Molecular dynamic simulation and characterization of self-assembled monolayer under sliding friction

The frictional mechanisms of a self-assembled monolayer (SAM) during nano-scale sliding are studied using molecular dynamics (MD) simulation. The MD model consists of a gold slider and gold substrate with n-hexadecanethiol SAM chemisorbed to the substrate. The trajectory, tilt angles, normal forces, frictional forces, friction coefficients and potential energies per molecular chain of the SAM molecules are evaluated during the frictional process for various parameters including as sliding height, sliding direction (i.e. pro- or anti- the SAM tilt angle), sliding velocity and system temperature. The various parameters are discussed with regard to frictional forces, mechanisms and SAM structural transition. Results show that stick-slip occurs and is related to the sliding period and tilt angle of the SAM molecules. Amplitude of the stick-slip cycle increases with decreasing sliding height until reaching a critical sliding height, which is characterized such that sliding below the critical height causes irreversible changes in the SAM molecular organization and cumulative loss of SAM lubricating efficiency. Different SAM recovery mechanisms were found for different sliding directions relative to SAM tilt angle (pro- or anti-tilt). In both cases, minimum friction occurred during the SAM tilt-angle recovery phase. The friction force curves for these two cases also showed a regular phase shift above the critical height. For stick-slip sliding above the critical height, anti-tilt sliding had significantly lower average friction, but this trend inverted below the critical height. Sliding lower than the critical height cause progressive disorder of the SAM structure and the characteristic differences between pro- and anti-tilt sliding were progressively lost.     

Nanotribology and nanomechanics in nano/biotechnology

Owing to larger surface area in micro/nanoelectromechanical systems (MEMS/NEMS), surface forces such as adhesion, friction, and meniscus and viscous drag forces become large when compared with inertial and electromagnetic forces. There is a need to develop lubricants and identify lubrication methods that are suitable for MEMS/NEMS. For BioMEMS/BioNEMS, adhesion between biological molecular layers and the substrate, and friction and wear of biological layers may be important, and methods to enhance adhesion between biomolecules and the device surface need to be developed. There is a need for development of a fundamental understanding of adhesion, friction/stiction, wear, the role of surface contamination and environment, and lubrication. MEMS/NEMS materials need to exhibit good mechanical and tribological properties on the micro/nanoscale. Most mechanical properties are known to be scale dependent. Therefore, the properties of nanoscale structures need to be measured. Component-level studies are required to provide a better understanding of the tribological phenomena occurring in MEMS/NEMS. The emergence of micro/nanotribology and atomic force microscopy-based techniques has provided researchers with a viable approach to address these problems. This paper presents an overview of micro/nanoscale adhesion, friction, and wear studies of materials and lubrication studies for MEMS/NEMS and BioMEMS/BioNEMS. It also presents a review of scale-dependent mechanical properties, and stress and deformation analysis of nanostructures.     

考虑接触点摩擦的索滑移行为分析

连续索在接触点处滑移时会受接触点摩擦的影响,导致连续索在接触点两侧索力不相等。张拉连续索施加预应力时,接触点处的摩擦力使远离张拉端的索段预应力存在一定程度的损失。针对索在接触点处滑移时的摩擦问题,该文采用有限质点法,将连续索离散成相互联系的质点集合,质点间通过索单元连接,滑移索单元内力根据接触点力传递系数和索单元原长不变原则求解。该文推导了考虑接触点摩擦的滑移索单元内力求解公式,提出了索滑移判定准则,并给出了接触点力传递系数计算方法,通过自编程序对算例进行计算分析,验证了考虑接触点摩擦的滑移索单元的正确性和合理性。     

The Molecular Dynamic Finite Element Method (MDFEM)

In order to understand the underlying mechanisms of inelastic material behavior and nonlinear surface interactions, which can be observed on macroscale as damping, softening, fracture, delamination, frictional contact etc., it is necessary to examine the molecular scale. Force fields can be applied to simulate the rearrangement of chemical and physical bonds. However, a simulation of the atomic interactions is very costly so that classical molecular dynamics (MD) is restricted to structures containing a low number of atoms such as carbon nanotubes. The objective of this paper is to show how MD simulations can be integrated into the finite element method (FEM) which is used to simulate engineering structures such as an aircraft panel or a vehicle chassis. A new type of finite element is required for force fields that include multi-body potentials. These elements take into account not only bond stretch but also bending, torsion and inversion without using rotational degrees of freedom. Since natural lengths and angles are implemented as intrinsic material parameters, the developed molecular dynamic finite element method (MDFEM) starts with a conformational analysis. By means of carbon nanotubes and elastomeric material it is demonstrated that this pre-step is needed to find an equilibrium configuration before the structure can be deformed in a succeeding loading step.     

芯轴摩擦系数对推压-拉拔复合缩径的影响

推压-拉拔复合缩径工艺是管坯减径的新方法,芯轴外表面与管坯内表面之间摩擦系数对工艺有重要的影响。通过建立推压-拉拔复合缩径变形过程中变形管坯的力学模型,分析了芯轴外表面与管坯内表面之间摩擦系数对成形的影响规律。针对某载重6.5 t胀压成形汽车桥壳管件的第一道次推压-拉拔复合缩径,设定不同的芯轴摩擦系数,进行了有限元仿真,得到了芯轴摩擦系数对管坯变形的影响规律,并基于管坯传力区不失稳以及变形所需凹模推力和芯轴拉拔力较小,给出了芯轴摩擦系数的设定范围。进行了缩径实验,实验和有限元模拟的结果接近。较小的芯轴摩擦系数可能造成管坯起皱失稳,而较大的芯轴摩擦系数,有利于降低管坯轴向压应力、凹模推力和芯轴拉拔力,但可能造成管坯表面划伤。     

Molecular dynamics simulation of asphalt-aggregate interface adhesion strength with moisture effect

This study developed an atomistic simulation framework based on the classical molecular dynamics (MD) method to study the moisture-induced damage at the asphalt-aggregate interface. The interface adhesion strength of the asphalt–quartz system was predicted using MD simulation for the first time. The interface stress-separation curve under tension that was obtained from MD simulation resembles the failure behaviour measured from the pull-off strength conducted at the macroscopic scale. The results show that the presence of moisture at the asphalt–quartz interface significantly reduces the interface adhesion strength. The interface failure process is affected by the chemical compositions of asphalt. The interface adhesion strength decreases as the moisture content increases or the temperature increases. It was found that the atomistic model size (number of atoms) and the loading rate in MD simulation have considerable effects on the predicted interface adhesion strength. The findings from MD simulation provide fundamental understanding of material failure at the atomistic scale that cannot be observed at the normal experimental testing environment for asphalt materials. The MD simulation results can be potentially calibrated and utilised as inputs for higher scale micromechanical models to predict bulk mechanical responses of asphalt mixtures.     

高速列车过隧道对弓网动力学影响分析

为研究高速列车通过隧道时产生的受电弓空气动力学效应对弓网动力学性能的影响,分别建立了受电弓/高速列车空气动力学仿真模型和弓网耦合系统动力学模型。采用滑移网格技术实现了高速列车运动,通过有限体积法求解三维瞬态可压缩Navier-Stokes方程和 两方程湍流模型,计算了列车速度为350km/h通过隧道时受电弓的气动抬升力,对考虑和未考虑列车通过隧道产生的受电弓气动抬升力作用时的弓网动力学响应进行了对比分析。计算结果表明,受电弓气动抬升力在隧道入口和出口时出现峰值,隧道内的气动抬升力较明线上大;通过隧道时产生的受电弓气抬升力变化对弓网接触压力和接触线抬升位移具有显著影响,导致受流质量变差。     

Atomistic simulations of shearing friction and dynamic adhesion of double-walled carbon nanotubes on Au substrates

Functional gecko-mimetic adhesives have attracted a lot of research interest in recent years. In this paper, the physical adhesion behavior of (5, 5)@(10, 10) double-walled carbon nanotubes (DWCNTs) on an Au substrate is investigated by performing detailed, fully atomistic molecular dynamics (MD) simulations. The effects of adhesion temperature, tube length, and peeling velocity on the binding energy, normal adhesion force, lateral shearing friction, and adhesion time are thoroughly analyzed. The simulation results indicate that the binding energy (per unit length) of the DWCNT–Au adhesive system is −26.7 × 10⁻² eV/Å, which is 7.2% higher than that of single-walled counterparts. The tip-surface adhesion force for a single DWCNT is calculated to be 1.4 nN, and thus the adhesive strength of a DWCNT array is about 1.4 × 10¹–1.4 × 10³ N/cm² (corresponding to an aerial density of 10¹⁰–10¹² tubes/cm²). Two distinctive friction modes, namely (i) sliding friction (by the nanotube wall) and (ii) sticking friction (by the nanotube tip), are elucidated in term of the phase relationship of atomic friction forces. Moreover, the effective Young’s moduli of double- and single-walled CNTs are obtained using MD simulations combined with Euler–Bernoulli beam theory. The calculation results show good agreement with previously reported numerical and experimental results.     

Investigation in the Effects of Configuration Parameters on the Thermal Behavior of Novel Conical Friction Plate in Continuously Sliding Condition

To investigate the effects of configuration parameters and operation condition on the thermal behavior of novel conical friction plate, a three-dimensional finite element model of conical friction plate is established for numerical simulation. The conical surface configuration and friction heat generation of novel conical friction surfaces are discussed. The results indicate that the thermal behavior of the conical friction plate during continuously sliding period is influenced by the conical surface configuration. Maximum temperature occurs in the conical friction plate with cone angle of 24°. The maximum temperature value of friction plate is increased 7.4°C, when cone depth increases from 3 mm to 4 mm. Thermal behavior investigation should be carried out when optimize conical surface configuration     

The effect of loading on surface roughness at the atomistic level

One of the key points to better understand the origins of friction is to know how two surfaces in contact adhere to one another. In this paper we present molecular dynamics (MD) simulations of two aluminium bodies in contact, exposed to a range of normal loads. The contact surfaces of both aluminium bodies have a self-affine fractal roughness, but the exact roughness varies from simulation to simulation. Both bodies are allowed to have an adhesive interaction and are fully deformable. Tracking important contact parameters (such as contact area, number of contact clusters, and contact pressure) during a simulation is challenging. We propose an algorithm (embedded within a parallel MD code) which is capable of accessing these contact statistics. As expected, our results show that contact area is increasing in proportion with applied load, and that a higher roughness reduces contact area. Contact pressure distributions are compared to theoretical models, and we show that they are shifted into the tensile regime due to the inclusion of adhesion in our model.     

高速滑靴瞬态摩擦温升研究

目的飞行器试验时,滑靴会因为与轨道间强烈的摩擦热、气动热以及接触表面微观粗糙峰间的相互高速冲击而产生磨损、温升、凿削等损伤,针对这种情况,利用数值模拟软件进行研究,以解决目前通过实际手段无法有效检测滑靴烧蚀的难题。方法利用Comsol Multiphysics高级多物理场有限元仿真软件对靴轨摩擦副建模,主要分析滑靴在不同速度、不同载荷下由于摩擦生热产生的温升现象。结果滑靴的具体烧蚀情况会随着施加速度、载荷的不同而有差异;其中,滑靴在最高速度为300 m/s时开始烧蚀的时间比最高速度为100 m/s时提前了1 s;而载荷对滑靴表面温度的大小略有影响。结论为滑靴的合理设计提供了理论依据。     

Molecular dynamics study of wearless friction in sub-micrometer size mechanisms and actuators based on an atomistic simplified model

Frictional resistance accompanying steady sliding motion is studied systematically as a function of various experimental parameters by means of molecular dynamics simulation using a two-dimensional model. Exploration of universal features lying in wearless friction in mesoscopic systems revealed unique dependence of the frictional force on sliding velocity, applied load, and mean temperature. Atomistic origin of this dependence is discussed in terms of “size effect” on the basis of a hypothesis that the frictional power corresponds to the rate of energy dissipation via forced vibration and subsequent energy transfer between normal phonon modes due to anharmonicity of inter-particle potential.     

Transverse compression behavior of textile rovings: finite element simulation and experimental study

Results from finite element simulation of the transverse compression of Polyamide 6.6 rovings made of 40 filaments are compared in this paper against experimental data. The finite element simulation, using a finite strain beam model to represent each filament of the roving, focuses on the modeling of contact–friction interactions between filaments. Experimental tests, consisting in crushing rovings between two rigid planes, varying the twist and the tensile force, are reproduced by simulation. Experimental and simulation results are compared with a good agreement. The simulation studies the influence of the roving twist, the applied tensile force, and the friction coefficient on the transverse compression behavior. The occurrence of plateaus in the transverse compression curve is highlighted and discussed.     

An adaptive FE–MD model coupling approach

This contribution introduces an adaptively coupled finite element (FE)–molecular dynamics (MD) model based on the Quasicontinuum (QC) method. The idea for obtaining constitutive laws from the underlying lattice structure (local QC model) will be discussed in detail. The outline of the formulation for the quasi-static MD model (nonlocal QC model) will also be derived in the same mathematical structure. A new type of element is proposed to solve the boundary problems and to couple the FE and MD models. The interpolation techniques for the atomic stress and strain fields are introduced. A two-step adaptive mechanism is applied to the multiscale model, including the mesh refinement step for the FE model and the FE–MD conversion step. A 3D nanoindentation example is used for demonstrating accuracy and the efficiency of the coupled FE–MD model at the end.     

Elastic crack propagation model for crystalline solids using a self-consistent coupled atomistic–continuum framework

Deformation and failure processes of crystalline materials are governed by complex phenomena at multiple scales. It is necessary to couple these scales for physics-based modeling of these phenomena, while overcoming limitations of modeling at individual scales. To address this issue, this paper develops self-consistent elastic constitutive and crack propagation relations of crystalline materials containing atomic scale cracks, from observations made in a concurrent multi-scale simulation system coupling atomistic and continuum domain models. The concurrent multi-scale model incorporates a finite temperature atomistic region containing the crack, a continuum region represented by a self-consistent crystal elasticity constitutive model, and a handshaking interphase region. Atomistic modeling is done by the molecular dynamics code LAMMPS, while continuum modeling is conducted by the finite element method. For single crystal nickel a nonlinear and nonlocal crystal elasticity constitutive relation is derived, consistent with the atomic potential function. An efficient, staggered solution scheme with parallel implementation is designed for the coupled problem. The atomistic–continuum coupling is achieved by enforcing geometric compatibility and force equilibrium in the interphase region. Quantitative analyses of the crack propagation process focuses on size dependence, strain energy release rate, crack propagation rate and degradation of the local stiffness. The self-consistent constitutive and crack propagation relations, derived from the concurrent model simulation results are validated by comparing results from the concurrent and full FE models. Excellent accuracy and enhanced efficiency are observed in comparison with pure MD and concurrent model results.     

高速列车交会时的风致振动研究

摘 要：为了阐明高速列车交会过程中气动力对列车的系统动力学行为的影响,分别建立了CRH-2动车组的简化几何模型和50个自由度的车辆系统动力学模型。采用有限体积法对三维瞬态可压缩雷诺时均Navier-Stokes方程和k-e 两方程湍流模型进行求解,并通过滑移网格技术实现列车的运动,对考虑和不考虑气动力时的列车系统力学响应进行了数值模拟,并对两种情况下列车的安全性和舒适性进行了分析讨论。研究发现：气动力在列车交会过程中变化剧烈,对列车系统动力学行为的影响非常明显,交会时列车振动剧烈,头车和尾车的安全性和舒适性明显降低。     

Selective damping method for the weak‐Arlequin coupling of molecular dynamics and finite element method

An artificial damping force is introduced in the weak coupling between the molecular dynamics (MD) and finite element (FE) models, to reduce the reflection of the high‐frequency motion that cannot be transmitted from the MD domain to the FE domain. We take advantage of the orthogonal property of the decomposed velocity in the weak coupling method and apply the damping force only to the high‐frequency part, therefore minimizing its effect on the low‐frequency part, which can be transmitted into the FE domain. The effectiveness of the damping method will be demonstrated by 1D numerical examples with linear force field applied to the atomistic model. In addition, we emphasize the importance of using the Arlequin energy interpolation, which is usually ignored in the weak coupling literature. Non‐uniform rational basis spline functions have been used to interpolate the MD data for the weak coupling method, and the influence of changing the number and order of basis functions on the interpolation accuracy has been investigated numerically. For this work, we restrict our discussion to mechanical problems only, involving only mechanical energy terms (e.g., strain potential and kinetic energy). Copyright © 2013 John Wiley & Sons, Ltd.     

表面张力作用下抗粘附的微齿轮结构设计原理研究

粘附是微机械中最常见的一种失效现象,一般的微机械结构都极力避免粘附的出现．表面张力是引起微结构粘附的最主要原因．深入研究了表面张力作用下使构件产生粘附的物理机理．以多晶硅微齿轮为例,考虑其在表面张力作用下发生的粘附现象,推导出表面张力作用下抗粘附多晶硅微齿轮结构参数设计公式．结果表明,微齿轮产生粘附与材料特性（弹性特性、表面特性）、微齿轮结构参数（半径、齿宽）、微齿轮与基体之间的间隙有关．     

基于有限元的三辊行星轧制力预测及分析

为预测三辊行星轧制力，建立了三维热力耦合有限元模型，用均匀设计法安排有限元模拟样本，采用基于LM算法的BP神经网络学习有限元轧制力结果，确定了轧制参数与轧制力的映射关系，并根据训练后的神经网络分析了摩擦系数、轧辊偏转角和轧辊转速对轧制力影响．预测结果表明：摩擦系数和轧辊偏转角对轧制力影响是多方面的，在高轧辊转速时，较大的摩擦系数有利于降低轧制力．     

Shear properties of organic films adsorbed on silica fibres

Contact adhesion and sliding friction of octadecylamine and silane molecules adsorbed on silica fibre were measured with orthogonally crossed silica fibres using an electronic microbalance. Interfacial shear strengths, τ, as a function of contact pressure, P, between organic films were deduced from the adhesional model of friction using the measured frictional force and the calculated real area of contact. The pressure dependence of the interfacial shear strength was then interpreted in terms of the molecular interaction of adsorbates with solvent and the surface energetics of molecules adsorbed on the silica fibre. This revised version was published online in November 2006 with corrections to the Cover Date.