On the dynamics of the friction coefficient

The paper deals with the principal wear mechanism in brake systems and introduces a new dynamical model of the friction coefficient, where necessarily both friction and wear are taken into account. This model explains many open questions on the principal functionality of brake systems.In brake systems, characteristic structures are formed in the contact area by the flow of wear particles. Modulated by the friction power the wear particles are used by the system to build up hard contact patches on the brake pad. Nearly all energetic dissipation of the system is concentrated on these patches. By wear, these contact patches are destroyed after some time.So the friction coefficient is given by the equilibrium of flow of birth and death of contact patches. The resulting dynamical model describes the dynamical behaviour of the friction coefficient and the dependence of the temperature in the friction layer.This theory explains the fading effect of brake systems as well as complex hysteretic effects in the diagram of the friction coefficient versus the velocity, known from instationary measurement procedures.The structure of this theory seems to be quite general to describe other frictional systems too.     

Contact damage of poly(methylmethacrylate) during complex microdisplacements

The wear behaviour of a PMMA substrate contacting against a steel ball has been investigated under small amplitude oscillating motions. A friction device has been developed, which allows the combination, to various extents, of linear sliding and torsional contact conditions. Using laser profilometry, the amounts of wear debris trapped within the contact or displaced from the contact have been quantified for the various loading configurations. The contact zone kinematics has been found to have a major influence upon the wear resistance of the PMMA. When the contact conditions progressed from torsional to linear sliding, a rapid decrease in the wear volumes was observed, which was associated with the progressive accumulation and compaction of an increased part of the detached PMMA particles within a central roll. These results have been interpreted by considering both the shape and the magnitude of the sliding paths that were generated within the contact under the various loading configurations. Using nano-indentation tests, it was also observed that the consolidated third body resulting from debris compaction within the rolls exhibits mechanical properties (hardness, modulus) similar to those of the original PMMA substrate. The achievement of a high level of compaction within the roll formation was explained by considering the effects of third body accumulation on the load carrying capacity of the contact.     

Friction and Wear of Fiber Composites with Abrasive Particles on Contact Surface

A friction and wear model of fiber composite material with abrasive particles on a contact surface has been proposed. The problem of a composite material friction in which fibers are perpendicular to a friction surface and a rigid counterbody separated by a layer of spherical rigid particles has been considered. The effect of the material and abrasive particles’ properties on the wear rate and surface degradation has been determined. A comparison of the obtained results with the experimental data has been performed.     

7075铝合金和QRO90模具钢在热成形工况下的摩擦磨损行为

铝合金在热成形制造过程中存在严重的模具磨损,不但缩短模具使用寿命,还造成工件表面拉毛和成形偏差。为模拟模具钢与7系铝合金在热成形工况下的摩擦过程,搭建单向高温摩擦磨损试验平台,对热成形工况下的摩擦磨损行为展开研究,并通过光学轮廓仪和扫描电镜分析模具钢与铝合金磨损表面形貌。结果发现,热成形过程中黏结磨损和磨粒磨损同时存在,磨损颗粒在粗糙表面上被压实和堆积,对后续的摩擦磨损行为有显著影响。对单向高温摩擦试验的接触副局部进行有限元建模,分析粗糙接触表面上的局部接触条件,探讨其对后续摩擦学行为的作用。分析表明,摩擦过程中存在黏滑现象,局部接触压力受表面形貌影响,显著大于名义载荷,最大等效应力出现在表面之下。这对进一步分析磨损行为、提出合适的表面工程方案提供了基础。     

曲线半径及轴重对轮轨摩擦功影响的研究

基于非Hertz滚动接触理论利用数值计算方法详细分析了静态接触情况下,轴重和曲线半径对轮轨接触质点间等效应力、接触斑粘滑区的分布、总滑动量和摩擦功的影响。分析计算表明,轴重增加引起轮轨接触质点间等效应力,接触质点间粘滑区的面积以及总滑动量的变化,同时对轮轨接触质点阍的摩擦功的变化有重要影响;小曲线半径处轮轨接触质点间的总滑动量,接触斑滑移区的面积以及摩擦功都明显增大,导致曲线上钢轨磨损加剧。因此曲线半径和轴重是影响轮轨滚动接触磨损的重要因素。     

WC颗粒增强钢基复合材料的制备及耐磨性研究

以0.076～0.1mmWC颗粒为增强相,45钢为基体,采用粉末冶金真空烧结法制备颗粒增强钢基表面复合材料,考察了常温压制烧结和温压烧结2种工艺条件下的摩擦磨损性能。结果表明,对于质量配比1∶1的WC和45钢粉末,采用硬脂酸锌作润滑剂,压制温度140℃,压制压力320kN,得到较为理想的复合材料,其耐磨性是Cr15的2.8倍,是同样工艺条件下常温压制试样的2.9倍。     

Formation of Low-Friction Particle/Polymer Composite Tribofilms by Tribopolymerization

Inorganic fullerene-like (IF) solid lubricant particles based on MoS₂ or WS₂ have recently gained attention in various tribological applications, for instance incorporated in coatings, porous materials, greases and dispersed in oils. However, their effect in oil can be limited often due to inadequate penetration into the contact zone. Meanwhile, tribopolymerization of monomer additives in oil have proven to be efficient to reduce wear, but without significantly reducing the friction. This investigation combines these two lubrication techniques in order to form particle/polymer composite tribofilms, aimed to give low friction and wear in high-pressure sliding contacts. A cyclic amine, caprolactam, was used as the monomer together with IF as well as normal 2H WS₂ particles. Zn-DTP was used as a reference antiwear additive and poly-alfa-olefin (PAO) was used as the base oil. Reciprocal ball-on-flat sliding test results found that monomer plus particles reduced the friction by 10–50%, depending on sliding speed and type of particles, compared with particles or monomer alone. And the scattering between different tests was also considerably lower. The wear rate was also substantially reduced to a level similar to that of Zn-DTP. The particle/polymer composite tribofilms were generally much larger than the actual contact area and its thickness varied from below a few nanometres in the contact centre to several micrometers in the outer parts. Consequently, the reduced friction is believed to come from two effects: one being a trapping of particles in the contact zone, which reduces the boundary friction level; and the other a shift in lubrication regime towards EHD-lubrication due to reduced contact pressures accompanied with the formation of the composite tribofilm.     

Wear rate testing in relation to airborne particles generated in a wheel–rail contact

This study examines the relationship between generated airborne particles and wear rate in the wheel–rail contact. The wheel–rail contact is experimentally simulated by using pin‐on‐disc testing to determine the difference in wear rate between selected contact conditions. Wear is discussed both in tribological terms and by using the wear categories prevalent in the railway industry, namely, mild, severe and catastrophic wear. The discussion is based on wear depth, the coefficient of friction, topographical measurements and measurements of airborne particles generated in the contact. The tests were performed under selected loading conditions representative of different contact conditions in a real wheel–rail contact. The results indicate that wear rates vary with the contact conditions arising from different types of triggered wear transitions. This is emphasised by the number and size of the airborne particles generated. Copyright © 2009 John Wiley & Sons, Ltd.     

A Study of the Effect of Wear Particles and Adhesive Wear at High Contact Pressures

The wear rate and friction characteristics were determined for certain combinations of cobalt base alloys and stainless steels when rubbed together at unit pressures up to 300,000 psi in an environment of demineralized water at room temperature. Wear rate and friction data were collected both in the presence and absence of wear particles. A study was also made on the effect of sliding-surface geometry in trapping wear particles.

The results suggest that an exponential relationship exists between wear rate and stress for the materials of test and this relationship is influenced by material combinations and sliding velocity. Entrapped wear particles also affect the wear rate.

Because the wear rate appears to increase sharply at some nominal contact stress when the data is plotted on Cartesian coordinates, the authors define an “Apparent Critical Stress”, using this term and a wear factor term to aid in the evaluation and discussion of results.     

Coating thickness effects on initial wear of nitrogen-doped amorphous carbon in nano-scale sliding contact: Part I—in situ examination

This paper, the first of a two-part series, presents the empirical data obtained from in situ examination on the generation of wear particles on carbon nitride coatings by a spherical diamond counter-face during repeated sliding contacts. In particular, the effect of coating thickness, varying from 1 to 500 nm, on the generation of wear particles was examined.Based on the in situ examination, the shape transition maps for generated wear particles were obtained for carbon nitride coatings of various thickness. The results show that the critical number of friction cycles, Nc, for the transition from “no observable wear particles” to “wear particle generation” generally increased with increasing coating thickness. It was noted that up to 20 friction cycles, the maximum Hertzian contact pressure, P_max, for “no observable wear particles” regime can be increased from 1.39Y to 1.53Y if silicon was coated with carbon nitride coating thicker than 10 nm, where Y is defined as the yield strength of silicon.     

重型汽车中桥减速器润滑油中磨损颗粒分析

重型汽车中桥减速器在工作过程中其摩擦副接触表面因相对运动而产生的磨损颗粒进入润滑油中,导致润滑油性能下降;磨损颗粒随润滑油进入摩擦副接触表面,摩擦副磨损加剧。根据重型汽车的运行特点,每隔一定里程数对减速器润滑油进行取样,综合分析在用润滑油中磨损颗粒物的大小、形貌及成分。结果表明：减速器在用润滑油中磨损颗粒主要成分是铁屑和铜屑,磨损颗粒主要来自于球面垫片（铜）及其配副的摩擦以及齿轮副间的摩擦。具体分析磨损颗粒产生过程及形成机制,为重型汽车中桥减速器中运动摩擦副的设计及制造提供理论依据。     

Effect of contact stress on friction and wear of ultra-high molecular weight polyethylene in total hip replacement

This paper studies the effect of contact stress on friction and wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups by means of friction and wear joint simulator testing under serum lubrication. For a given applied load, increasing the contact stress by increasing the ball/socket radial clearance decreased both the coefficient of friction and the wear rate. Friction and wear were highly correlated. The dependence of friction on contact stress for the UHMWPE socket under serum lubrication was similar to that of semi-crystalline polymers under dry sliding. This finding indicates the occurrence of partial dry contact at asperity levels for the metal-polyethylene ball-in-socket joint under serum lubrication.     

Simulation of friction and wear in the contact between the valve bridge and rocker arm pad in a cam mechanism

In this paper the surface velocities obtained from a rigid body model are used to simulate friction and wear in the contact between the rocker arm pad and valve bridge in the cam mechanism of a diesel engine. The friction is simulated with two different friction models, a 3D brush model capable of handling transient conditions such as an varying normal load and varying surface velocities and a Coulombian friction model. The wear simulations are based on a generalised form of Archard's wear model.The results presented here show that both the maximum wear depths and the wear distributions are influenced significantly by the combination of wear pad radius and the position of the wear pad radius centre relative to the rocker arm bearing centre. A combination with wear pad radius of 20 mm and centre position of 5 mm is found to give the least wear depths on both the wear pad and the valve bridge. It is also seen that the contact between the wear pad and the valve bridge is mainly a sliding contact and that the transitions from sliding in one direction to the opposite are very rapid. The change of the surface shapes due to wear has a negative effect on the contact situation causing very high contact pressures.     

Tribological Characteristics of Binder Resins for Brake Friction Materials at Elevated Temperatures

The heat resistance of the multiphase friction materials made with straight phenolic resin (ST), boron–phosphorous (B–P)-modified phenolic resin (BP), or polyimide (PI) was investigated using a Krauss-type friction tester. Thermal analysis of the cured resins was also carried out to examine the chemical and mechanical properties of the resin and their effects on the wear rate and the coefficient of friction. The results showed that the fade and wear of the friction materials were closely related to the thermal decomposition of the binder resin and durability of the contact plateaus, which were produced by the compaction of wear debris around hard ingredients on the rubbing surface. The friction material containing the B–P modified resin was more resistant to wear and fade than the other binder resins. On the other hand, the friction material with polyimide showed good fade resistance but exhibited excessive wear despite its good thermal resistance.     

原位制备CaCO3微纳米流体及摩擦学性能研究

选用菜籽油、失水山梨糖醇脂肪酸酯(Span 80)、壬基酚聚氧乙烯醚(TX-7)、正丁醇制备W/O型微乳液，并将其作为微反应器原位合成CaCO₃微纳米粒子。对CaCO₃粒子形貌的表征表明，随含水量增加，CaCO₃粒子由球形转变为梭形薄片状，CaCO₃粒子尺寸也可受含水量控制。通过四球摩擦磨损试验机评价CaCO₃微纳米流体的摩擦学性能，结果表明，该微乳液体系相较于基础油的摩擦因数降低了约55.74%,CaCO₃粒子使微乳液P_B值提高了约10.88%,摩擦面粗糙度降低了约16.01%。在摩擦过程中，胶团在表面活性剂的作用下平行吸附于摩擦面且易于剪切，实现优异的减摩效果；梭形薄片状的CaCO₃微纳米粒子可以抑制表面微凸体之间的直接接触，减少黏着磨损与磨粒磨损，从而发挥极压作用并显著改善表面质量。     

Friction and wear of powder coatings of epoxy composites with alumosilicate nanoparticles

The paper studies the effect of alumosilicate nanofillers of tubular and lamellar shape on the friction and wear of epoxy composites. It is shown that the influence of concentration and shape of the fillers on the tribological behavior of the composites is due to variations in their viscoelastic properties and shielding of the contact area of the matrix material with the metallic counterbody by the filler particles. The data evidence that at equal concentrations of alumosilicate fillers in the epoxy matrix, the best tribological characteristics are provided in the case of tubular-shaped particles.     

磨料粒度对表面微织构纯钛干摩擦性能的影响

为了提高钛及钛合金钻具在超深钻探、深海钻探和外太空钻探工程中的减摩抗磨性能。利用激光表面加工技术在工业纯钛（TA2）表面制备了不同参数的点阵微织构。采用MS-T3000摩擦磨损试验机测试了微织构钛合金在不同粒度模拟月壤作用下的摩擦学性能。利用扫描电子显微镜和能谱分析仪分析磨痕形貌及元素含量。研究结果表明：当磨料粒度小于微织构点阵的直径时,磨料压入微织构点阵里,磨料具有滚动和滑动两种运动方式。当粒度大于微织构点阵的直径时,磨料不能完全压入微织构点阵里,磨料对微织构TA2表面产生了滑动犁削作用。由于两种磨料磨损的作用机理不同,同等条件下,小粒度的磨料作用下的微织构TA2的摩擦因数和磨损率较大粒度磨粒作用下的最大减少量分别为50%和53%。考虑磨料粒度与微结构的匹配性,可以大大降低摩擦减少磨损。     

The role of triboparticulates in dry sliding wear

In this paper, wear processes and mechanisms for wear transitions with sliding time and temperature during sliding of a nickel-based alloy, N80A, in oxygen at temperatures to 250°C are discussed. Transitions in wear from high rates to low rates with sliding time were always observed at all the temperatures investigated. The transitions in wear were usually accompanied by transitions in contact resistance between the rubbing surfaces from nearly zero to positive high values. It was found that wear debris particles were heavily involved in the wear processes. The transitions in wear and contact resistance with sliding time mainly resulted from the development of wear-protective layers following the compaction of wear debris particles on the rubbing surfaces. The adhesion of triboparticulates to each other and to the rubbing surfaces played an important role in the rapid decrease in wear rate with sliding time and with increase in temperature. Processes involved in the development of the wear-protective particle layers and mechanisms for the wear transitions have been described on the basis of experimental observations. The importance of triboparticulates in wear and its implications for wear protection are discussed.     

Effect of deformation of the contact surface on wear and friction—a study on clarification of the mechanism of abrasive wear

The cutting action due to a protuberance between the rubbing surfaces in abrasive wear might be susceptible to adhesion at the rake surface in the direction of sliding.

In the present paper we discuss an approach to clarify the abrasive wear mechanism, which is dominated by the effect of the difference in the contact deformation between both surfaces on wear and friction related with abrasive wear, on the basis of experiments conducted under ultrahigh vacuum.

Although in practice it is generally very difficult to obtain a characteristic surface during sliding, we carried out experiments on surfaces that were as clean as possible to investigate the effect of the size of the deformed contact surface on the abrasive wear.

Consequently, we were able to confirm that the friction differs according to which of the test pieces is the harder and, if the friction pair is composed of the same material, because the contact surface is deformed differently in each case.     

碳化硼增强铝基复合材料的摩擦磨损性能

为了比较两种含量不同的碳化硼颗粒增强铝基复合材料的摩擦学性能,将其加工成销试样,在多功能摩擦磨损试验机上分别与钢盘试样进行对比摩擦磨损试验,重点研究了接触载荷和相对滑动速度对两种复合材料摩擦磨损性能的影响.结果表明:碳化硼增强铝基复合材料的磨损量随载荷与相对滑动速度的增大而增大,而摩擦因数随载荷与相对滑动速度的增大而减小,较高碳化硼含量的复合材料的耐磨性能比较低含量的复合材料好.