Investigating Alkylsilane Monolayer Tribology at a Single-Asperity Contact with Molecular Dynamics Simulation.
Investigating Alkylsilane Monolayer Tribology at a Single-Asperity Contact with Molecular Dynamics Simulation.
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DOI：
10.1021/acs.langmuir.7b02479
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发表时间：
2017-10
期刊：
影响因子：
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通讯作者：
Andrew Z. Summers;C. Iacovella;P. Cummings;C. McCabe
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文献类型：
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作者：
Andrew Z. Summers;C. Iacovella;P. Cummings;C. McCabe
化学吸附单层膜具有良好的纳米级润滑特性的微和纳机电系统（MEMS/NEMS）。先前的研究表明，所观察到的单层涂层表面的摩擦具有很强的依赖于接触的几何形状。具体而言，尖端状的几何形状已被证明渗透到单层膜中，诱导单层链中的缺陷，并导致在剪切过程中的犁机制，这导致更高的摩擦系数（COF）比那些观察到的平面几何形状。在这项工作中，我们使用分子动力学模拟来研究模型二氧化硅单粗糙接触下剪切与单层涂层基板具有不同的膜密度的摩擦学。据观察，较低的单层密度导致减少COF，在平面系统中，COF被发现是几乎独立的单层密度的结果相反。这是由于类似液体的响应剪切，从而较少的缺陷被赋予在单层链从凹凸不平，和链很容易被尖端位移的结果，较高的自由体积。这种过渡的分子犁耕机制表明，液体状薄膜应提供良好的润滑在单粗糙接触。
Chemisorbed monolayer films are known to possess favorable characteristics for nanoscale lubrication of micro- and nanoelectromechanical systems (MEMS/NEMS). Prior studies have shown that the friction observed for monolayer-coated surfaces features a strong dependence on the geometry of contact. Specifically, tip-like geometries have been shown to penetrate into monolayer films, inducing defects in the monolayer chains and leading to plowing mechanisms during shear, which result in higher coefficients of friction (COF) than those observed for planar geometries. In this work, we use molecular dynamics simulations to examine the tribology of model silica single-asperity contacts under shear with monolayer-coated substrates featuring various film densities. It is observed that lower monolayer densities lead to reduced COFs, in contrast to results for planar systems where COF is found to be nearly independent of monolayer density. This is attributed to a liquid-like response to shear, whereby fewer defects are imparted in monolayer chains from the asperity, and chains are easily displaced by the tip as a result of the higher free volume. This transition in the mechanism of molecular plowing suggests that liquid-like films should provide favorable lubrication at single-asperity contacts.