Adhesion Friction Model Research Articles

Prediction of Adhesion Friction Coefficient Using Two Different Models for Tire Tread Rubber Compounds

ABSTRACT Two proposed methods to determine the adhesion friction coefficient were validated by experimental results of two types of rubber compounds at different sliding velocities under dry conditions. The experimental results were measured from a linear friction tester, while the viscoelastic friction coefficient was estimated using the Persson's contact theory. Adhesive friction (model 1) was derived from the deconvolution of dry friction coefficient in two Gaussian-like curves. Interesting results were obtained using the deconvoluted method in the range of intermediate sliding velocities where preponderant contribution to the adhesion friction is replaced by the viscoelastic friction. Fitting parameter results were in good general agreement with values derived from the literature, confirming the influence of the mechanical properties of the compound and substrate texture on the proposed adhesion frictional method. The second adhesive friction model (model 2) was based on the confinement rheology of rubber chains on the contact with the asperities of the road surface. We demonstrated that acceptable adhesion friction results were achieved from a dynamic viscosity test at low frequencies, confirming the applicability of the proposed rheological model. Moreover, the relationship between the rubber composition and the modified contact layer along with the likely interphase reaction are also discussed.

On the presence of a critical detachment angle in gecko spatula peeling - a numerical investigation using an adhesive friction model

ABSTRACT A continuum-based computational contact model is employed to study coupled adhesion and friction in gecko spatulae. Nonlinear finite element analysis is carried out to simulate spatula peeling from a rigid substrate. It is shown that the “frictional adhesion” behavior, until now only observed from seta to toe levels, is also present at the spatula level. It is shown that for sufficiently small spatula pad thickness, the spatula detaches at a constant angle known as the critical detachment angle irrespective of the peeling and shaft angles. The spatula reaches the same energy states at the jump-off contact point, which directly relates to the invariance of the critical detachment angle. This study also reveals that there is an optimum pad thickness associated with the invariance of the critical detachment angle. It is further observed that the sliding of the spatula pad is essential for the invariance of the critical detachment angle. Further, it is found that the invariance of the critical detachment angle is unaffected for a wide range of spatula shaft lengths.

On the Formation Mechanisms of Adhering Layer during Machining Metal Material

Built-up Layer (BUL)/Built-up Edge (BUE) formed on the tool surface can be treated as a protective, thermal barrier or lubricant films especially in the extreme severe conditions when machining the metal materials, which can sustain the tool effective and wear resistance. In order to have a thorough understanding of the adhesion effect during machining, experiments have been carried out to investigate the performance and the formation mechanisms of adhering layer on the carbide tool in machining of aluminium alloys A6063, carbon steel S45C and difficult-to-cut hardened steel S45C (H-S45C). The morphology of tool adhered surface was examined by employing Scanning Electron Microscopy (SEM), the dimensions of adhering layer were measured by Laser Scanning Microscopy (LSM) and the elements on the tool were analyzed by Electron Probe Micro Analyser (EPMA), respectively. The atomic-scale cluster adhesive friction model is proposed to explain the tool-chip contact conditions, which considers the nature of the shear strain, shear strain rate and temperature distribution in the secondary deformation zone. The model is a dynamic model and the rate equation approach can be applied to estimate the formation process of adhering layer during machining. Results have shown that the adhering layer will give rise to BUL on the tool rake face and the BUE on the cutting edge and clearance face.

Tunable Friction Through Microwrinkle Formation on a Reinforced Rubber Surface

A new system is presented for controlling of friction through manipulating the contact between a rigid indenter and a porous-film-embedded polydimethylsiloxane (PDMS) rubber surface that can buckle to form regular microwrinkles depending on the applied lateral compression. The effect of this change in topography on friction is investigated with a fixed surface chemistry at the sliding interface to ensure a consistent value of adhesive shear strength per unit area of contact. With a wrinkled surface, a shorter stick–slip period and corresponding lower average force of friction are observed when compared to a flat PDMS surface. Based on the framework of Bowden–Tabor’s adhesive friction model, this result is attributed to a decrease in the contact area caused by an increase in the effective modulus and the distribution of interfacial contacts on the wrinkles. These results therefore imply that a shape-tunable microstructure is an effective approach to developing surfaces with dynamically tunable tribological properties.

Effect of normal load on friction coefficient for sliding contact between rough rubber surface and rigid smooth plane

This study focused on the normal load dependence of the friction coefficient for the sliding friction of a rubber material with a rough surface. A developed friction tester was used to visualize the real contact regions distributed within the transparent contact interface between poly-dimethyl siloxane (PDMS) and glass surfaces. Based on experimental results, an adhesion friction model was developed to explain the normal load dependence of the friction coefficient. This model provides a simple technique that can roughly but easily estimate the real contact area and shear stress without in situ observation of the contact interface.

Relationship Between the Friction and Microscopic Contact Behavior of a Medical Compression Stocking at Different Strains

The contact and friction behavior of a medical compression stocking (MCS) under different strains was investigated in friction and compression experiments against a mechanical skin model. In addition, the 3D topography of the MCS surfaces was analyzed in order to study the relationship between macroscopic friction and microscopic surface properties. The load dependence of friction coefficients was found to be in accordance with the adhesion friction model. The surface structure of MCS samples was considerably changed when varying the strain state, while friction coefficients remained comparable, indicating real contact areas independent of strain on the microscopic level. The experimental findings could be confirmed and explained on the basis of the microscopic surface analyses, when interpreting the fabric surfaces to be composed of numerous individual round asperities obeying the Hertz contact model.

Understanding the variation of friction coefficients of human skin as a function of skin hydration and interfacial water films

Human skin is characterised by a complex and highly variable friction behaviour. Although the variation of friction coefficients measured for skin depends on numerous parameters related to the skin itself, the surface in contact as well as contact conditions, water or sweat – either bound in the stratum corneum and manifest as skin hydration or in form of liquid films lubricating the interface – is the most important factor. Here, we analyse the variation of previous experimental data on the basis of the adhesion friction model and show how lower and upper bounds, i.e. envelope functions, can be derived for measured skin-friction coefficients. From these envelope functions, essential tribological parameters such as the interfacial shear strength and the real contact area of skin are estimated.

Stick–slip phenomena in the friction of human skin

The stick–slip behaviour of the index finger pad sliding on wet, smooth glass was investigated as a function of normal force and sliding velocity in friction measurements using a tri-axial force plate. The friction coefficients during stick–slip (0.53±0.21) were found to be typically 30% lower than those of stationary sliding (0.78±0.35). Stick–slip friction either occurred continuously throughout entire friction measurements or alternated with stationary sliding phases. During stick–slip the amplitude of the friction coefficient generally varied more than ±25% about the mean, whereas stationary sliding was characterized by variations smaller than ±10%. The load-dependence of the overall friction coefficients as well as those for stationary sliding and stick–slip friction was in agreement with the adhesion friction model. In addition, the friction coefficients decreasing with increasing sliding velocity indicated friction in the mixed lubrication regime. Results obtained on stick–slip friction of the finger pad might be useful in connection with the development of artificial skin in robotics and for the control of stick–slip friction between precision tools and soft tissue.

Influence of variations in the pressure distribution on the friction of the finger pad

We investigated the influence of the contact pressure distribution on the friction of the index finger pad. The skin contact pressure distributions were characterised by round profiles for low forces. At higher loads, the pressure distributions of the finger pad became asymmetric and conical. Additional experiments with the knuckle revealed pressure distributions with sharp peaks. The friction coefficients of the finger pad and the knuckle indicated a common behaviour in accordance with the adhesion friction model. Varying pressure distributions were found to influence the friction coefficients of skin, contributing to the variation of measurement results at comparable normal loads.

Dissipation characteristics of adhesive kinetic friction on amorphous polymer surfaces

The results of adhesion and friction experiments are compared with those of a simplified formula based on power-law or logarithmic approximations of a previous adhesive friction model. The velocity dependence of the adhesive shear stress at low sliding velocities follows a power law, and the exponents for adhesive contact and sliding are similar. The primary and secondary contributors to adhesive friction are the adhesive shear resistance of the interface and the dissipation of reattachment folds near the trailing edge. The results provide additional evidence supporting the assumptions in our earlier frictional model.

Tribology of Skin: Review and Analysis of Experimental Results for the Friction Coefficient of Human Skin

In this review, we discuss the current knowledge on the tribology of human skin and present an analysis of the available experimental results for skin friction coefficients. Starting with an overview on the factors influencing the friction behaviour of skin, we discuss the up-to-date existing experimental data and compare the results for different anatomical skin areas and friction measurement techniques. For this purpose, we also estimated and analysed skin contact pressures applied during the various friction measurements. The detailed analyses show that substantial variations are a characteristic feature of friction coefficients measured for skin and that differences in skin hydration are the main cause thereof, followed by the influences of surface and material properties of the contacting materials. When the friction coefficients of skin are plotted as a function of the contact pressure, the majority of the literature data scatter over a wide range that can be explained by the adhesion friction model. The case of dry skin is reflected by relatively low and pressure-independent friction coefficients (greater than 0.2 and typically around 0.5), comparable to the dry friction of solids with rough surfaces. In contrast, the case of moist or wet skin is characterised by significantly higher (typically >1) friction coefficients that increase strongly with decreasing contact pressure and are essentially determined by the mechanical shear properties of wet skin. In several studies, effects of skin deformation mechanisms contributing to the total friction are evident from friction coefficients increasing with contact pressure. However, the corresponding friction coefficients still lie within the range delimited by the adhesion friction model. Further research effort towards the analysis of the microscopic contact area and mechanical properties of the upper skin layers is needed to improve our so far limited understanding of the complex tribological behaviour of human skin.

Normal and Lateral Interactions between Thermosensitive Nanoparticle Monolayers in Water

Static and dynamic interaction forces between two thermosensitive polymeric nanoparticle monolayers grafted onto mica surfaces and immersed in water were studied using a surface forces apparatus. The polymeric nanoparticles (NPs) were made of N,N-diethylacrylamide and had a hydrodynamic diameter of ca. 780 nm at 20 degrees C in aqueous suspension. They were irreversibly grafted onto chemically modified mica surfaces at a constant surface coverage of 2.6 NPs/mum(2). The measured normal forces between two opposing NP monolayers were found to be strongly dependent on the temperature. At temperatures lower than the lower critical solution temperature (LCST), the grafted NPs were swollen, and the normal interaction forces between the two NP monolayers were repulsive. Above the LCST, the NPs collapsed, and attractive forces between the NP layers were measured. The swollen NPs were found to exhibit very low friction forces compared to the collapsed ones. The effect of the sliding velocity on the shear stress was investigated, and the results are in agreement with the so-called adhesive friction model developed for rubber friction. Our results suggest that the water content in the contact area and the interdiffusion of polymer chains are important parameters in determining the friction between polymer-bearing surfaces.

Electric Field Effect on the Sliding Friction of a Charged Gel

Sliding friction of a charged gel on a substrate was investigated by applying electric voltage between gel and the substrate. It is found that the sliding friction increases markedly with the increase of the applied voltage; the friction coefficient increased by a factor of about 10 when 70 V voltage is applied. This voltage dependence of frictional force is explained by using the adhesion friction model proposed by Schallamach.

Friction and deformation behaviour of human skin

The skin friction and deformation behaviour both in-plane and perpendicular to the sliding direction have been measured simultaneously during in vivo reciprocating-sliding of a smooth steel ball probe against human forearm skin. In these experiments, the skin is compressed in front of the probe, forming a “bow-wave”, and stretched behind as a consequence of friction and lateral skin displacement. The effects of two probe diameters and two normal loads on the skin friction and deformation behaviours were investigated for nominally dry skin in a 22 °C, 50% relative humidity environment. Friction force “stick” and “slip” phases could be distinguished during the probe motion cycle and these could be related to the skin deformation behaviour, allowing the effective lateral skin stiffness to be estimated for the different experimental conditions. Comparison to measurements of the effective indentation stiffness of the skin showed differences in the effects of load and probe diameter that may be attributable to differences in the extent of the deformation field within the skin and the complex structure and anisotropic mechanical properties of the different skin layers. The different friction results observed for different loads and probe diameters could be interpreted using an adhesion friction model for smooth contact with hydrated skin, taking into account the skin effective elastic modulus. For the experimental conditions applied here, deformation friction appeared to play only a minor role in determining the total skin friction force.