Instabilities generated by friction in a pad–disc system during the braking process

This paper presents a numerical and experimental study of a pad–disc tribometer. The explicit dynamic finite element software PLAST 3 in 3-D is used to simulate the behaviour of the two bodies involved. Coulomb's friction law is used at the contact surface with a constant coefficient. For this application, temporal simulations show that separation occurs between surfaces, proof of instabilities. This unstable state is characterized by a stick–slip–separation wave. We show that instabilities describe a periodic shock phenomenon at the contact interface. Consequently, the acceleration spectrum recorded on the surface of the pad reveals periodicity in the frequency domain. It shows also that, in this case, the vibrations responsible for the instability are localized in the pad. The mode responsible for squealing can be obtained by a modal analysis of the pad–disc system by assuming that the interface is stuck. We highlight the importance of the pad Poisson's ratio in the occurrence of this unstable state. A numerical/experimental comparison has been performed and the fundamental frequency of squeal obtained experimentally and its magnitude agree with those calculated numerically with PLAST 3.     

Simulations on atomic-scale friction between self-assembled monolayers: Phononic energy dissipation

Atomic-scale friction between self-assembled monolayers (SAMs) on Au (1 1 1) has been studied through molecular dynamics simulations, with emphasis on the mechanism of energy dissipation. Results show that the shear stress and chain angle on commensurate SAMs exhibit a clean periodic pattern and atomic stick–slip friction, which manifests a gradual storage and sudden release of energy. Using a simple model of two atoms, analysis shows that the atomic stick–slip originates from the dynamic instability of molecule motion. Energy has been built up during the stick, followed by a sudden separation as the equilibrium becomes unstable, and most energy dissipates at the time of slip. Moreover, the simulations reveal that more energy is stored and released in commensurate sliding, resulting in much higher friction than that in incommensurate cases. The contradictory frictional behavior can be traced to the difference in the number and strength of the Van der Waals bonds, formed in the two types of contacts.     

Friction-induced vibration of oscillating multi-degree of freedom polymeric sliding systems

The purpose of this study is to investigate friction-induced vibration of oscillating systems. Special attention is focused on modeling polymer-on-metal systems. Past experimental and analytical friction results from non-oscillating or unidirectional translational sliding modes are extended into the oscillating sliding mode. Specifically, this refers to the incorporation of a representative functional shape of the friction–velocity relationships estimated from experimental results. Dependent on the relative sliding velocity between the frictionally coupled subsystems, the modeled system exhibits behavior such as a single stick–slip at the beginning of oscillatory motion, a single stick–slip at each motion reversal, or multiple stick-slip events during each half cycle of motion. Additionally, the boundary of incipient friction-induced vibration was identified via a rigorous definition of friction-induced vibration.     

Spanning Time Scales in Dynamic Simulations of Atomic-Scale Friction

We investigate the sliding dynamics of atomic-scale friction at different time scales. Depending on the dynamic inertia and sliding velocity of a cantilever, different dynamic behaviors are observed from both conventional molecular dynamics (MD) simulation and temporally hybrid molecular simulation methods. The mechanism of friction dissipation is also investigated. For either smooth sliding or stick–slip, a non-zero work of friction is obtained, while surface forces are no longer conservative.     

Thermomechanical analysis of elastoplastic medium in sliding contact with fractal surface

The effects of mechanical and thermal surface loadings on deformation of elastic–plastic semi-infinite medium were analyzed simultaneously by using the finite element method. Rigid rough surface of a magnetic head and smooth surface of an elastic–plastic hard disk were chosen to perform a comprehensive thermo-elastic–plastic contact analysis at the head–disk interface (HDI). A two-dimensional finite element model of a rigid rough surface characterized by fractal geometry sliding over an elastic–plastic medium was then developed. The evolution of deformation in the semi-infinite medium due to thermomechanical surface loading is interpreted in terms of temperature, von Mises equivalent stress, and equivalent plastic strain. In addition to this, the effects of friction coefficient, sliding, and interference distance on deformation behavior were also analyzed. It is shown that frictional heating increases not only the contact area but also the contact pressure and stresses.     

Modification of Boundary Lubrication by Oil-Soluble Friction Modifier Additives

The molecular-level function of model and commercial friction modifier additives in lubricants of the type used at the wet clutch interface in automatic transmissions has been studied using a surface forces apparatus (SFA) modified for oscillatory shear. The nanorheological properties of tetradecane with and without a model friction modifier additive (1-hexadecylamine) were examined in the boundary lubrication regime and compared to a fully-formulated automatic transmission fluid (ATF). 1-Hexadecylamine adsorbed as a single layer on the sliding surfaces, reduced the static frictional force and the limiting shear stress, and eliminated the stick–slip transition that exists in pure tetradecane. The ATF, which contains commercial-grade friction modifiers, showed nanorheological properties similar to those observed for tetradecane containing 0.1–0.2 wt% 1-hexadecylamine.     

Measurements of friction-induced surface strains in a steel/polymer contact

The paper describes an experimental study combined with analyses and numerical simulations of the surface strains developed in a metal–polymer contact under a variety of loading configurations. Specifically, a steel ball is caused to slide over a poly(methylmethacrylate) flat counterface under a fixed normal load where the imposed motions are small and consist of sliding and rotation and the combination of both. The surface strains have been measured directly using conventional strain gauges in two types of configurations specifically designed to monitor the strains for sliding and rotation. Calculations of frictional forces provide friction coefficients which are self-consistent and the computed ‘friction displacement loops’ correspond closely to those measured. In addition, the surface strain measurements provide a convenient and accurate insight into the stick–slip transitions in fretting contacts.     

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.     

A study of fiber-capstan friction. 2. Stick–slip phenomena

A study was made of the friction of nylon fiber (monofilament fishing line) rubbing against spinning aluminum and nylon capstans (cylinders), in the presence of various lubricants. Use of variable speed drives with appropriate gear reducers allowed variation of sliding speed over four orders of magnitude and monitoring of the entire Stribeck curves, from boundary through mixed to hydrodynamic sliding, for these systems. The effects on friction of seven system variables—cylinder material, sliding speed, system elasticity, input tension, lubricant viscosity, lubricant additives and cylinder surface roughness—were investigated. Special attention was given to the effects of elasticity on friction and the character of sliding. Factors which determine the critical speed where stick–slip sliding just appears or disappears are defined.     

Sliding friction at elastomer/glass contact: Influence of the wetting conditions and instability analysis

The aim of this paper is to simulate and give a better understanding of the behavior of rubber blade/windscreen contact: the friction behavior of the rubber/glass contact, the wiping quality and the characterization of friction instabilities are investigated. Therefore, the tribological prospect of a rough natural compound rubber sliding over a smooth glass in various wetting conditions is analyzed. This study is carried out using a modified elastohydrodynamic tribometer that allows us to investigate the rubber dynamical behavior under controlled conditions, thanks to the coupling of dedicated dynamical sensors and the contact visualization. Wetting conditions are varied as the impact of velocity and normal load is studied and a specific interest has been given to the transition between wet and dry regimes. Frictional instabilities are observed and correlated to the contact vibrations.     

Wear resistance of coating materials under the frictional heating conditions

The problem of the wear of an elastic coating due to a rigid body sliding over the coating surface and heating due to contact friction has been considered. The solution of the quasi-static problem has been constructed in the form of a series over eigenvalues. The area of unstable solutions of the problem, where the thermoelastic instability of a sliding contact takes place, has been determined in the dimensionless parameter space. The wear resistance of a coating has been studied for different kinds of materials depending on the following parameters: the relative sliding velocity of contact surfaces, the mode of the contact interaction of the friction surfaces, the coating thickness, etc. taking into account the temperature and stresses developing at the contact interface.     

Velocity dependent friction laws in contact mode atomic force microscopy

Friction forces in the tip–sample contact govern the dynamics of contact mode atomic force microscopy. In ambient conditions typical contact radii between tip and sample are in the order of a few nanometers. In order to account for the large interaction area the dynamics of contact mode atomic force microscope (AFM) is investigated under the assumption of a multi-asperity contact interface between tip and sample. Thus, the kinetic friction force between tip and sample is the product of the real contact area between both solids and the interfacial shear strength. The velocity strengthening of the lateral force is modeled assuming a logarithmic relationship between shear-strength and velocity. Numerical simulations of the system dynamics with this empirical model show the existence of two different regimes in contact mode AFM: steady sliding and stick–slip where the tip undergoes periodically stiction and kinetic friction. The state of the system depends on the scan velocity as well as on the velocity dependence of the interfacial friction force between tip and sample. Already small viscous damping contributions in the tip–sample contact are sufficient to suppress stick–slip oscillations.     

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.     

Stick–slip motion in the atomic force microscope

Measurements of atomic friction in the atomic force microscope frequently show periodic variations at the lattice spacing of the surface being scanned, which have the saw‐tooth wave form characteristic of “stick–slip” motion. Simple models of this behaviour have been proposed, in which the “dynamic element” of the system is provided by the elastic stiffness and inertia of the cantilever which supports the tip of the microscope, in its lateral, i.e., torsional mode of vibration. These models have been successful in predicting the observed motion, but only by assuming that the cantilever is heavily damped. However, the source of this damping in a highly elastic cantilever is not explained. To resolve the paradox, it is shown in this note that it is necessary to introduce the elastic stiffness of the contact into the model. The relationship between the contact stiffness, the cantilever stiffness and the amplitude of the periodic friction force is derived in order for stick–slip motion at lattice spacing to be achieved. This revised version was published online in July 2006 with corrections to the Cover Date.     

An integrated approach to evaluating the tribo-contact for coated cutting inserts

The orthogonal machining process when end turning medium carbon and austenitic stainless steels with cemented WC-Co tools coated with single-layer (TiC), two-layer (TiC/TiN), and three-layer (TiC/Al₂O₃/TiN) hard thin films was investigated. Extensive experimental investigations including the thermal, mechanical and tribological responses of the tribo-contact between the coating–substrate system and the chip, under different cutting conditions, were carried out. The study sheds light on the cutting forces, the interface temperatures and the tribo-contact conditions, including the friction energy dissipated at the tool–chip interface, the frictional heat flux conducting into either the chip or the insert, the mean coefficient of sliding friction and the contact loads exerted on the tool rake face. Finally, it was demonstrated how the intrinsic coating properties control the heat flux flowing into both components of such a closed tribo-system and the mechanical stresses on the contact area.     

Seizure mechanisms of wheel–rail contacts under lubricated conditions using a transient ball-on-disc test method

This study focuses on the transition from mild to severe wear in the wheel and rail contact. Such a transition has been observed at increased loading (normal load, sliding velocity, or bulk temperature) which can be compared to a change from a wheel thread–rail head contact to a wheel flange–rail gauge contact. This transition was experimentally studied using a transient test method of ball-on-disc type at different sliding velocities, contact pressures, and lubricants. It can be seen in the results that different seizure mechanisms are active for different sliding velocities. Also the amount of applied lubricant clearly affects the transition to seizure.     

Thermal Behaviour of a Slipping Wet Clutch Contact

Wet clutches are used in automatic transmissions to enable gear changes and also to reduce energy loss in the torque converter. These friction devices are susceptible to stick–slip effects, which result in the vehicle giving an unsteady ride. Stick–slip effects can be avoided by ensuring the wet clutch and lubricant combination produces a friction coefficient that increases with sliding speed. Although wet clutches have been studied throughout the industry for many decades, the mechanism of the generated friction is still not fully understood. It is known that, because of the fibrous nature and thus very large roughness of friction material, the overall contact between clutch plates actually consists of many small, independent, contact units, which are the sites of the generated friction. Some authors have suggested that a temperature rise due to friction either at these contact units or of the overall clutch plate may be important in controlling friction behaviour. In this study, the flash temperatures at the contact units formed in the wet clutch contact have been measured using an infrared camera. Three friction materials have been tested. It was found that measured flash temperature in a pure sliding system similar to that present in a slipping clutch remain well below 5 °C, and are therefore unlikely to play a major role in the observed friction-speed dependency of slipping wet clutches at speeds below 2 m/s.     

Stick–slip friction in dissimilar polymer pairs used in automobile interiors

The static and dynamic friction of dissimilar pairs of plastics used in automotive interiors was measured as a function of normal load, system stiffness, and surface roughness. Glass fiber filled polypropylene (FPP) was slid on polycarbonate (PC) and glass fiber filled styrene–maleic–anhydride copolymer (SMAC) in a single pass, unidirectional sliding test. The friction was characterized by the value of static coefficient of friction (COF) and the number of stick–slip cycles during sliding. It was found that the FPP/PC and FPP/SMAC pairs had fewer instances of stick slip than FPP/FPP, PC/PC, and SMAC/SMAC pairs except for one of the SMAC polymers. The surface texture which had the smallest average radius of peak curvature, had the lowest value of static COF. The decrease in the static COF of polypropylene (PP) caused by the addition of glass fiber was most likely caused by the increase in elastic modulus and hardness.     

Effect of the Abrasive Size on the Friction Effectiveness and Instability of Brake Friction Materials: A Case Study with Zircon

The friction-induced vibration triggered at the sliding interface between the gray iron disk and brake friction material was studied by changing the size of the zircon particles in the friction material. The friction tests were performed using a reduced brake dynamometer and the friction characteristics of the friction materials containing zircon particles with sizes of 3, 50, and 100 μm were analyzed. Our results show that the properties of the sliding surface were strongly affected by the entrenchment of the abrasive particles in the friction layers during sliding. The friction effectiveness was inversely proportional to the size of the abrasive, while friction instability was pronounced when smaller zircon particles were used. The smaller zircon particles produced larger plateaus on the sliding surface with low contact stiffness. However, the contact plateaus with the low contact stiffness showed higher amplitudes of the friction oscillations, suggesting a surface with low stiffness also can produce high propensity of friction instability during sliding. Based on the friction stability diagram and surface properties, such as contact stiffness and surface roughness, it was suggested that the static coefficient of friction, which was changed as a function of dwell time, was crucial to understand the cause of friction-induced force oscillations and propensity of friction instability of brake friction materials.     

Influence of temperature on the friction performance of gear oils in rolling–sliding and pure sliding contacts

The friction behaviour of five different gear oils in rolling–sliding and pure sliding contacts and how temperature influences their friction properties were investigated. It is found that increasing temperature decreases boundary friction with gear oils that contain friction modifiers while not for other gear oils, at all contact pressures investigated. In mixed lubrication region, temperature decreases friction at low contact pressures while increases friction at high contact pressures. The effect of slide–roll ratio on friction is significant in boundary lubrication region especially at higher temperature while less significant in mixed lubrication region at both low and high temperatures. The ranking of gear oils for friction in boundary and mixed lubrication regimes is similar both in rolling–sliding and pure sliding contacts, regardless of temperature. Copyright © 2013 John Wiley & Sons, Ltd.