基于分形理论的滑动摩擦表面接触力学模型

依据分形理论,考虑微凸体变形特征及摩擦作用的影响建立滑动摩擦表面接触力学模型。采用一个三次多项式来表达弹塑性变形微凸体的接触压力与接触面积的关系,从而满足在变形状态转变临界点处的微凸体接触面积与接触压力转化皆是连续和光滑的条件。推导出滑动摩擦表面临界弹性变形微接触面积、临界塑性变形微接触面积、量纲一真实接触面积的数学表达式。理论计算结果表明,表面形貌一定时,真实接触面积随着载荷的增大而增大;载荷一定时,真实接触面积随着特征尺度系数的增大而减小,随着分形维数的增大先增大后减小;当表面较粗糙时,摩擦因数对真实接触面积的影响很小;随着表面光滑程度的增大,摩擦因数对真实接触面积的影响增大,真实接触面积随着摩擦因数的增大而增大,特别是当摩擦因数较大时,真实接触面积增大的幅度也较大。接触力学模型的建立,为研究滑动摩擦表面间的摩擦磨损性能提供了依据。     

Fretting behavior of a rubber coating: Friction characteristics of rubber debris

This paper presents friction characteristics of a rubber coating undergoing fretting-like, oscillatory motion. Results are presented for rubber coated stainless steel as well as uncoated stainless steel experiencing fretting motion. Friction coefficients are experimentally investigated before and after the rubber coating is fully worn out. The effect of load, velocity and displacement amplitude on the friction coefficients of both uncoated and coated stainless steel is discussed.     

冰面上橡胶滑动摩擦特性的数值分析

基于摩擦熔化理论,考虑流体动力润滑和热传导的耦合作用,采用打靶法和线性插值法获得了冰面上滑动橡胶块摩擦界面上的压力、流体膜厚及温度的无量纲分布规律,研究了胶块的摩擦特性。结果表明,摩擦界面上的压力峰值随载荷的减小向界面中部移动,摩擦因数与滑动速度的平方根成正比,而与载荷和试样长度的平方根成反比。所用方法可应用于其它材料在冰面上的摩擦及其它情况下的摩擦熔化问题。     

Microstructural investigation of ice surfaces after rubber–ice and sand–ice sliding friction tests

The dual process of etching and replicating technique revealing lattice dislocations, dislocation cells, grain and sub-grain boundaries, etc., in ice has been applied to investigate the microstructure of freshly deformed ice surfaces after rubber–ice and sand–ice sliding friction tests. Features related to the generation, multiplication and mobility of dislocations were observed in ice subjected to rubber–ice friction even at high speeds and high ice temperatures, involving low friction. During sand–ice friction, deformation took place at the surfaces as well as deeper within the ice by ploughing sand particle, and was accompanied with recrystallization. The deformation features in ice found in the laboratory were also observed in full-scale tire–ice interactions on ice covered runway pavements.     

Role of Rubber Stiffness and Surface Roughness in the Tribological Performance on Ice

The aim of this work is to investigate the effect of slider surface roughness and stiffness on the friction between rubber compounds and ice surfaces in order to provide insight into rubber–ice friction generation mechanisms. For this purpose, rubber compounds were designed to have different levels of macroscopic roughness and cured stiffness by modifying the filler system and plasticizer loading. In order to accurately evaluate the effects of surface and bulk rubber property on ice friction, linear friction tests were performed on laboratory ice with varied frictional heat buildup by modifying the friction test protocol. The results showed that the friction force was in general increased through the ploughing effect of a rough rubber block sliding on smooth ice. The increase in friction by ploughing was more pronounced when the contacting rubber block had sufficiently low stiffness and when the accumulation of frictional heat on ice was sufficiently high. It was also evidenced that a sufficiently hard rubber with test conditions leading to low heat buildup on ice could nevertheless lead to an opposite influence of roughness on rubber–ice friction; that is, lower friction force with a slider with a higher roughness.     

基于分形理论的圆弧齿轮滑动摩擦接触力学模型

考虑到圆弧齿线圆柱齿轮传动接触之间的滑动摩擦与微凸体的连续性变形,结合分形理论和Hertz接触理论建立圆弧齿线圆柱齿轮的滑动摩擦接触力学模型,通过模型数值分析与ANSYS WORKBENCH分析的最大接触应力结果对比,证明该模型所反映圆弧齿线圆柱齿轮接触应力状态的正确性。该模型中,载荷与真实接触面积之间关系不仅与分形维数和特征尺度系数有关,还与齿轮节点曲率和齿轮齿线半径有关。同时,理论计算表明,分形维数一定时,真实接触面积随着载荷的增大而增大;载荷一定时,真实接触面积随着分形维数的增大先增大后减小,随着特征尺度系数的增大而减小;摩擦因数对真实接触面积的影响不大。该模型的建立为圆弧齿线圆柱齿轮工作状态的研究及强度分析提供了理论依据。     

The contact and friction between flat belts and pulleys

Rubber coated nylon flat belts running over pulleys in practice display friction coefficients between 0·3 and 0·8. This paper studies the causes of the friction variations. Adhesive friction theory considers the friction force to be the product of the real areas of contact between the sliding surfaces and the shear stress at the contacts: these two quantities have been separately measured by running belts over transparent perspex pulleys and directly observing the contacts. It has been found that variations in contact from one belt to another due to their method of manufacture are as significant in explaining differences in their friction behaviour as are variations in shear stress caused by their different rubber formulations. Real areas of contact were less than one third of the apparent area and varied with load, elastic modulus and roughness of the belt surface in a way broadly understandable in terms of elastic contact mechanics. Shear stress were about 0·5 Nmm⁻², perhaps determined by hydrocarbon films. Some belts showed real areas of contact not directly proportional to load. This led to their friction coefficients being load dependent.     

Laboratory studies of the friction of rubber on ice

Information regarding the frictional characteristics of rubber on ice surfaces is helpful in the study of skid behaviour of vehicles on icy roads. This paper describes an experimental test rig developed to study the frictional characteristics of rubber on ice surfaces under controlled conditions in a laboratory. The effect of various operating parameters such as load, speed, temperature, type of ice surface, etc., have been studied and the results discussed.     

Comparison between rubber–ice and sand–ice friction and the effect of loose snow contamination

The application of sand particles is a common method to improve the friction of aircraft tires on snow or ice covered runways. Hence, an understanding of the prevailing rubber–ice and sand–ice friction mechanisms is of practical interest. Rubber–ice and sand–ice friction measurements were made with a British Pendulum Tester at temperatures between ?22 and 0 °C and the effect of loose snow contamination on top of the ice was investigated. The results (the response of the instrument) were expressed in a sliding length averaged friction coefficient μ_BP. Close to the melting point the friction of rubber on ice was low and increased with decreasing ice temperature. Below ?5 °C, reasonably high friction levels (0.2<μ_BP<0.5) were obtained between rubber and ice, but the friction level dropped drastically by the presence of a very thin layer of snow. The sand–ice friction level was less dependent on ice temperature and clearly not as much affected by the presence of snow, compared to rubber–ice friction. The micromechanisms involved in rubber–ice and sand–ice frictions were investigated by the application of etching and replicating technique (ERT) developed for the examinations of the dynamics of dislocations in ice during deformation.     

汽车轮胎橡胶摩擦试验研究

介绍了所开发的轮胎橡胶摩擦试验台,提出了轮胎橡胶摩擦试验方法。在该试验台上进行了干水泥路面、冰面等工况下的轮胎橡胶块摩擦试验,对所得试验结果进行了分析、处理。初步掌握了路面、温度、垂直压力和滑移速度等因素对摩擦因数的影响。在此基础上把动摩擦的概念引入轮胎半经验模型,对现有模型进行了改进。提出了应用轮胎低速试验数据预测高速下轮胎特性的方法。     

Contact properties of a wet clutch friction material

Wet clutches are required to transmit torque and also prevent motion in automatic transmissions. Their performance is critically dependent on a friction material which comprises one of the contacting surfaces. Friction materials are usually a composite of fibres, naturally occurring minerals and particles of silicon and graphite, which are all bonded together with a resin. The material formed has very rough surfaces with much steeper slopes than normally-finished steel surfaces. When the friction material is loaded against a relatively flat counterface the real area of contact is only a small percentage of the nominal area and consists of many small, independent “contact units”. It is important to know the conditions present in the contact units (spatial dimensions and pressure) in order to understand and model wet clutch lubrication.In this study, the contact units formed between a paper based friction material and a glass counterface have been investigated under different pressures and during rubbing. A contact visualisation technique is used to directly view and capture images of the contact. The real area of contact and the number of individual units is subsequently determined by image analysis. It is found that the real area of contact increases approximately linearly with applied load, and increases rapidly with rubbing, due to wear. As the load is increased, the number of individual contact units increases up to a critical pressure, suggesting more parts of the material support the load. Above the critical pressure the contact units may be deforming elastically and/or plastically to form larger units. After rubbing, large contact units are formed by flat areas on the tops of the contacting fibres, which are formed during wear. The topography of individual fibres is studied before and after the wearing process using atomic force microscopy, and the results support the truncating wear mechanism.     

Variations in the properties of the friction zone of metallic composites under the effect of electric current and Pb-Sn melt in the contact gap

The paper deals with study of the wear rate and specific surface electric resistance of the contact of metallic composites in dry friction at a high current density. Friction induced structures are shown to appear in the surface layer of the composites. The specific electric resistance of the friction induced structures is estimated quantitatively. It is found that their specific electric resistance is similar to that of the known electrographites and carbon graphites, resulting in a high contact resistance. The electric resistance and wear rate of the friction zone are diminished by the introduction of a Pb-Sn melt. The possibility of developing of an efficient sliding electric contact of two metallic composites with a metal melt between their friction surfaces is noted.     

Sliding friction of polyethylene on ice: tribometer measurements

It is well known that the low kinetic friction experienced when sliding on snow and ice is due to water films generated through frictional heating. There is, however, uncertainty concerning the thickness and the distribution of these water films. Since direct observation of the water films is difficult, tribometer studies coupled with temperature measurements have been carried out on a large-scale, pin-on-disc tribometer (diameter 1.80 m). IR sensors were used to measure the temperature of the ice track in front of and behind the contact region. In addition, thermocouples integrated into the polyethylene slider measured the temperature close to the interface. The kinetic friction between polyethylene and ice has been measured as a function of temperature, velocity, load, and apparent contact area. The friction coefficient, as well as the temperature increase of the slider and the ice track, depends on all of these parameters. Interpretation of the results is given on the basis of hydrodynamic lubrication, taking into account the generation and shearing of thin water films in the contact regions.     

A theory of viscous hydroplaning

The traction developed in the contact area of a pneumatic tyre rolling and accelerating on a wet road surface is simulated by a block of rubber slipping on a two-dimensional sinusoidal surface covered with a thin water film. The resulting pressure-wedge formation on individual asperities of the surface provides a net uplift which tends to force tyre-tread and surface apart, thus promoting the onset of skidding or viscous hydroplaning. The important factor in minimizing skidding hazards is the provision of an adequate micro-roughness at asperity tips which contributes an adhesional mechanism to the overall friction coefficient. It is shown that in comparison with the adhesional contribution to friction, the contribution of hysteresis and viscous shear of the film are insignificant at low slip speeds.     

Tribological Behavior of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> Sliding Against Ni-based Alloys at Elevated Temperatures

Understanding the mechanism of “rubbing” noise and low-amplitude friction exited vibration generation in steady sliding can be helped by models describing the contact interactions. In the current article, we consider a simple microscopic contact model for surfaces in sliding, which is based on the adhesion theory of friction. In the proposed model, we consider that the formation and shearing of a junction contributes to a small change in the real contact area. The model incorporates random size and random spacing between junctions. We investigate the dependence of the instantaneous real contact area on the average size and number of junctions. We find that from the viewpoint of vibration reduction, it is advantageous if the real contact area needed to support the given load is obtained as a sum of many small-sized micro-contacts, instead of few large-sized micro-contacts. The above result is in agreement with experimentally observed reduction of vibrations of a hard-disk slider after texturing.     

封隔器胶筒大变形的粘-滑摩擦接触分析

采用罚函数方法,结合橡胶大变形问题的增量分析过程,考虑封隔器胶筒与套管之间的粘-滑摩擦接触问题,研究摩擦因数变化对接触压力的影响规律。给出解决封隔器胶筒摩擦接触问题的数值方法,并在此基础上对胶筒与套管之间的摩擦接触进行有限元分析,分析得到采用大变形非线性粘弹性理论和接触摩擦描述的有限元模型,可以比较准确模拟封隔器在坐封和工作过程中胶筒接触压力和变彤的情况,结果表明,摩擦因数变化对封隔器胶筒的接触压力有着较明显的影响。     

Effect of consolidation in boundary lubrication

This paper is concerned with the effects of the colloidal aspect in boundary lubrication. The newly developed method for surface forces measurement has clearly shown the consolidation effect during contact loading. The paper also shows that, in some cases, the friction coefficient varies with the compaction of wear particles in the contact. Friction experiments between a ball and flat (both sapphire) in which the contact is immersed in water, have been performed to understand the effect of compaction of worn material on friction. During friction, wear debris is piled up in the contact and shear behaviour depends on pressure and volume fraction of wear particles.     

Thermomechanical spectrometry of friction products formed in friction of compatibilized PA-6/HDPE blend with steel

Friction products (surface layers of the polymer sample, wear products, layers of polymer material transfer to the metal counterface) from the friction on steel of PA-6/HDPE blend, compatibilized HDPE with grafted methylene-butane-dione acid with the composition PA-6/(HDPE/HDPE-gr-MA-25 mass %)-20 mass % *, are studied using thermomechanical spectrometry. A considerable influence of contact load on the molecular- topological structure of these products is revealed. It is established that component redistribution takes place in the friction products, and this redistribution changes their ratio in the blend and their molecular-topological structure; in this case, none of the wear products has the structure of any of the components of the blend for the studied contact load range (1–6 MPa). It is shown that one of the following mechano-chemical processes, depending on the contact load, takes precedence in the polymer material during friction on steel: macromolecule destruction accompanied by decrease in their molecular mass, increase in molecular mass due to formation of linked and presumably copolymer structures, decrease or increase in crystallinity, and change of relaxation properties.     

Frictional behaviour of Pb‐Sn thin‐film lubrication

The frictional properties of lead‐tin thin films (thickness of 0.05–0.19 μm) with two types of copper interlayer were investigated. The thin film and the interlayer were formed on a silicon wafer surface by vacuum deposition. Friction tests were carried out using a ball‐on‐disc apparatus in a vacuum chamber. The thin copper interlayer reduced the friction coefficient and prolonged the film life. The effect of load on the friction coefficient is explained by an equation derived using the Hertzian contact area between a sphere and a plate. The thicker copper interlayer did not reduce the friction coefficient but markedly extended the life of the film. In this case, the dependence of the friction coefficient on the load is explained by an equation derived using the Hertzian contact area between a sphere with surface roughness of second order and a plate.     

Tribological study of peroxide-cured EPDM rubber filled with electron beam irradiated PTFE powder

The aim of this work is the evaluation of the effects of electron beam modification of Polytetrafluoroethylene (PTFE) powder on the friction and wear properties of PTFE filled Ethylene-Propylene-Diene-Monomer (EPDM) rubber cured by a radical-initiated peroxide system. Friction and wear properties of EPDM vulcanizates were determined with the help of pin-on-disc tribometer in sliding contact with a steel-ball at room temperature without lubrication. PTFE powder was modified in atmospheric conditions with low (20 kGy) and high (500 kGy) irradiation doses using electron beam accelerator. The spectroscopic investigations reveal that the increasing concentration of reactive free radicals and functional groups with irradiation dose has a drastic influence on crosslinking efficiency due to their interference with peroxide radicals in curing process. As a result, non-irradiated and low-irradiated PTFE filled EPDM with higher crosslinking densities showed remarkably enhanced friction and wear properties. On the contrary, 500 kGy PTFE filled EPDM having significantly lower crosslinking density resulted in poor friction and wear characteristics. The apparent crosslinking density measured directly from the curing curves was found to be the most dominating parameter influencing friction, wear as well as the physical properties.