Contact and wear of paper-based friction materials for oil-immersed clutches—wear model for composite materials

Paper-based friction materials are widely used for oil-immersed clutches in automatic transmissions for passenger cars. It is known that repetition of engagements of the clutches causes wear of the friction materials leading to running-in with an increase in contact area. Observation of the contact of a typical paper-based friction material has been made by contact microscopy using the reflection of polarized light. The results have shown that contact is made at the top of its particulate and fibrous constituents, and comparison with the results of laser microscopy has revealed that the contact area observed by the contact microscopy is the contour contact area. An analysis has been made to describe this behavior by employing a mechanical model in which spherical and columnar asperities are supported by an elastic halfspace and wear under the loads they support. Analytical results describe the observed change in the contour contact area and demonstrate applicability of the model to wear of composite materials.     

The contact problem for a viscoelastic layer and rigid cylinder during regular sliding

The contact problem is formulated for a rigid cylinder and viscoelastic layer in sliding. The solution of the contact problem is reduced to the solution of an integral equation the core of which possesses a logarithmic feature and contains a sum of nonintrinsic integrals. The method of solution of this integral equation is disclosed. The effect of the viscoelastic properties of the layer on the contact pressure distribution is analyzed. It is shown that the function of the contact pressure distribution can have two maxima. The obtained solution of the problem is compared with the analytical solutions for two limiting cases: the solution of the contact problem for a rigid cylinder and viscoelastic semispace described in the works of L.A. Galin and I.G. Goryacheva and the approximate solution of the problem for a thin viscoelastic layer. The developed method of solving the contact problem for a viscoelastic layer can be used to calculate the contact parameters in friction units with coatings from composite materials having viscoelastic properties.     

Evaluation of Tribological Behaviour of Polymeric Materials for Hip Prostheses Application

The tribological (friction and wear) behaviour of different polymeric materials was evaluated for hip prostheses applications. First, three polymeric materials were tested with fixed conditions of contact stress and sliding velocity. In a second phase, the material which presented the best results was selected. Its frictional behaviour under dynamic conditions of contact stress and sliding velocity was evaluated.     

Numerical analysis of viscous elastohydrodynamic point contact under stationary conditions

A model is proposed for study of the effect of nonelastic qualities of contacting bodies separated by a fine lubricant film on the contact characteristics. The problem is studied of the movement of the fine lubricant film between the rigid spherical surface and the mobile viscoelastic-layer surface rigidly adhering to the base. A unidimensional Kelvin model of the viscoelastic medium serves as the rheological model of the viscoelastic layer. The calculation results show that the pressure is distributed in the viscous elastohydrodynamic point contact film very much differently from the pressure distribution in the UHD contact, particularly at slow sliding velocities. The friction coefficient is a nonmonotonous function of the sliding velocity. The friction coefficient drops at slow velocities to a minimum as the velocity accelerates, and then it grows.     

On the decrease in contact area for spheres and cylinders rolling on a viscoelastic plane

A theory is presented for predicting the variation with speed of the contact area for spheres and cylinders rolling on a dry, viscoelastic plane. Two distinct effects are shown to occur, due to size and shape variations respectively. The size effect is an overall reduction in contact area because of elastomer stiffening at very high forward speeds, whereas the shape effect produces contact asymmetry at lower speeds. Both effects are a manifestation of the hysteresis properties of the viscoelastic plane. A complex Voigt model with variable parameters is used to simulate the dynamic performance of the plane, and there is close agreement with experimental data. An important application is the rolling behaviour of pneumatic tyres on rough road surfaces.     

Use of phase imaging in atomic force microscopy for measurement of viscoelastic contrast in polymer nanocomposites and molecularly thick lubricant films

Phase contrast microscopy, using an atomic force microscope, is used to detect and quantify changes in composition across polymer nanocomposites and molecularly thick lubricated surfaces. The technique takes advantage of the contrast in viscoelastic (viscous energy dissipation) properties of the different materials across the surface. Some materials, especially polymers, are found to display viscoelastic behavior. For such materials, the strain response lags the stress by a phase angle that is characteristic of the material. In tapping (or intermittent contact) mode, phase angle contrast is found to be highly dependent on vibration amplitude and mean tip-to-sample distance (setpoint). Phase angle contrast seems to be a stronger function of viscoelastic properties at relatively high vibration amplitude and low mean tip-to-sample distance. In this regime the effects of sample deformation, and thus viscoelastic properties, are dominant. In these contrast images, low phase angle corresponds to materials with low viscoelastic properties. This technique was used to find fairly reproducible phase angle contrast for polyethylene terephthalate (PET) films with embedded ceramic particles, metal particle (MP) magnetic tape, and Si(100) with a nonuniform Z-15 lubricant film. Very little correlation is found between phase angle images and friction force images for PET films with embedded ceramic particles and MP tape; phase angle images give information that cannot be obtained from topography or friction images. A numerical vibration model verifies that viscoelastic properties are dominant for high vibration amplitude and low mean tip-to-sample distance. For these conditions, the model also verifies that low phase angle corresponds to low viscoelastic properties.     

Investigation of disc/pad interface temperatures in friction braking

Maintaining appropriate levels of friction interface temperature is important for the overall operating effectiveness of modern friction brakes, and implicitly the safety of the vehicle. Measurement and prediction of the distribution and magnitude of brake friction interface temperatures are difficult. A thermocouple method with an exposed hot junction configuration was used for interface temperature measurement, and the magnitude and distribution of the friction interface temperature were investigated in this study. Using a designed experiment approach, the factors affecting the interface temperature, including the number of braking applications, sliding speed, braking load and type of friction material were studied. It was found that the number of braking applications had the strongest effect on the friction interface temperature. The real contact area between the disc and pad, i.e. pad regions where the bulk of the kinetic energy is dissipated via friction, had a significant effect on the braking interface temperature. For understanding the effect of real contact area on local interface temperatures and friction coefficient, finite element analysis (FEA) was conducted, and it was found that the maximum temperature at the friction interface does not increase linearly with decreasing contact area ratio. This finding is potentially significant in optimising the design and formulation of friction materials for stable friction and wear performance.     

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.     

Deformational contact and friction on viscoelastic substrates

The deformation of a rigid slider of arbitrary shape sliding smoothly over the surface of a general linear viscoelastic layer is studied. A solution of the two-dimensional problem is presented in order to calculate rigorously the contact area, deformational friction and frictional heating. For a rigid cylinder moving on a standard linear viscoelastic substrate the distinctive effects of varying the size and shape on the contact area are calculated exactly as a special case of our general solution. The results, given in terms of the slider speed and the hysteresis properties of a viscoelastic solid, agree well with the excellent qualitative prediction of Moore's theory. More importantly, the calculation concludes that all the sudden changes in the size and shape of contact, the deformational friction and heat generation correspond to the maximum loss tangent of the viscoelastic material.     

Influence of load and elastic properties on the rolling and sliding friction of lubricated compliant contacts

Lubricated “soft” contacts, where one or both contacting solids have a low elastic modulus, are present in many practical engineering and biological applications including windscreen wipers, wet tyres, elastomeric seals, contact lenses and the tongue/palate system. In such contacts, the prevailing lubrication mode is “isoviscous EHL” (elastohydrodynamic lubrication). Unlike in steel–steel contacts, rolling friction can be considerable and this originates in part from the viscoelastic properties of the compliant surfaces.In this paper the influence on friction of both applied load and the elastic properties of the solids is studied using a mini traction machine. In this machine, the rolling and sliding friction can be separately determined. The viscoelastic properties of the polymers employed are measured using a dynamic mechanical analysis apparatus. The measured friction is compared to theoretical models for soft EHL and the viscoelastic energy losses arising from the contact deformation. Consideration of the frequency dependence of the substrate viscoelasticity enables reasonably accurate predictions of the rolling friction coefficient, especially within the mixed and boundary lubrication regimes.     

Friction-Induced Vibration by Stribeck’s Law: Application to Wiper Blade Squeal Noise

This paper is concerned with the squeal noise of a wiper/windscreen contact. It is shown that squeal noise stems from friction-induced self-excited vibrations in the context of Stribeck’s law for friction coefficient. The study is specifically focussed on the instability range of velocities and not on the amplitude of limit cycles. The studied dynamic system consists of a single degree-of-freedom mass-spring-damper oscillator submitted to a velocity-dependent frictional force which follows the Stribeck law. The local stability is analyzed by the first Lyapunov method and results in a stability criterion. Experiments have been performed on a glass/elastomer contact lubricated with water. The tribometer ‘LUG’ provides measurements of the vibrational velocity and friction force versus sliding speed. It is found that the instability appears during the transition between boundary and elastohydrodynamic regimes where the negative gradient of the friction versus velocity curve is steep. The apparition and vanishing of instability are correctly predicted by the steady-state stability criterion.     

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.     

Contact problem of die regular relief motion over viscoelastic base

The numerical and analytical model is constructed to calculate the deformation friction force component when the regular relief die moves over the viscoelastic base, which is modeled with the Kelvin body with the relation time spectrum. The developed friction model is applicable to both the discrete and full contact between interacting surfaces. The die regular relief is modeled with a set of surface points. The results obtained with the developed model are compared for the case of full contact with the analytic calculation. The sliding velocity and the die regular relief shape are studied to identify the influence on the contact characteristics and the deformation componentof the coefficient of friction.     

Rolling Friction: Comparison of Analytical Theory with Exact Numerical Results

We study the contact mechanics of a smooth hard cylinder rolling on a flat surface of a linear viscoelastic solid. Using the measured viscoelastic modulus of unfilled and filled (with carbon black) nitrile rubber, we compare numerically exact results for the rolling friction with the prediction of a simple analytical theory. For the unfilled rubber, the two theories agree perfectly while some small difference exists for the filled rubber. The rolling friction coefficient depends nonlinearly on the normal load and the rolling velocity.     

Study of stress state of viscoelastic half-space in sliding contact with smooth indenter

A method of calculating internal stresses in linearly viscoelastic bodies has been substantiated based on the solution to the contact problem of the sliding of a smooth indenter with friction against the boundary of a viscoelastic half-space. The effects of the coefficient of friction, Poisson’s ratio, and the velocity of sliding on the localization and the maximum values of tensile stresses, as well as maximum tangential stresses, have been analyzed.     

The frictional behaviour of materials

A review of friction is made and a general equation of friction, based on dimensional analysis, is presented. The equation takes into account the environment, material properties, surface roughness, the area of real contact, rubbing velocity, deformation and surface energy, and suggests friction to be system dependent. By imposing certain conditions, it was shown that the equation satisfies equations based on the adhesion and agrees with deformation and surface energy theories. A model of friction is presented which suggests that the friction force results from adhesion, plowing and interlocking of asperities, the magnitude of which is governed by surface energy and the nature and magnitude of surface deformation.The experimental results presented suggest the dependence of friction on load and velocity to be heavily influenced by the oxidation characteristics of the materials in rubbing contact.     

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.     

Cryogenic Friction Behavior of PTFE based Solid Lubricant Composites

Solid lubricants used in aerospace applications must provide low friction and a predictable operation life over an extreme range of temperatures, environments and contact conditions. PTFE and PTFE composites have shown favorable tribological performance as solid lubricants. This study evaluates the effect of temperature on the friction coefficient of neat PTFE, a PTFE/PEEK composite and an expanded PTFE (ePTFE)/epoxy coating. These experiments evaluate friction coefficient over a temperature span which, to the investigators’ knowledge, has not been previously examined. Results show a monotonic increase in friction coefficient as sample surface temperature was decreased from 317 to 173 K for all three samples. The frictional performance of these and other published solid lubricant polymers was modeled using an adjusted Arrhenius equation, which correlates the coefficient of friction of the polymer materials to their viscoelastic behavior. A model fit of all the polymer data from 173 to 450 K gives an activation energy of 3.7 kJ/mol. This value suggests that breaking of van der Waals bonds is the likely mechanism responsible for the frictional behavior over this temperature range.     

Theory of viscoelastic lubrication

We study the lubricated (wet) contact mechanics of a smooth hard cylinder sliding on a randomly rough nominally flat surface of a linear viscoelastic solid. We calculate the rolling and sliding friction, and study the transition from the boundary lubrication to the elasto-hydrodynamic lubrication regime. For the viscoelastic contact the minimum (average) separation does not monotonically increase with the sliding velocity, and the Stribeck curve exhibits new structures not shown for elastic solids.     

Development of contact sliders with nanotextures by femtosecond laser processing

The information on the frictional resistance of a self-propelled robotic capsule endoscope moving inside the body is very important for the design and the performance enhancement of such parameters of the capsule endoscope as power consumption, motion control and positioning accuracy. Based on this motivation, the ultimate goal of this research was to develop an analytical model that can predict the frictional resistance of the capsule endoscope moving inside the living body. In this work, experimental investigations of the fundamental frictional characteristics and the viscoelastic behaviors of the small intestine were performed by using custom-built testers and various capsule dummies. The small intestine of a pig was used for the experiments. Experimental results showed that the average frictional force was 10–50 mN and higher moving speed of the capsule dummy resulted in larger frictional resistance of the capsule. In addition, the friction coefficient did not change significantly with respect to the apparent area of contact between the capsule dummy and the intestine, and also the friction coefficients decreased with an increase in the normal load and varied from 0.08 to 0.2. Such frictional behaviors could be explained by the lubrication characteristics of the intestine surface and typical viscoelastic characteristics of the small intestine material. Also, based on the experimental results, a viscoelasticity model for the stress relaxation of the small intestine could be derived.