【作者】 李国杰；

【导师】 白敏丽；

【作者基本信息】 大连理工大学 ， 工程热物理， 2012， 硕士

【摘要】 摩擦和磨损现象普遍存在于人们的生活和生产实践中,尤其是在工业生产中,摩擦和磨损不仅损失掉了大量的能源,而且导致机器的精度和效率降低,甚至使机器损坏,很大程度地提高了生产成本,造成安全事故。在摩擦副间应用润滑油是减少摩擦磨损的最重要手段。但是在边界润滑情况下的润滑油膜十分容易破裂,造成摩擦副直接接触,导致更严重的摩擦和磨损。因此,提高边界润滑状态下润滑膜的性能就具有十分重要的意义。润滑油添加剂就能够很好地改善润滑油的性能,将纳米级的颗粒作为添加剂添加到润滑油中可以有效地提高润滑油的抗磨减摩性能和承载能力,纳米颗粒对摩擦副表面还有一定的修复作用。目前,纳米流体润滑的实验研究较多,学者们也提出了一些纳米颗粒改善润滑的机理,但这些机理多是在实验的基础上得出的,理论依据不足。鉴于此,本文应用分子动力学方法对边界润滑情况下纳米流体的润滑性能及纳米颗粒改善润滑的机理进行探讨。本文对纳米流体和基础流体的Couette流进行分子动力学模拟,以探讨剪切流动时纳米流体的流动特征和纳米颗粒的运动状态,结果表明：纳米流体流动过程中颗粒的旋转运动和平移运动会造成纳米流体速度分布的非线性,说明纳米粒子在一定程度上改变了流体的流动特性；壁面和纳米颗粒表面都会形成厚度与剪切速度无关的液体吸附层,且吸附层具有“类固”特性,对提高润滑油膜的承载能力有一定的作用。为探讨边界润滑情况下,纳米流体的润滑性能及纳米颗粒改善润滑油性能的机理,本文构建了纳米流体和基础流体在摩擦副间边界润滑的分子动力学模型并进行分子动力学计算,获得以下结论：在边界润滑状态下,纳米流体比基础流体有更好的承载能力,且高压情况下纳米流体有更好的抗压性能,这是由于纳米颗粒的固态性质引起的。剪切速度对纳米流体的减摩性能造成一定的影响,当剪切速度比较大时,纳米流体减摩的性能发挥的更好。纳米流体的流动状态受压力的影响比较大,在压力较大的边界润滑状态时有液-固相变的趋势。正压力严重地削弱了纳米颗粒的旋转运动,但是对纳米颗粒的平移运动的影响的规律性并不明显。软质的纳米颗粒能够在一定程度上减小软质材料摩擦副的粗糙度并在摩擦副表面形成沉积膜,沉积膜起到保护摩擦副的作用；软质纳米颗粒在硬质摩擦副表面主要起到填充凹槽的作用。添加硬质材料纳米颗粒的纳米流体有更好的承载能力,硬质纳米颗粒在减小壁面粗糙度时主要靠的是它的抛光作用。更多还原

【Abstract】 Friction and wear phenomena generally exist in human’s life and production. Especially in industrial production, friction and wear not only waste a great deal of energy, but also reduce precision and efficiency of machineries, even damage the machineries. This will raise the production costs and cause accidents. Adding lubricant oil between friction pairs is the most significant method to reduce friction and wear. Lubricant film under boundary lubrication condition is crackly. The rupture of lubricant film would make the friction pairs contact to each other directly, which will course extremely serious friction and wear. Therefore, enhancing the lubricant properties of lubricant film under boundary lubrication is necessary. Lubricant additives can effectively improve the performance of lubricant oil. Adding nanoparticles in lubricant oil as lubricant additive can enhance anti-wear and friction-reduction properties and bearing capability of lubricant oil, nanoparticles can repair friction surfaces.At present, researchers have performed lots of experimental studies on lubrication of nanofluids and proposed some mechanisms for nanoparticles improve the properties of oil, however, most of these mechanisms are obtained by experiments, and the theoretical bases for them are insufficient. Therefore, in this paper, molecular dynamics simulation is used to study lubricant properties of nanofluids under boundary lubrication. Also, mechanism for nanoparticles improve the properties of oil is discussed.To analyze flow behaviors of nanofluids and motion characteristics of nanoparticles in shear flow, molecular dynamics simulations for Couette flow of nanofluids and base fluid are performed. The results show that:Rotation and translation of nanoparticles in flowing nanofluids cause nonlinear velocity profile of nanofluids, which indicates that nanoparticles change flow behavior of nanofluids to some extent. Absorbed layers whose thicknesses are independent of shear velocity are formed on the surfaces of nanoparticles and walls, these "solid-like" layers play a role in enhancing bearing capability of nanofluids.To discuss the lubricant properties of nanofluids under boundary lubrication and mechanism for nanoparticles improve the properties of oil, molecular dynamics simulation models for boundary lubrication of nanofluids and base fluid are constructed and the simulations are performed. Conclusions as following are obtained:under boundary lubrication, bearing capability of nanofluids is better than that of base fluid, and the bearing capability is more excellent under higher pressure. The outstanding bearing capability of nanofluids is caused by the solid nanoparticles. Shear velocity influences the friction-reduction property of nanofluids to some extent. When the shear velocity is higher, the friction-reduction property of nanofluids is more obvious. Normal pressure largely influences the flow behaviors of nanofluids, and tendency of liquid-solid phase transition exists in flowing nanofluids under boundary lubrication with high pressure. Normal pressure also seriously weakens the rotation of nanoparticles, however, its influence on translation of nanoparticles has no obvious regularity. Soft nanoparticles can reduce the roughness of soft friction surfaces to some extent and form deposited films on these surfaces. Soft nanoparticles mainly play a role in filling grooves on hard frication surfaces. Nanofluids with hard nanoparticles have more excellent bearing capability than that with soft nanoparticles. Hard nanoparticles primarily play a polishing role in reducing the roughness of friction surfaces.更多还原