Modelling of plowing and shear friction coefficients during high-temperature ball-on-disc tests

During high-temperature ball-on-disc tests of aluminium against steel, hot aluminium deforms and wear track evolves. The individual contributions of plowing and shearing to the apparent friction and the contact pressure are unknown. The aim of the research was to develop a model capable of determining these parameters. It was found that during high-temperature ball-on-disc tests, the plowing friction accounted for only about 1% of the apparent friction, although the plowing friction coefficient increased with increasing wear lap, while the shear friction played a dominant role in determining the apparent friction measured. The mean contact pressure decreased significantly as the test proceeded.     

The ploughing friction: analytical model with elastic recovery for a conical tip with a blunted spherical extremity

The current analytical models are insufficient to describe the ploughing part of the friction coefficient when there is elastic recovery at the rear part of the contact after passage of a moving tip. A solution for the case of a perfectly conical tip was proposed recently. This paper presents a model to describe the ploughing friction for a perfectly spherical tip and then develops this analysis for the case of a conical tip with a blunted spherical extremity. The major difficulty resides in taking into account the shape of the contact area in relation to the elastic, elastoplastic or plastic behaviour of the contact.     

Substrate dependence of the scratch resistance of CrN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> coatings on steel

Hard CrN_x coatings were sputter deposited on hot work tool steel (HWTS) and high speed steel (HSS) in an industrial PVD reactor. Coatings were deposited under various nitrogen flows. The thickness, density, hardness, elastic modulus, composition, and stress were determined for the coatings. The specimens were subjected to scratch testing. Two different failure mechanisms were investigated: chipping and complete coating removal. For all specimens, the coating-to-substrate adhesion was that good that adhesion did not limit the scratch resistance. Therefore, the minimum loads at which a given type of failure was initiated were not a measure for the coating-to-substrate adhesion. The scratch resistance was better for coatings on HSS than for coatings on HWTS. This is due to the higher hardness of the HSS. Substrate independent measures for the scratch resistance of the coating were obtained by considering critical track widths instead of critical loads. The hardening of the coating--substrate systems due to the coating was investigated. The uncoated substrates exhibited track width independent scratch hardness. For the coated specimens the scratch hardness increased with increasing track width until chipping of the coating occurred. Complete coating removal coincided with a decrease in hardness. Although the elastic properties, hardness, and thickness of all coatings were more or less equal, CrN_1.0 coatings outperformed CrN_0.6 coatings in scratch tests both on HSS and on HWTS.     

The scratch hardness and friction of a soft rigid-plastic solid

The paper describes an experimental and analytical study of the normal and scratch hardnesses of a model soft rigid-plastic solid. The material known as ‘Plasticine’, a mixture of dry particles and a mineral oil, has been deformed with a range of rigid conical indentors with included angles of between 30° and 170°. The sliding velocity dependence of the computed scratch hardness and friction has been examined in the velocity range 0.19 mm/s to 7.3 m/s. Data are also described for the time dependence of the normal hardness and also the estimated rate dependence of the intrinsic flow stress. The latter values were estimated from data obtained during the upsetting of right cylinders. Three major conclusions are drawn from these data and the associated analysis. (1) A first-order account of the scratching force may be provided by adopting a model which sums the computed plastic deformation and interfacial sliding contributions to the total sliding work. This is tantamount to the adoption of the two-term non-interacting model of friction. (2) For this system during sliding, at high sliding velocities at least, the interface shear stress which defines the boundary condition is not directly related to the bulk shear stress. The interface rheological characteristics indicate an appreciable dependence on the imposed strain or strain rate. In particular, the relative contributions of the slip and stick boundary conditions appear to be a function of the imposed sliding velocity. (3) The computed normal and scratch hardness values are not simply interrelated primarily because of the evolving boundary conditions which appear to exist in the scratching experiments.     

摩擦轮传动中滑动问题及其对传动比的影响

对摩擦轮传动中的滑动现象进行研究。介绍弹性滑动和几何滑动现象的机理,并以无级变速摩擦轮机构为例,分析和计算了两对摩擦轮接触处的弹性滑动和几何滑动对无级变速摩擦轮机构传动比的影响,提出了无级变速摩擦轮机构传动比及输出扭矩的计算公式。     

CrN和CrN/Ag涂层的真空高温摩擦磨损性能*

为改善涂层在真空、高温等苛刻条件下的摩擦学性能,利用中频直流磁控溅射技术在硅片和316L不锈钢上沉积了CrN和CrN/Ag涂层,利用扫描电镜、透射电镜和X射线衍射仪对涂层的成分及相结构进行了表征,通过划痕测试仪、纳米压痕仪和摩擦磨损试验机测试了涂层的力学及摩擦学性能。结果表明：添加Ag元素以后,CrN/Ag涂层硬度及承载能力有所减小,但结合强度增加;真空高温环境下CrN与CrN/Ag涂层摩擦因数随温度升高呈下降趋势,其中CrN涂层通过软化镀层减小剪切强度和阻力,从而减小摩擦因数,CrN/Ag涂层主要通过高温产生的热驱动力诱导表面Ag润滑膜的形成来减小摩擦因数;CrN涂层依靠自身剪切特性参与摩擦,而CrN/Ag涂层在真空高温下具有自润滑和持续润滑性能,作为自润滑零部件具有潜在的应用价值。     

Experimental and theoretical evaluation of the deformation component of the coefficient of friction

An analytical model for the calculation of the deformation component of a friction force in sliding of a hard spherical indenter on a viscoelastic material that is modeled as a Kelvin solid is constructed. The influence of mechanical properties of a material and slip velocity of an indenter on the contact characteristics and deformation component of the coefficient of friction is studied. An experimental method for estimation of deformation loss under friction of a high-elasticity material is proposed. A comparison of the results obtained with the use of theoretical model with experimental data is performed.     

Study of plastic shear deformation in surface layer at friction. Simulation results. Part II. Effect of friction parameters

Deformation and temperature in the surface layer of plastic material and thickness of the plasticized layer are calculated using the one-dimensional macroscopic friction model developed in the first part of the paper [1]. We have studied the effect of sliding velocity, contact spot area, hardening ability, and thermal conductivity of material on the frictional heating and plastic deformation of the surface layer during a single touch between the contact spot and counterbody. Repeated contacts of the specimen and counterbody are simulated and the accumulation of plastic shear is considered for different contact spot sizes.     

Studies on friction and transfer layer: role of surface texture

Friction influences the nature of transfer layer formed at the interface between tool and metal during sliding. In the present investigation, experiments were conducted using “Inclined Scratch Tester” to understand the effect of surface texture of hard surfaces on coefficient of friction and transfer layer formation. EN8 steel flats were ground to attain surfaces of different textures with different roughness. Then super purity aluminium pins were scratched against the prepared steel flats. Scanning electron micrographs of the contact surfaces of pins and flats were used to reveal the morphology of transfer layer. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the texture of hard surfaces, but independent of surface roughness of hard surfaces. It was observed that on surfaces that promote plane strain conditions near the surface, the transfer of material takes place due to the plowing action of the asperities. But, on a surface that promotes plane stress conditions the transfer layer was more due to the adhesion component of friction. It was observed that the adhesion component increases for surfaces that have random texture but was constant for the other surfaces.     

An investigation of the fretting behaviour of low friction coatings on steel

Three different low friction coatings on steel (electrolytically deposited cadmium, PTFE solid lubricant in epoxy resin and PTFE solid lubricant in polyimide resin) were studied in order to relate their fretting behaviour with mechanical properties. Particular importance was given to adhesion which was measured using a scratch test. Fretting tests were carried out on the steel substrate and on the coatings under the same conditions. The major parameters of the tribological system were identified and then quantified. The values of the parameters obtained for each coating were compared with the corresponding values for uncoated steel. The mechanical characteristics of the coatings and their fretting parameters were plotted using a polar diagram in order to give an overview of the fretting behaviour of each coating. Differences were noted and the corresponding parameters were identified. The influence of the adherence of the coating and of the fretting test load on the lifetime of the coating was determined.     

Experimental and theoretical evaluation of friction at contacting magnetic storage slider–disk interfaces

To understand better the friction force and wear processes at contacting slider–disk interfaces, we have developed an experimental method for measuring and a theoretical method for calculating the friction force. For this study, a slider with a 1500 μm² contact pad located at the recording head is burnished against a relatively rough disk (～12 Å rms), which ensures smooth sliding. In the experimental method, the friction force is measured as the disk is spun-down to bring the slider–disk interface into an increasing degree of contact. A modified air bearing code is used to determine the experimental normal contact force for each friction measurement. In the theoretical method, the friction force and other relevant interfacial forces are calculated using an improved sub-boundary lubrication (ISBL) rough surface model. The friction force calculation in this model is based on the force needed to induce yielding of the individual disk asperities contacting the flat surface of the contact pad without any assumption of the coefficient of friction. Good agreement is found between the measured and theoretical friction vs. normal contact force curves, indicating that the model is capturing the essential origins of friction at this interface. The model also provides valuable insights into how wear particles may be generated at this contacting slider–disk interface.     

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.     

二维粗糙面间滑动摩擦过程中平稳性研究

基于现有的分形模型,考虑到黏着现象的普遍性,通过在界面引入黏着剪切强度并考虑材料断裂,建立了考虑黏着、弹塑性变形影响的二维分形金属粗糙面间的滑动摩擦模型,并运用有限元软件进行数值求解.通过设置不同界面剪切强度及滑动速度参数,研究了两粗糙面间滑动过程中的平稳性.对建立的二维双粗糙弹塑性分形模型的实例进行分析,结果表明:当界面剪切强度较小时,法向速度波动幅值增大;当界面剪切强度增大到一定值后,法向速度波动幅值迅速的减小,使滑动较为平稳.滑动速度对平稳性的影响较大,滑动速度越大,滑动过程中振动越剧烈.     

Ceramic-ceramic composite materials with improved friction and wear properties

Dry friction and wear tests were performed with self-mated couples of SiC containing 50% TiC, Si₃N₄---BN, SiC---TiB₂ and Si₃N₄ with 32% TiN at room temperature and 400°C or 800°C.Under room temperature conditions, the friction coefficient of the couple SiC---TiC/SiC---TiC is only half of that of the couple SiC/SiC and the wear is one order of magnitude smaller. At 400°C, it exceeds the friction coefficient of SiC/SiC except at the highest sliding velocity of 3 m s⁻¹. At lower sliding velocities the wear coefficient of SiC---TiC/SiC---TiC is lower than that of SiC/SiC.The couple Si₃N₄---TiN/Si₃N₄---TiN exhibits high friction coefficients under all test conditions. At room temperature the wear volume of the self-mated couples of Si₃N₄ and Si₃N₄---TiN after a sliding distance of 1000 m is similar, but Si₃N₄---TiN shows a running-in behaviour. At 800°C the wear coefficient of Si₃N₄---TiN/Si₃N₄---TiN is approximately two orders of magnitude smaller than that of Si₃N₄/Si₃N₄, and equal to those at room temperature. At 22°C the addition of BN reduces the friction of Si₃N₄. The wear coefficient is independent of sliding velocity and the self-mated couples showing running-in. Friction and wear increase with increasing temperature. The wear coefficient of SiC---TiB₂ above 0.5 m s⁻¹ at 400°C is advantageously near 10⁻⁶ mm³ (Nm)⁻¹. With the other test conditions the wear behaviour is similar to SSiC.     

A combined analytical‐experimental investigation of friction in cylinder liner inserts under mixed and boundary regimes of lubrication

It is necessary to develop an analytical solution in order to combine predictions with measured tribological parameters and fundamentally understand the mechanism of lubrication in a typical region of engine cycle, using tribometric studies. This paper deals with the development of such a representative approach. An analytical, rather than a numerical approach is expounded, as it is shown to suffice for the purpose of precise time‐efficient predictions, which conform well to the measurements. The effect of surface topography, material and operating conditions are ascertained for the representative case of top compression ring—cylinder liner contact at the top dead centre reversal in transition from the compression to power stroke. Stainless steel uncoated surface used as press fit cylinder liners for niche original equipment manufacturer applications are compared with those furnished with a Nickel‐Silicon Carbide wear‐resistant coating of choice in high performance motorsport. © 2017 The Authors Lubrication Science Published by John Wiley & Sons Ltd.     

圆锥轴承内圈大挡边与滚子基面摩擦和噪声分析

通过对设计和设备的改进,内圈大挡边与滚子球基面之间的滑动摩擦及滑动摩擦部位的振动噪声都得到很大地改善。     

Wear and friction of filled polyamide coatings

The aim of this article is to report wear and friction studies of cylindrical test steel pins coated with polyamide (PA6) incorporating fillers (iron, copper, aluminium, tin, lead) and solid lubricants (graphite, molybdenum disulphide (MoS₂)) to improve the mechanical, thermal, and frictional properties of polyamide (PA6). The coated testpins were loaded against a rotating cylinder of bearing steel in dry, lubricated, abrasive conditions at constant load and sliding speed. The test results show a significant reduction in wear and friction for both copper-filled polyamides and lead-filled polyamides.     

考虑热弹流润滑作用的滚珠丝杠副摩擦力矩建模与分析

针对以往研究中滚珠丝杠副摩擦力矩计算方法中未考虑润滑作用对其影响,或未考虑接触弹性滞后效应以及滑移摩擦效应对其影响的情况。首先基于单个滚珠接触效应建立其热弹流润滑方程;然后在考虑润滑效应的基础上建立其黏性摩擦力、弹性滞后摩擦力及滑移摩擦力方程;最后提出一种基于全滚珠载荷分布与热弹流润滑耦合的滚珠丝杠副摩擦力矩计算模型,建立了预紧力、转速与摩擦力矩之间的耦合关系。试验结果表明,当预紧力从1 kN变化到6 kN,在滚珠丝杠低转速(100 r/min)时,摩擦因数变化范围为0.005 6～0.006 5。随转速的提髙摩擦因数升高且变化量逐渐增大,说明国际标准DIN ISO3408-3:2006中将滚珠丝杠副空载摩擦力矩测量速度定为100 r/min具有合理性,验证了所提出的摩擦力矩计算方法的准确性。     

基于ANSYS的轮轨摩擦滑动接触应力分析

以Hertz接触理论为依据,利用ANSYS建立2D有限元计算模型,模拟原地打滑、完全制动等轮轨滑动摩擦接触行为。分析了轮轨静接触和滑动接触时接触应力分布情况,研究了接触状态、轴重、滑动速度、载荷类型和钢轨轨顶圆半径对接触应力的影响。结合Hertz接触理论计算结果、剥离损伤理论和自激振动理论进行了轮轨损伤分析。