Occurrence of Wall Slip in Elastohydrodynamic Lubrication Contacts

Preliminary experimental work has been carried out to identify some of the boundary slip phenomena of highly pressurised polybutenes in an elastohydrodynamic lubrication (EHL) conjunction. The movement of the oil is signified using an entrapment that can be readily formed by the impact of a steel ball against a layer of oil on a glass block in an optical EHL test apparatus. The post-impact lateral movement of the entrapment was investigated under the conditions: (i) pure rolling, (ii) pure glass block sliding (steel ball stationary) and (iii) pure ball sliding (glass block stationary). It was observed that under pure rolling the entrapped oil travels within the contact region at the entrainment speed, which is correlated with EHL theory. Under pure glass block sliding conditions, the speed of the entrapped oil core is less than the entrainment speed, and in the extreme cases, this core can be nearly stationary. Under pure ball sliding conditions, the oil core moves at a speed greater than the entrainment speed. The observation indicates that the oil/steel ball interface can sustain higher shear stress than the oil/glass (chromium coated) interface and there is a boundary slip in terms of relative sliding at the latter interface under the experimental conditions. Furthermore, the amount of slip increases with an increase in the pressure. These experiments provide evidence of the existence of wall slippage, which leads to the abnormal EHL film profile characterised with an inlet dimple as reported earlier.     

表面织构凹坑对弹流润滑点接触摩擦力和成膜能力影响研究

零件之间形成的润滑接触在工程中广泛存在,在零件表面人工织构特定的形貌,改善接触的摩擦磨损性能,是目前工程表面设计的热点和前沿。建立等温条件下的润滑点接触分析模型,并考虑润滑粘度和密度随压力变化以及接触弹性变形,研究了表面圆柱形凹坑的直径、间距和排列方向对弹流润滑接触摩擦系数的影响。结果表明,过大或过小的凹坑直径都不利于形成油膜,降低摩擦;凹坑间距越小,对油膜形成有利,摩擦系数就越低。     

Investigating the Effect of Surface Finish on Mixed EHL in Rolling and Rolling-Sliding Contacts

Surface finish may significantly affect the lubrication performance of a tribological interface through the influence of topography on micro/nanoscale fluid flows around localized contacts at surface asperities. This paper aims to study the mixed lubrication performance of a group of engineered surfaces, including turned, isotropically finished, ground, and dimpled surfaces, under different operation conditions by means of a deterministic mixed elastohydrodynamic lubrication (EHL) model. The honed surface was used to mate with other surfaces. The results indicate that a longitudinal contact ellipse favors longitudinally oriented mating surface roughness and that a transverse contact ellipse, as well as a line contact, prefers a transversely orientated mating surface roughness for lubrication enhancement.     

Effect of Metallic-Coating Properties on the Tribology of Coated and Oil-Lubricated Ceramics

Friction and wear behavior was determined for zirconia ceramics lubricated with solid coatings (Ag, Au, and Nb) deposited by ion-beam-assisted-deposition (IBAD) techniques, and a polyol-ester-based synthetic oil. Although the use of soft Ag and Au coatings as solid lubricants in conjunction with the synthetic oil significantly reduced the fiction and wear under boundary lubrication at temperatures up to 250°C, these films had poor durability. In contrast, the Nb coating was more durable in terms of chemical reactivity and adhesion during the tribo-tests than were the Ag or Au films. However, the friction and wear behavior of the Nb-coated zirconia was poorer than that of the ceramics coated with Ag or Au.     

Friction and Wear on Single-Layer Epitaxial Graphene in Multi-Asperity Contacts

Friction and wear of single layers of graphene have been studied at the micrometer scale. Epitaxial graphene grown by thermal decomposition on SiC-6H(0001) is found to have an initial friction coefficient of 0.02, significantly lower than graphite under the same experimental conditions. During reciprocal sliding the graphene layer is damaged. The evolving friction coefficient of 0.08 for the carbon-rich interface layer terminating the SiC layer is still lower than that of graphite and five times lower than that of the hydrogen-etched SiC substrate. Micrometer-sized patches within the sliding track retain the low friction coefficient of graphene even after hundred sliding cycles.     

基于滑动与摩擦效应下金属V带系统瞬态动力学特性研究

为了进一步研究无级变速器金属V带传动过程中瞬态动力学特性,建立了基于金属传动带与推块相对滑动力学特性以及推块与带轮相对摩擦特性的金属V带传动系统瞬态动力学模型,研究了金属V带在运动过程中各对象速度、加速度和作用力的变化规律。分析结果表明,推块与从动带轮之间的相对切向速度变化区间较推块与驱动带轮大,相对径向速度变化上两者变化区间基本相同;从动带轮上传动带张紧力整体呈现下降趋势,推块压缩力整体也呈现下降趋势;驱动轮上有用弧段较从动轮大,驱动带轮与从动带轮上推块与传动带均存在相对滑动现象,且从动带轮上随时间推移两者速度差将会增加。     

Effects of Friction on the Contact and Deformation Behavior in Sliding Asperity Contacts

Finite-element analyses are carried out to study the effects of friction on the contact and deformation behavior of sliding asperity contacts. In the analysis, on elastic-perfectly-plastic asperity is brought in contact with a rigid flat at a given normal approach. Two critical values of the normal approach are used to describe the asperity deformation. One is the approach corresponding to the point of initial plastic yielding, and the other at the point of full plastic flow. Additional variables used to characterize the deformation behavior include the shape and size of the plastic zone and the asperity contact size, pressure, and load capacity. Results from the finite-element analysis show that the two values of critical normal approach decrease significantly as the friction in the contact increases, particularly the approach that causes plastic flow of the asperity. The size of the plastically deformed zone is reduced by the friction when the contact becomes fully plastic. The reduction is very considerable with a high friction coefficient, and the plastic deformation is largely confined to a small thin surface layer. For a low friction coefficient, the contact size, pressure and load capacity of the asperity are not very sensitive to the friction coefficient. For a moderate friction coefficient, the contact pressure is reduced and the junction size increased; the load capacity of the asperity is not significantly affected due to the compensating effects of the pressure reduction and the junction growth. For a high friction coefficient, the pressure-junction compensation is not longer sufficient and the asperity load capacity is reduced. The degree of the friction effects on these contact variables depends on the applied force or the normal approach. Although the analyses are conducted using a line-contact model, the authors believe that the effects of friction in sliding asperity contacts of three-dimensional geometry are essentially the same and the same conclusions would have been reached. These results may provide some guidance to the modeling of rough surfaces in boundary lubrication, in which the asperity friction coefficient can be high and vary significantly both in time and from one micro-contact to another.     

离合器摩擦副表面温度对摩擦因数的影响

通过对某型离合器摩擦副的摩擦学小样试验,研究了离合器在结合的滑动摩擦过程中,摩擦面温度对离合器摩擦材料摩擦因数的影响.采用扫描电子显微镜(SEM),分析了样件的摩擦表面形貌,探讨了产生影响的机制,并从摩擦因数角度探讨了微车离合器起步发抖和烧蚀的主要原因.微车离合器摩擦材料摩擦因数随着摩擦面温度先升高,然后趋于稳定,最后再降低,其稳定工作的温度区间为130～220℃;在摩擦面温度较低的工况下,摩擦因数较低,微车起步时,离合器传递的扭矩不足以克服道路阻力,引起微车起步发抖的现象;而在摩擦面温度过高的工况下,离合过程中,摩擦因数较低,传递扭矩效率低,导致离合器滑磨时间过长,引起烧蚀现象.     

Noise Effect on Ice Surface Softening During Friction

The ice surface softening by friction is investigated considering the additive non-correlated fluctuations of the shear strain and stress, and the temperature. The premelting is construed by the Kelvin–Voigt equation for shear strain and by the relaxation equations of Landau–Khalatnikov type for shear stress and temperature. Taking into account the noises in these equations, the Langevin and Fokker–Planck equations are derived. Their analysis is based on the investigation of extrema of the distribution function, i.e., steady-state values of the shear strain using the Stratonovich interpretation. The phase diagrams are constructed, where the noises intensities and thermostat temperature determine the regions of ice, softened ice and their mixture (stick–slip rubbing). We present that domain of ice friction is bounded by relatively small background sliding block temperatures and fluctuation intensities of the stress and temperature. The ice film softens with growth of the stress noise intensity even at small thermostat temperatures. The friction force time series for all rubbing modes are calculated and compared with experimentally observed ones.     

表面织构特征对其摩擦润滑特性的影响

为研究织构阵列的形貌、排布方式对织构化滑动摩擦副摩擦学特性的影响,利用三维CFD仿真方法,基于Navier-Stokes(N-S)方程建立不同形貌、排布的织构阵列摩擦副的润滑模型,分析不同形貌及排布方式对压力分布和承载特性的影响;在此基础上,研究6种形貌、2种排布方式织构的润滑特性及摩擦性能。结果表明,流体流入收敛楔产生正的流体动压,流入发散楔产生负的流体动压,规则排布的织构单元摩擦性能优于错开排布的织构单元。在最优的雷诺数条件下,不同形貌的织构单元平均摩擦系数随面积率的增加而减少,菱形织构和球冠形织构面积率从4.5%增加到12.5%时,摩擦系数分别降低了81%和76%。因此,合理的选择织构形貌、尺寸有助于改善织构化滑动摩擦副的摩擦学特性。     

沟槽织构化表面影响摩擦振动噪声机理

在列车制动盘蠕墨铸铁材料表面上制备出平行间隔分布的沟槽表面织构,将其和光滑表面进行摩擦噪声对比试验,并利用有限元软件ABAQUS/Explicit(显式动态求解器)对试验进行数值模拟分析,研究沟槽织构化表面影响摩擦振动噪声的机理。结果表明,本试验条件下的光滑表面会产生较高强度的噪声而沟槽表面几乎不产生噪声,利用有限元软件ABAQUS/Explicit可以很好地模拟试验现象并揭示沟槽表面织构影响界面摩擦振动噪声的机理,即沟槽织构表面在对磨球滑过并碰击沟槽时引起的摩擦力波动能有效的打断摩擦界面的连续接触,作为不连续激励扰乱系统的自激振动,抑制界面摩擦力和振动加速度高频成分的形成并最终降低摩擦噪声。     

From Continuous to Molecular Scale in Modelling Elastohydrodynamic Lubrication: Nanoscale Surface Slip Effects on Film Thickness and Friction

The aim of this article is to propose an original approach in modelling elastohydrodynamic lubrication (EHL), combining a classical model derived from continuum mechanics and a nanoscale investigation carried out by Molecular Dynamics simulations. In particular, nanoscale slip is numerically quantified and a semi-analytical model for surface slip variation with pressure, film thickness and sliding velocity is presented. A composite model involving both continuum mechanics and nanoscale effects allows for a better understanding of dimple formation and offers a new basis towards a physically based friction prediction. This tentative represents a new direction towards a more realistic modelling of lubricated contacts with ultra thin film for the present industrial needs.     

The Combined Role of Soot Aggregation and Surface Effect on the Friction of a Lubricated Contact

One of the considered research paths to reduce friction loss consists in optimizing the interactions between surfaces and lubricants. The latter may significantly change with the lubricant ageing. In this framework, the tribological behaviour of aged formulated lubricant is analysed for various low-speed reciprocating motions and with different nature of surfaces. This paper focuses on soot aggregate formation processes in a lubricated contact and on their correlation to friction. Although no aggregates have been observed in pure rolling conditions, pure sliding conditions may lead to the appearance of aggregates moving through the contact as a function of the nature of the surfaces. The analysis of their displacement within the contact is used to discuss their interactions with the surfaces. Moreover, we show that the velocity and the dwell time of the aggregate depend on the sliding speed. The morphology of these aggregates evolves over time, affecting friction behaviour. An additive law combining a contribution from the shear of the aggregates with another one due to the shear of a thin lubricant film surrounding the aggregate is then proposed to interpret friction origin and friction evolution with time of shear. The aggregate motion also varies with the nature of the surfaces: in particular, DLC–DLC couple reduces aggregation phenomena and maintains a low friction without apparent wear.     

Effect of Reciprocation Frequency on Friction and Wear of Vibrating Contacts Lubricated with Soybean-Based B100 Biodiesel

The use of renewable, bio-based fuels has become increasingly widespread in recent years, with a major example being biodiesel, a bio-derived alternative to Number 2 diesel fuel. The increased usage of biodiesel gives rise to an augmented need to understand its tribological effects on critical engine components. This study focused on determining the tribological performance of soybean-based B100 (i.e., pure) biodiesel within a fuel injector with varying oscillating frequency by performing a series of linear reciprocating tribological tests of biodiesel-lubricated interfaces with varying reciprocating frequency. Comparison of friction coefficient variation with reciprocating frequency indicated a transition from boundary lubrication to hydrodynamic lubrication as the frequency increased, while hysteresis loop and energy loss observations showed a transition between full stick and partial slip contact with increasing frequency. However, observations of induced wear showed the wear to increase with increasing frequency, most likely due to the augmented number of sliding cycles as well as an increased degree of interfacial slip.     

Fast High-Resolution Simulation of the Gross Slip Wear of Axially Symmetric Contacts

In the present article, we study the development of a wear profile in an axially symmetric contact under conditions of gross slip and assumption of the Reye-Archard wear criterion. Simulations are carried out using the method of dimensionality reduction and a full finite element method (FEM) formulation. The calculation time of the proposed model is several orders lower than that of FEM-based models and allows for much higher spatial resolution.     

表面粗糙度对碳/铜载流摩擦副摩擦磨损性能的影响

通过环-块式摩擦磨损试验研究了表面粗糙度对碳/铜载流摩擦副摩擦磨损性能的影响,并分析了磨损形貌及机制。结果表明:其摩擦因数与电弧行为密切相关,无电弧时摩擦因数曲线平滑;对磨环的表面粗糙度越大越容易产生电弧,电弧的烧蚀导致块试样的磨损加剧;不同表面粗糙度下均存在临界起弧法向压力,且随着表面粗糙度的增大而增大;载流摩擦副的磨损机制主要为磨粒磨损、粘着磨损、电弧烧蚀及材料转移。     

边界润滑条件下表面微细织构减摩特性的研究

表面织构(Surface texture)已被证明是一种提高表面承载力和改善表面摩擦学特性的有效方法。然而在边界润滑条件下,织构对表面摩擦性能的影响机制仍未明确。利用纳米压痕仪在碳钢表面制作了具有不同密度和深度(125~500nm)的划痕的点阵,并通过改进的四球试验机对其在边界润滑下的摩擦性能进行了评价。试验载荷为100~300N,相对滑动速度为0.19~1.33m/s。研究发现:在边界润滑条件下,深度为125nm的低密度"划痕"点阵具有良好的减摩效果。     

摩擦片表面沟槽对调速起动影响的数值模拟

摩擦片表面沟槽对油膜传递扭矩和承载力有较大影响,从而影响液体粘性传动调速起动性能.同时在调速起动过程中,油膜的挤压效应也会对调速起动性能产生影响.为揭示摩擦片表面沟槽对调速起动性能影响的机制,采用修正瞬态雷诺方程、能量方程及润滑油粘温方程对目前常用的几种表面沟槽对油膜传递扭矩和承载力的影响进行了数值计算和分析.结果表明宽和深的沟槽并不适用于调速起动装置,为获得更好的调速起动性能,沟槽的尺寸和布置应使油膜承载力在起动过程中一直呈上升趋势.     

The Influence of Surface Dents and Grooves on Traction in Sliding EHD Point Contacts

Changes in traction, caused by dents and grooves on a highly polished ball, are investigated as these defects approach and go through sliding elastohydrodynamic (EHD) point contacts. The contacts are formed with the ball loaded against a transparent disk. The ball and thus the topographical features are held stationary at various locations in the vicinity and within the contact while the disk is rotating. It is shown that these topographical features can cause substantial changes in the traction when compared to traction obtained with smooth surfaces.