Two dimensional nanoscale reciprocating sliding contacts of textured surfaces

Detailed behaviors of nanoscale textured surfaces during the reciprocating sliding contacts are still unknown although they are widely used in mechanical components to improve tribological characteristics. The current research of sliding contacts of textured surfaces mainly focuses on the experimental studies, while the cost is too high. Molecular dynamics(MD) simulation is widely used in the studies of nanoscale single-pass sliding contacts, but the CPU cost of MD simulation is also too high to simulate the reciprocating sliding contacts. In this paper, employing multiscale method which couples molecular dynamics simulation and finite element method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. Four textured surfaces with different texture shapes are designed, and a rigid cylindrical tip is used to slide on these textured surfaces. For different textured surfaces, average potential energies and average friction forces of the corresponding sliding processes are analyzed. The analyzing results show that “running-in” stages are different for each texture, and steady friction processes are discovered for textured surfaces II, III and IV. Texture shape and sliding direction play important roles in reciprocating sliding contacts, which influence average friction forces greatly. This research can help to design textured surfaces to improve tribological behaviors in nanoscale reciprocating sliding contacts.     

纳观纹理表面相关参数对滑动摩擦过程的影响

采用分子动力学方法研究了半球形刚性压头在单晶铜纹理表面上的纳观黏着滑动摩擦过程。对不同纹理密度下纹理形状和纹理深度对黏滑摩擦性能的影响进行了全面研究,通过对比分析不同纹理参数下的滑动摩擦力和基体变形,揭示了上述参数对纹理表面黏滑摩擦的影响规律。模拟结果表明:在相同的纹理密度下,柱状纹理表面的滑动摩擦力小于矩形纹理表面。相比矩形纹理,柱状纹理表面的结构稳定性较差,但纹理表面的结构稳定性随着纹理密度的增加而加强。在相同的纹理密度下,矩形纹理表面的滑动摩擦力随着纹理深度的增加而减小。     

Computer simulation of sliding hydroxylated alumina surfaces

A molecular dynamics simulation is performed to investigate the frictional force and energy transfer dynamics associated with sliding hydroxylated alumina surfaces. The calculated coefficient of friction is in good agreement with a recent experimental study. The dynamics of energy transfer from the interface of the sliding surface is investigated by calculating the surface–surface intermolecular potential and the energy in surface hydroxyl groups. The simulations indicate the experimental friction force arises from energy relaxation. A transition from stick–slip to smooth sliding is observed as the sliding velocity is increased. This revised version was published online in September 2006 with corrections to the Cover Date.     

Molecular dynamics studies of atomic-scale tribological characteristics for different sliding systems

A slider-slab sliding model for hard-to-soft and soft-to-soft sliding systems with abrasive and non-abrasive wear conditions is used to investigate atomic-scale friction. The molecular dynamics simulation uses the Morse potential to calculate interatomic forces between atoms. Separation distance between the slider and the slab is changed to simulate repulsive and attractive interactive force fields exerted on interface between two sliding components. Effects of the interaction potential parameters on the sliding friction are investigated. The relationship of frictional force, normal force and temperature rise of the slider and the slab during sliding are established. Comparison of the hard-to-soft and the soft-to-soft sliding system are carried out and shows different tribological phenomena.     

Low-Speed Atomistic Simulation of Stick–Slip Friction using Parallel Replica Dynamics

Atomic stick–slip friction has been predicted by molecular dynamics simulation and observed in experiments. However, direct quantitative comparison of the two has thus far not been possible because of the large difference between scanning velocities accessible to simulations and experiments. In general, the slowest sliding speeds in MD simulations are at least five orders of magnitude larger than the upper limit available to experimentalists. To take a step toward bridging this gap, we have applied parallel replica dynamics, an accelerated molecular dynamics method, to the simulation of atomic stick–slip. The method allows molecular simulations to run parallel in time in order to extend their duration, thereby enabling lower scanning velocities. We show here that this method is able to predict atomic stick–slip friction accurately and efficiently at scanning speeds several orders of magnitude slower than standard molecular dynamics simulations. The accuracy and usefulness of this method is illustrated by correct prediction of the logarithmic dependence of friction on velocity.     

Atomistic Simulation of Frictional Sliding Between Cellulose Iβ Nanocrystals

Sliding friction between cellulose Iβ nanocrystals is studied using molecular dynamics simulation. The effects of sliding velocity, normal load, and relative angle between sliding surface are predicted, and the results analyzed in terms of the number of hydrogen bonds within and between the cellulose chains. We find that although the observed friction trends can be correlated with hydrogen bonding, it may not be the most significant factor in determining frictional behavior on cellulose nanocrystal surfaces.     

Modelling lubricated friction for dynamic contact systems

Multi-valued friction is often observed in lubricated contact systems operating under unsteady sliding conditions. In the past, this behaviour has been modelled using a measured time delay between changes in sliding velocity and resulting changes in friction. More recent work has revealed that fluctuations in normal separation are the principal cause of this deviation from steady friction behaviour. This paper demonstrates that tangential contact dynamics can result in apparent deviations from steady friction that can resemble true deviations found from fluctuations in normal separation. Two different dynamic models are considered. One model accommodates the tangential rider dynamics of a friction test apparatus. The other model considers the possibility of tangential dynamics associated with asperity contacts.     

滑动轴承动力学温度及压力分布规律研究

从系统动力学的角度出发分析了滑动轴承在运动状态下的内部温度变化规律和压力分布规律,建立了滑动轴承内部在系统动力学下的温度场模型和动态油膜压力分布模型,并对模型进行了数值仿真.仿真结果表明:滑动轴承在系统动力学状态下,其内部的温度分布规律是沿轴承套中心轴线方向,往外温度升高较快;动态油膜压力的大小与转子的位移有关,并且油膜压力在动力学状态下呈不对称分布,与轴承的摩擦磨损有着紧密的关系.这些工作对于进一步掌握滑动轴承内部在系统动力学下的温度和压力分布规律有一定的借鉴意义,为滑动轴承内部的动态摩擦磨损的研究提供了理论研究支撑.     

A Local Region Molecular Dynamics Simulation Method for Nanoscale Sliding Contacts

Computational efficiency and accuracy always conflict with each other in molecular dynamics (MD) simulations. How to enhance the computational efficiency and keep accuracy at the same time is concerned by each corresponding researcher. However, most of the current studies focus on MD algorithms, and if the scale of MD model could be reduced, the algorithms would be more meaningful. A local region molecular dynamics (LRMD) simulation method which can meet these two factors concurrently in nanoscale sliding contacts is developed in this paper. Full MD simulation is used to simulate indentation process before sliding. A criterion called contribution of displacement is presented, which is used to determine the effective local region in the MD model after indentation. By using the local region, nanoscale sliding contact between a rigid cylindrical tip and an elastic substrate is investigated. Two two-dimensional MD models are presented, and the friction forces from LRMD simulations agree well with that from full MD simulations, which testifies the effectiveness of the LRMD simulation method for two-dimensional cases. A three-dimensional MD model for sliding contacts is developed then to show the validity of the LRMD simulation method further. Finally, a discussion is carried out by the principles of tribology. In the discussion, two two-dimensional full MD models are used to simulate the nanoscale sliding contact problems. The results indicate that original smaller model will induce higher equivalent scratching depth, and then results in higher friction forces, which will help to explain the mechanism how the LRMD simulation method works. This method can be used to reduce the scale of MD model in large scale simulations, and it will enhance the computational efficiency without losing accuracy during the simulation of nanoscale sliding contacts.     

A Local Region Molecular Dynamics Simulation Method for Nanoscale Sliding Contacts

Computational e ciency and accuracy always conflict with each other in molecular dynamics(MD) simulations. How to enhance the computational e ciency and keep accuracy at the same time is concerned by each corresponding researcher. However, most of the current studies focus on MD algorithms, and if the scale of MD model could be reduced, the algorithms would be more meaningful. A local region molecular dynamics(LRMD) simulation method which can meet these two factors concurrently in nanoscale sliding contacts is developed in this paper. Full MD simulation is used to simulate indentation process before sliding. A criterion called contribution of displacement is presented, which is used to determine the e ective local region in the MD model after indentation. By using the local region, nanoscale sliding contact between a rigid cylindrical tip and an elastic substrate is investigated. Two two?dimensional MD models are presented, and the friction forces from LRMD simulations agree well with that from full MD simulations, which testifies the e ectiveness of the LRMD simulation method for two?dimensional cases. A three?dimensional MD model for sliding contacts is developed then to show the validity of the LRMD simulation method further. Finally, a discussion is carried out by the principles of tribology. In the discussion, two two?dimensional full MD models are used to simulate the nanoscale sliding contact problems. The results indicate that original smaller model will induce higher equivalent scratching depth, and then results in higher friction forces, which will help to explain the mechanism how the LRMD simulation method works. This method can be used to reduce the scale of MD model in large scale simulations, and it will enhance the computational e ciency without losing accuracy during the simula?tion of nanoscale sliding contacts.     

考虑变工况冲击的齿轮动态啮合力分析

为研究变工况冲击对齿轮传动系统动特性影响,基于弹塑性接触理论,给出一种可以考虑变工况冲击、啮入冲击、节点冲击的齿轮接触碰撞力参数预估算法,并结合多体动力学软件建立柔性齿轮传动系统动力学模型。该模型和算法可用于大接触变形和承受频繁冲击齿轮传动系统稳态和瞬态动特性分析。研究表明:与啮入冲击导致的稳态动态啮合力相比,变工况冲击引起的瞬态动态啮合力具有幅值大、冲击时间短等特点;在不同工况下,啮入冲击会引起不同周期的齿轮动态啮合力波动;滑动摩擦系数对齿轮切向摩擦力的节点冲击影响更大。研究结果对齿轮的接触碰撞力参数预估及全面认识齿轮传动系统瞬态动特性等研究具有积极的意义。     

The Roles of Statics and Dynamics in Determining Transitions Between Atomic Friction Regimes

We introduce a model AFM tip/substrate system that includes full atomistic detail as well as system compliance to study the transitions between three regimes of atomic friction: smooth sliding, stick-single slip, and stick-multiple slip. We characterize these atomic friction regimes in terms of static and dynamic effects, and investigate how the slip modes affect the mean friction. Molecular statics calculations show that reduced-order model predictions of possible transitions between slip regimes are generally adequate for a fully atomistic system, even for complex reaction coordinates. However, molecular dynamics simulations demonstrate that, while static features of the system govern possible slip regimes, dynamic effects ultimately determine actual transitions between slip regimes.     

Estimation of Road Friction Coefficient in Different Road Conditions Based on Vehicle Braking Dynamics

The accurate estimation of road friction coefficient in the active safety control system has become increasingly prominent. Most previous studies on road friction estimation have only used vehicle longitudinal or lateral dynamics and often ignored the load transfer, which tends to cause inaccurate of the actual road friction coefficient. A novel method considering load transfer of front and rear axles is proposed to estimate road friction coefficient based on braking dynamic model of two-wheeled vehicle. Sliding mode control technique is used to build the ideal braking torque controller, which control target is to control the actual wheel slip ratio of front and rear wheels tracking the ideal wheel slip ratio. In order to eliminate the chattering problem of the sliding mode controller, integral switching surface is used to design the sliding mode surface. A second order linear extended state observer is designed to observe road friction coefficient based on wheel speed and braking torque of front and rear wheels.The proposed road friction coefficient estimation schemes are evaluated by simulation in ADAMS/Car. The results show that the estimated values can well agree with the actual values in different road conditions. The observer can estimate road friction coefficient exactly in real-time andresist external disturbance. The proposed research provides a novel method to estimate road friction coefficient with strong robustness and more accurate.     

Dry Sliding Contact Between Rough Surfaces at the Atomistic Scale

Although, a lot is known about the factors contributing to friction, a complete physical understanding of the origins of friction is still lacking. At the macroscale several laws have long since described the relation between load (Amontons, Coulomb), apparent and real area of contact (Bowden and Tabor), and frictional forces. But it is not yet completely understood if these laws of friction extend all the way down to the atomistic level. Some current research suggests that a linear dependence of friction on the real contact area is observed at the atomistic level, but only for specific cases (indentors and rigid substrates). Because continuum models are not applicable at the atomic scale, other modeling techniques (such as molecular dynamics simulations) are necessary to elucidate the physics of friction at the small scale. We use molecular dynamics simulations to model the friction of two rough deformable surfaces, while changing the surface roughness, the sliding speed, and the applied normal load. We find that friction increases with roughness. Also all sliding cases show considerable surface flattening, reducing the friction close to zero after repetitive sliding. This questions the current view of (static) roughness at the atomistic scale, and possibly indicates that the macroscopic laws of friction break down several orders of magnitude before reaching the atomic scale.     

Enhanced friction model for high-speed right-angle gear dynamics

The modeling of elastohydrodynamic lubrication friction and the analysis of its dynamic effect on right-angle gears, such as hypoid and spiral bevel types are performed in the present study. Unlike the classically applied empirical constant coefficient of friction at the contacting tooth surfaces, the enhanced physics-based gear mesh friction model is both spatial and time-varying. The underlying formulation assumes mixed elastohydrodynamic lubrication (EHL) condition in which the division and load distribution between the full film and asperity contact zones are determined by the film thickness ratio and load sharing coefficient. In the proposed time-varying friction model, the calculation of friction coefficient is performed at each contact grid inside the instantaneous contact area that is being subjected to mineral oil lubrication. The effective friction coefficient and directional parameters synthesized from the net frictional and normal contact forces are then incorporated into a nonlinear time-varying right-angle gear dynamic model. Using this model, the effect of friction on the gear dynamic response due to the transmission error and mesh excitations is analyzed. Also, parametric studies are performed by varying torque, surface roughness and lubrication properties to understand the salient role of tooth sliding friction in gear dynamics. The simulation results are included. But experimental verification is needed.     

单壁碳纳米管与石墨基底间摩擦的模拟研究

采用分子动力学模拟方法研究了常温300 K时,公度、不公度情况下,单壁碳纳米管CNT(10,10)在石墨基底上的运动、摩擦行为。计算中首先使碳纳米管在基底上弛豫平衡,而后施加持续时间500 fs的固定外力,撤去外力后碳纳米管在基底上减速至相对基底静止。结果表明,碳纳米管在石墨基底上不同的放置位置决定了它与基底接触面的微观构型,从而决定了碳纳米管的运动、摩擦规律。公度时,碳纳米管先在基底上滑动,摩擦力、平动能均呈现周期性起伏,之后碳纳米管在基底上滚动、滑动、翻转,滑动、转动之间运动形式的转变提高了能量耗散,增大了摩擦力,非公度时摩擦力约为公度时的70%。非公度时碳纳米管一直在基底上滑动,平动能和摩擦力不具有周期性。     

Friction characteristics of nanoscale sliding contacts between multi-asperity tips and textured surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.     

Consequence of contact local kinematics of sliding bodies on the surface temperatures generated

When studying contact with friction between two bodies, it is not possible to obtain data on real contact conditions on the basis of steady-state situations. Indeed, contacts with friction usually lead to dynamic instabilities generated at the contact interface. It is therefore necessary to take into account contact dynamics in order to better understand the phenomena involved during sliding with friction. The explicit dynamic finite element code PlastD in 2D is used to simulate the contact between two bodies. A constant Coulomb friction coefficient is imposed at the interface. The simulations carried out permitted identifying local contact conditions (kinematics, tribological state, stresses, etc.). They revealed that different instability regimes can be generated (stick–slip, slip–separation, stick–slip–separation, etc.). Local contact stresses and the sliding velocity oscillate through time when instabilities are generated and their maximum values can be much higher than those expected for steady-state conditions. The aim of this paper is to analyse the frictional instabilities and their consequences on the heat generated in the contact. First, the influence of the different instability regimes is studied on a simple contact. Then, an industrial mechanism is studied (wheel–rail contact) to investigate the influence of local contact conditions on the temperature of the rail surface.     

Internal synthesis of motion and the dynamic characteristics of external friction

On the example of a simple tribosystem in the form of a body sliding (rolling) over an inclined plane, it is demonstrated that the kinematic concept of external friction based on the Amonton-Coulomb law of modifications is not appropriate as a base for the study of the dynamics of external friction. The obtained dynamic friction characteristics in respect to the velocity and acceleration prove that the velocity of motion varies and that the dynamic equations are nonlinear both in respect to velocity and acceleration. The mechanism of dynamic feedback in tribosystems ensures both subordination to external effects and the capacity for effective synthesis of relevant motion types. The problem is analyzed of selecting adequate design types of motion elements of a tribosystem in relation to the frictional self-excited vibrations and the metrology of measuring the coefficients of sliding and rolling friction. In view of the high complexity of a priori modeling of external friction, it is concluded that methods of experimental tribodynamics should be developed.     

Atomic-scale study of friction and energy dissipation

This paper presents an analysis of the interaction energy and various forces between two surfaces, and the microscopic study of friction. Atomic-scale simulations of dry sliding friction and boundary lubrication are based on the classical molecular dynamics (CMD) calculations using realistic empirical potentials. The dry sliding of a single metal asperity on an incommensurate substrate surface exhibits a quasi-periodic variation of the lateral force with two different stick-slip stage involving two structural transformation followed by a wear. The contact area of the asperity increases discontinuously with increasing normal force. Xe atoms placed between two atomically flat Ni surfaces screen the Ni-Ni interaction, decrease the corrugation of the potential energy as well as the friction force at submonolayer coverage. We present a phononic model of energy dissipation from an asperity to the substrates.