Department of Mechanical Engineering, Imperial College London , London SW7 2AZ, England.
Shell Global Solutions UK Ltd, Manchester M22 0RR, Lancashire, England.
Langmuir. 2016 May 10;32(18):4450-63. doi: 10.1021/acs.langmuir.6b00586. Epub 2016 Apr 26.
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.
为了成功开发和应用润滑剂，需要充分了解复杂摩擦学系统的纳米尺度行为，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）模拟来研究商业相关有机摩擦改性剂（OFM）单层在氧化铁表面上的吸附以及由十六烷薄分离层润滑时的原子结构和摩擦特性。具体来说，模拟了具有饱和和 Z-不饱和烃尾基团的酸、酰胺和甘油酯 OFM，在不同的表面覆盖率和滑动速度下。在低和中覆盖率下，OFM 分别形成液态和无定形单层，这些单层与十六烷润滑剂明显交错，导致相对较高的摩擦系数。在高覆盖率下，所有 OFM 都形成固态单层，在滑动过程中，在明确的 OFM 和十六烷层之间形成滑移面，导致摩擦系数显著降低。当存在相等的表面覆盖率时，发现具有饱和和 Z-不饱和尾基团的 OFM 产生相似的结构和摩擦行为。具有甘油酯头部基团的 OFM 产生的摩擦系数明显低于酰胺和特别是羧酸头部基团。对于模拟的所有 OFM 和覆盖率，摩擦系数被发现与滑动速度的对数呈线性增加；然而，这种增加的梯度取决于覆盖率。这些模拟中获得的结构和摩擦细节与实验结果非常吻合，并且还通过纳米尺度结构变化揭示了这些 OFM 的相对摩擦性能。这对于 NEMD 应用于辅助开发控制摩擦的新配方具有重要意义。
