Deformation and wear behaviour of the junction in quasi-scratch friction

The initial scratching of soft metals by relatively hard metallic asperities involves considerable plastic deformation and wear of the harder metal. Thus the penetration effect on friction is reduced successively as sliding proceeds, leading to the shearing type of friction. Such a transition state of sliding can be defined as a quasi-scratch friction process because ploughing precedes the steady sliding condition.The deformation and wear behaviour at a friction junction was investigated using model experiments between a mild steel conical rider and a flat copper surface. Changes in geometry of the rider and pile-up of the flat metal were examined metallographically and with a microscope. It was found that a stable value of the friction force is determined from the geometric shape of the junction attained after the completion of transient sliding and the effect of initial asperity shape on the friction force becomes insignificant.     

Mathematical slip-line field solutions for ploughing a hard particle over a softer material

Wear mechanisms and friction in metals can be investigated by the analysis of the unit event represented by the interaction of a hard particle or asperity with a softer surface. Effective friction is the result of the interaction of many such asperities which constitute the roughness of the harder of the solid surfaces. Three types of plastic deformation at the metal surface can be identified: ploughing, edge formation and chip formation. Each mode of plastic deformation can be analysed using the slip-line field plasticity theory which requires as inputs the geometry of the hard particle and some information on the interface between the harder and the softer surfaces. The classical and the recent chord solution by Oxley assumes a sharp edge sliding against a metal surface but does not consider a curved roughness profile. However, the profiles of real asperities are more like waves with rounded summits. In the present work a new model for the asperities interaction is shown, using the slip-line field theory to calculate the friction forces, depth of sheared layer, average contact pressure and friction coefficient for a cylindrical hard particle sliding over a softer surface. The theoretical results are presented as friction graphs and maps in which the regions of elastic deformations are shown using the Hertz theory while the region of plastic strains is obtained from the present analysis. Present model results are in good agreement with experimental data obtained by Busquet et al. and are quite different from the Oxley chord model for sliding a circular particle.     

Metal transfer in the frictional contact of a rough hard surface

The frictional behaviour between a hard rough surface and a soft smooth surface was examined under lubricated and unlubricated conditions. Transfer of soft metal to hard asperities in contact with it caused significant changes in the shape, size and height distributions of the asperities. Thus metal transfer reduced the effect of the initial surface roughness of the hard metal on friction.     

Initial wear rates of soft metallic films

A theoretical expression was derived for the initial wear rates of hard conical asperities ploughing a soft metallic film. The theory yields a wear equation similar to the well-known existing empirical formula, although the new wear coefficient K appears to be dominated by the surface topography of the system rather than by the material properties. The initial wear rates of the film appear to be determined by the system rather than by the material. The general characteristics of the model are confirmed by experimental results for the initial wear rates of ion-plated tin and lead films.     

Effects of particle size, polishing pad and contact pressure in free abrasive polishing

The objective of this research is to better understand the mechanisms of material removal in the free abrasive polishing process. Experiments were carried out to understand the effects of particle size, polishing pad and nominal contact pressure on the wear rate and surface roughness of the polished surface. A theoretical model was developed to predict the relationship between the polishing parameters and the wear rate for the case of hard abrasive particles sandwiched between a soft pad and a workpiece (softer than the abrasive particles). Experimental results and theoretical predictions indicate that the wear rate increases with an increase in particle size, hardness of polishing pad and nominal contact pressure, and with a decrease in elastic modulus of the polishing pad. Surface roughness increases with an increase in particle size and hardness of polishing pad, and nominal contact pressure has little effect on the roughness. A dimensionless parameter, wear index which combines all of the preceding parameters, was introduced to give a semi-quantitative prediction for the wear rate in free abrasive polishing. It is also suggested that when polishing hard material, in order to achieve a high materials removal rate and a smooth surface, it is preferable to use diamond as the polishing particles because of their high deformation resistance.     

Explaining the mechanics of metallic sliding friction and wear in terms of slipline field models of asperity deformation

It is shown that the force which opposes the sliding of a hard relatively smooth surface over a softer surface can be explained as the force needed to push waves of plastically deformed material along the soft surface ahead of asperities on the hard surface. For rougher surfaces and/or poorer lubrication it is shown how the wave can be torn off or material removed by a chip formation process and wear particles formed. Coefficients of friction predicted from the corresponding asperity deformation models are shown to give good agreement with experimental results. For smooth well lubricated surfaces the wear of the softer surface is shown to occur as a result of the progressive damage to this surface brought about by the repeated passage of waves across it. Equations for predicting wear are derived from the asperity deformation models and a comparison made between predicted and experimental wear results. The paper ends by considering possible future trends in research into the mechanics of friction and wear.     

Prediction of Archard's wear coefficient for metallic sliding friction assuming a low cycle fatigue wear mechanism

A recently developed model for sliding friction, in which the frictional force is assumed to result from the pushing of waves of plastically deformed material in the soft surface ahead of asperities on the hard surface, is used to calculate the magnitude of the resulting plastic strain increments which progressively deform the soft surface. On the assumption that a low cycle fatigue mechanism eventually results in detachment of wear particles from the soft surface as a result of this cyclic working of the surface, the calculated strain increments (the magnitudes of which vary with the roughness of the hard surface and the boundary lubrication conditions) are used to estimate wear rates. The results are expressed in terms of Archard's wear coefficient and, for very smooth surfaces and good lubrication, this is predicted to have extremely low values as observed in wear tests for such conditions. For rougher surfaces and less efficient lubrication it is shown that the wear coefficient can increase dramatically.     

Wear of boundary-lubricated metal surfaces

In the past, the friction and wear of boundary-lubricated metallic surfaces were attributed to adhesion and shearing of lubricants. However, examination of the friction and wear of pure metals lubricated with mineral oil indicates that while the friction coefficient was typical of the values obtained in boundary-lubricated sliding, the predominant wear mechanism was an abrasive-type mechanism. Scanning electron microscopy and surface profilometry revealed many ploughing grooves on the surfaces. These results indicate that the ploughing mechanism may be the predominant factor in controlling friction and wear of boundary-lubricated surfaces. On the basis of slip line field analysis and surface topography statistics an approximate expression for the wear coefficient was derived. It was found that the wear coefficient depends on the sharpness of the surface asperities (or the entrapped wear debris), the interfacial “friction” and the extent of the plastically deformed zone below the surface.     

Brake features enhancing the wear debris bimodal distribution

Investigated is wear debris separation from dry friction interface of a vehicle brake system. Owing to structural and operational properties of the interface between the disc hard surface and the pad soft surface, the study focuses on the abrasive wear process of the pad profile. A theoretical dependence is sought between debris size distribution and governing parameters such as surface roughness, unevenness, and levels of normal loading.The analysis describes interacting surface profiles in terms of discrete probabilities of their heights. Enforced by motion constraints, the disc hard profile is monitored while clearing its way through the stationary pad overlapped profile by shearing of likely encountered asperities as estimated by their conditional probability. As a result, a single tangential stroke leads to a pad redistributed profile along with a collection of separated debris. A pre-stroke approach of the pad closer to the disc simulates the effect of a normal loading increment. Accordingly, three types of shear strokes may develop, namely; the superficial, the shallow subsurface, and the deep subsurface.It is found that the range of debris size only extends up to the mean height of surface profile. The debris size distribution is generally bimodal with large-size and small-size peaks, both being distinct during superficial strokes. The position and level of either peak depend on the two interacting profiles just before each individual stroke. For a disc Gaussian profile, sharply wavy profile of the pad surface makes the debris large-size peak to lie evidently near the complementary of the disc mean height. An originally Gaussian or a disc-induced profile of the pad surface enhances the level of the debris small-size peak whose position is constant. In addition, deep subsurface strokes yield an invariable debris distribution with a single small-size peak.     

Sliding wear and friction of electroplated and clad connector contact materials: effect of surface roughness

A modeling study with bench apparatus was conducted with connector contact materials consisting of electroplated hard gold and gold flashed palladium on nickel underplatings mated to a clad noble metal. The clad metal contact was the ‘rider’, i.e., had the smaller surface involved in sliding compared to the plated ‘flat’ surface. This configuration is consistent with good engineering practice in commercial products. It was found that although a lubricant could reduce friction significantly and virtually eliminate adhesive transfer of metal, a requirement for negligible wear, i.e., the absence of loose particles, was that the plated surface be very smooth. If this contact was rough, such as might occur due to it having a nodular nickel underplate, tool marks, or burrsm abrasive wear of the cladding occurred in proportion to the magnitude of roughness. However, when both contacts were plated, lubricated wear was low and surface roughness was not a significant factor. Unlubricated plated contacts were severely with high friction by the prow formation adhesive wear mechanism regardless of their surface roughness.     

Influence of surface roughness and work-hardened layers on the contact between a rough and a flat surface

The validity of a theory which was derived considering the distribution of the surface slopes of conical asperities and the variation of the flow pressure of each contact due to the work-hardened layer of the softer surface was checked by comparing the theoretical and experimental results. The number, the real area and the distribution of the radii of contact points produced by impressing hard and rough surfaces into soft and flat surfaces with work-hardened layers were measured. The distribution of the surface slopes of contact asperities and the variation of the flow pressure with increasing penetration of the hard asperities were also obtained experimentally.     

硬脆性材料用柔性磨具研磨的加工表面粗糙度建模

通过分析磨粒与工件表面的作用过程,建立了硬脆性材料柔性磨具加工表面粗糙度的理论预测模型.以橡胶结合剂金刚石研磨盘为柔性磨具、蓝宝石衬底为工件,在不同弹性模量、磨粒浓度、磨粒粒度和研磨压力下开展研磨试验,将不同研磨条件下的表面粗糙度试验值与理论预测值进行比较,发现试验结果与理论模型预测结果的趋势一致,且预测误差为7.71...     

Analyses of wear rate due to micro-asperities and transferred particles on hard asperities

Assuming that micro-asperities and transferred particles on semicylindrical harder asperities with spherical ends ploughed a softer surface and that transferred particles at the front of harder asperities supported normal load, the effects on the wear rate of the parameters of surface topography, such as the mean height of micro-asperities, the radii of curvature at the tips of semicylindrical asperities and the length of an asperity, were analysed. The theoretical trends agreed well with the experimental wear rate trends for the polymers rubbed on harder surfaces with various roughnesses and those for various metals rubbed on the virgin surfaces of abrasive papers. The theoretical results showed that the wear rate decreased with a decrease in mean height of the micro-asperities and with increases in the radii of curvature at the tips of semicylindrical asperities and in the lengths of asperities. Furthermore, it was found, under the condition of abrasive wear, that the decrease in wear rate resulted from wear debris adhering to the front faces of the abrasive grains.     

Mechanism of Abrasive Wear in Nanomachining

In this study, nanomachining is utilized to investigate the abrasive wear mechanism that produces a nano scale groove on a bulk material. Two different tools (Berkovich and Conical) with the same tip radius (100 nm) but different edge geometries were used for machining both Cu- and Ni-coated materials with a nanoindenter that was equipped with a nano scratching attachment. It was found that the generated forces (normal and cutting) increased with the increase of depth of cut; however, the generated normal force at the minimum depth of cut (50 nm) was more than the critical force for all machining conditions. Therefore, at the minimum depth of cut, groove formation started with the ploughing mode of abrasive wear mechanism, then the cutting mechanism activated along with the ploughing mechanism above a 100 nm depth of cut. The percentage values of these two mechanisms were determined and utilized to determine the dominant mode of the abrasive wear mechanism for producing a nano scale groove on a metal surface and, to correlate this, abrasive wear mechanism with the co-efficient of friction (μ) at different machining conditions. The results also showed that the co-efficient of friction (μ) increased when ploughing was the dominant mode of abrasive wear mechanism to produce a nano scale groove. Thus, μ was found to be proportional to the ploughing mode of abrasive wear mechanism in nanomachining.     

Influence of abrasive grain geometry on friction coefficient and wear rate in belt finishing

We focus our study on belt finishing process using a 3D model with multi-asperity abrasive wear on real rough surfaces. The established model allows determining the effect of the local geometry of abrasive grain on the friction coefficient and wear rate. This study shows that the friction coefficient and wear rate increase with the local slopes of the roughness. With the increase of the macroscopic normal load, the wear rate increases rapidly. Such effect is related to the increase of the cutting force of each grain leading to the transition in dominant wear mode from ploughing to wedging and cutting.     

Two-body abrasion of some polymers against 6–50 μm SiC abrasives

Single- and multiple-pass two-body abrasion tests were run on Nylon $\text{6}\text{6}\text{+ 20\%}$ polytetrafluoroethylene (PTFE) and polycarbonate + 10% PTFE sliding dry against 6–50 μm SiC abrasives. A functional relationship was developed between the single-pass wear rate and the abrasive particle size for abrasive particle sizes less than or equal to 10.4 μm. The single-pass abrasive wear rate was 20–40 times greater than the multiple-pass wear rate for each material when it was slid against abrasive grains with a mean size not exceeding 10.4 μm. This was due to the formation of loose polymer fibril wear debris in single-pass sliding and of transferred plateaux of polymer in multiple-pass sliding. The rate of increase in wear with particle size was about 20 times greater for single-pass sliding than for multiple-pass sliding. Above a mean abrasive particle size of 10.4 μm the type of mechanism in both single-pass and multiple-pass sliding was that of ploughing.     

Surface coatings materials conservation and optimum tribological performance

Surface coating technology has received considerable attention in recent years. It improves corrosion, friction, wear and fatigue resistance of components with a minimum demand on scarce raw materials. It is shown that the behaviour of surface films depends on material properties, surface finish and film thickness and that friction may vary through two orders of magnitude due to such effects; the friction of soft metal films being low and that of hard films being high. Such effects are enhanced for very smooth surfaces and very thin films. It is also shown that the wear resistance is greater for thin soft metal films and less for thin hard metal films. Some experimental evidence is available to justify the theoretical proposition but much further work is required and the paper identifies the gaps in knowledge     

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。     

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。     

Roughness variations in cylinder liners induced by honing tools’ wear

The manufacturing and finishing (honing) of cylinder liners for the automotive industry is a constant challenge in order to reduce friction losses and oil consumption. A better knowledge of surfaces generated during plateau honing is then required for optimization of the process. Despite a well-known and controlled honing process, variations in surface roughness appear due to honing tool wear and need to be mapped and analyzed. The following paper proposes to map the variations in roughness by using confocal 3D measuring equipment able to inspect any area of a cylinder liner. Six motor blocks, each with five cylinder liners, were evaluated with 20 topography measurements per liner (giving six hundred 3D measurements in total). In addition to standard 3D roughness parameters, tailor made parameters extracting honing texture information are computed. The results show that only a few parameters (Spk, Ssc and Sk) do correlate with the honing tool wear specific to each cylinder. Tailor made parameters indicate similar results. Indeed, as the honing tool wears down, the cylinder liner surface gets rougher plateau or peaks and sharper asperities indicating that ploughing occurs instead of cutting. In future, experimental models could be built in order to perform production and functional optimizations.