Surfactant molecules, known as organic friction modifiers (OFMs), are routinely added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases; however, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we investigate various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. We show that the differences between the friction–coverage relations from macroscale and nanoscale experiments are due to molecular plowing in the latter. For our small tip radii, the friction coefficient and indentation depth both have a nonmonotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. We rationalize the nonmonotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, we find that friction predominately originates from plowing of the monolayers by the leading edge of the tip, where gauche defects are created, while thermal dissipation is mostly localized in molecules toward the trailing edge of the tip, where the chains return to a more extended conformation.

中文翻译：

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表面活性剂单层中尺度依赖的摩擦-覆盖关系和非局部耗散

通常将表面活性剂分子（称为有机摩擦改性剂（OFM））添加到润滑剂中，以减少滑动表面之间的摩擦和磨损。在宏观实验中，摩擦通常随着OFM分子在滑动表面上的覆盖率的增加而减小。但是，最近使用尖锐原子力显微镜（AFM）尖端进行的纳米级实验显示出摩擦力在增加。为了阐明这些相反趋势的起源，我们使用非平衡分子动力学（NEMD）模拟并研究了OFM单层与缩进的纳米级凹凸之间的动摩擦。为此，我们研究了具有不同曲率半径的硬脂酰胺OFM在氧化铁表面和二氧化硅AFM尖端上的各种覆盖范围。我们表明，宏观实验和纳米实验的摩擦覆盖率关系之间的差异是由于后者中的分子耕作造成的。对于我们的小尖端半径，摩擦系数和压痕深度都与OFM表面覆盖率具有非单调依赖性，最大值出现在中间覆盖率处。我们通过改变表面覆盖率对OFM单层的有效刚度的两个影响（约束和堆积密度）的竞争来合理化非单调关系。我们还表明，动摩擦对所研究范围内的滑动速度不是很敏感，表明它是由不稳定性引起的。实际上，我们发现，摩擦主要是由尖端的前边缘对单层的犁削而产生的，在该尖端上形成了纱疵，