Study of plastic shear deformation in surface layer at friction. Simulation results. Part I. Model description

A one-dimensional macroscopic model of friction is presented which considers changes in elastic characteristics of the material caused by frictional heating and strain hardening. A single act of contact between a microasperity made of a ductile hardenable material and the counterbody was calculated for various contact spot sizes. It is shown that the contact spot size affects considerably the temperature, plastic deformation, and changes in the material elastic characteristics. Shears of surface layers relatively to the base material observed experimentally are explained proceeding from the modeling results.     

Analytic and FE based estimations of the coefficient of friction of composite surfaces

In this work the coefficient of friction (COF) of a body with a composite surface layer (CSL) in sliding contact with a homogeneous counterbody is studied. With an analytic model the upper and lower bounds of the COF are estimated, whereas the material and surface properties of the CSL components are known and no information about their geometrical arrangement is at hand. An analogy with the Voigt and Reuss bounds is invoked. The analytic results are compared to FE computations. Additionally the influence of plastic deformation and geometric properties of the CSL are treated.     

The influence of friction and wear on the generation of surface microprofile of steel–sintered friction powder based on copper in the course of operation in lubricated medium

The development of new compositions of sintered friction powders based on copper that operate in lubricated states should be based on deep knowledge of peculiar features of break-in and wear mechanisms of both counterbody and friction material. The obtained data have enabled the determination of the existence of the transferred bronze layer of friction material, the peculiarities of the generation of the surface microprofile of the counterbody during operation, these data can be useful upon assessing the thermal loading of the friction zone.     

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.     

Autowave friction and nonequilibrium dynamic tribosystem self-regulation

The transition to the condition of autowave motion is illustrated on the example of a dynamic system consisting of weights linked to a pulley moving with friction in the pulley axis under the force of gravity. The coherent interaction between the external modes of nonstationary directed motion and the internal wave modes under the studied conditions induces the effects of negative dynamic friction. The tangential intensification of the counterbody friction in the intervals of positive feedback actuates both the tangential and normal component of the full contact response that has no counterparts in the equilibrium tribodynamics. This effect is explained by the resonance automodulation of contact friction by elastic waves of stresses. The qualitative parameters controlling the changes in external friction conditions are introduced, similar to the Reynolds number in hydroaeromechanics. The general problems of contact tribodynamics and hydroaerodymics of the boundary layer are analyzed in respect to the coherent interaction in the friction system and the wave impulse transfer.     

Tribosynthesis of lead fluoride in friction of fluroplastic lead-containing composites and its effect on wear resistance

The processes running in the friction contact zone of lead-containing fluoroplastic composites and steel counterbody are studied. The results of X-ray phase examination of the transfer films on the steel counterbody are presented for the case of friction of lead-fluoride-containing fluroplastic composites. In friction of the given composites, the secondary structure, namely lead fluoride, is shown to arise in the contact zone. It is found that the fluorite crystalline structure β-PbF₂ possesses lubricity and increases the wear resistance of PTFE-based composites.     

Calculation of the actual contact area between a single microasperity and the smooth surface of a part when the hardnesses of their materials are similar

The paper presents the dependences for determination of the interface, the radius of curvature of the loaded contact surface, and the actual contact area between a single microasperity and a part for different relationships between the hardnesses of their materials. To the previously used regularities of the indentation or flattening of a microasperity in contact with a counterbody, we add the case where the microasperity is flattened and simultaneously indented into the surface of the counterbody. This case is typical for the situation where a spherical indenter and flat counterbody of similar hardnesses contact. The experimental data corroborate the generalized analytical dependence for determination of the actual contact area.     

Relationship between the propagation of fatigue cracks and the behaviour of plastic flow under rolling/sliding contacts - effect of slip ratio

The relationship between the propagation of fatigue cracks and the behaviour of plastic flow in the surface layer was experimentally investigated under lubricated rolling-sliding contact. The test specimen material was medium carbon steel (S45C) with hardness H_v 187 after annealing. Results show that the displacement of plastic flow in the surface layer of the follower increased with increasing number of revolutions. Crack propagation occurred in the surface layer of the follower when the displacement of plastic flow on the centre of the contact reached a saturated value. On the contact surface of the follower, the propagating angle of the fatigue crack was found to be correlated to the angle of plastic flow, which was independent of the slip ratio. The rate of plastic flow increased with increasing slip ratio, resulted in a reduction of fatigue life. The displacement of plastic flow beneath the contact surface could be evaluated from the profile change of the plastic flow on the contact surface.     

Influence of counterbody material on fretting wear behaviour of surface mechanical attrition treated Ti–6Al–4V

Surface mechanical attrition treatment (SMAT) was carried out on Ti–6Al–4V. Fretting wear tests were conducted using two counterbody materials (alumina and steel). SMAT resulted in surface nanocrystallization. Due to high hardness, low tangential force coefficient (TFC) and more TiO₂ layer, fretting wear resistance of SMAT treated samples was higher than that of the untreated samples. TFC values obtained with alumina counterbody were higher than those obtained with steel counterbody. The fretting wear resistance of untreated and treated samples fretted against alumina was lower than that of the samples fretted against steel due to tribochemical reactions at the contact zone.     

On the role of copper in brake friction materials

Copper is a major ingredient in friction materials used for automotive braking. The purpose of this study was to find out how copper contributes to good brake performance properties in addition to providing good thermal conductivity. Microstructural investigations of copper chips at the surfaces of brake pads revealed a zone of severe plastic deformation which provides high hardness, but there is also evidence of recrystallized copper nano-particles which are incorporated into friction layers as soft ingredient once detached from the pad surface. Thus copper seems to play a dual role, firstly as reinforcing element of the brake pad providing primary contact sites, and secondly as solid lubricant by contributing to the formation of a layer of granular material providing velocity accommodation between the rotating disc and fixed pad. Confirmation for this hypothesis was obtained by modelling contact sites on the nanometre scale with the method of movable cellular automata. Results show both, the similarity of steel fibres and copper macro-particles in respect to providing primary contact sites, as well as similar sliding behaviours of friction layers containing either copper or graphite as soft inclusions. Furthermore, it is shown that not only material properties, but also the concentration of solid lubricant particles in the friction layers, determine conditions for friction force stabilization and smooth sliding behaviour.     

Numerical simulation of operation heat modes of multidisc oil-cooled vehicle brake. Part 1. Heat problem solution

The paper considers the operating heat modes of multidisc oil-cooled vehicle brakes under single and multiple braking, as well as under deceleration over a long downgrade. The initially boundary-value heat problem for the given operating modes of the brake is formulated and solved by the finite-element method. The regularities of formation of nonstationary temperature fields both on the contact surfaces and in the bulk of the brake friction elements are obtained for preset braking modes. The dependences are obtained for the maximal contact surface temperature on the metal counterbody and frictional material vs. the load and velocity parameters of vehicle motion. A method for forecasting the critical operating regimes of multidisc oil-cooled brakes is proposed for the given operation conditions of the vehicle.     

Tribological behavior and topography of friction surfaces of diamond-like coatings in contact with steel,silicon nitride,and quartz glass

The paper presents the study results of the tribological behavior and surface topography formed at friction of diamond-like coatings against indenters made of silicon nitride, quartz glass, and steel. It is shown that the tribological behavior depends on the nature and hardness of the counterbody material whose wear causes changes in the surface topography of the diamond-like coating at the nanometer level. At friction of the diamond-like coating against the silicon nitride indenter surface asperities are deformed plastically and the deformation rate is governed by the coating structure.     

热力耦合下微接触点塑性应变沿深度变化分析

在考虑粗糙实体弹塑性变形、热力耦合、微凸体间相互作用和摩擦热流耦合等影响下,运用有限元法数值模拟具有三维分形特性的粗糙面与刚性平面间滑动摩擦过程,分析了粗糙实体接触凸点塑性变形随深度变化情况。发现：在速度的突变和闪点温度形成时,摩擦接触表层等效塑性应变增大明显;在这一摩擦表层,过不同接触点的纵向剖面塑性应变沿深度分布不同：有的是接触表面塑性变形最大,有的是在接触微凸体表面下某一深度塑性变形最严重,而接触凸点表面的塑性应变稍小些。这与相关文献用SEM研究干摩擦后金属摩擦表层变形照片后发现的结果一致。滑动摩擦过程中,金属粗糙摩擦接触表层塑性变形的不断累积,将会导致材料表层中的夹杂或微观缺陷周围萌生微孔和裂纹源。     

What role for contact spots and dislocations in friction and wear?

Sixteen years of research on the properties of 'contact spots' of ductile materials in 'adhesive' wear are surveyed. They are the locations (typically occupying less than 1% of the macroscopic area of contact) at which solids ale in load-bearing contact during wear. Their properties determine the coefficient of friction, and friction and Joule heat are generated at them. It was found that the mechanical behavior of contact spots is governed by ordinary dislocation plasticity. They remain elastic if the local pressure falls short of the indentation hardness of the softer side. In that case (almost) no wear occurs. Elastic contact spots are promoted by light loads and fine polishing. Ordinarily, wear particles are detached through tangential shearing-off where, statistically, the two sides at contact spots momentarily interlock. The Holm-Archard wear 'law' is a simple direct result of this mechanism. However, quite typically the bulk of sliding takes place within adsorbed moisture films which are ubiquitous in our daily surroundings. Outside of contact spots the adsorbed moisture behaves much like ordinary water. At the typical contact spot the moisture is squeezed down to but two monomolecular layers; and the relative motion between sliding solids overwhelmingly takes place between these. This causes the prevalence of friction coefficients about μ 0.3 in our surroundings. The complex behavior of the adsorbed water can cause stick-slip. As water is desorbed at 170 °C, the determination of flash temperatures at contact spots is critical. A corresponding theory was developed and verified in connection with experiments on graphite lubrication which depends on the presence of adsorbed moisture. Further, plastic contact spots and solid lubrication have been simulated by means of a Bridgman anvil apparatus. The results show that the same work-hardening behavior applies at contact spots as known from the bulk. They also proved that at contact spots intimate mixing of the materials of the two sides gives rise to a finely mixed layer which can amorphize by much the same dislocation mechanism as is believed to cause melting. The amorphous material promptly recrystallizes at least in the Cu-Ag system. It is proposed to try and inhibit such recrystallization by selected lubricants, so as to promote beneficial wear-resistant tribo-films.     

Analysis of friction contact between wavy surfaces

The objective of the present work is to study the tribological behaviour of two contact bodies coated with solid lubricant films. The suggested model relaxes two restrictions of the classical Hertz theory by introducing the friction effect and the elastic-plastic behaviour as irreversible effects.The analysis is based on the automated direct procedure developed by Mahmoud and Salamon and an extension of that model to deal with plastic flow and Coulomb friction between contact surfaces. The proposed model is applied to a typical example of a wavy surface pressed against a flat strata. Both the wavy surface and the flat strata are coated with a hard lubricant film. The history of the advancing contact and the initiation of the plastic flow are determined through the procedure of the model.     

An upper-bound analysis of friction in metal forming

In the analysis of metal forming processes, a knowledge of friction is important, especially when the microstructure evolution and criteria for limiting phenomena are predicted by numerical simulation. The friction wave model has been studied by several researchers. Their analyses are mainly based on the assumption that there is no plastic deformation of the bulk material. However, it is necessary to clarify the influence of bulk material deformation on the surface asperity deformation. This paper deals with the development of a friction wave model by considering the influence of bulk material on the surface asperity deformation. The situation of rough tool—smooth workpiece (RT—SW) contact during forming process has been investigated. Based on this condition, an admissible velocity field is constructed for the upper bound analysis. The relationship between the normal pressure and the sliding resistance is established over a large range of pressure. The role of surface roughness, bulk displacement and bulk strain on metal forming friction is analysed.     

激光喷丸技术及其应用

激光喷丸技术是一项新技术，它是用短脉冲（ns级）的强激光辐照在表面覆盖着能量吸收层和约束层的材料上产生冲击波，当激光冲击波诱导的应力波的峰值超过材料的动态屈服极限时，材料的表层将会发生塑性变形，不可回复的塑性变形导致靶材内残余应力的产生。激光喷丸的效果与脉冲的能量、光斑直径的大小、材料的力学性能等因素有关。激光喷丸不仅可以对材料表面进行改性，还可实现板材的塑性成形，并且表面留有有益残余的压缩应力，从而降低了材料的疲劳断裂和应力腐蚀的比率和延长其疲劳寿命。文章简要介绍了该技术在发达国家的应用，也指出该技术在我国走向实用化阶段必须要解决的关键问题。     

渗氮钢粗糙表面的弹塑性接触研究

利用弹塑性有限元和单纯形法求解弹塑性接触模型，分别模拟了屈服强度呈梯度变化的渗氮钢、未经处理的匀质材料和硬涂层材料粗糙表面的弹塑性接触行为。与未经处理的匀质材料相比，渗氮钢可承受更大接触载荷。在相同载荷作用下，渗氮钢表面粗糙峰接触面积较小，平均间距较大，接触体内材料不易发生屈服，从而显著提高接触性能。和硬涂层材料相比，渗氮钢接触体内等效von Mises应力分布平缓，没有应力突变。最后讨论了渗氮层和硬涂层的厚度对粗糙表面接触特性的影响。     

A friction model evaluated with results from a bending-under-tension test

The friction in stamping is an important process parameter to control the flow of material in the tool. Consequently, it is also an important parameter in the design process of new stamping tools when numerical simulations of the forming operations are performed. In this work an advanced friction model is evaluated, which considers properties of surface topography, lubricant, sheet material, and process parameters such as sliding speed and pressure. The evaluation is made by comparing theoretical results with experimental ones obtained in a bending-under-tension friction test. The results show conformance in behaviour between the friction model and the experimental work. Furthermore, a model, which considers the influence of bulk plastic strains on the real area of contact, is investigated. The developed model predicts that the effective hardness of a surface is reduced by the presence of underlying plastic flow. It is found that when the strain rates are increased, the Stribeck curve becomes flatter and mixed lubrication is introduced at lower Hersey values. The friction model clearly shows the potential of improving the FE simulations of sheet metal forming operations, in comparison to the use of the classical Coulomb's friction model.     

Role of contact frequency on the wear rate of steel in discontinuous sliding contact conditions

This work presents a novel approach of sliding ball-on-disk wear tests where the disc material is investigated. Each part of the wear track on the disc is in discontinuous contact with the counterbody. The contact frequency at each part of the wear track on the disc with the counterbody is defined by the rotation frequency of the disc. The sliding speed is however a function of both the rotation frequency and wear track diameter. In this work, the effect of the contact frequency on friction and wear was investigated on carbon steel in discontinuous sliding contact with corundum balls. Various sliding speeds were used while maintaining the contact frequency at a fixed value, and various contact frequencies were applied at constant sliding speeds.The wear rate of the disk material is shown to depend not only on the usual wear test parameters, namely sliding speed and contact load, but also on contact frequency. Moreover, contact frequency is shown to be a key factor determining the wear mode even at constant sliding speed and load. At contact frequencies above 9 Hz, the dominant wear mechanism is oxidational wear, while at frequencies below 4 Hz the dominant wear mechanism is adhesive wear. This transition from adhesive to oxidational wear takes place together with a change in the type of debris generated and in the value of the coefficient of friction.The validity of the Garcia-Ramil-Celis model proposed earlier for discontinous sliding contact conditions, is demonstrated for the case of carbon steel disks sliding against a chemically inert counterbody.