Gao Hongyu, Ewen James P, Hartkamp Remco, Müser Martin H, Dini Daniele
Department of Materials Science and Engineering, Universität des Saarlandes, 66123 Saarbrücken, Germany.
Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, U.K.
Langmuir. 2021 Feb 23;37(7):2406-2418. doi: 10.1021/acs.langmuir.0c03403. Epub 2021 Feb 5.
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.
表面活性剂分子，即有机摩擦改性剂（OFMs），通常被添加到润滑剂中，以减少滑动表面之间的摩擦和磨损。在宏观实验中，随着OFM分子在滑动表面上的覆盖率增加，摩擦力通常会降低；然而，最近使用尖锐原子力显微镜（AFM）探针进行的纳米级实验表明摩擦力在增加。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OFM单分子层与压痕纳米级粗糙表面之间的动摩擦。为此，我们研究了硬脂酰胺OFMs在具有不同曲率半径的氧化铁表面和二氧化硅AFM探针上的各种覆盖率。我们表明，宏观和纳米级实验中摩擦 - 覆盖率关系的差异是由于后者中的分子犁削。对于我们较小的探针半径，摩擦系数和压痕深度对OFM表面覆盖率都有非单调依赖性，最大值出现在中间覆盖率处。我们通过两种效应（限制和堆积密度）的竞争来解释这种非单调关系，这两种效应是表面覆盖率对OFM单分子层有效刚度的影响。我们还表明，在所研究的速度范围内，动摩擦对滑动速度不太敏感，这表明它起源于不稳定性。实际上，我们发现摩擦力主要源于探针前沿对单分子层的犁削，在那里会产生gauche缺陷，而热耗散大多集中在靠近探针后缘的分子中，在那里链会恢复到更伸展的构象。
