纳观纹理表面相关参数对滑动摩擦过程的影响

采用分子动力学方法研究了半球形刚性压头在单晶铜纹理表面上的纳观黏着滑动摩擦过程。对不同纹理密度下纹理形状和纹理深度对黏滑摩擦性能的影响进行了全面研究,通过对比分析不同纹理参数下的滑动摩擦力和基体变形,揭示了上述参数对纹理表面黏滑摩擦的影响规律。模拟结果表明:在相同的纹理密度下,柱状纹理表面的滑动摩擦力小于矩形纹理表面。相比矩形纹理,柱状纹理表面的结构稳定性较差,但纹理表面的结构稳定性随着纹理密度的增加而加强。在相同的纹理密度下,矩形纹理表面的滑动摩擦力随着纹理深度的增加而减小。     

Friction characteristics of nanoscale sliding contacts between multi-asperity tips and textured surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.     

Surface lay effect on rough friction in roller contact

Surface lay describes the direction of the predominant surface pattern. A properly designed surface texture configuration has been recognised as a vital issue affecting lubrication and sliding in machinery applications in the literature. Gaining understanding of this tribological phenomenon is no doubt beneficial in facilitating the production of more efficient machine parts and thus reduces production cost. This paper describes an experimental method to investigate the effect of surface lay on lubricated rolling/sliding of ground roller surfaces. By using the rough friction test rig, different surface lay contacts can be simulated and the friction can be measured. Friction behaviour was interpreted in terms of Stribeck curves (friction coefficient as the function of Hersey parameter [ην/p]). Results show that an optimal contact lay angle that provides a minimum friction value is achievable through rig testing. The relative sliding speed direction has a symmetrical effect on friction at the same lay orientation; for sliding speed angles less than about 80°, the larger the angle, the lower the friction, and vice versa.     

非对称结构表面织构对滑动轴承摩擦性能的影响

为揭示织构凹坑方式对径向滑动轴承摩擦性能的影响,基于气液两相流理论,建立径向滑动轴承腔内油气两相流数值计算模型.将传统普通织构改进为出入口呈阶梯状的非对称织构,定义出入口壁面高度比值H为非对称因数,采用SIMPLEC算法进行求解,分析非对称因数对凹槽织构单元流体域内流场的影响,探讨径向滑动轴承不同位置织构、不同转速下非...     

Impact of Wire-EDM on Tribological Characteristics of ZrO<Subscript>2</Subscript>-based Composites in Dry Sliding Contact with WC–Co-Cemented Carbide

In the present investigation, experiments were conducted by unidirectional sliding of pins made of FCC metals (Pb, Al, and Cu) with significantly different hardness values against the steel plates of various surface textures and roughness using an inclined pin-on-plate sliding apparatus in ambient conditions under both the dry and lubricated conditions. For a given material pair, it was observed that transfer layer formation and the coefficient of friction along with its two components, namely adhesion and plowing, are controlled by the surface texture of the harder mating surfaces and are less dependent of surface roughness (R _a) of the harder mating surfaces. The effect of surface texture on the friction was attributed to the variation of the plowing component of friction for different surfaces. It was also observed that the variation of plowing friction as a function of hardness depends on surface textures. More specifically, the plowing friction varies with hardness of the soft materials for a given type of surface texture and it is independent of hardness of soft materials for other type of surface texture. These variations could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding. It was also observed that among the surface roughness parameters, the mean slope of the profile, Δ _a, correlated best with the friction. Furthermore, dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing taking place at the asperity level.     

Effects of entraining velocity of lubricant and sliding velocity on friction behavior in stainless steel sheet rolling

A series of experiments was carried out using a rolling-type tribometer to investigate the effects on friction behavior of the entraining velocity of the lubricant at the inlet to the contact zone (V) and sliding velocity during deformation (ΔV). Experiments with stainless steel sheets of two different surface roughnesses showed that the variations in the friction coefficient with entraining velocity V and sliding velocity ΔV are largely dependent on the initial surface texture of the workpiece. For a smooth workpiece, the friction coefficient decreases with increasing sliding velocity ΔV but keeps almost constant with increasing entraining velocity V. However, for a rough workpiece, the friction coefficient initially decreases slowly and increases largely with increasing sliding velocity ΔV or decreasing entraining velocity V. Observation of the rolled surface for a smooth workpiece shows that, with increasing entraining velocity V, the slip band becomes more marked, and with increasing sliding velocity ΔV, the rubbed portions become more conspicuous. For a rough workpiece, galling occurs at high sliding velocity ΔV. The critical condition for galling outbreak is shown on the V-ΔV graph. The galling outbreak process is observed by interrupting the rolling process.     

Rolling and sliding: Separation of adhesion and deformation friction and their relative contribution to total friction

This study is concerned with determining the relative contribution of adhesion and deformation friction using rolling and sliding method. The challenges associated with in-vivo friction testing were overcome by utilising a novel substrate that mimics the viscoelastic behaviour and surface texture of human skin combined with a repeatable and reproducible test setup. The results show that in the dry state, deformation friction contributes 20% of the total friction while the remaining proportion is due to adhesion. These proportions are affected by probe material where for PTFE, deformation friction contributes 30% of the total friction. For the lubricated state, the contribution of deformation friction to total friction increases approaching 50–50% at the higher sliding speeds and normal loads investigated.     

Effects of groove textures on fully lubricated sliding with cavitation

A two-dimensional computational fluid dynamics (CFD) model was developed to investigate the effects of groove textures on fully lubricated sliding with cavitation. The effects of cavitation pressure, sliding speed, sliding pitch angle and texture scale were discussed. It was found that the hydrodynamic pressure effect becomes more pronounced with higher cavitation pressure, and the hydrodynamic pressure decreases with the reduction of the sliding speed. Also with a sliding pitch angle, the hydrodynamic pressure is dependent on both pocket and wedge effects. Increasing groove number and reducing the groove size enhances the overall load capacity, but has a little effect on friction coefficient.     

Surface Engineering Design of Alumina/Molybdenum Fibrous Monolithic Ceramic to Achieve Excellent Lubrication in a High Vacuum Environment

Al₂O₃/Mo fibrous monolithic ceramics are potential candidates for space applications because of their excellent mechanical properties and low density. This study aims at achieving low friction and long life of this material in a high vacuum environment. Three-dimensional composite-lubricating layers were fabricated by considering texture pattern as storage dimples and MoS₂ synthesized via hydrothermal method as lubricant. The tribological properties were studied sliding against Si₃N₄ ceramic and GCr15 bearing steel balls under high vacuum condition. Results showed that the lubricating properties of the Al₂O₃/Mo fibrous monolithic ceramics were improved greatly by the micro-texture and MoS₂ solid lubricant; the friction coefficients were as low as approximately 0.08 and 0.04, respectively, when Si₃N₄ ceramic and GCr15 bearing steel balls acted as the pairing materials. It was also demonstrated that the low friction coefficient can be realized with various normal loads and sliding speeds, indicating the composite-lubricating layers have good adaptation of working conditions. This excellent performance of the material is mainly because of MoS₂ stored in dimples can be easily dragged onto the friction surface to form lubricating and transferring films during the friction process. This work is an extension of studies that were previously published in Tribology Letters journal.     

Effects of groove textures on fully lubricated sliding with cavitation

A two-dimensional computational fluid dynamics (CFD) model was developed to investigate the effects of groove textures on fully lubricated sliding with cavitation. The effects of cavitation pressure, sliding speed, sliding pitch angle and texture scale were discussed. It was found that the hydrodynamic pressure effect becomes more pronounced with higher cavitation pressure, and the hydrodynamic pressure decreases with the reduction of the sliding speed. Also with a sliding pitch angle, the hydrodynamic pressure is dependent on both pocket and wedge effects. Increasing groove number and reducing the groove size enhances the overall load capacity, but has a little effect on friction coefficient.     

The genesis of friction

The genesis of friction is explained in terms of a new theory. Contrary to the postulates of the adhesion theory of friction, this theory postulates that the frictional force (and thus the friction coefficient μ) is affected by the sliding distance and the environment because of the changing contributions of three components of friction, i.e. that due to the deformation of surface asperities (denoted μ_d), that due to plowing by wear particles and hard asperities (denoted μ_p) and that due to the adhesion of the flat portions of the sliding surface (denoted μ_a). Therefore the coefficient of friction is not a simple material property. There are four or six stages of the friction regime depending on the sliding conditions. The initial friction coefficient μ_i can range from 0.1 to 0.2 for most machined surfaces. μ_i is largely independent of environmental conditions (including lubricants), materials and surface topography. μ_p varies from 0 to 1.0 and μ_a from 0 to 0.4. Only μ_a depends on the quality of surface adhesion. The history-dependent frictional behavior of materials is represented in the “friction space” diagram. According to this theory, the compatibility of sliding surfaces is dictated more by the mechanical properties of materials such as hardness than by their relative solubility at low temperatures.     

Preliminary investigation of the effect of dimple size on friction in line contacts

The scale of surface texture is becoming an important issue of surface texture design, particularly for the condition of low speed and high load. Experiments were carried out to investigate the effect of dimple size on friction under line contact condition. The patterns of dimples distributed as square array were fabricated on the surface of brass disks. Each pattern has the same area density of 7%, the same depth over diameter ratio h/d of 0.03, and dimple diameter d varying from 20 to 60 μm. The frictional tests of the brass disk sliding against a stationary cylindrical surface of bearing roller were conducted. It was found that the pattern with dimple diameter of 20 μm presented the effect of friction reduction. For the further understanding of the effect of dimple size under line contact condition, numerical simulations were also carried out to evaluate the hydrodynamic pressure within the contact of cylindrical and plane surfaces. The effects of dimple size and radius of the cylinder on the load carrying capacity were evaluated and discussed.     

表面凹痕织构动压承载性能的CFD分析

为研究表面凹痕织构的动压承载性能,在建立其几何模型的基础上,采用基于N-S方程的CFD方法研究不同参数下表面凹痕织构的平均承载力变化规律,并考察雷诺数、织构宽度和深度对动压承载性能的影响。结果表明:雷诺数的增加能够大幅提升表面凹痕织构的平均承载力;随着织构宽度的增加,表面凹痕织构的动压承载性能趋于优良;当量纲一织构深度处于0.5～1时,表面凹痕织构具有最佳的动压承载性能。     

The effect of sliding time and speed on the wear of composite materials

The friction and wear properties of an Al 201 alloy and a unidirectionally oriented graphite fiber-aluminum matrix composite (T50-Al 201) were investigated. The experiments were conducted on a pin-on-disc type friction machine. The diameter of the pin was 0.22 cm and the load 4.46 N. The sliding velocity varied between 0.17 and 0.43 m s^?1. The disc counterface was of commercially pure iron. It has been found that the friction coefficient μ and the wear rate W_L of the composite material decrease as the sliding time is increased until a steady state value is reached. The steady state wear rate is proportional to the reciprocal of the sliding speed in accord with a recently proposed model. Scanning electron microscopy and Auger electron spectroscopy observations indicate that the high initial values of μ and W_L are due to a high degree of matrix adhesion to the counterface accompanied by fiber breaking and transfer. The low steady state values of μ and W_L are due to the formation of a film that impedes adhesion and confers some degree of self-lubrication. It is suggested that the observed variation of W_L with sliding speed is related to changes in the degree of subsurface damage as the velocity is varied.     

Experimental Study on the Friction Characteristics of Textured Steel Surface with Ring-Shaped Pits under Lubricated Sliding Conditions

Surface texturing has been recognized to be an effective approach to modify the tribological performances of sliding surfaces. A ring-shaped texture has been fabricated on the surface of AISI1045 steel using a pulse laser, and reciprocating sliding tests were performed on a variable load tribometer (VLT). Frictional force of the textured surfaces was investigated under various geometrical parameters (inner radius, external radius, pitch, and offset ratio of texture units) and operating conditions (velocity and load). The results show that the width of the ring-shaped texture has a significant influence on antifriction, an optimum width for minimum friction is about 200 µm, and the staggered array of texture units is an effective way of reducing frictional force and wear.     

Tribocorrosion behaviour of Ni–SiC nano-structured composite coatings obtained by electrodeposition

The combined corrosion-wear degradation of nano-structured Ni–SiC coatings in sliding contacts immersed in electrically conductive solutions is investigated in situ by electrochemical techniques (open-circuit potential measurements, E_OC, the potentiodynamic polarization measurements, PD, and the electrochemical impedance spectroscopy). The coating thickness was 50 μm, with an average volume of dispersed phases inside nickel of 20%. The samples were tested in a cell, containing the electrolyte and electrodes, and mounted on a pin-on-disk tribometer, with the working surface of the specimen facing upwards. Both continuous and intermittent friction tests were carried out. In the intermittent tests, friction was applied periodically: during each cycle, friction was first applied for 2 s at constant sliding speed under constant normal load and then stopped during a latency time of 20 s or 0.5 s. Without friction, the free potential reaches a passive value after immersion in the test solution. When friction force is applied the free potential is shutting down to active values. Under friction the measured current, I can be considered as the sum of two partial currents: one generated by the wear track areas, where the passive film is destroyed and the surface is active; the other one linked to the surface not subjected to friction and that remains in the passive state. A localised corrosion process when subjected to friction in 0.5 M K₂SO₄ was not observed on nano-structured Ni–SiC composite coatings. The mechanical destruction of the passive film occurs in the wear track by friction and subsequent restoration of the film (repassivation) when friction stops. The wear volume loss increases with sliding forces.     

Dependence of polymer sliding friction on normal load and contact pressure

The sliding friction of bulk polymers was studied varying the normal load, contact pressure and sliding velocity. The variation of the area of apparent contact A with normal load W was also measured both under the sliding and unloaded conditions. For the sliding condition A ∝ W, while for the unloaded condition A ∝ Wⁿ where n is less than unity. The friction measurements were performed on a tribometer in the low load range and on a lathe using a strain gage dynamometer in the high load range. It was found that the coefficient of friction depends upon the velocity and pressure and the variation can be explained by the adhesion theory of friction in the light of the conditions at the interface. The measurement of sliding friction in an extrusion process shows that the coefficient of friction decreases with contact pressure and the interface friction shear stress is almost equal to the bulk shear strength of the material. All of these findings support the adhesion theory of friction for polymeric materials.     

Fundamental aspects and technological implications of the solubility concept for the prediction of running properties

The first Goodzeit rule, which relates resistance to cold welding and seizure with mutual solubility of metal sliding couples, has been investigated. The sliding properties of Au-Ag and Pd-Ag alloys against SAE 1045 carbon steel were observed in ultrahigh vacuum and in argon. The abrupt change in friction and wear coinciding with the composition of the alloys at which the solubility of iron into the alloy vanishes found by De Gee was not confirmed. Instead a continuous decrease in friction coefficient and wear was observed with increasing silver content. Pure silver showed a very limited tendency to metal transfer, although in some cases the transferred layer was removed spontaneously after 200–300 cycles. Electrical conduction measurements made during sliding revealed a correlation between the momentary friction force and the electrical conductivity which indicated that variations in the friction force are synchronous with variations in the contact area.A model is proposed in which the conditions of friction and metal transfer are described in terms of one system and two material properties, i.e. the interfacial shear strength τ_i, the shear strength τ¹ and the strain hardening exponent n of the junction materials. The friction behaviour is described by a modified adhesion theory: instead of a contaminating layer it is the specific atomic interaction of the contacting metals acting over small regions of atomic contact which controls τ_i. The junction growth mechanism is correlated with strain hardening.It is suggested that the first Goodzeit rule must be interpreted as a result of atomic interaction in the interface between the contacting metals. In this way prediction of the running behaviour of both metals and alloys would become possible.     

Frictional response of precision finished surfaces in pure sliding

The functional performance of a component is closely tied to the geometric properties of its surface, which is an outcome of the finishing process used to make the component. This paper investigates the role of three-dimensional (3-D) surface topographic features of four precision finished surfaces (Ground, Hard Turned, Honed, and Isotropic) commonly used in the bearing industry on their frictional response. Experiments are conducted to investigate the effect of surface type, sliding speed, and normal contact load on the coefficient of kinetic friction under point contact. The effect of lay is also analyzed for the strongly anisotropic ground surface. Based on a detailed 3-D surface characterization, it is shown that the amplitude parameter S_q (rms deviation of surface), and spatial parameters S_ds (density of summits) and S_td (texture direction) play an important role in determining the frictional behavior of the surfaces studied. An empirical model for the coefficient of kinetic friction as a function of these parameters is proposed.     

Toughness and the transition between cutting and rubbing in abrasive contacts

The transition between rubbing and cutting when a facet-first pyramid is slid along the surface of a ductile material is investigated with new experiments under both constant depth of groove and constant load, and with a new ductile fracture mechanics analysis for the cutting regime. The critical attack angle at which the transition occurs is shown to be controlled by the toughness/strength (R/τ_y) ratio of the material, the depth of groove, friction and the face angle of the tool. The new analysis is contrasted with the Bowden and Tabor approach in Sedriks and Mulhearn [A.J. Sedriks, T.O. Mulhearn, Mechanics of cutting and rubbing in simulated abrasive processes, Wear 6 (1963) 457–466; A.J. Sedriks, T.O. Mulhearn, The effect of work-hardening on the mechanics of cutting in simulated abrasive processes, Wear 7 (1964) 451–459] and with other approaches for the same events, and it explains why certain assumptions in Sedriks and Mulhearn can be justified.The analysis shows that the vertical load during sliding is different from that required for indentation to the same depth below the surface. Consequently the depth of the groove during sliding under deadweight loading is different from the depth resulting from initial indentation. At very small attack angles, the depth can be smaller than the static indentation depth, but as the attack angle increases, so does the depth of groove formed by cutting.The relevance of the model to abrasive wear and polishing is discussed, along with implications for Krushschov–Babichev wear resistance diagrams and the Archard equation. The analysis explains why the specific energy in grinding increases at small depths of cut.Measurement of the critical attack angle in scratching with a facet-first indenter may possibly be a way to estimate the fracture toughness of small samples when other, more conventional, methods are difficult to carry out.