薄膜润滑是指用一层流体薄膜分开两个固体基底的表面避免由于基底表面的直接接触发生干摩擦而造成磨损。精密机械工作中，摩擦副之间的润滑膜常处于十几个到几十个纳米厚度的薄膜润滑状态，宏观的摩擦行为将追溯到分子接触过程，传统的摩擦分析不再适用。因此，研究薄膜润滑理论在微纳米领域和纳米摩擦领域的基础理论研究和工程应用中都是很有意义的。

由于计算机的计算速度和存储量的限制，包含从宏观到微观摩擦过程的全原子模拟难以实现，必须考虑将传统的连续介质表象和纯分子表象耦合起来，形成多尺度耦合方法。本文提出和发展了三维薄膜润滑系统具有自由能修正的混合原子与粗粒化的多尺度模拟方法，采用蒙特卡罗方法研究了三维面心立方LJ晶体薄膜润滑系统的可逆滑移过程。首先，采用全原子模拟方法，通过改变固体基底的厚度验证了固体基底的弹性变形对薄膜润滑系统的摩擦应力具有显著影响。其次，通过将固体基底近区域的全原子描述和远区域的有限元描述耦合起来，建立含有弹性固体基底的多尺度模型。在固体基底远区域有限元粗粒化过程中，分别采用局域和非局域的有限元单元进行粗粒化。研究表明，这两种粗粒化处理方法计算所得的摩擦应力曲线与平均分离曲线均能够很好地再现全原子模拟的结果，并且能够很大程度地提高计算效率。最后，基于上述对单层流体原子薄膜润滑的有效模拟，进一步采用局域单元覆盖的HACG方法研究了多层流体原子的三维薄膜润滑系统的摩擦特性。通过分析计算结果发现，最大摩擦力随流体原子层数的增加而降低，随系统正压力的增大而增大。

从节约资源和保护环境的层面来看，用水作为一种绿色清洁的润滑剂代替传统的油润滑剂是一个值得探索和研究的问题。然而，水的低粘性和易于与金属发生化学作用的特点限制了它成为一种有效的润滑剂。国内外研究发现，由于石墨烯的各项优异性能，用石墨烯对金属进行涂层能够有效地阻止金属与水发生化学作用，使得水能够作为清洁的润滑剂被使用。本文进一步将上述的混合原子与粗粒化的多尺度模拟方法应用到水作为润滑剂石墨烯涂层作为防腐措施的单晶铜薄膜润滑系统中，研究该系统的摩擦润滑特性。研究表明，石墨烯涂层能够有效减弱水分子与单晶铜基底之间的相互作用，进而使系统的摩擦力有很大程度的降低。

关键词：准连续方法，薄膜润滑系统，多尺度模拟，石墨烯涂层

Film lubrication refers to the use of a thin film of fluid to separate the surfaces of two solid substrates to avoid wear due to dry friction caused by direct contact with the substrate surface. In precision machinery work, the lubrication film between the friction pairs is often in the lubrication state of a dozen to several tens of nanometers in thickness. The macroscopic friction behavior will be traced back to the molecular contact process, and the traditional friction analysis is no longer applicable. Therefore, it is of great significance to study the theory of thin film lubrication in basic theoretical research and engineering applications in the fields of micro-nano technology and nano-tribology.

Due to the computational speed and storage limitations of computers, using the fully-atomistic simulation to treat systems that involve macroscopic to microscopic tribological processes is difficult to achieve. It is necessary to couple the traditional continuum methods with the fully-atomistic methods to create a multi-scale coupling methods. In this thesis, we developed a multi-scale simulation method , which is named as free-energy-corrected hybrid atomistic coarse-graining method of three-dimensional thin-film lubrication system. Monte Carlo simulation is used to investigate the reversible slip process of three-dimensional thin-film lubrication system with face-centered cubic LJ crystal structures. Firstly, the fully-atomistic simulation was used to verify that the elastic deformation of the solid substrate has a significant effect on the shear stress of the film lubrication system by changing the thickness of the solid substrate. Second, a multi-scale model containing an elastic solid substrate is established by coupling the fully-atomistic description of the near region of the solid substrate with the finite element description of the far region. In the process of finite element coarse graining in the far area of the solid substrate, local and non-local finite element units were respectively used. Studies have shown that both the shear  stress profiles and the average separation profiles calculated by the two coarse-grained treatment methods can reproduce the results calculated by the fully-atomistic simulation, and can greatly improve the computational efficiency. Finally, based on the effective simulation of mono-layer fluid atom film lubrication, a three-dimensional thin film lubrication system with multi-layer fluid atoms was further studied by using this multi-scale simulation method. Through the analysis of the calculation results, it is found that the maximum shear stress decreases with the increasing of the number of fluid atomic layers, and increases with the increasing of positive pressure of the system.

In order to saving resources and protecting the environment, using water as a green and clean lubricant to replace the traditional oil lubricants is a problem which is worth to explore and research. However, the low viscosity of water and its tendency to chemically interact with metals have limited it to be an effective lubricant. Both the domestic and foreign research shows that due to the excellent properties of graphene, coating the metal with graphene can effectively prevent the metal from reacting with water, so that water can be used as a clean lubricant. This paper further applies the hybrid atomistic and coarse-grained multi-scale simulation method developed in this thesis to the single-crystal copper thin-film lubrication system with water as a lubricant and graphene as the coatings, and researched the tribological and  lubricant characteristics of this system. Results shown that the graphene coating can effectively reduce the interaction between water molecules and the copper substrate, so that the friction of the system is greatly reduced.

Key words: Quasicontinum method, Thin-film lubrication system, Multi-scale simulation, Graphene coating