-24
期刊：
影响因子：
64.8
通讯作者：
Li, Ju
中科院分区：
文献类型：
--
作者：
Li, Suzhi;Li, Qunyang;Li, Ju
石墨和其他层状材料用作大型金属滑动部件和高压触点的干润滑剂。实验表明，单层石墨烯比多层石墨烯和石墨具有更高的摩擦力，并且这种摩擦力随着持续滑动而增加，但其背后的机制仍然存在争议。长期以来，人们一直推测两个粗糙体之间的真正接触面积控制着界面摩擦。真正的接触面积，例如由原子间作用力范围内的原子数量来定义，是很难直接可视化的，但它表征了接触的数量。然而，越来越多的证据表明，对于给定的一对材料，接触的质量可以改变，这也会强烈影响界面摩擦，，，，，。最近，人们发现二维材料的摩擦行为表现出与传统块状材料不同的，，，，，特征。这包括上面提到的发现，对于几层二维材料，静摩擦力在达到恒定值之前的几个初始原子周期内逐渐增强。这种瞬态行为，以及与之相关的稳态摩擦增强，随着二维层数的增加而减少，并且只有当二维材料松散地粘附在衬底上时才会观察到。任何模拟都没有捕捉到这种依赖于层的瞬态现象。在这里，利用原子模拟，我们重现了石墨烯上层相关摩擦和瞬态摩擦强化的实验观察。原子力分析表明，静摩擦的演变是更薄、约束更少的石墨烯重新调整其结构的自然趋势的一种表现，这是其更大灵活性的直接结果。也就是说，尖端原子变得更强地固定，并且在粘滑行为中表现出更大的同步性。当原子尺度接触的数量（真正的接触面积）发生变化时，石墨烯摩擦滑动的质量（在这种情况下，单个原子的局部钉住状态和整体可通约性）也在变化。此外，这种效果可以通过预皱来调整。不断变化的接触质量对于解释构型柔性界面的时变摩擦是至关重要的。
Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction. The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction,,,,,. Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits,,,,,unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases, and was observed only when the two-dimensional material was loosely adhering to a substrate. This layer-dependent transient phenomenon has not been captured by any simulations,. Here, using atomistic simulations, we reproduce the experimental observations of layer-dependent friction and transient frictional strengthening on graphene. Atomic force analysis reveals that the evolution of static friction is a manifestation of the natural tendency for thinner and less-constrained graphene to re-adjust its configuration as a direct consequence of its greater flexibility. That is, the tip atoms become more strongly pinned, and show greater synchrony in their stick–slip behaviour. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. Moreover, the effects can be tuned by pre-wrinkling. The evolving contact quality is critical for explaining the time-dependent friction of configurationally flexible interfaces.