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.     

原子尺度上的摩擦

J.Krim首先提出纳米摩擦学的概念,章根据前人的研究讨论了摩擦的根源,其中包括晶格振动和声波,对摩擦根源的研究有助于机械设计概念的更新。     

接触式干涉仪定格数的调整方法

有的初学量块检定的人员,对接触式干涉仪的调整,由于陌生,往往容易出现差错。本文谈一谈该仪器定格数调整的简便方法。接触式干涉仪使用前,须先进行定度,确定分划板上一个刻度间距所代表的测杆位移量。这时干涉滤光片置入光路内,用单色光调整到清晰的干涉带后,任意取一定数量的干涉条纹间隔,使它和刻度尺一定的刻度间格数相重合,以此来定刻度值。其公式为:n=λk/2i式中:n—k个干涉条纹间隔中所包含的刻度尺的刻度间隔数;λ—所用的单色光波长;i—仪器的分度值;k—干涉带条纹间隔数。这类仪器所推荐使用的i与k值列于下表:这样,根据所采用的波…     

Friction and Wear of Pyrophyllite on the Atomic Scale

Pyrophyllite, Al₂Si₄O₁₀(OH)₂, a layered aluminosilicate material, has several properties which suggest that it might exhibit desirable frictional properties for solid-state lubrication. These include a lamellar structure and a low-energy basal plane. Here, we utilize atomic force microscopy to investigate the tribological performance of mineralogical pyrophyllite samples and compare the results to those of other minerals and solid lubricants. The results indicate that pyrophyllite exhibits low friction forces and shear stresses, on the same order of other commonly used solid lubricant materials, and a high resistance to deformation within a single layer. These fundamental findings highlight the potential use of pyrophyllite as a possible tribological material and alternative solid lubricant.     

Defining True Tribological Contact Through Application of the Morphological Method to Surface Topography

In tribological functions high peaks (summits) in the surface topography play a dominant role in that they determine the position of first contact and how the contact will occur. Both statistic-based methods and feature-based methods address the characterization of a single surface, while neglecting the interacting surface. A morphological method is proposed to simulate the contact of two mating surfaces. The surface under evaluation is rolled by a ball with radius meant to simulate the largest reasonable peak curvature at a contact. In such a situation the contact points of the rolling ball may serve as an identification of those surface portions that are in real contact. The morphological closing operation could then be applied to detect the contact points of the rolling ball, however, the traditional computation method does not lead to an accurate result. To overcome this deficiency, a geometrical computation approach has been developed to capture the contact points based on four searching procedures. The resulting method has been verified through experimentation and then applied to a case study in which the underlying form of the surface of a hip replacement taper junction is analyzed to remove the effect of the dominant threaded structure.     

Roughness and Lubricant Effect on 3D Atomic Asperity Contact

Large-scale molecular dynamics simulations were performed to study the sliding process of rough surfaces with and without lubricant. In the dry contact, a linear relationship has been observed between the load and the contact area for surfaces with large root mean square (RMS) roughness. However, it becomes nonlinear when the RMS is small. In the presence of adhesion, small roughness results in a large friction force when the surfaces are flattened and the contact area reaches 60 %. In order to confirm this observation, nonadhesive models have been established with an observation that under the combined influence from roughness and adhesion, the contact area plays a crucial role to determine whether the dry sliding is under the domination of roughness or adhesion. In the lubricated sliding, an increase in friction force has been found for the partially lubricated condition because the asperity contact still accounts for a great deal of resisting force. Besides, the lubricant exerts a comparable resisting force to the sliding.     

用复合振子模型计算纳米尺度滑动摩擦力的研究

以光滑界面摩擦为研究对象,探讨用复合振子模型计算纳米尺度滑动摩擦力的原理和方法,推导出滑动摩擦力和摩擦因数的计算公式,并用原子力显微镜探针在硅试样和云母试样上做扫描实验进行验证。实验值与理论计算值的对比结果表明,两者所反映的规律基本一致,表明所提出的理论和方法可行。研究表明,滑动摩擦力与接触面积成线性增长关系,并随宏观振子横向刚度的增大而减小,由于可通过改变摩擦表面的几何形貌来改变宏观振子的横向刚度,因此这一结论将为摩擦控制提供一种新途径。     

The Scale Effect of Roughness on Hydrodynamic Contact Friction

Multistage abrasive finishing processes (grinding, polishing, honing, etc.) are commonly used to produce the geometrical properties of a surface to meet its technical functionalities in the operating characteristics of contacting parts in friction, relating to their durability and reliability (running-in performance, wear resistance, load-carrying capacity, etc.). Coarse abrasive grits followed progressively finer ones are used, which leads to a multiscale stratified surface texture.

In this article, a numerical model of elastohydrodynamic (EHD) contact coupled to a multiscale surface texture model was developed that allows tracking the scale effect of surface features and their interactions on friction performance and lubricant flow under hydrodynamic lubrication conditions. Because the simulation model has as an input the surface topography and to overcome the variability in surface finish formation, textured surfaces at different stages of the finishing process were simulated (virtual texturing method). Surface topography can be decomposed into two principal components: superficial roughness and valleys. Superficial roughness was modeled using a fractal model and a scaling factor was introduced to model valley patterns. The results show the relationship between friction and surface scales.     

尺度相关的分形结合面法向接触刚度模型

基于分形理论,利用双变量Weierstrass-Mandelbrot函数模拟三维分形结合面,建立尺度相关的三维分形结合面法向接触刚度模型。推导出各等级微凸体发生弹性、弹塑性以及完全塑性变形的存在条件。确定结合面上各等级微凸体的面积分布密度函数,推导出法向接触刚度和法向接触载荷的解析表达式。计算结果表明：当结合面上的微凸体只能发生弹性变形,即自身等级小于弹性临界等级的微凸体,该部分微凸体引起的法向接触刚度和对应法向载荷关系呈非线性。当微凸体的等级大于弹性临界等级,在结合面接触过程中,微凸体弹性变形引起的法向接触刚度与对应的法向载荷关系为线性,非弹性变形引起的法向接触刚度与法向载荷关系为非线性。微凸体的等级范围对结合面的刚度影响较大,在相同的法向载荷作用下,高等级微凸体的结合面产生较高的法向接触刚度,即结合面越平整,结合面的法向刚度越高。     

Temperature-Dependent Atomic Scale Friction and Wear on PbS(100)

Friction and wear on PbS(100) surfaces have been investigated on the atomic scale as a function of temperature with atomic force microscopy. At room temperature and above, the PbS(100) surface exhibited low friction (μ < 0.05) in contact with a silicon nitride probe tip, provided that interfacial wear was not encountered. In the absence of wear, friction increased exponentially with decreasing temperature, transitioning to an athermal behavior near 200 K. An Arrhenius analysis of the temperature dependence of friction yielded an activation energy ∆E = 0.32 ± 0.02 eV for the sliding contact of a silicon nitride tip on PbS(100).     

Contact conditions at the chip-tool interface in machining

Direct observation of the chip-tool interface was made using transparent sapphire tools in combination with a CCD-based high-speed imagining system. The observations made for various nonferrous workpiece materials suggest that the contact conditions at the chip-tool interface be classified into three types depending on the nature of the zone of stagnant material — negligible zone of stagnant material (Type 1), zone of stagnant material that is stable and confined to the vicinity of the cutting edge (Type 2), and zone of stagnant material that expands upward from the cutting edge as cutting progresses (Type 3). Velocity profiles obtained using the particle image velocimetry (PIV) show that retardation of the chip underside occurs in the intimate contact region for the Type 2 materials while it is negligible for the Type 1 materials. Nanoindentation hardness profiles measured with depth into the chip from the chip underside indicate that the expansion of the zone of stagnant material observed for the Type 3 materials could be related to the work-hardenability of the chip material in the secondary deformation zone.     

渐开线环形齿球齿轮副接触点共轭曲率半径及接触强度计算研究

从渐开线环形齿球齿轮齿廓曲面基本方程出发,根据理论论证和工程计算的不同需要,首先采用两种不同方法推导得出了其齿廓曲面上任意接触点处的主曲率计算公式,然后在此基础上进一步分析研究,完整地提出了渐开线环形齿球齿轮副接触承载能力计算方法,并进一步给出了有限元对比实例计算分析验证结果,解决了渐开线环形齿球齿轮自发明以来长期缺乏轮齿接触强度计算方法的问题。     

线接触零卷吸弹流条件下的急停分析

对滚珠丝杠、滚珠蜗杆、无保持架滚柱轴承中常见的零卷吸状态下的急停问题进行热弹性流体动力润滑模拟;采用不同的初始零卷吸表面速度和急停时间,探讨线接触零卷吸条件下发生急停时油膜压力、膜厚和温升的变化。结果表明：相同急停时间下,中心压力和中心膜厚在急停过程中逐渐增大,随初始零卷吸表面速度的增加而轻微减小;初始零卷吸表面速度相同时,中心压力和中心膜厚均随急停时间的增加而逐渐增加;油膜的温度峰在急停前期缓慢下降,而在急停后期急剧下降。零卷吸工况下的急停会造成运动过程中接触区中心压力的急剧增加,会导致两接触固体间的应力远超过材料的屈服极限,使两接触表面会发生塑性变形,造成表面损伤。     

钢卷尺零位动态瞄准技术

当前工量具企业在进行钢卷尺切带生产中,普遍使用半自动切零位系统。该系统由于使用人眼瞄准,所以存在着劳动强度高,切带精度不稳定等问题。为此提出了基于视频分划的动态瞄准技术。动态瞄准技术以视频行场同步信号为基础生成视频分划横竖线,从而可实现钢卷尺零位的动态自动瞄准。瞄准控制系统包括视频分划线的动态生成,被测对象的位置识别和动态视频瞄准功能。目前,该技术已应用于钢卷尺切带的生产实践中,切带精度优于±0.1mm,切带速度为2~3s/条。     

基于振子模型的镍钛形状记忆合金微摩擦力研究

为研究接触表面之间的微摩擦力,建立振子模型。采用该模型计算镍钛形状记忆合金2个原子之间的摩擦力,并用原子力显微镜对镍钛形状记忆合金马氏体和奥氏体相的摩擦力大小进行测量。结果表明,在纳米条件下摩擦力与外载荷和表面物理性质,如弹性模量、泊松比、晶格常数等关系密切;镍钛形状记忆合金奥氏体相的摩擦因数小于马氏体相。理论分析和实验结果基本一致,证明采用振子模型计算微摩擦力符合实际情况。     

高速线接触热弹流分析

应用多重网格法和多重网格积分法,开发了线接触弹性流体动力润滑C 计算程序.针对航空领域中无量纲速度为10-9的高速线接触弹流情况,应用该程序求得了高速情况下Newton流体和Ree-Eyring流体的弹流数值等温解及热解,分析了热效应和非牛顿性对润滑的影响,验证了计算程序的可靠性.在相同无量纲速度下,研究了滚滑比和滑油粘度对润滑性能的影响.结果表明:随着滚滑比的增加压力峰逐渐变小,油膜厚度变薄,油膜温度显著增加;高速情况下,粘度大的润滑油可以增加油膜厚度,但是压力峰增大,油膜温度也升高很多.     

Friction Force Measurement at Windscreen Wiper/Glass Contact

In the present study, a novel windscreen wiper-on-cylinder machine has been used to investigate the influence of sliding speed and normal force on the coefficient of friction. Using this machine it is possible to measure the friction force not only on specimen level, as in former studies to be available in the literature, but also on structural level by considering the whole windscreen wiper. As measurement results are strongly influenced by both the real, non-circular cross-section, and the eccentricity of the rotating glass cylinder an analytical model has been developed to explain the measurement results. The good agreement to be found between theory and experiment confirms the validity of the model. Majority of the results belongs to partial contact where the wiper blade is not in contact with the glass countersurface along its total length. After the discussion of experimental results, as a last step, authors made an attempt to compare quantitatively the predictive capability of two different contact models widely used in mixed friction model of sliding rubber components. The results show that the difference in film thickness due to solid–solid contact can be larger than three orders of magnitude in case of a typical windscreen wiper.     

Experimental Investigation of Lubrication Properties at High Contact Pressure

A new test rig with an ability to obtain clear interference images at high pressure contacts has been developed. The technique of relative optical interference intensity has been used to obtain film thickness profiles and then lubrication properties of some base oils including six kinds of polyalphaolefin and four kinds of silicone oil have been studied at different pressures ranging from 1 to 3 GPa. The results show that viscosities of these lubricants have notable effect on slopes of film thickness curves (speed versus film thickness in log–log form), and the observed phenomenon is attributed to fluidity and molecular structure. A comparison of experimental central film thicknesses with computational work shows that at high loads the relationship between load and film thickness usually go against prediction given by Hamrock and Dowson. In addition, when more pressure is applied, the profiles of film thickness become more and more flat while rolling speed do little to change the shape of profiles.