基于分子动力学方法的多层石墨烯超滑失效机理分析

本文作者采用混合势函数(Lennard-Jones势和REBO势),基于Verlet算法动态模拟了金刚石探针(100)与多层石墨烯间的压入和滑动摩擦过程,分析了不同压深下多层石墨烯的摩擦力及平均摩擦系数变化的特点,统计了不同压深下的层间键合作用、层内断键数量以及实际原子接触面积,阐述了多层石墨烯超滑失效的机理.结果表明:压深对多层石墨烯间的超滑失效有着重要影响,探针压深增加导致的摩擦力振幅上升与悬空石墨烯压缩应变增加产生的影响非常相似;6.1(A)是石墨烯超滑失效的临界压入深度值,压深小于6.1(A)时,石墨烯有着明显的超滑特性,且摩擦力变化周期不受压深的影响,并等于石墨烯晶格常数;压深大于6.1(A)后,摩擦力变化完全失去周期性,探针在滑动中需要克服的势垒急剧上升,摩擦力显著增大,超滑明显失效.当压深为6.1(A)时,石墨烯层间开始出现键合作用,层内也出现断键现象,且随压深的增加越加剧烈,层内的断键和层间键的形成是引起石墨烯超滑失效和摩擦力突变的主要原因,而实际原子接触面积与此突变的相关性很小.     

Difference of Directions Between External Driving Force and Movement Direction of Center of Mass

Driven dynamics of a two-dimensional Frenkel-Kontorova model is studied in the paper. In our numerical simulations, it is found that the movement direction of the center of mass is not consistent with that of the external driving force except for some special symmetric directions at the lower driving force. Our results also indicate that the movement direction of the center of mass strongly depends on both the magnitude and the direction of the external driving force as well as the misfit angle between two layers.     

超滑和界面摩擦及耗散过程——关于摩擦机理微观研究的思考与展望

评述了近年来关于摩擦现象基础研究的进展与成果,包括超滑的分子动力学模型,界面摩擦试验,表面微观结构的相对称性以及摩擦的能量耗散过程等。肯定了超滑的模拟结果及其理论解释对于摩擦机理和控制研究的启迪和促进作用,分析了模拟结果与现实世界差异的深层次原因,并指出了这一差别将促使人们超越经典力学的框架,为探索和发现尚未认识的摩擦机制创造新的机遇。     

Application of Two-Dimensional Frenkel-Kontorova Model to Nanotribology

The dry friction force between two contacting surface layers is studied. The upper layer is arranged on a two-dimensional square lattice and driven by an external driving force. The lower layer is approximated by a two-dimensional periodic substrate potential. This model, usually called two-dimensional Frenkel-Kontorova model, is applied to study the friction forces in this paper. The behaviors of different substrate potential strongly affect the static friction force. It is found that the system has strong anisotropic character. The possibility to obtain superlubricity is suggested.     

Difference of Directions Between External Driving Force and MovementDirection of Center of Mass

Driven dynamics of a two-dimensional Frenkel-Kontorova model is studied in the paper. In our numerical simulations, it is found that the movement direction of the center of mass is not consistent with that of the external driving force except for some special symmetric directions at the lower driving force. Our results also indicate that the movement direction of the center of mass strongly depends on both the magnitude and the direction of the external driving force as well as the misfit angle between two layers.     

The Registry Index: A Quantitative Measure of Materials′ Interfacial Commensurability

Nanoscale tribology is an active and rapidly developing area of research that poses fundamental scientific questions that, if answered, may offer great technological potential in the fields of friction, wear, and lubrication. When considering nanoscale material′s junctions, surface commensurability often plays a crucial rule in dictating the tribological properties of the interface. This Review surveys recent theoretical work in this area, with the aim of providing a quantitative measure of the crystal lattice commensurability at interfaces between rigid materials and relating it to the tribological properties of the junction. By considering a variety of hexagonal layered materials, including graphene, hexagonal boron nitride, and molybdenum disulfide, we show how a simple geometrical parameter, termed the “registry index” (RI), can capture the interlayer sliding energy landscape as calculated using advanced electronic structure methods. The predictive power of this method is further demonstrated by showing how the RI is able to fully reproduce the experimentally measured frictional behavior of a graphene nanoflake sliding over a graphite surface. It is shown that generalizations towards heterogeneous junctions and non‐planar structures (e.g., nanotubes) provide a route for designing nanoscale systems with unique tribological properties, such as robust superlubricity. Future extension of this method towards nonparallel interfaces, bulk‐material junctions, molecular surface diffusion barriers, and dynamic simulations are discussed.     

Molecular origin of friction

The wearless friction originating from molecular interactions has been discussed in this paper. We find that the frictional properties are closely related to the structural match of two surfaces in relative motion. For the surfaces with incommensurate structure and week inter-surface interaction, zero static and kinetic friction can be achieved. In a sliding considered as in a quasi-static state, the energy dissipation initiates when interfacial particles move in a discontinuous fashion, which gives rise to a finite kinetic friction. The state of superlubricity is a result of computer simulations, but the prediction will encourage people to look for a technical approach to realizing the state of super low friction.     

二十年跨界创新研究的反思与建议

近十年来,人类发展进入到了一个前所未有的急剧变化期——即将跨过人工智能超越人类智能的临界点。在此大背景下,过去两个世纪缘由知识爆炸和人类智能局限而发生的学科越分越细的趋势正发生根本性扭转,即转向以“发现未知–创造未有”大问题为牵引,跨越学科的创新。其中,所谓 “从0到1”创新是指最终能产生全球历史性影响,甚至改变世界的源头创新。任何一项成功的从0到1创新在其从0到1和之后的“从1到N”阶段都难免遭遇“死亡谷”,且每一段都是长达10余年的周期。自2000年起,我的研究从力争在传统学科（力学、应用数学、材料科学）中做得“最好”,全面转向了“跨界”源头创新。经过20年的不懈努力,我和团队在“结构超滑技术”和“拔尖创新人才培养模式”两方面,从一定意义上讲,都跨越了从0到1阶段的死亡谷并步入了从1到N的发展新阶段,现正以双螺旋的方式跋涉迈进。在这一过程中发挥了独特且关键作用的是我60年人生前20年的受教育经历、中间20年的数学与力学研究积淀,和近20年学生直面挑战的勇敢参与,以及与多学科著名教授学者的深度合作。我的研究经历,不仅横跨了STEM(科学、技术、工程、数学),也扩展到更前端的创新教育和更后端的技术产品研发。希望我的分享对怀抱从0到1创新梦想的年轻研究者和学子们,能有所借鉴和启发。

     

无处不在的摩擦学：从宏观到单原子尺度

摩擦,是一个人类文明发展史和日常生活中至关重要、无处不在、离开它人类将无法正常生活和生产的科学问题。原子制造也不得不考虑摩擦问题。从宏观到微观,摩擦现象非常复杂、神秘和精彩！在宏观领域,早在 500 年前,就建立起著名的达·芬奇—阿蒙顿经典定律;在微观领域,摩擦具有诸多新奇的特性,比如摩擦力被降低至接近于零的超润滑现象以及摩擦力随着载荷的增加而减少的负摩擦系数现象等。微观摩擦和宏观摩擦表现出的不同规律为我们从微观和单原子尺度上理解摩擦起源,进而调控摩擦力的大小提供了重要的理论基础和途径。文章简要介绍摩擦学及其发展历史,并重点介绍摩擦起源的微观物理机制以及在单原子尺度上调控摩擦力大小的可能途径。     

Application of Two-Dimensional Frenkel-Kontorova Model to Nanotribology

The dry friction force between two contacting surface layers is studied. The upper layer is arranged on a two-dimensional square lattice and driven by an external driving force. The lower layer is approximated by a two-dimensional periodic substrate potential. This model, usually called two-dimensional Frenkel-Kontorova model, is applied to study the friction forces in this paper. The behaviors of different substrate potential strongly affect the static frictionforce. It is found that the system has strong anisotropic character. The possibility to obtain superlubricity is suggested.