The elastohydrodynamic transition speed for spheres sliding on lubricated rubber

An elastohydrodynamic number derived elsewhere in the literature [1] characterizes the onset of hydrodynamic support for a rigid sphere sliding on a lubricated viscoelastic base. This number includes elastic properties of the base track, in contrast with previous studies where such have been neglected. A generalized coefficient of sliding friction has been defined as the actual coefficient of friction divided by the tangent modulus of the viscoelastic material. Experimental plots of the coefficient of friction versus sliding speed for spheres sliding on lubricated rubber are shown to produce a relatively sudden decay in coefficient at the transition speed from “dry” to elastohydrodynamic contact. These plots in turn fit closely on a master curve of generalized coefficient of friction versus the elastohydrodynamic number.The inclusion of surface roughness on the sphere produces both a higher value of the generalized coefficient prior to the transition speed and a higher sliding velocity at which the transition itself occurs. Furthermore, the rate of decay for the generalized coefficient of friction appears distinctly greater for rough spheres. The overall effect of roughness is to reduce the difference between the dry and wet coefficients of sliding friction. Random abrasion of the spheres with emery paper of known grit size appears to be an effective method of inducing surface roughness on the spheres. The nature of all the experimental curves may be satisfactorily explained by squeeze-film theory.An important application of the sliding of smooth and rough spheres on a lubricated flexible base is the sliding/slipping behaviour of automobile tyres on a wet road surface during normal rolling.     

Sliding wear behavior of Fe-Al and Fe-Al/WC coatings prepared by high velocity arc spraying

A comparative study was carried out to investigate the microstructure and tribological behavior of Fe-Al and Fe-Al/WC iron aluminide based coatings against Si₃N₄ under dry sliding at room temperature using a pin-on-disc tribotester. The coatings were prepared by high velocity arc spraying (HVAS) and cored wires. The effect of normal load on friction coefficient and wear rate of the coatings was studied. The microstructure and the worn surfaces of the coatings were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscope (EDS). The results showed that, the main phases in both coatings were iron aluminide (Fe₃Al and FeAl) and α. WC/W₂C particles were embedded in the matrix of the composite coating. With adding WC hard particles, the Fe-Al/WC composite coating exhibited higher wear-resistance than Fe-Al coating. But the friction coefficient of both coatings showed little difference. As the load increased, the friction coefficient decreases slightly due to a rise of friction contact temperature and larger areas of oxide film formation on the worn surface, which act as a solid lubricant. Increasing load causes the maximum shear stress occurring at the deeper position below the surface, thereby aggravating the wear. The coating surface is subjected to alternately tensile stress and compression stress during sliding, and the predominant wear mechanism of the coatings appears to be delamination.     

Laboratory test simulation of galling in cold forming of aluminium

Cold forming of aluminium is a group of very efficient methods, which are successfully used in a number of industrial applications. Two of the major factors limiting the tool life and restricting the shapes and sizes possible to produce, are galling, i.e. transfer of work material to the tool surface, and high stresses occurring when forming complex shapes. Both phenomena are closely related to the friction and adhesion in the tool to workpiece interface.The present paper investigates the influence of several surface parameters to the tendency to galling. This is done by forming aluminium using tool steel in a geometrically simplified lab test. The test scans over a wide load interval while monitoring the coefficient of friction. The corresponding transfer of work material to the tool surface is studied in the SEM after testing. The test is focused on the initial tool contact and also on the number of contacts before a critical friction level is reached.The test set-up comprises two crossed cylinders in sliding contact, one made of tool steel and one of work material. Three commercial tool materials were included, each prepared to two surface finishes. The aluminium workpiece cylinders of AA6082 were prepared by two different pre-treatments, solid lubrication followed by soft annealing and pickling, respectively.The respective importance of the tool material, preparation of the tool surface finish and the aluminium surface pre-treatment are compared with respect to initial galling tendencies and friction stability. The practical implications for real forming applications are discussed.     

Measurements of friction in strip drawing under thin film lubrication

Strip drawing is used to investigate the friction behaviour under thin film lubrication in metal forming with plastic deformation. Friction coefficients are measured under a wide range of tribological conditions. The surface roughness is measured on an interferometric profilometer. The results show that the friction coefficient decreases with increasing oil film thickness h_w, as estimated using a formula appropriate for smooth tool and workpiece. Measurements of the surface topography show that change in friction is associated with a change in contact ratio between the tool and strip. The effect of strip reduction, strip roughness and die roughness on the friction coefficient is also investigated.     

Influence of sliding velocity on friction force in rubber seal rings under reciprocal motion

The influence of sliding velocity on the value of maximum and steady friction force in rubber seal rings under reciprocal motion is studied. It is established that the nature of the change in a static friction force and a steady-state friction force as a function of the sliding velocity is the same all other conditions being equal. The greater the sliding velocity, the faster the decrease and stabilization of a friction force after endurance of rubber seal rings in fixed contact with a cylinder. The maximum of the friction force dependence on the velocity in an investigated pair decreases with increasing temperature of a sealed medium.     

Experimental Study on the Friction Characteristics of Lasertex Steel Sheets During Metal Forming Process

The surface of steel sheets used in the metal-forming process discussed in this article was textured by a laser-ablation technique. Differently shaped craters are formed in a patterned structure on the steel surface by controlling the pulsed-laser power density, pulse-repetition rate, and pulse duration. Lasertex sheets formed by this process have unique friction characteristics because of the uniform surface roughness and valley-biased topography. The friction of lasertex sheets was studied using a metal-forming bench test rig. Influencing factors, including surface roughness and sliding velocity, were studied under lubricated conditions. The friction of lasertex sheets was compared with that of shot-blasted sheets. The results showed that the coefficient of friction of the lasertex sheet under dry friction decreases with an increase in surface roughness and changes little with varying sliding velocity. With lubrication, the coefficient of friction of the lasertex sheet rises with an increase in surface roughness and decreases with an increase in sliding velocity. Lasertex sheets were found to have lower friction coefficients than shot-blasted sheets over the rage of surface roughness and sliding velocity investigated.     

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.     

Sliding velocity dependency of the friction coefficient of Si-containing diamond-like carbon film under oil lubricated condition

In this study we investigated the sliding velocity dependency of the coefficient of friction for a Si-containing diamond-like carbon (DLC-Si) film in an automatic transmission fluid (ATF) under a wide range of contact pressures. The DLC-Si film and a nitrided steel with a surface roughness, Rz_JIS, of around 3.0 μm were used as disk specimens. A high-carbon chromium steel (JIS-SUJ2) bearing ball was used as a ball specimen. Friction tests were conducted using a ball-on-disk friction apparatus under a wide range of sliding velocites (0.1-2.0 m/s) and contact pressures (P_max: 0.42-3.61 GPa) in ATF. The friction coefficients for the nitrided steel had a tendency to decrease with an increase in sliding veloicity under all the contact pressure conditions; however, the friction coefficients for the DLC-Si film were stable with respect to sliding velocities under all the contatct pressures. These results indicate that the DLC-Si film suppresses the stick-slip motion during sliding againt steel in ATF, which is a desired frictional characteristic for the electromagnetic clutch disks used under lubrication. Furthermore, the DLC-Si film showed a higher wear resistance and lower aggression on the steel ball specimen than the nitrided steel. There were less hydrodynamic effects on the friction coefficient for the DLC-Si film possibly due to maintenance of the initial surface roughness and its poorer wettability with the fluid. X-ray photoelectron spectroscopy (XPS) analysis of the sliding surfaces revealed that the adsorption film derived from the succinimide on the sliding surfaces of the DLC-Si film and the mating steel ball also contributed to the sufficient and less sliding-velocity-dependant friction coefficients.     

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.     

Friction and wear of sintered iron

Using a wear test rig of the disc and pin type the influence of the sliding velocity V and contact pressure P on the friction and wear of sintered iron under unlubricated (dry) conditions was studied. The results showed that for a fixed test duration the coefficient of friction and wear varied according to the law KP^aV^b.     

Surface Roughness Effects on Metallic Contact and Friction

A study was made of surface roughness effects on metallic contact and friction in the transition zone between hydrodynamic and boundary lubrication. The system used was one of pure sliding and relatively high contact stress, namely a fixed steel ball riding on a rotating steel cylinder.

It was found that very smooth and very rough surfaces gave less metallic contact than surfaces of intermediate roughness; very smooth surfaces also gave less friction.

Four different types of antiwear/antifriction additives (including tricresyl phosphate) were studied and although they were found to reduce metallic contact and friction, they had little effect in reducing surface roughness. Rather, the additives merely slowed down the wearing-in process of the base oil. Thus, the “chemical polishing” mechanism advanced for the antiwear behavior of tricresyl phosphate appears to be incorrect.

With rough surfaces, the improvement in load-carrying capacity with increasing viscosity was less than that shown previously with smooth surfaces. Also, oils with a large pressure-viscosity coefficient did not show the expected beneficial effect with rougher surfaces.     

Effect of surface roughness and surface texture on scuffing

This paper discusses the results of disk tests designed to examine the effect of surface roughness and surface texture on scuffing and related behavior. AISI 9310 steel disk of varying surface roughness and surface texture, along with a MIL-L-7808G lubricant, were used. It is shown that, under otherwise comparable situations, (a) an increase in the composite surface roughness increases the coefficient of friction at scuffing, decreases the scuff failure load, and decreases the critical temperature, (b) the cross-ground disks give a lower coefficient of friction at scuffing and a higher scuff failure load than the circumferentially-ground disks, but nearly the same critical temperature, and (c) an increase in the sliding velocity or sum velocity, at a constant sliding-to-sum velocity ratio, decreases the coefficient of friction at scuffing, decreases the scuff failure load, and decreases the critical temperature.The superior performance of the cross-ground disks compared with the circumferentially-ground disks is attributed to the effect of surface texture on microelastohydrodynamic action. The variations of the oil film thickness ratio at scuffing, the coefficient of friction at scuffing, and the critical temperature with respect to surface roughness, surface texture, and operating variables have been shown to correlate well with a dimensionless parameter $\text{ξ}{}_{\mathrm{?}}$.     

The tribochemistry of silicon nitride: effects of friction, temperature and sliding velocity

In order to obtain information on the mechanisms of tribochemistry in silicon nitride, we studied the effects of chemical parameters (temperature and concentration of reagent) and tribological parameters (load and sliding speed) on the kinetics of the reaction, i.e. the rate of material removal. The temperature dependence of the wear rate of silicon nitride has been studied in several solutions. In CrO₃ and in KOH, the removal rate increases with temperature; the apparent activation energy is 20 kJ/mole in CrO₃ and 22 kJ/mole in KOH. In water, material removal is temperature independent, in KMnO₄, its rate decreases with increasing temperature. These changes are accompanied by parallel variations in the coefficient of friction. The reaction rate presents a complex dependence on the concentration of CrO₃ solutions. In water and CrO₃ solutions, we observed a strong dependence of friction and material removal rate with the load. With the changes in temperature, concentration and load, it is found that the reaction rate (in mm³/(N·m)) is linear with the coefficient of friction above a threshold value μ_th≈0.2. The velocity dependence is complicated by the phenomena of mixed lubrication. In all cases, the lack of solid wear particles and the production of ammonia have verified the tribochemical nature of the material removal. The mechanism of stimulation of the chemical reaction by friction is a quasi-static stretching of the bonds at the interface and a high local vibration energy of the atoms at the sliding contact.     

Frictional characteristics of ceramics under water‐lubricated conditions

Friction experiments were conducted on four kinds of ceramics (SiC, Si₃N₄, Al₂O₃ and ZrO₂) against themselves in water under different contact pressures and sliding velocities. The variations of friction coefficients as a function of sliding distance, and the effects of mean contact pressure and sliding velocity on friction coefficients were shown. Friction coefficients lower than 0.03 were observed under a certain combination of mean contact pressure and sliding velocity for each material. The friction coefficient suddenly increased when the mean contact pressure was above a threshold value, which depended on both sliding velocity and the material of sliding pairs. SiC had a higher threshold value than the other three ceramics at every sliding velocity investigated. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Effect of interfacial friction force on material removal in full aperture continuous polishing process

Full aperture continuous polishing using pitch lap is a key process of finishing large flat optical workpiece. The friction force of the workpiece and pitch lap interface significantly affects material removal. In this work, the friction force was determined by a measurement system that uses force transducers to support the workpiece. Experimental and theoretical analyses have been carried out to investigate the evolution of friction force with polishing time and its effect on material removal. Our results show that the friction coefficient of the workpiece/lap interface decreases during polishing, which is due to surface smoothing of the viscoelastic pitch lap by loading conditioner. In addition, the spatial average and uniformity of material removal rate (removal coefficient) increases with the increase of friction coefficient, which is due to rough lap surface, provides more sharp asperities to charge the polishing particles.     

Effect of Oxide Film on the Friction of 52100 Steel in Vacuum

To solve the anomalous behaviors of friction coefficient in reciprocated slidings of 52100 steel in ultrahigh vacuum, studies were made if the wear track, friction level, and number of traverses for a 52100 steel test ball sliding on a 52100 steel flat over a pressure range of 760 to 3 × 10^?9 torr. Plots of the friction coefficient versus number of traverses show a maximum (f = 1.0–1.2) followed by a drop (f = 0.5–0.6). Both the friction force-time curves and microscopic friction area change from smooth to rough at the point of the drop in friction coefficient. The shape of the friction plot is ascribed to a build-up and rupture of the oxide film that forms on the surface of the 52100 steel specimen in atmosphere and is affected by applied load, direction of finishing marks and pressure level of the environment. Further, above a critical pressure of 10^?6 torr the drop was not experienced. It was also found that higher friction does not always mean that the sliding surfaces are clear or more oxide-free.     

Enhancement of abrasive wear resistance in consolidated Al matrix composites via extrusion process

Abstract

The present study addresses the dry wear behaviour of aluminium matrix composites under different sliding speeds and applied loads. Values of the friction coefficient of the matrix alloy and composite materials were in expected range for light metals in dry sliding conditions. The higher coefficient of friction was the consequence of established contact between hard SiC particles and the counter body material. The rough and smooth regions are distinguished on the worn surface of the composites similar to the unreinforced Al alloy. Plastic deformation occurred when the applied specific load was higher than the critical value. The high shear stresses on the sliding surface cause initiation and propagation of the cracks in the subsurface, leading to the loss of material from the worn surface in the form of flakes. The debrises of the composites at low wear rate comprise a mixture of the fine particles and small shiny metallic plate-like flakes and are associated with the formation of more iron rich layers on the contact surfaces.     

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.     

Tribological Behavior of Self-mated Ti3SiC2 in Short-Chain n-Alcohols,Glycol and Glycerol under Boundary Lubrication

The tribological behavior of self-mated Ti₃SiC₂ in bath of alcohols are investigated at sliding velocity ranging from 0.005 to 0.3 m/s. The results show that the friction coefficient and wear rates of self-mated Ti₃SiC₂ are reduced greatly under lubrication of alcohols compared to that under dry condition. In alcohols, smooth worn surfaces of Ti₃SiC₂ can be obtained. The mechanical wear is inhibited and the oxidized Ti and Si species on the worn surface are TiO₂ and silica gel. The friction coefficients decrease with carbon numbers and sliding velocity in the n-alcohols. The friction coefficient decrease with sliding velocity in glycol and glycerol as well. The decrease is determined by the increase of the viscosity of alcohols. In glycerol, at 0.1 m/s and 5 N, the friction coefficient of self-mated Ti₃SiC₂ is 0.08 which is the lowest in this paper. The lubrication regimes are calculated according to classical lubrication theory. At velocity from 0.005 to 0.3 m/s, in all of the alcohols except glycerol, the λ are below 1 which indicates that the lubrication regime is BL. While in glycerol, the lubrication regimes vary from BL to BL and EHL, then to EHL as the velocity increases.