基底势函数的无序性对静摩擦力的影响

在二维Frenkel-Kontorova模型中, 为了更好地模拟真实的物理系统, 采用高斯波的平移叠加作为基底, 运用分子动力学模拟方法, 讨论了有相互作用的原子平面在公度、 不公度两种情况下, 原子在无序基底上运动的摩擦机理, 以及基底的无序对静摩擦力F_s 的影响.     

Friction phenomena in a two-dimensional Frenkel-Kontorova model

By using the molecular dynamic simulation method with a fourth-order Runge--Kutta algorithm, a two-dimensional dc- and ac-driven Frenkel--Kontorova (FK) model with a square symmetry substrate potential for a square lattice layer has been investigated in this paper. For this system, the effects of many different parameters on the average velocity and the static friction force have been studied. It is found that not only the amplitude and frequency of ac-driven force, but also the direction of the external driving force and the misfit angle between two layers have some strong influences on the static friction force. It can be concluded that the superlubricity phenomenon appears easily with a larger ac amplitude and lower ac frequency for some special direction of the external force and misfit angle.     

Frenkel-Kontorova模型中垫底势对最大静摩擦力的影响

用一维Frenkel-Kontorova模型,对相互接触的两个单原子分子链具有相对运动趋势时所产生的最大静摩擦力进行了研究.分别在相邻原子的距离与周期势场的周期比b/a为可公度(commensurate)、黄金分割(golden mean)、螺旋分割(spiral mean)三种情况下,描述了特殊垫底势力的振幅A与分子链静摩擦力的关系,在特殊垫底势力的作用下上层原子链弹性系数K对静摩擦力的影响.研究表明,垫底势力的形式对静摩擦力的大小有很重要的影响.     

Frenkel-Kontorova模型中垫底势对最大静摩擦力的影响

用一维Frenkel-Kontorova模型,对相互接触的两个单原子分子链具有相对运动趋势时所产生的最大静摩擦力进行了研究.分别在相邻原子的距离与周期势场的周期比b/a为可公度(commensurate)、黄金分割(golden mean)、螺旋分割(spiral mean)三种情况下,描述了特殊垫底势力的振幅A与分子链静摩擦力的关系,在特殊垫底势力的作用下上层原子链弹性系数K对静摩擦力的影响.研究表明,垫底势力的形式对静摩擦力的大小有很重要的影响. 关键词： Frenkel-Kontorova模型 静摩擦力     

Friction behavior of monolayer molybdenum diselenide nanosheet under normal electric field

The friction behavior of monolayer molybdenum diselenide (MoSe₂) under normal electric field was studied by the atomic force microscope. The friction coefficients of MoSe₂ are increasing with bias voltage applied on the Si substrate. The results show that the adhesion and electrostatic forces increase with bias and approximately follow a parabolic law. The friction force and surface potential are of the same tendency with bias application time, and the contribution of charges accumulation to friction is considerable. The mechanisms of the friction behavior under external normal electric field were explained with electrostatic force and adsorption. This study reveals a possibility of electronically controlling friction in two-dimensional MoSe₂ system, with potential applications in solid lubricant and moving parts for MEMS devices.     

Anisotropic character of atoms in a two-dimensional Frenkel–Kontorova model

The dynamics of a certain density of interacting atoms arranged on a two-dimensional square lattice, which is made to slide over a two-dimensional periodic substrate potential with also the quare lattice symmetry, in the presence of dissipation, by an externally applied driving force, is studied. By rotating the misfit angle θ, the dynamical behaviour displays two different tribological regimes: one is smooth, the other becomes intermittent. We comment both on the nature of the atomic dynamics in the locked-to-sliding transition, and on the dynamical states displayed during the atom motion at different values of the driving force. In tribological applications, we also investigate how the main model parameters such as the stiffness strength and the magnitude of the adhesive force affect the static friction of the system. In particular, our simulation indicates that the superlubricity will appear.     

Towards a statistical theory of solid dry friction

Wearless dry friction of an elastic block of weight N, driven by an external force F over a rigid substrate, is investigated. The slider and substrate surfaces are both microscopically rough, interacting via a repulsive potential that depends on the local overlap. The model reproduces Amontons’s laws which state that the friction force is proportional to the normal loading force N and independent of the nominal surface area. In this model, the dynamic friction force decays for large velocities and approaches a finite static friction for small velocities if the surface profiles are self-affine on small length scales.     

不同基底石墨烯涂层的层间滑移减磨性能研究

石墨烯薄膜作为一种二维材料,是提高微/纳机电系统(MEMS/NEMS)摩擦力学性能的优异润滑剂.为了探究基底材料和石墨烯层数对其减磨性能的影响,本文通过在不同基底制备了不同层数的石墨烯涂层,利用原子力显微镜(AFM)实验和分子动力学(MD)仿真结合的方法,研究了石墨烯层数对减磨效应的影响.并且通过建立不同层数石墨烯涂层的摩擦性能分析模型,探究出石墨烯层间滑移是产生减磨的主要因素.结果表明:在不同载荷下,石墨烯涂层对硅基底和铜基底均有优异的减磨效果,摩擦力随着石墨烯层数的增加逐渐降低,当石墨烯层数大于10层时,达到最优99.3%的减磨效果.通过仿真分析发现,随着层数增加,石墨烯与基底的干摩擦转变为石墨烯的层间摩擦,并产生层间剪切滑移,石墨烯层间滑移是导致多层石墨烯优异减磨性能的主要因素.     

二维Frenkel-Kontorova模型中六角对称结构的摩擦现象

基于二维 Frenkel-Kontorova 模型, 运用分子动力学模拟方法, 研究了具有六角对称结构的系统从 locked 态到 sliding 态的相变, 并数值分析了上层原子采用六角对称结构时, 系统的不同参数对静摩擦力F_s和动摩擦力F_c的影响. 关键词： 二维 Frenkel-Kontorova 模型 超润滑 分子动力学 摩擦     

Mechanical properties of hexagonal boron nitride synthesized from film of Cu/BN mixture by surface segregation

Hexagonal boron nitride (h-BN) has a low friction coefficient and weak surface attractive force similar to graphite. Furthermore, while graphite is conductive, BN is a good insulator. These properties make it suitable for application like lubricating coating or as an insulator/buffer layer in electronic devices. The synthesize of h-BN layer by surface segregation phenomena and mechanical properties of the h-BN surface segregated on Cu substrate have been investigated. During in situ annealing, the surface segregation of BN occurred on Cu/BN film deposited by deposition process with a rf magnetron co-sputtering system. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analysis showed that though the h-BN layer synthesized was not covered whole area of substrate but the h-BN layers partially covered substrate. And the concentration of oxygen on the surface after exposure in air is decreased with increase of BN concentration. The topography of atomic forces microscopy (AFM) showed that h-BN phases surface segregated are discontinuous droplet shape. The force curves of AFM and friction force of lateral force microscopy (LFM) showed that the h-BN droplet surface segregated have very weak attractive force and low friction coefficient equal to h-BN sintered plate.     

Frenkel-Kontorova模型中基底势振动的影响

基于一维Frenkel-Kontorova模型, 研究了振动的基底势对系统纳米摩擦现象的影响. 分别在相邻原子间的距离与周期势场的周期比为不公度(incommensurate)、可公度(commensurate)两种情形下, 探讨了基底势振动的振幅和频率对滞回现象(hysteresis)、最大静摩擦力以及超滑现象的作用机理. 两种情形下, 固定频率, 随着振幅的增大, 滞回区域的面积以及最大静摩擦力都将减小, 对于不同的频率, 减小的趋势不同. 系统甚至产生了超滑现象. 但当频率过大时, 振幅的改变不会影响滞回区域的面积以及最大静摩擦力的大小, 此时与基底不加振动时的情形一致; 当振幅固定, 随着频率的增大, 滞回区域的面积将增大, 对于不同振幅, 增大的趋势不同. 特别地, 对于某些固定的振幅, 最大静摩擦力随着振动频率的增大先逐步减小直至出现超滑现象, 再进一步增大频率, 最大静摩擦力又转而逐步增大. 这一现象类似于共振, 表明存在最佳的振动频率促进系统内所有原子的共同运动, 使得整个系统的最大静摩擦力几乎消失. 另外, 两种情形的区别是, 对于某些固定的频率(如ω= 0.5)和不同的小振幅, 不可公度情形往往具有相同的平均终止速度, 而可公度情形则不同, 表明相同前提下后者具有更复杂的动力学行为. 关键词： Frenkel-Kontorova模型 滞回 最大静摩擦力 超润滑     

Brownian motors in the low-energy approximation: Classification and properties

We classify Brownian motors based on the expansion of their velocity in terms of the reciprocal friction coefficient. The two main classes of motors (with dichotomic fluctuations in homogeneous force and periodic potential energy) are characterized by different analytical dependences of their mean velocity on the spatial and temporal asymmetry coefficients and by different adiabatic limits. The competition between the spatial and temporal asymmetries gives rise to stopping points. The transition through these points can be achieved by varying the asymmetry coefficients, temperature, and other motor parameters, which can be used, for example, for nanoparticle segregation. The proposed classification separates out a new type of motors based on synchronous fluctuations in symmetric potential and applied homogeneous force. As an example of this type of motors, we consider a near-surface motor whose two-dimensional motion (parallel and perpendicular to the substrate plane) results from fluctuations in external force inclined to the surface.     

纳米级随机粗糙表面微观滑动摩擦力的计算研究

表面形貌很大程度上决定了摩擦副的摩擦性能, 而所有的表面都不可能是绝对光滑的.由于摩擦表面形貌的随机性, 决定了实际的摩擦过程具有随机性的特点, 因此为了获得与随机形貌对应的摩擦特性, 建立合理的随机摩擦模型是必要的. 本文基于Lennard-Jones势能建立了纳米级随机粗糙表面和原子级光滑的刚性平面间的随机摩擦模型; 模型中, 界面势能由法向载荷和界面间平衡距离决定.通过数值计算的方法, 推导了微观滑动摩擦力的计算公式和摩擦力与法向载荷之间的关系. 研究结果表明摩擦力随着法向载荷的增加而增加, 但不是线性增长. 结果也说明界面间的表面势能可能是微观摩擦力的本质起源. 关键词： 随机粗糙表面 Lennard-Jones势能 微滑动摩擦力 微摩擦     

基底支撑刚度梯度变化对石墨烯层间摩擦力的影响

基于纳米摩擦能耗理论,利用分子动力学方法建立了公度接触下支撑刚度梯度变化的石墨烯层间摩擦力模型,分析了基底质心刚度和支撑刚度梯度变化对基底和薄片各接触区摩擦能耗的贡献.结果表明:软边界区始终贡献驱动力;硬边界区贡献的摩擦力最大,且随着支撑刚度的增大,硬边界区对总摩擦的贡献比也越高.各接触区的摩擦力是薄片和基底之间的褶皱势和接触区产生的法向变形差两部分的共同作用.前者是公度接触下阻碍滑移的界面势垒和刚度梯度方向上不同刚度支撑原子热振动引起的势梯度;后者是接触边界过渡区两侧原子的非对称变形和自由度约束突变引起的非平衡边界势垒相耦合的结果.本文对研究公度接触下刚度梯度支撑的纳米器件的相对运动规律有指导意义.     

摩擦导致的聚合物表层微观结构改变

应用大规模分子动力学方法, 模拟了锥形探头在非晶态聚合物薄膜表面的滑动摩擦过程, 研究了摩擦导致的聚合物薄膜表层微观结构改变, 以及探头与基体间黏着作用、滑动速度和分子链长度对基体表层微观结构改变的影响. 当探头与基体之间为黏着作用时, 摩擦导致基体表面滑痕区域的键取向沿滑动方向重新取向, 导致表层分子链回转半径沿滑动方向伸长, 并且这些表层微观结构的改变程度随滑动速度的减小而增大. 在摩擦导致结构改变的过程中, 链端单体和链中单体的贡献作用不同, 形成了不同的分子链拉伸变形机制. 当样本缠结度较大或探头滑动速度较小时, 相比于链中单体, 探头对链端单体的拖曳作用使更多分子链发生拉伸变形. 研究还发现, 在探头与聚合物薄膜系统中, 使薄膜表层微观结构发生改变是摩擦能量耗散的重要途径.     

Nano-friction phenomenon of Frenkel—Kontorova model under Gaussian colored noise

The nano-friction phenomenon in a one-dimensional Frenkel-Kontorova (FK) model under Gaussian colored noise is investigated by using the molecular dynamic simulation method. The role of colored noise is analyzed through the inclusion of a stochastic force via a Langevin molecular dynamics method. Via the stochastic Runge-Kutta algorithm, the relationship between different parameter values of the Gaussian colored noise (the noise intensity and the correlation time) and the nano-friction phenomena such as hysteresis, the maximum static friction force is separately studied here. Similar results are obtained from the two geometrically opposed ideal cases: incommensurate and commensurate interfaces. It was found that the noise strongly influences the hysteresis and maximum static friction force and with an appropriate external driving force, the introduction of noise can accelerate the motion of the system, making the atoms escape from the substrate potential well more easily. Interestingly, suitable correlation time and noise intensity give rise to super-lubricity. It is noteworthy that the difference between the two circumstances lies in the fact that the effect of the noise is much stronger on triggering the motion of the FK model for the commensurate interface than that for the incommensurate interface.     

Giant enhancement of noncontact friction between closely spaced bodies by dielectric films and two-dimensional systems

The effect of an external bias voltage and spatial variations of the surface potential on the damping of cantilever vibrations in an atomic force microscope (AFM) is considered. The damping is due to an electrostatic friction that arises due to dissipation of the energy of an electromagnetic field generated in the sample by oscillating static charges induced on the surface of the AFM probe tip by the bias voltage or spatial variations of the surface potential. A similar effect appears when the tip is oscillating in an electrostatic field created by charged defects present in the dielectric sample. The electrostatic friction is compared to the van der Waals (vdW) friction between closely spaced bodies, which is caused by a fluctuating electromagnetic field related to the quantum and thermal fluctuations of current density inside the bodies. It is shown that the electrostatic friction and the vdW friction can be strongly enhanced in the presence of dielectric films or two-dimensional (2D) structures—such as a 2D electron system or an incommensurate layer of adsorbed ions exhibiting acoustic oscillations—on the probe tip and sample surfaces. It is also shown that the damping of cantilever oscillations caused by the electrostatic friction in the presence of such 2D structures can have the same order of magnitude and the same dependence on the distance as observed in experiment by Stipe et al. [Phys. Rev. Lett. 87, 096801 (2001)]. At small distances, the vdW friction can be large enough to be measured in experiment. In interpreting the experimental data that obey a quadratic dependence on the bias voltage, one can reject a phonon mechanism according to which the friction depends on the fourth power of the voltage.     

Adsorbate-induced enhancement of electrostatic noncontact friction

We study the noncontact friction between an atomic force microscope tip and a metal substrate in the presence of bias voltage. The friction is due to energy losses in the sample created by the electromagnetic field from the oscillating charges induced on the tip surface by the bias voltage. We show that the friction can be enhanced by many orders of magnitude if the adsorbate layer can support acoustic vibrations. The theory predicts the magnitude and the distance dependence of friction in good agreement with recent puzzling noncontact friction experiment [B. C. Stipe, H. J. Mamin, T. D. Stowe, T. W. Kenny, and D. Rugar, Phys. Rev. Lett. 87, 096801 (2001).]. We demonstrate that even an isolated adsorbate can produce high enough friction to be measured experimentally.     

Detachment of stretched viscoelastic fibrils

New experimental results are presented about the final stage of failure of soft viscoelastic adhesives. A microscopic view of the detachment of the adhesive shows that after cavity growth and expansion, well adhered soft adhesives form a network of fibrils connected to expanded contacting feet which fail via a sliding mechanism, sensitive to interfacial shear stresses rather than by a fracture mechanism as sometimes suggested in earlier work. A mechanical model of this stretching and sliding failure phenomenon is presented which treats the fibril as a nonlinear elastic or viscoelastic rod and the foot as an elastic layer subject to a friction force proportional to the local displacement rate. The force on the stretched rod drives the sliding of the foot against the substrate. The main experimental parameter controlling the failure strain and stress during the sliding process is identified by the model as the normalized probe pull speed, which also depends on the magnitude of the friction and PSA modulus. In addition, the material properties, viscoelasticity and finite extensibility of the polymer chains, are shown to have an important effect on both the details of the sliding process and the ultimate failure strain and stress. Electronic supplementary material Appendix B is only available in electronic form at and are accessible for authorised users.     

The effect of substrate roughness on the static friction of CuO nanowires

The dependence of static friction on surface roughness was measured for copper oxide nanowires on silicon wafers coated with amorphous silicon. The surface roughness of the substrate was varied to different extent by the chemical etching of the substrates. For friction measurements, the nanowires (NWs) were pushed by an atomic-force microscope (AFM) tip at one end of the NW until complete displacement of the NW was achieved. The elastic bending profile of a NW during this manipulation process was used to calculate the ultimate static friction force. A strong dependence of static friction on surface roughness was demonstrated. The real contact area and interfacial shear strength were estimated using a multiple elastic asperity model, which is based on the Derjaguin–Muller–Toporov (DMT) contact mechanics. The model included vertical elastic flexure of NW rested on high asperities due to van der Waals force.