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Machine elements and mechanical components have often surfaces with anisotropic roughness, which may result from the machining processes, e.g. grinding, or from wear. Hence, it is important to understand how surface roughness anisotropy affects the contact mechanics properties, such as friction and the interface separation, which is important for lubricated contacts. Here we extend a multiscale mean-field model to the lubricated contact between a soft (e.g. rubber) elastic solid and a rigid countersurface. We consider surfaces with anisotropic surface roughness, and discuss how the fluid flow factors and friction factors depend on the roughness power spectral density, as well as on the location of roughness on the interacting solids. Finally, we present an experimental study of the lubricated sliding contact between a nitrile butadiene rubber O-ring and steel surfaces with different kinds of isotropic and anisotropic surface roughness. The good quantitative comparison between the experimental results and the theory predictions suggests that the multiscale lubrication mechanisms are accurately captured by the theory. [ABSTRACT FROM AUTHOR]

Rubber friction depends on the surface roughness of the counter surface, e.g. a road surface. We have measured the friction for two rubber compounds sliding on randomly rough surfaces produced by 3D-printing. The surfaces had different macro-roughness and the friction experiments were performed over six decades in speeds, from 10 −6 m s −1 to 1 m s −1 , on both dry and lubricated substrates. The viscoelastic modulus master curves of the rubber compounds, and their large-strain effective modulus were obtained from dynamic mechanical analysis (DMA). The measured friction coefficients were compared to the calculations of rubber friction using Persson's contact mechanics theory. The role of the macro-level roughness is discussed. [ABSTRACT FROM AUTHOR]

This paper presents a new contribution to model tire/road friction. It proposes a calculation of hysteretic component of the rubber friction based on a simplified viscoelastic-rough contact model. The mechanical behavior of rubber elements is represented through “Kelvin–Voigt” model. The hysteretic friction force is calculated from the dissymmetry of the contact pressure due to the rubber viscoelastic behavior in given operating conditions. To validate the model, a series of texture and friction data are measured using two machines respectively, the Circular Track Meter and the Dynamic Friction Tester, through various test surfaces and compared to model results. The Circular Track Meter is a complementary measurement device to the Dynamic Friction Tester as texture measurements can be done on the exact same physical surface tracks where the Dynamic Friction Tester measuring pads have been in contact with the test surface. The obtained results show a very encouraging correlation between the model predictions to the experiment results, thus opening a promising and attractive way to predict tire–road friction. Some indications are given at the end to improve the modeling and to extend to it to a whole tire/road friction model. [ABSTRACT FROM AUTHOR]

This article presents the development of a method for manufacturing durable countersurfaces for testing the friction of rubber samples in a laboratory. The surface sample was designed to replicate the surface roughness characteristics of a predefined road surface. Four different types of samples were manufactured: two bitumen bound stone mastic asphalt samples, one concrete bound sample, and one epoxy bound sample. In addition, the surfaces were sandblasted to remove binder from the top of the surface. The similarity in surface roughness between the predefined target road surface and the laboratory surface samples was evaluated using optical 3D surface roughness measurements and surface roughness power spectrums. The durability of the surfaces was investigated by repeated dry rubber sliding friction tests. It was found that out of the studied surface samples the best one to both replicate the surface characteristics of the target surface and to produce a durable surface in terms of wear was the one using epoxy resin as a binder. By replacing bitumen with epoxy, more friction tests could be run without significant changes in the surface topography or rubber friction. In addition a reasonable correlation between the surface roughness power spectrums and the friction results was found, which supports the use of roughness power spectrums as a roughness metric for road surfaces in the context of rubber friction. [ABSTRACT FROM AUTHOR]

Abstract: The constant need to improve road safety and the driving performance of winter tires imposes challenging requirements on tire development, especially when considering that one of the key steps, the evaluation of the product itself, is limited by the seasonal changes. This explains why the industry is moving towards indoor testing. An example is the ice performance of tires, which is very frequently characterized indoors or even in the laboratory. The quality of the data thus obtained, however, depends strongly on how well the artificial surface approximates real life. Today, several methods are available to assure the consistency of the test data by controlling the creation and the break-in of the ice track used for testing. At the same time what happens at the surface during its preparation and how this affects the physics of the rubber–ice contact is not yet fully understood. To build this understanding, in the work reported here we were focusing on one of the simplest experiments available. Series of linear friction tests were performed in the laboratory with strict environmental and procedural control. The quasi-real-time microstructural monitoring of the ice surface, together with the friction measurements, allowed the main physical processes that were ongoing during the break-in of the fresh or aged ice surfaces and led to changes in the contact conditions to be identified. The results indicated that there was no large-scale melting of the ice when the rubber was sliding over it. The deformation of the ice and its strong dependency on temperature, on the other hand, may play a significant role in ice friction. [Copyright &y& Elsevier]

Abstract: We study the role of rubber friction in tire traction with special emphasis on the load and velocity dependence of the friction coefficient. In the first part, we present some basic concepts of contact mechanics of slipping tires and analyze the influence of energy dissipation due to tread deformation on the friction force. Then, we apply a recently developed model of hysteresis and adhesion friction of rubber on self-affine road surfaces for estimating the load dependence of the kinetic friction coefficient in the contact area of slipping tires. In this context the impact of track morphology (sharp or blunt) on the kinetic friction characteristics is demonstrated. Finally, using the brush model for slipping tires we discuss consequences of the load dependence of the kinetic friction coefficient with respect to the overall tire friction and slip characteristics. We show that due to the presence of a load dependence of the local rubber-road friction coefficient the tread contact patch is globally never entirely in a fully sliding situation. The presented results yield a contribution to an improved physical understanding of the dynamic frictional contact of tires with road tracks during cornering and braking, especially in the case of cars equipped with Anti-Blocking Systems (ABSs). [Copyright &y& Elsevier]

Abstract: The paper presents an analytical multi-scale approach for the modelling of rough surfaces in regard to rubber friction properties. In addition to a fractal description of the micrometer range within the length-scale interval [5–100μm], the macrotexture is considered via the introduction of a second scaling regime for the contact interval [300–1000μm]. As a result, the hysteresis friction coefficient is decomposed in two distinct contributions. The macrotexture mostly contributes to hysteresis friction, while the microtexture is found to be determining for the amount of contact patches, e.g. the real area of contact. Experimental friction tests carried out for carbon black and silica filled S-SBR 5025 on rough surfaces and road pavement under wet conditions point out the relevance of a two-scaling-ranges approach for the modelling of hysteresis friction. Finally, predictions of the load dependent hysteresis friction coefficient are formulated for rough surfaces and give a deeper insight into the physical mechanisms involved in rubber friction properties. [Copyright &y& Elsevier]

The authors propose a new philosophy and theory for rubber friction and wear that are significantly different from the existing classical theories. Several distinctive features of rubber friction such as the exceedingly high friction coefficient and the intense stick-slip motion during frictional sliding all result from the sticky surface behavior exhibited by a cross-linked rubber, where there is a meniscus force brought about at the interface between the rubber and the rigid surface. The total friction coefficient μ all incorporates three factors including an adhesion term μ adh , a deformation term μ def and a crack formation term μ crac. This generates an equation μ all = μ adh + μ def + μ crac ≒ K 1 ηv [ 1 + K 2 ( tanδ 2 ) + 2 K ε c) E - 7 / 6 W 1 / 6 ] where η is the viscosity of the uncross-linked phase, E the modulus of the cross-linked rubber, v sliding velocity, c crack length, W normal load, K 1 , K 2 , K ε are all coefficients whose characteristics also govern rubber wear. The adhesion term is the most dominant factor during rubber friction, which roughly contributes about 70–80% of the total friction coefficient according to a very rough estimation. The close relationships between the observed stick-slip motion, abrasion pattern formation and wear have been verified experimentally. The abrasion pattern is initiated by the high frequency vibration and the steady abrasion pattern together with steady wear is promoted by the stick-slip motion. Steady wear rate V ˙ could be estimated theoretically as a function of the steady abrasion pattern distance D ab using an equation V ˙ = k'D ab 3, which indicates that many of the characteristics observed in rubber wear are also fundamentally governed by the intense stick-slip motion induced by the sticky rubber surface. This figure shows schematically the meniscus formation by the uncross-linked phase and the Van der Waals contact by the cross-linked phase in the interface between the cross-linked rubber and solid, where the meniscus force generates the most significant adhesion friction coefficient for rubber friction together with high surface stickiness and violent stick-slip motion. Image 1 • A new theory of rubber friction, one that differs significantly from the existing classical theories, is proposed. • The adhesive component dominates the friction coefficient of rubber, comprising almost 70–80% of its magnitude. • The close relationship between stick-slip motion, abrasion pattern formation, and wear were validated by both experiments and theory. [ABSTRACT FROM AUTHOR]