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.     

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.     

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。     

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。     

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.     

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.     

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.     

Performance Characteristics at the Rib Roller End Contact in Tapered Roller Bearings

Analytical and experimental work is presented to determine the nature of the coefficient of friction at the large rib roller end of a tapered roller bearing. The analytical work considers the dynamic equilibrium of a roller, and it is shown that for normal bearing speeds the inertial forces have negligible effect on the performance at the rib contact. Mathematical expressions for the frictional torque on a detached rib have been derived for the case of axially applied bearing loads.

Experimental work has been carried out to obtain the rib torque values for the detached rib of a tapered roller bearing. Experimental information has been presented to show the variation in the value of the coefficient of friction with the sliding speed and the Hertzian stress at the rib contact. It is observed that the condition of lubrication is an important factor influencing the performance at the rib and that for the hydrodynamic condition of lubrication, the value of the rib torque is very low.     

热轧润滑剂用基础油的点接触和线接触高温减摩性能研究

用多功能SRV试验机评价了3种热轧润滑剂用基础油在点接触和线接触形式下的高温减摩性能。试验结果表明：在试验负荷范围内,当试验温度不大于300℃时,3种基础油的点接触和线接触摩擦因数都非常小,其摩擦因数在0．10—0．14之间,而且摩擦因数在试验过程中变化非常平稳;除了合成油和矿物油在试验负荷为20N的情况外,3种基础油在400℃时的点接触摩擦因数和线接触摩擦因数之间存在非常大的差异,点接触的摩擦因数非常高,而线接触摩擦因数则非常低;试验温度为500℃时,3种基础油的点接触摩擦因数和线接触摩擦因数都比较高,但线接触摩擦因数要比点接触摩擦因数低。     

化学机械抛光中的纳米级薄膜流动

在简述化学机械抛光技术的基础上,提出化学机械抛光过程中,受载的粗糙峰和被抛光的晶片表面之间存在一纳米量级的薄流体膜,形成了纳米级薄膜流动系统。指出对纳米级流动规律进行研究将有助于了解化学机械抛光的作用机理,其中,在极薄的膜厚情况下的温度场分析是一项迫切任务。     

Study on Rolling Contact Fatigue in Hydrogen Environment at a Contact Pressure below Basic Static Load Capacity

A method to determine the shoulder height in an angular contact ball bearing using a 3D contact analysis is proposed. The load analysis was performed by calculating the distributions of internal loads and contact angles for each rolling element. From the results of the bearing load analysis and the contact geometry between the ball and raceways, 3D contact analyses using an influence function were conducted. The algorithm developed was applied to an angular contact ball bearing for an automotive wheel. The effects of axial load on contact pressure at the inner and outer raceways were evaluated and the critical axial loads in the present shoulder height were calculated. The critical shoulder heights were also determined when the bearing was subjected to a practical load for a steel ball bearing. The proposed methodology is generally applicable for the purpose of reducing the material cost and improving the efficiency of the bearing design process.