Nonequilibrium Molecular Dynamics Simulations of Organic Friction Modifiers Adsorbed on Iron Oxide Surfaces
Nonequilibrium Molecular Dynamics Simulations of Organic Friction Modifiers Adsorbed on Iron Oxide Surfaces
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DOI：
10.1021/acs.langmuir.6b00586
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发表时间：
2016-05-10
期刊：
影响因子：
3.9
通讯作者：
Dini, Daniele
中科院分区：
文献类型：
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作者：
Ewen, James P.;Gattinoni, Chiara;Dini, Daniele
为了成功开发和应用润滑剂，需要充分了解复杂摩擦系统的纳米级行为，但这很难通过实验获得。在这项研究中，我们使用非平衡分子动力学（NEMD）模拟来检查吸附在氧化铁表面并由薄薄的十六烷分离层润滑的商业相关有机摩擦改性剂（OFM）单分子层的原子结构和摩擦特性。具体来说，具有饱和和 Z-不饱和烃尾基的酸、酰胺和甘油酯 OFM 在不同的表面覆盖度和滑动速度下进行模拟。在低和中等覆盖率下，OFM 分别形成液体状和无定形单层，它们与十六烷润滑剂显着交错，从而产生相对较高的摩擦系数。在高覆盖率下，所有 OFM 均形成固体状单分子层，在滑动过程中，会在明确的 OFM 和十六烷层之间产生滑移面，从而显着降低摩擦系数。当存在相同的表面覆盖度时，具有饱和尾部基团和 Z 不饱和尾部基团的 OFM 被发现会产生相似的结构和摩擦行为。具有甘油酯头基的 OFM 产生的摩擦系数明显低于酰胺，特别是羧酸头基。对于所有模拟的 OFM 和覆盖范围，发现摩擦系数随着滑动速度的对数线性增加；然而，这种增加的梯度取决于覆盖范围。从这些模拟中获得的结构和摩擦细节与实验结果非常吻合，并且还通过纳米级结构变化揭示了这些 OFM 的相对摩擦学性能。这对于 NEMD 在帮助开发控制摩擦的新配方方面的适用性具有重要意义。
For the successful development and application of lubricants, a full understanding of the nanoscale behavior of complex tribological systems is required, but this is difficult to obtain experimentally. In this study, we use nonequilibrium molecular dynamics (NEMD) simulations to examine the atomistic structure and friction properties of commercially relevant organic friction modifier (OFM) monolayers adsorbed on iron oxide surfaces and lubricated by a thin, separating layer of hexadecane. Specifically, acid, amide, and glyceride OFMs, with saturated and Z-unsaturated hydrocarbon tail groups, are simulated at various surface coverages and sliding velocities. At low and medium coverage, the OFMs form liquidlike and amorphous monolayers, respectively, which are significantly interdigitated with the hexadecane lubricant, resulting in relatively high friction coefficients. At high coverage, solidlike monolayers are formed for all of the OFMs, which, during sliding, results in slip planes between well-defined OFM and hexadecane layers, yielding a marked reduction in the friction coefficient. When present at equal surface coverage, OFMs with saturated and Z-unsaturated tail groups are found to yield similar structure and friction behavior. OFMs with glyceride head groups yield significantly lower friction coefficients than amide and particularly carboxylic acid head groups. For all of the OFMs and coverages simulated, the friction coefficient is found to increase linearly with the logarithm of sliding velocity; however, the gradient of this increase depends on the coverage. The structure and friction details obtained from these simulations agree well with experimental results and also shed light on the relative tribological performance of these OFMs through nanoscale structural variations. This has important implications in terms of the applicability of NEMD to aid the development of new formulations to control friction.