Ewen James P, Gattinoni Chiara, Thakkar Foram M, Morgan Neal, Spikes Hugh A, Dini Daniele
Department of Mechanical Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK.
Shell India Markets Private Limited, 8B RMZ Centennial Building, Kundanahalli Main Road, Bangalore 560048, India.
Materials (Basel). 2016 Aug 2;9(8):651. doi: 10.3390/ma9080651.
For the successful development and application of lubricants, a full understanding of their complex nanoscale behavior under a wide range of external conditions is required, but this is difficult to obtain experimentally. Nonequilibrium molecular dynamics (NEMD) simulations can be used to yield unique insights into the atomic-scale structure and friction of lubricants and additives; however, the accuracy of the results depend on the chosen force-field. In this study, we demonstrate that the use of an accurate, all-atom force-field is critical in order to; (i) accurately predict important properties of long-chain, linear molecules; and (ii) reproduce experimental friction behavior of multi-component tribological systems. In particular, we focus on -hexadecane, an important model lubricant with a wide range of industrial applications. Moreover, simulating conditions common in tribological systems, i.e., high temperatures and pressures (HTHP), allows the limits of the selected force-fields to be tested. In the first section, a large number of united-atom and all-atom force-fields are benchmarked in terms of their density and viscosity prediction accuracy of -hexadecane using equilibrium molecular dynamics (EMD) simulations at ambient and HTHP conditions. Whilst united-atom force-fields accurately reproduce experimental density, the viscosity is significantly under-predicted compared to all-atom force-fields and experiments. Moreover, some all-atom force-fields yield elevated melting points, leading to significant overestimation of both the density and viscosity. In the second section, the most accurate united-atom and all-atom force-field are compared in confined NEMD simulations which probe the structure and friction of stearic acid adsorbed on iron oxide and separated by a thin layer of -hexadecane. The united-atom force-field provides an accurate representation of the structure of the confined stearic acid film; however, friction coefficients are consistently under-predicted and the friction-coverage and friction-velocity behavior deviates from that observed using all-atom force-fields and experimentally. This has important implications regarding force-field selection for NEMD simulations of systems containing long-chain, linear molecules; specifically, it is recommended that accurate all-atom potentials, such as L-OPLS-AA, are employed.
为了成功开发和应用润滑剂，需要全面了解其在各种外部条件下复杂的纳米级行为，但这很难通过实验获得。非平衡分子动力学（NEMD）模拟可用于深入了解润滑剂和添加剂的原子尺度结构及摩擦；然而，结果的准确性取决于所选的力场。在本研究中，我们证明使用准确的全原子力场对于：（i）准确预测长链线性分子的重要性质；以及（ii）重现多组分摩擦学系统的实验摩擦行为至关重要。特别是，我们聚焦于十六烷，一种具有广泛工业应用的重要模型润滑剂。此外，模拟摩擦学系统中常见的条件，即高温高压（HTHP），可以测试所选力场的极限。在第一部分中，使用平衡分子动力学（EMD）模拟在环境条件和高温高压条件下，根据十六烷的密度和粘度预测准确性对大量联合原子力场和全原子力场进行了基准测试。虽然联合原子力场能准确重现实验密度，但与全原子力场和实验相比，粘度被显著低估。此外，一些全原子力场给出的熔点升高，导致密度和粘度都被严重高估。在第二部分中，在受限NEMD模拟中比较了最准确的联合原子力场和全原子力场，该模拟探究了吸附在氧化铁上并被一薄层十六烷隔开的硬脂酸的结构和摩擦。联合原子力场能准确表示受限硬脂酸膜的结构；然而，摩擦系数一直被低估，且摩擦覆盖率和摩擦速度行为与使用全原子力场观察到的以及实验结果不同。这对于包含长链线性分子的系统的NEMD模拟中力场的选择具有重要意义；具体而言，建议使用准确的全原子势，如L - OPLS - AA。
