The fundamental questions of how lubricant molecules organize into a layered structure under nanometers confinement and what is the interplay between layering and friction are still not well answered in the field of nanotribology. While the phase transition of lubricants during a squeeze-out process under compression is a long-standing controversial debate (i.e., liquid-like to solid-like phase transition versus amorphous glass-like transition), recent different interpretations to the stick–slip friction of lubricants in boundary lubrication present new challenges in this field. We carry out molecular dynamics simulations of a model lubricant film (cyclohexane) confined between molecularly smooth surfaces (mica)––a prototypical model system studied in surface force apparatus or surface force balance experiments. Through fully atomistic simulations, we find that repulsive force between two solid surfaces starts at about seven lubricant layers (n = 7) and the lubricant film undergoes a sudden liquid-like to solid-like phase transition at n < 6 monolayers thickness. Shear of solidified lubricant films at three- or four-monolayer thickness results in stick–slip friction. The sliding friction simulation shows that instead of shear melting of the film during the slip of the surface, boundary slips at solid–lubricant interfaces happen, while the solidified structure of the lubricant film is well maintained during repeated stick–slip friction cycles. Moreover, no dilation of the lubricant film during the slip is observed, which is surprisingly consistent with recent surface force balance experimental measurements.

在纳米摩擦学领域，关于润滑剂分子如何在纳米范围内组织成层状结构以及层与摩擦之间的相互作用是什么的基本问题仍然没有得到很好的回答。尽管润滑剂在挤压过程中的相变是一个长期存在争议的争论（例如，从液态到固态的相变与无定形玻璃状的转变），但对粘滑摩擦的最新解释却不同边界润滑中润滑剂的使用在该领域提出了新的挑战。我们对限制在分子光滑表面（云母）之间的模型润滑膜（环己烷）进行了分子动力学模拟-在表面力装置或表面力平衡实验中研究的原型模型系统。通过完全原子模拟，n = 7），并且润滑膜在n <6单层厚度下经历突然的液体状到固体状的相变。固化的润滑剂膜在三层或四层的剪切作用下会产生粘滑摩擦。滑动摩擦模拟表明，在表面滑移过程中，薄膜没有发生剪切熔融，而是在固体-润滑剂界面处发生边界滑移，而在反复的粘滑摩擦过程中，润滑膜的固化结构得到了很好的维持。此外，在滑动过程中未观察到润滑膜的膨胀，这与最近的表面力平衡实验测量令人惊讶地一致。