纳米黏着接触行为及其压力分布和相变过程分析

运用分子动力模拟方法研究刚性球型探头与原子级光滑表面的黏着接触过程,分析接触过程的载荷-位移曲线、压力分布和相变情况;得到加载过程黏着接触的"突跳点"与"引力最大点",分别是由于探头与基体原子之间的范德华作用和键合作用导致;在卸载过程中引力最大时刻,探头与基体发生"颈缩"分离。在"突跳"点时,接触区原子处于受拉状态,在"突跳"过程后,接触区中心原子逐渐处于受压状态,接触边缘处于受拉状态;随着探头不断压入基底,接触区中心原子所受压力越来越大。在卸载过程中的引力最大时刻,接触区原子重新处于受拉状态。在加载过程中接触区内有高压相的产生,在卸载过程"颈缩"分离阶段,配位数为0的原子数目迅速增加,更多的原子粘附在探头上被带走。     

Velocity-Dependent Adhesion with Lubricants on Thin-Film Disks

The well-known problem of stiction in a magnetic disk drive largely depends on the forces induced by the presence of a thin liquid film. It is commonly recognized that both adhesive and viscous effects contribute to the magnitude of the stiction force, but is is not known what relative roles the two effects have in a lubricated contact. In the present work, the nature of adhesive and viscous effects is investigated for the slider/disk interface under conditions of constant-speed sliding.

Friction measurements are conducted over a range of sliding speeds, 0.25-250 mm/s, with eight perfluoropolyether (PFPE) lubricants applied in various thicknesses, 0-6.6 nm, to carbon-coated magnetic thin-film disks. The lubricants were selected to cover a broad range of viscosities. For several sliding speeds and lubricant film thicknesses, the friction force is found to decrease significantly with increasing sliding speed for all lubricants. In several instances, large friction forces are observed at the lowest sliding speeds, indicating stiction-like behavior, whereas, at higher speeds, the friction is reduced to even below unlubricated friction levels. At the highest film thickness and sliding speed, the friction was found to increase with speed for some lubricants. The implications of these results on current models of lubricant-mediated adhesion are discussed.     

Nano-scale roughness effects on hysteresis in micro-scale adhesive contact

A multi-scale model enables investigation of the effect of surface roughness on energy loss in adhesive contact. Fully atomistic simulation of nano-scale indentation predicts size-dependent force–distance trends that are introduced as roughness into a micro-scale finite element model through randomly distributed non-linear hysteretic springs. The multi-scale model predicts that the energy loss, quantified by the hysteresis loop formed by loading and unloading data, increases with increasing indentation depth and surface roughness. This behavior is discussed in terms of an analytical model of a simply connected system.     

Modulus characterization of cells with submicron colloidal probes by atomic force microscope

Colloidal probes have been increasingly demanded for the characterization of cellular modulus in atomic force microscope because of their well-defined geometry and large contact area with cell. In this work, submicron colloidal probes are prepared by scanning electron microscope/focused ion beam and compared with sharp tip and micron colloidal probe, in conjunction with loading velocity and indentation depth on the apparent elastic modulus. NIM and cartilage cells are used as specimens. The results show that modulus value measured by sharp tip changes significantly with loading velocity while remains almost stable by colloidal probes. Also, submicron colloidal probe is superior in characterizing the modulus with increasing indentation depth, which could help reveal the mechanical details of cellular membrane and the modulus of the whole cell. To test the submicron colloidal probe further, the modulus distribution map of cell is scanned with submicron colloidal probe of 50 nm radius during small and large indentation depths with high spatial resolution. The outcome of this work will provide the effective submicron colloidal probe according to the effect of loading velocity and indentation depth, characterizing the mechanical properties of the cells.     

Intermolecular Forces, Adhesion, and the Elastic Foundation

A model for the elastic contact between a rigid sphere and an ideal elastic foundation with adhesion has been developed. The model was derived by integrating the full Lennard-Jones potential to arrive at a closed-form equilibrium condition that balances surface energy with strain energy. It was found that the separation height is not a function of the penetration. Using this energy criterion for separation of contact in an elastic foundation, a model for the force displacement relationship was then developed. In this derivation there exists a tensile zone of deformation along the perimeter of the contact. The model also reveals a number of unique aspects of the adhesive contact, including: the maximum adhesion occurs when the apex of the sphere is tangent to the plane of the undeformed surface, the maximum adhesion force $ F_{\text{adh}} = - 2\pi R\Updelta \gamma $ , and the contact area is linearly dependent on penetration. The ability to fit high fidelity indentation data from finite-element analysis and molecular dynamics simulation for thin films was demonstrated. Additionally, experiments were performed on thin films (~40 μm) of PDMS using a custom-built microtribometer with in situ optical interferometry that enabled simultaneous measurements of contact area, penetration depths, externally applied force, and the detailed measurements of the free-surface deformations, which include the predicted tensile zone along the perimeter of contact.     

干膜润滑剂附着性能评价方法的试验探讨

本文对润滑涂层附着力的测定方法进行了全面的评价,通过干膜润滑剂多项性能试验测定结果的讨论,认为评价润滑涂层真地着力应考虑其它性能测定结果。     

磨屑对润滑油摩擦性能的影响

通过实验和模拟研究磨粒对润滑油摩擦性能的影响。首先通过微纳米压/划痕试验测量含磨屑润滑油的摩擦因数。同时,建立边界润滑体系模型,采用分子动力学方法模拟含磨屑润滑油膜在不同载荷下沿膜厚方向的压缩率和密度分布;对体系的上下固体壁面施加方向相反的剪切速度,计算出壁面原子的应力、摩擦力、正压力和摩擦因数;分析不同粒径磨屑的动态行为特征;通过减少润滑油分子数量,探究乏油工况下含磨屑润滑体系的摩擦性能。结果表明,润滑体系摩擦因数的模拟值与试验值一致;磨屑的存在会降低油膜的压缩率,同时在高载下磨屑的存在会对油膜的分层产生破坏,影响磨屑附近的密度分布;含小粒径磨屑的润滑体系的摩擦因数比含大粒径磨屑的润滑体系的小,表明磨粒聚集长大现象会恶化润滑油的润滑性能;磨屑在剪切过程中同时存在滚动和滑动,含小粒径磨屑的润滑体系剪切过程中表现出波动幅度更大的角速度;随着载荷的增大,磨屑角速度减小,波动幅度降低;在乏油工况下,磨屑会在剪切过程中出现变形破碎现象。     

Ionic liquids with ammonium cations as lubricants or additives

A new class of more effective lubricants could lead to huge energy savings. Limited recent literature has suggested potential for using room-temperature ionic liquids as lubricants, however, only a few out of millions (or more) of species possible have been evaluated. In this study, a series of new protic alkylammonium ionic liquids were synthesized by neutralization and metathesis reactions, and have demonstrated promising lubricating properties as neat lubricants or lubricant additives, particularly for use with difficult-to-lubricate metals like aluminum. More than a 30% friction reduction was observed with ammonium-based ionic liquids compared to conventional hydrocarbon oils in reciprocating sliding tests of 52100 bearing steel on aluminum alloy 6061-T6511. The inherent polarity of ionic liquids is believed to provide strong adsorption to contact surfaces and can form a boundary lubricating film leading to friction and wear reductions. Other advantages of ionic liquids include (1) negligible volatility, (2) high thermal stability, (3) non-flammability, and (4) better intrinsic properties that may eliminate the need for more complex and expensive additive packages. With very flexible molecular structures, this new class of lubricants, particularly ammonium-based ionic liquids, can be tailored to fit a variety of applications.     

Effect of tetraalkylphosphonium based ionic liquids as lubricants on the tribological performance of a steel-on-steel system

A series of asymmetrical tetraalkylphosphonium ionic liquids were synthesized and evaluated as a new kind of lubricant for the contact of steel/steel using an Optimol SRV oscillating friction and wear tester in ambient condition. The phosphonium ionic liquid shows excellent tribological performance when being used as the lubricating oil, and is superior to the conventional high temperature lubricants X-1P and perfluoropolyether (PFPE) in terms of anti-wear performance and load-carrying capacity. The chemical compositions of the boundary film generated on different contact surfaces were analyzed on a scanning electron microscope with a Kevex energy dispersive X-ray analyzer attachment (SEM/EDS) and X-ray photoelectron spectrometer (XPS). The friction–reduction and anti-wear mechanism of tetraalkylphosphonium as the lubricant were proposed to originate from the active elements P in the tetraalkylphosphonium ionic liquids reacting with the fresh surface to form a reaction film onto specimen surface, an extreme-pressure film with lower shearing strength, which leads to lower friction coefficient, and good wear resistance.     

Multiscale analysis on two-dimensional nanoscale adhesive contacts

Nanoscale adhesive contacts play a key role in micro/nano-electro-mechanical systems as the dimension of the components come to nanometer.Experimental studies on nanoscale adhesive contacts are limited by some uncertain factors and the cost of experiments is too high.Besides,nanoscale textured surfaces are difficult to process and nanoscale adhesive contacts of textured surfaces are still lack of investigation.By using multiscale method,which couples molecular dynamics simulation and finite element method,two-dimensional nanoscale adhesive contacts between a rigid cylindrical tip and an elastic substrate are investigated.For the contacts between the rigid cylindrical tip and smooth surface,Von Mises stress distributions,the maximum Von Mises stresses,and contact forces are compared for different radii to show the size effects and adhesive effects.The phenomena of hysteresis are observed and more obvious as the radii of the tip increase.The influences of indentation depth and indentation speed are also discussed.Then two series of textured surfaces are employed,and the influences of the texture asperity shape,asperity height,and asperity spacing on contact forces are studied.The contact forces comparisons show that textured surfaces can reduce contact forces effectively in the range of the two series.Contact forces of textured surfaces increase as the asperity heights increase,and textured surfaces with smaller asperity spacing will get higher contact forces.Contact forces may be controlled through textured surfaces in the future.The obtained results will help to improve contact condition and provide theory basis for texture design.     

Lubrication potential of boron compounds: An overview

Boron compounds are emerging as promising materials for a wide range of applications in automotive and industrial lubrication systems. Several studies conducted on boron compounds have revealed that they exhibit desirable properties for preparing stable and compatible lubricant components for a new generation of lubricating oil formulations. Boron‐containing lubricants have major tribological advantages, such as antiwear efficiency, good film strength, high‐temperature resistance, and self‐lubricating properties. The increasing number of patents concerning boron‐containing lubricants illustrates commercial interest in this area. Boron lubricants can be used in many forms, such as oxides, esters, and boric acid. Therefore, it can be expected that a new generation of lubricant formulations includes boron compounds. This paper presents an overview of various solid and liquid lubricants containing boron as an important ingredient, and is intended to aid the development of new lubricants.     

Effect of electrical current on the tribological behavior of the Cu-WS2-G composites in air and vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.     

固液两相流体弹流润滑研究

应用微极流体理论,考虑流体的可压缩性,建立线接触微极流体动力润滑的基本方程,进行固液两相流体稳态流动弹流润滑数值分析,获得了润滑油膜压力、形状以及摩擦力分布,分析了微极参数对润滑性能的影响,并与不可压缩流体结果进行比较。结果表明,固液两相润滑流体较单相牛顿流体,在增加膜厚、提高承载力方面有显著的作用,而接触表面的摩擦因数有所降低,流体的可压缩性降低了油膜压力与油膜厚度。     

纳米粗糙间隙中季戊四醇四酯的薄膜润滑行为

采用分子动力学模拟方法建立光滑和粗糙2种固体壁面结构,研究季戊四醇四酯润滑剂在不同压力、薄膜厚度下,在恒定剪切速度和温度下的薄膜润滑行为。分析壁面间润滑薄膜的密度分布,以及剪切过程中润滑剂的速度分布。输出固体壁面在x向和z向的力学响应,并计算摩擦因数。结果表明：表面纳米结构降低了润滑薄膜的厚度,减弱了润滑薄膜分层现象;当润滑薄膜厚度较大时,V形纳米沟槽有助于减小薄膜润滑系统的摩擦因数;润滑薄膜厚度较小时,V形纳米沟槽表面润滑状态容易从流体润滑转变到边界润滑状态,摩擦因数增大。     

界面滑移条件下弹流油膜的试验观察

在极低速纯滑动条件下完成高粘度聚丁烯弹流油膜的光干涉测量试验,观察到明显的油膜入口凹陷现象。试验结果表明,入口凹陷的深度随载荷的增加而增加,随卷吸速度的增加存在一个极大值。试验中润滑剂的粘度越高,入口凹陷越容易出现。纯滑动条件下,油膜厚度对速度和载荷的依赖关系明显偏离了经典的弹流理论。试验中观察到的入口凹陷证明了已有入口凹陷弹流数值分析的正确性。     

Adhesion Mechanics between Nanoscale Silicon Oxide Tips and Few-Layer Graphene

Finite element method (FEM) simulations of the adhesive contact between a nanoscale tip and a silicon oxide substrate covered with graphene were performed, modelling experimental atomic force microscopy pull-off measurements. Simulations showed a slight increase in the pull-off force as layer number increased. This small enhancement was within reported experimental error, agreeing with the experimental findings of layer-independent adhesion forces. Pull-off forces did not vary with the elastic strain in the system for a given number of layers, but were influenced by the greater adhesive stresses for tip–graphene interaction compared with tip–substrate interactions. FEM simulations were also performed on suspended graphene and showed that the adhesive forces increased slightly beyond one layer of graphene, but then varied little from two to four layers of graphene. The results indicate that while there is some local delamination of the graphene sheets from the substrate, the adhesive stresses between the graphene layers in multilayer graphene effectively prevent out-of-plane mechanical deformation of the graphene layers that could result from tip–graphene interactions. Thus, the increased pull-off forces observed beyond one monolayer results from a change in the amount of material between the tip and substrate, or in this case the number of graphene layers, thus increasing the van der Waals force between tip and graphene.     

Numerical simulation of elastohydrodynamic lubrication for an elliptical contact

An elastohydrodynamic problem on elliptical contact is formulated and numerically solved. The mathematical model is described by a system of integro-differential equations and inequalities with boundary conditions. The direction of the rolling velocity vector with respect to the axes of the ellipse has a marked influence on the distribution of pressure and the lubricating film thickness in the contact area. The elastohydrodynamic contact parameters are determined depending on the compression force applied thereto. These results are useful for analyzing processes occurring in lubricated concentrated contacts.     

A New Method for the Measurement of Real Area of Contact by the Adhesive Transfer of Thin Au film

In this study, a new experimental method is proposed to measure the real area of contact between a ceramic sphere and an Al surface based on the adhesive transfer of the Au film and the scanning electron microscope (SEM) in the back-scattered mode. A thin film of Au is sputtered on the ceramic sphere before the indentation with the Al surface. The success of this method relies on the fundamental assumption that the adhesive transfer of Au only occurs everywhere inside the contact area. A thin polymer (PMMA) film is deposited between gold film and the ceramic surface to further reduce adhesive strength. After indentation, the interfaces of the ceramic sphere and Al surface are observed by SEM. Experimental evidence that the adhesive transfer of the Au film occurs inside the contact area is given. The entire contact regions on the ceramic sphere and the Al surface are captured in the second electron and back-scattered images with a magnification of 220× (resolution: 432 nm, i.e., distance between neighboring pixels). The contact area can be identified based on both the distributions of the ceramic and Au on the ceramic sphere and Al surface, respectively. The back-scattered images with the magnifications of 5000× and 10,000× (resolution: 20 and 4 nm) are captured at four different locations along the radial direction (starting from the contact center), respectively. The real area of contact decreases from the center to the contact edge.     

微量供油条件下润滑油液滴的生长与脱附

润滑油液滴的生长与脱附性能对于微量供油过程和微量润滑效果有重要影响。采用试验和数值仿真相结合的方法,研究微量供油条件下、在重力环境中的润滑油液滴在毛细管出口端的生长与脱附行为,考察毛细管管径和表面润湿特性变化对润滑油液滴脱附性能的影响。结果表明,润滑油液滴的生长与脱附是毛细力、黏性力、表面张力和重力等共同作用的结果;减小毛细管管径或增大润滑油液滴在毛细管表面的接触角,均可有效减弱毛细效应,降低润滑油液的爬移高度和脱附粒径,改善液滴脱附性能;毛细管管径由1.2 mm减小至0.7 mm过程中,液滴脱附粒径减小了4.5%;接触角由5°逐渐增加至90°的过程中,液滴脱附粒径减小了9.3%;通过选用低表面能材料制作微量供油的毛细管可以显著增大接触角。     

Tribological properties of nanoscale GeSb2Te4 film in the ambient air

In this paper, lateral force microscope (LFM) is used to investigate the frictional behavior of GeSb₂Te₄ film of 20 nm thickness deposited by magnetron sputtering. The effect of relative humidity, scanning rate, scanning area on friction is concerned. The JKRS (Johnson–Kendall–Roberts) theory considering the energy of adhesion between tip and GeSb₂Te₄ (at.%) film is introduced. Experimental results indicate that high humidity leads to high adhesive force, low humidity to low adhesion. When the tip's surface energy is unchanged, minute change of surface energy of GeSb₂Te₄ film will affect the adhesive force. Experiment also shows that variance of friction level goes unanimous at the different scanning area and the same normal load. Next, the effect of scanning rate on friction is attributed to the surface absorbate and liquid film. Finally, it suggests that relative humidity and protective coating should be considered for the promising ultra-high density probe storage medium.