Contributions of adhesion and hysteresis to coefficient of friction between shoe and floor surfaces: effects of floor roughness and sliding speed

Abstract

Improving shoe–floor friction in order to reduce slip and fall accidents requires thorough understanding of the factors that contribute to friction. The friction between a sliding viscoelastic material (shoe) and a hard surface (floor) has two major components: adhesion and hysteresis. This study aimed to quantify the effects of floor roughness and sliding speed on adhesion and hysteresis to determine how each component contributes to the coefficient of friction. Experiments were conducted on a pin on disc tribometer using ceramic tiles with three levels of roughness, six sliding speeds, two common shoe materials and four liquid lubricants. Hysteresis was measured using a lubricant that minimised adhesion. Dry and lubricated adhesion was measured by subtracting hysteresis from the coefficient of friction. Analysis of variance regression models were used to determine the contributions of hysteresis, dry adhesion, sliding speed and fluid to lubricated coefficient of friction. Increased floor roughness led to increased hysteresis, while increased sliding speed reduced both adhesion and hysteresis. These findings are consistent with theory that states that larger asperities increase hysteretic deformation and that sliding speed affects deformation and real area of contact between a viscoelastic material and a hard surface. The model correctly predicted 83% of variation in coefficient of friction based on dry adhesion, hysteresis and fluid dependent constants. The sensitivity of hysteresis friction to shoe material and floor roughness indicates that optimising these parameters may be effective at reducing slip accidents on oily floor surfaces.     

Adhesion and Friction Enhancement of Film-Terminated Structures against Rough Surfaces

A number of biomimetic and bioinspired micron-scale surface structures have been developed in recent years with unique surface properties such as highly enhanced and switchable adhesion and friction against smooth surfaces. However, few studies have examined the effect of roughness on mechanisms for property enhancement, although this is of critical importance for applications. Here, we investigate the effect of roughness on adhesion and friction of a family of film-terminated fibrillar and ridge/channel microstructures. Although increasing roughness uniformly attenuates adhesion and friction, we find that the film-terminated structures maintain their enhancement compared to flat controls against a variety of rough surfaces (including natural stones). The principal mechanisms underlying property enhancement against smooth surfaces remain operative against rough surfaces. We show how the effect of roughness on surface mechanical properties of structured surfaces can be understood as a combination of known effects due to structure and roughness.     

Tribological study on the surface modification of metal-on-polymer bioimplants

The tribological performance of artificial joints is regarded as the main factor of the lifespan of implanted prostheses. The relationship between surface roughness and coefficient of friction (COF) under dry and lubricated conditions is studied. Results show that under dry test, friction coefficient is not reduced all the time with a decrease in surface roughness. On the contrary, a threshold of roughness value is observed, and frictional force increases again below this value. This critical value lies between 40 and 100 nm in S_a (roughness). This phenomenon is due to the transfer of friction mechanisms from abrasion to adhesion. Under wet test, COF always decreases with reduction in surface roughness. This result is mainly attributed to the existence of a thin layer of lubricant film that prevents the intimate contact of two articulating surfaces, thus greatly alleviating adhesion friction. Furthermore, surface texturing technology is successful in improving the corresponding tribological performance by decreasing friction force and mitigating surface deterioration. The even-distribution mode of texturing patterns is most suitable for artificial joints. By obtaining the optimal surface roughness and applying texturing technology, the tribological performance of polymer-based bioimplants can be greatly enhanced.     

Studies on friction and transfer layer: role of surface texture

Friction influences the nature of transfer layer formed at the interface between tool and metal during sliding. In the present investigation, experiments were conducted using “Inclined Scratch Tester” to understand the effect of surface texture of hard surfaces on coefficient of friction and transfer layer formation. EN8 steel flats were ground to attain surfaces of different textures with different roughness. Then super purity aluminium pins were scratched against the prepared steel flats. Scanning electron micrographs of the contact surfaces of pins and flats were used to reveal the morphology of transfer layer. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the texture of hard surfaces, but independent of surface roughness of hard surfaces. It was observed that on surfaces that promote plane strain conditions near the surface, the transfer of material takes place due to the plowing action of the asperities. But, on a surface that promotes plane stress conditions the transfer layer was more due to the adhesion component of friction. It was observed that the adhesion component increases for surfaces that have random texture but was constant for the other surfaces.     

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.     

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.     

Effect of surface structure on frictional behaviour of a tongue/palate tribological system

The effect of surface structure on the friction and lubrication properties of a model tribological system representing the tongue/palate contact was investigated under dry conditions and in the presence of oil and aqueous solution having the same viscosity at mouth temperature. To this end, several soft silicone surfaces with well-defined surface structures, based on hemisphere pillars of different dimensions in the sub-millimetre range, were fabricated by a moulding technique in order to cover the different scales roughness of the human tongue. The friction experiments were performed on a reciprocating motion sliding tribometer involving contact between a relatively hard ball (steel or PCTFE) representing the palate and one of the soft silicone surfaces simulating the tongue. Test conditions were designed to represent those encountered in the mouth when thin films of food residues coat the oral mucosa surfaces. The results show that the frictional behaviour of the investigated model tribosystem is strongly affected by the topographical structure of the contacting surfaces. Under dry conditions, the coefficient of friction decreases significantly with increase of hemispherical pillar density. For lubricated surfaces, higher pillars with an optimal high density increase the friction coefficient. Further, it was observed that a minimal wetting of at least one contacting surface is essential for establishing effective lubrication.     

Stress intensity in viscoelastic contacts

Dynamic mechanical contacts with nanometer-to-micrometer dimensions are important in scanned probe microscopy, ultra-low load indentation, microelectromechanical systems, compact discs, etc. The response of these contacts is poorly understood if they involve adhesive viscoelastic materials, such as polymers and self-assembled monolayers. We have studied dynamic contacts to styrene–butadiene latex films. Plots of load vs. displacement show substantial adhesion hysteresis between the loading and unloading portions. The hysteresis is at least partially due to creep, as indicated by the continued increase in penetration after the start of unloading. Thermodynamic works of adhesion were estimated from fits to the loading–unloading data obtained at small loading and unloading rates. Theoretical models that include adhesion but neglect long-range creep effects could not fit the data at all loading rates. Creep tests were carried out under constant load. A model due to Hui, Baney, and Kramer (HBK), which predicts the response of an adhesive viscoelastic contact under increasing load, was used to extract a mode I stress-intensity functional from the data. When this functional is normalized by the square root of the displacement rate, it is shown empirically, to have a simple, nearly universal time-dependence. Variations of this universal form due to load, range of interaction potential, glass transition temperature, and probe shape are weak. This result supports the suggestion of HBK that the stress-intensity functional provides a useful way to characterize adhesive contacts to viscoelastic materials.     

Surface characterization and adhesion and friction properties of hydrophobic leaf surfaces

Super-hydrophobic surfaces as well as low adhesion and friction are desirable for various industrial applications. Certain plant leaves are known to be hydrophobic in nature. These leaves are hydrophobic due to the presence of microbumps and a thin wax film on the surface of the leaf. The purpose of this study is to fully characterize the leaf surface and to separate out the effects of the microbumps and the wax on the hydrophobicity. Furthermore, the adhesion and friction properties of the leaves, with and without wax, are studied. Using an optical profiler and an atomic/friction force microscope (AFM/FFM), measurements on the hydrophobic leaves, both with and without wax, were made to fully characterize the leaf surface. Using a model that predicts contact angle as a function of roughness, the roughness factor for the hydrophobic leaves has been calculated, which is used to calculate the contact angle for a flat leaf surface. It is shown that both the microbumps and the wax play an equally important role in the hydrophobic nature as well as adhesion and friction of the leaf. This study will be useful in developing super-hydrophobic surfaces.     

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.     

Effect of deformation of the contact surface on wear and friction—a study on clarification of the mechanism of abrasive wear

The cutting action due to a protuberance between the rubbing surfaces in abrasive wear might be susceptible to adhesion at the rake surface in the direction of sliding.

In the present paper we discuss an approach to clarify the abrasive wear mechanism, which is dominated by the effect of the difference in the contact deformation between both surfaces on wear and friction related with abrasive wear, on the basis of experiments conducted under ultrahigh vacuum.

Although in practice it is generally very difficult to obtain a characteristic surface during sliding, we carried out experiments on surfaces that were as clean as possible to investigate the effect of the size of the deformed contact surface on the abrasive wear.

Consequently, we were able to confirm that the friction differs according to which of the test pieces is the harder and, if the friction pair is composed of the same material, because the contact surface is deformed differently in each case.     

A study on the effect of surface topography on rough friction in roller contact

The friction behaviour of gear teeth in the context of tribology can have a strong effect on housing vibration, noise and efficiency. One of the parameters that greatly influences the friction under certain running conditions is surface roughness. In this work, rough friction was studied in lubricated sliding of roller surfaces, which were manufactured to simulate the real gear surfaces. By examining 3D surface topography of two mating bodies, both surface roughness and its effect on friction behaviour can be studied. In a previous study, a rough-friction test rig has been designed, constructed and initially verified. The types of surfaces involved in this study are ground, shot-peened, phosphated and electrochemically deburred. These rollers were subjected to the same friction testing procedures. Roller surfaces were then examined, and correlation between the topography and the frictional behaviour was analysed. Friction behaviour was interpreted in terms of Stribeck curves (friction coefficient as the function of Hersey parameter (ην/p)). The results showed that electrochemically deburred and certain phosphated surfaces provide lower friction coefficient values which are competitive to fine-ground surfaces in lubricated rolling/sliding contact.     

Wetting study of patterned surfaces for superhydrophobicity

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water-repellent properties. A number of studies have been carried out to produce artificial biomimetic roughness-induced hydrophobic surfaces. In general, both homogeneous and composite interfaces are possible on the produced surface. Silicon surfaces patterned with pillars of two different diameters and heights with varying pitch values were fabricated. We show how static contact angles vary with different pitch values on the patterned silicon surfaces. Based on the experimental data and a numerical model, the trends are explained. We show that superhydrophobic surfaces have low hysteresis and tilt angle. Tribological properties play an important role in many applications requiring water-repellent properties. Therefore, it is important to study the adhesion and friction properties of these surfaces that mimic nature. An atomic/friction force microscope (AFM/FFM) is used for surface characterization and adhesion and friction measurements.     

Adhesion and micromechanical properties of metal surfaces

This paper is part of a long-standing attack on problems of friction and adhesion. It falls into two parts. The first discusses the general mechanism of adhesion and then describes an experimental study of the adhesion and deformation of a model microasperity. This consists of the contact formed between a fine pointed stylus of tungsten and a single crystal of a softer metal (nickel). The experiments are carried out in ultrahigh vacuum and the surfaces are characterized in situ using Auger spectroscopy. The loads range from 0.5 to 1000 μN and contact during loading and unloading is monitored using electrical resistance measurements. With clean surfaces the results suggest that surface forces alone are able to initiate plastic deformation. Oxide monolayers reduce the adhesion but the general behaviour is little changed. In contrast, oxide layers 5 nm thick greatly modify the adhesion and deformation behaviour.

The second part describes microhardness measurements carried out over approximately the same load range. A Berkovitch triangular pyramid is used as the indenter. Some of the indents are studied in the electron microscope but the major hardness determinations are based on depth measurements of the indents formed. These measurements are extremely sensitive and may be conveniently monitored so that the plastic indentation process during loading and the elastic relaxation which occurs on unloading may be recorded. Both the adhesion measurements and the microindentation hardness measurements show that the plastic yield stress of very small volumes may be three or four times larger than the bulk values. The results also show that detailed contact models may be of limited value when dealing with contacts involving plastic deformation.     

A Combined Molecular Dynamics and Finite Element Analysis of Contact and Adhesion of a Rough Sphere and a Flat Surface

A combined molecular dynamics and finite element model and simulation of contact and adhesion between a rough sphere and a flat surface has been developed. This model uses the results of molecular dynamics (MD) simulations, obtained using an embedded atom potential, of a nanoscale Ru-Ru asperity contact. A continuum finite element model of an elastic–plastic microscale Ru-Ru contact bump is then created. In this model, the surface roughness is represented by a system of nanoscale asperities, each of which is represented by a nonlinear hysteretic force vs. distance relationship. The nonlinear hysteretic character of these relations is determined from curve-fits of the MD results. Load vs. interference and contact area vs. interference are determined using this two-scale model for loading and unloading. Comparisons with a single-scale continuum model show that the effect of the nanoscale asperities is to reduce both the adhesion and the real area of contact. The choice of Ru as the material for this work is due to its relevance in microswitches.     

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.     

Influence of surface roughness and running-in on the lubrication of steel surfaces with oil containing MoS2 nanotubes in all lubrication regimes

Despite several studies that have confirmed the beneficial effect of MoS₂ and WS₂ nanoparticle-assisted lubrication, an understanding of how the nanoparticles behave in different, even very common contact conditions, such as roughness, is still missing. As a result we have focused on a comparison of the lubrication behaviour of MoS₂ nanotubes mixed with PAO oil using steel surfaces with different roughnesses. Moreover, we have investigated the MoS₂-nanotubes-assisted lubrication of steel/steel contacts in all lubrication regimes and also the effect of the running-in of these contacts. It was realized that the friction with the nanotubes-containing oil was 40–65% lower compared to the base oil, depending on the different contact conditions used. Furthermore, we showed that by using MoS₂ nanotubes in the oil the friction is the same for rough and smooth steel surfaces, meaning that the nanotubes completely govern the lubrication behaviour in self-mated steel contacts in the boundary- and mixed-lubrication regimes, irrespective of the surface roughness or the running-in.     

Adhesion and Friction Evaluation of Textured Slider Surfaces in Ultra-Low Flying Head-Disk Interfaces

To achieve extremely high-density magnetic recording of 1Tbit per square inch using conventional technologies, the distance between the recording slider and the rotating disk needs to be less than 5nm. For successful operation, disk and slider surfaces must also be extremely smooth with root-mean-square roughness values of few angstroms. However, ultra-low flying super smooth head-disk interfaces may be exposed to a significant amount of intermittent contact, adhesion, stiction and friction that can cause the interface to collapse. In order to circumvent such problems, many novel techniques have been proposed, such as laser zone texturing, contact pads and surface microtexturing. A reliable method to reduce adhesion and friction in ultra-low flying head-disk interfaces is to control the area of contact and roughen the interface, which allows the slider to fly at sub-5nm with minimal contact. A technique known as preferential texturing provides a unique roughening of the air-bearing surface, where parts of the surface are removed, i.e., subtractive texturing process. In this paper, the effect of preferential texturing (roughening) of slider air-bearing surfaces on the adhesion and friction forces are investigated using quasi-dynamic models. The simulation results show that surface texturing reduces adhesion and friction by reducing the effective area of contact between the slider and media surfaces and by preferentially roughening the interface. The simulation results of friction compare favorably with experimental data.     

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.     

Demonstration of topography modification by friction processes and vice versa

Contact formation and development are the basis of friction and wear modelling and understanding. Unanimously topography formation and development in friction contacts are regarded of highest importance for understanding and modelling friction processes. The frequently found running in behaviour of sliding contacts is—aside from the build up of reaction and transfer layers-at least partly caused by the topography development due to friction processes until a stable equilibrium state is reached.Experimental results of friction and topography measurements are presented which demonstrate the mutual modification of friction and contact topography.A special experimental set up with an AFM allowed to correlate the measured friction forces with the contact position and the topography at this point. In this way, friction force transitions and changes can be assigned to topography changes due to abrasion, adhesion and wear particle agglomeration.Contact surfaces with artificial regular structures have been prepared to avoid problems with topography and friction correlation due to the statistical nature of roughness on technical contact surfaces. The friction effects of roughness were simulated by etched ditches of defined width, depth and distance on silicon or metal surfaces. This allowed to explain the mutual influences of topography and friction. The effect of a single ‘asperity’ and of the ‘roughness structures’ could be demonstrated.Topography measurements with an AFM correlated with the friction force could help to understand friction changes without changing any parameter.