Analysis of the Contribution of Adhesion and Hysteresis to Shoe–Floor Lubricated Friction in the Boundary Lubrication Regime

Slip and fall accidents cause frequent occupational injuries. Despite recent evidence that boundary lubrication is relevant to slipping, few studies have examined the mechanisms that contribute to shoe?Cfloor friction in this lubrication regime. This study aims to identify the contributions of adhesion and hysteresis to friction in boundary lubrication. Three shoe materials (40 Shore A hardness polyurethane, 60 Shore A hardness rubber, and 70 Shore A hardness rubber), two floor materials (vinyl and marble), and six lubricants (water, 1.5?% detergent, 25?% glycerol?C75?% water, 50?% glycerol?C50?% water, 75?% glycerol?C25?% water, and canola oil) were tested at a single sliding speed (0.01?m?s^?1). Dry adhesion and hysteresis were quantified for each of the shoe?Cfloor combinations and lubricated adhesion was quantified for all shoe?Cfloor-fluid combinations. The contribution of adhesion and hysteresis to shoe?Cfloor-lubricant friction was affected by both the shoe and floor material due to differences in hardness and roughness. Lubricated adhesion was complex and multifactorial with contributions from the shoe, fluid, shoe?Cfloor interaction, floor-lubricant interaction, and shoe-lubricant interactions. A simple regression model including two fluid coefficients and the dry adhesion friction force was able to predict 49?% of the lubricated adhesion friction variability.     

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.     

Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate

Surface texture of harder mating surfaces plays an important role during sliding against softer materials and hence the importance of characterizing the surfaces in terms of roughness parameters. In the present investigation, basic studies were conducted using inclined pin-on-plate sliding tester to understand the surface texture effect of hard surfaces on coefficient of friction and transfer layer formation. A tribological couple made of a super purity aluminium pin against steel plate was used in the tests. Two surface parameters of steel plates, namely roughness and texture, were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture and are independent of surface roughness (R_a). Among the various surface roughness parameters, the average or the mean slope of the profile was found to explain the variations best. Under lubricated conditions, stick–slip phenomena was observed, the amplitude of which depends on the plowing component of friction. The presence of stick–slip motion under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities.     

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.     

Experimental and theoretical evaluation of the deformation component of the coefficient of friction

An analytical model for the calculation of the deformation component of a friction force in sliding of a hard spherical indenter on a viscoelastic material that is modeled as a Kelvin solid is constructed. The influence of mechanical properties of a material and slip velocity of an indenter on the contact characteristics and deformation component of the coefficient of friction is studied. An experimental method for estimation of deformation loss under friction of a high-elasticity material is proposed. A comparison of the results obtained with the use of theoretical model with experimental data is performed.     

Estimation of rubber sliding friction from asperity interaction modeling

Interaction between a soft rubber asperity and its hard counterpart is traced with the help of a finite element computation. The analysis is aimed to estimate the influence of adhesion between rubber and rigid surfaces and the energy losses arising from the deformation of rubber bulk to the sliding resistance. At the contact zone, interfacial bonds are formed due to adhesion and their resistance to sliding is represented by the shear strength of the contact interface. In the rubber bulk, the hysteresis loss is calculated using an appropriate model of the viscoelastic mechanical behavior of rubber for large strains. Dependence of friction on sliding speeds and temperature is hence detected. Influence of surface roughness and contact pressure on friction is also examined.     

Roughness and Lubricant Effect on 3D Atomic Asperity Contact

Large-scale molecular dynamics simulations were performed to study the sliding process of rough surfaces with and without lubricant. In the dry contact, a linear relationship has been observed between the load and the contact area for surfaces with large root mean square (RMS) roughness. However, it becomes nonlinear when the RMS is small. In the presence of adhesion, small roughness results in a large friction force when the surfaces are flattened and the contact area reaches 60 %. In order to confirm this observation, nonadhesive models have been established with an observation that under the combined influence from roughness and adhesion, the contact area plays a crucial role to determine whether the dry sliding is under the domination of roughness or adhesion. In the lubricated sliding, an increase in friction force has been found for the partially lubricated condition because the asperity contact still accounts for a great deal of resisting force. Besides, the lubricant exerts a comparable resisting force to the sliding.     

Effects of Interfacial Strength and Roughness on the Static Friction Coefficient

A finite element model is used to simulate sliding inception of a rigid flat on a deformable sphere under combined normal and tangential loading. Sliding inception is treated as the loss of tangential contact stiffness under combined effects of plasticity, crack propagation and interfacial slip. Energy dissipation distribution is used to quantify the relative contribution of these mechanisms on the increased compliance during tangential loading. Materials with different strength and toughness properties, and varying local interface conditions ranging from fully adhered to finite friction, are studied to relate variations in plastic deformations, crack and slip to the sliding inception. For fully adhered contact condition, crack and fracture toughness have no effect on sliding inception, with plasticity, the dominant failure mechanism. A measure of recoverable strain (yield strength to Young’s modulus ratio) is found to be the most influential parameter in sliding inception. Interfacial slip is expectedly the dominant mechanism for sliding inception for lower coefficient of friction, modeling lubricated contacts. Interplay of plasticity and interfacial slip is found to govern the onset of sliding for higher local friction coefficients. Furthermore, the single asperity results are incorporated in a statistical model for nominally flat contacting rough surfaces under combined normal and tangential loading to investigate the stochastic effects due to surface roughness and material property uncertainties. The results show that the static coefficient of friction strongly depends on the normal load, material properties, local interfacial strength and roughness parameters.     

多因素影响的格莱圈材料摩擦磨损试验研究*

格莱圈由聚四氟乙烯(PTFE)矩形滑环和丁腈橡胶(NBR)O形圈组成。为了研究不同因素对于格莱圈密封材料摩擦磨损性能的影响,利用UMT-3多功能摩擦磨损试验机,通过改变往复频率、粗糙度、润滑状态研究格莱圈材料与45钢配副时的摩擦磨损性能,利用SEM对试块试验前后表面形貌进行观测,并对摩擦磨损机制进行分析。试验结果表明:在干摩擦和滴油润滑条件下PTFE材料相比NBR材料具有更为优异的摩擦磨损性能;NBR材料表面粗糙度过高或过低都会导致摩擦因数升高,表面粗糙度对具有自润滑性能的PTFE材料的摩擦因数影响不大;高往复频率会使NBR材料摩擦因数降低,过高或过低的往复频率都会使PTFE材料摩擦因数降低;NBR材料的磨损形式以磨粒磨损和黏着磨损为主,PTFE材料以黏着磨损和疲劳磨损为主。     

The influence of surface roughness on abrasive wear

Analyses are given of the mechanism of friction and abrasive wear and of the effect of surface roughness on them. Theoretical expressions are derived for ploughing, adhesion and the total friction coefficient of hard conical asperities ploughing a soft metal surface, with the assumption that the asperities of the hard metal are cones with randomly distributed slopes, the mean value of which varies with surface roughness. Simple expressions for the abrasive wear rate and the mean wear particle size are also derived on the basis of a ploughing mechanism of the hard conical asperities on the soft metal surface.A comparison of calculated values based on these theories with experimental data of single-pass wear tests for various soft metals such as copper, cadmium, lead and zinc sliding on low carbon steel plates shows good agreement. The effects of surface roughness on the tangential forces under unlubricated and lubricated conditions as well as the mean wear particle size are theoretically discussed and the theoretical results are compared with experimental data.     

Super low friction of DLC applied to engine cam follower lubricated with ester-containing oil

This paper presents a material combination that reduces the friction coefficient markedly to a superlow friction regime (below 0.01) under boundary lubrication. A state approaching superlubricity was obtained by sliding hardened steel pins on a hydrogen-free diamond-like carbon (DLC) film (ta-C) lubricated with a poly-alpha-olefin (PAO) oil containing 1 mass% of an ester additive. This ta-C/steel material combination showed a superlow friction coefficient of 0.006 at a sliding speed of 0.1 m/s. A hydrogencontaining DLC coating/steel combination also showed a lower friction coefficient in air than a steel/steel combination, 0.1 vs. 0.8, but no large reduction was observed when the sliding surfaces were lubricated with ordinary 5W-30 engine oil and the PAO oil containing an ester additive. The friction coefficient of the hydrogen containing DLC/steel combination lubricated with the PAO containing an ester additive was above 0.05. On the other hand, the superlow friction performance demonstrates that the rolling contact friction level of needle roller bearings can be obtained in sliding contact under a boundary lubrication condition. It is planned to apply this advanced DLC coating technology to valve lifters lubricated with a newly formulated engine oil in actual mass-produced gasoline engines. A larger friction reduction of more than 45% is expected to be obtained at an engine speed of 2000 rpm.     

Influence of roughness parameters and surface texture on friction during sliding of pure lead over 080 M40 steel

In the present investigation, tests were conducted on a tribological couple made of cylindrical lead pin with spherical tip against 080 M40 steel plates of different textures with varying roughness under both dry and lubricated conditions using an inclined pin-on-plate sliding tester. Surface roughness parameters of the steel plates were measured using optical profilometer. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the surface texture of hard surfaces. A newly formulated non-dimensional hybrid roughness parameter called ‘ξ’ (a product of number of peaks and maximum profile peak height) of the tool surface plays an important role in determining the frictional behaviour of the surfaces studied. The effect of surfaces texture on coefficient of friction was attributed to the variation of plowing component of friction, which in turn depends on the roughness parameter ‘ξ’.     

滑动速度对油润滑表面胶合磨损的影响

在油润滑条件下,钢对钢摩擦副的胶合摩损不仅取决于润滑油膜是否破裂,而且取决于在摩擦表面上化学反应膜的形成情况。本文研究了在油润滑条件下滑动速度对钢摩擦副胶合的影响。在低滑动速度下摩擦表面易于形成反应膜,油膜破裂后并不直接发生胶合。胶合发生在高温、高摩擦系数的恶劣条件下。在高滑动速度下油膜破裂后很容易发生胶合,发生胶合前的表面温度和摩擦系数都比较低。     

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.     

The influence of surface roughness and the contact pressure distribution on friction in rolling/sliding contacts

A numerical contact model is used to study the influence of surface roughness and the pressure distribution on the frictional behaviour in rolling/sliding contacts. Double-crowned roller surfaces are measured and used as input for the contact analysis. The contact pressure distribution is calculated for dry static contacts and the results are compared with friction measurements in a lubricated rolling/sliding contact made with a rough friction test rig. The mean pressure is suggested as a parameter that can be used to predict the influence of surface roughness on the friction coefficient in such contacts. The results show two important properties of the friction coefficient for the friction regime studied in this paper: (1) there is a linear decrease in friction coefficient as a function of the slide-to-roll ratio, and (2) the friction coefficient increases linearly with increasing mean contact pressure up to a maximum limit above which the friction coefficient is constant. The absolute deviation of experimental results from the derived theory is for most cases within 0.005.     

Influence of testing method on the contact pressure distribution and its effect on coefficient of friction in polymeric bearings

Influence of contact pressure distribution on the coefficient of friction was investigated for some polymeric bearings in dry and lubricated conditions. Reciprocal pin on plate sliding test and Soda pendulum type sliding tests were carried out. It was found that the contact pressure distribution had a significant effect on the coefficient of friction in lubricated condition. The coefficient of friction became smaller when the pin was sliding with the lower contact pressure side ahead. The coefficient of friction in dry condition does not depend on sliding direction regardless of contact pressure distribution. Furthermore a parametric equation was derived to calculate contact pressure in polymeric journal bearings. The accuracy of the equation was confirmed by Finite Element Method (FEM).     

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.     

An Interfacial Friction Law for a Circular EHL Contact Under Free Sliding Oscillating Motion

The friction response of a lubricated interface under free sliding oscillating motion is investigated as a function of the contact pressure and the rheology of the lubricant in terms of viscosity and piezoviscosity. For loaded contacts, both velocity dependent friction, referred to as viscous damping, and friction independent of the instantaneous sliding velocity contribute to the energy dissipation. Viscous damping mainly corresponds to the dissipation in the lubricant meniscus surrounding the contact, while dissipation within the confined lubricated interface is mainly independent of the instantaneous sliding velocity. The friction coefficient independent of the instantaneous sliding velocity falls on a master curve for the wide range of tested operating conditions and lubricant rheological properties. The master curve is a logarithmic function of a dimensionless parameter corresponding to the ratio of the viscosity of the confined lubricant to the product of the pressure and a characteristic time. The physical meaning of this latter and the friction law are discussed considering the confined interface as a viscoelastic fluid or a non-Newtonian Eyring fluid.     

Effects of Contact Pressure and Sliding Speed on the Unlubricated Friction and Wear Properties of Zn-15Al-3Cu-1Si Alloy

The unlubricated friction and wear properties of Zn-15Al-3Cu-1Si alloy were studied over a range of contact pressure (1–5 MPa) and sliding speed (0.5–2.5 ms^?1) for a sliding distance of 2,500 m using a block-on-disc type test machine. It was observed that as the contact pressure increased, the friction coefficient of the alloy decreased but its working temperature, surface roughness, and wear volume increased. Sliding speed had no significant effect on the friction coefficient of the alloy but increased its working temperature, surface roughness, and wear volume. It was also observed that the formation of a hard and brittle surface layer had a great influence on the wear behavior of the experimental alloy. The hardness and thickness of this layer increased with increasing contact pressure and sliding speed. However, contact pressure was found to be much more effective on the hardness of the surface layer of this alloy. Both adhesion and abrasion were observed to be the dominant wear mechanisms for the alloy under the given sliding conditions. The results obtained from the friction and wear tests are discussed in terms of the test conditions and microstructural changes that take place during sliding.     

Response of materials as a function of grinding angle on friction and transfer layer formation

Surface texture influences friction and transfer layer formation during sliding contact. In the present investigation, basic studies were conducted using an inclined pin-on-plate sliding apparatus to understand the effect of grinding mark directionality on the coefficient of friction and transfer layer formation. In the experiments, 080 M40 steel plates were ground to attain different surface roughness with unidirectional grinding marks. Pins consisting of soft materials (pure Al, pure Mg, and Al–4Mg alloy) were then slid against the prepared steel plates. The grinding angle (angle between direction of sliding and grinding marks) was varied between 0° and 90° in the tests. The experiments were conducted under both dry and lubricated conditions in an ambient environment. It was observed that the transfer layer formation and the coefficient of friction depend primarily on the directionality of the plate grinding marks. For the case of pure Mg pins, a stick-slip friction phenomenon was observed for all grinding angles under dry conditions and for grinding angles over 25° under lubricated conditions. In the case of Al pins, the stick-slip phenomenon was observed only under lubricated conditions for angles exceeding 25°. The stick-slip phenomena did not occur in any of the conditions studied with Al–4Mg alloy pins. Based on the results, it was concluded that the magnitudes of the friction and the stick-slip motion amplitude (for Al and Mg pins) were primarily controlled by changes in the level of plowing friction.