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Soft-hard matter friction is a long-standing tribology problem that remains unclarified, requiring engineers to empirically predict the wear life. To clarify this issue, this study examines the transient running-in regime of rubber friction on a hard rough substrate and models the temporal wear progression using the spectrum curves of surface roughness for both materials. Performing a series of friction tests and three-dimensional surface-height measurements, the time-dependent behaviours of the power spectral densities (PSDs) are divided into two phases, namely the initial non-steady and long-term steady phases. The detailed spectral analyses of worn rubber surfaces in the initial phase lead to a blended PSD function between self-affine and K-correlation surface models, consisting of one variable (the Hurst exponent) that is saturated by the substrate self-affinity. Supported by the Greenwood--Williamson theory concerning rough contact mechanics, the volumetric estimate with the blended PSD function is used to assess the volume rate of wear debris in the steady phase, which is validated experimentally. These findings not only improve the wear predictions of soft materials from the initial measurements of worn surfaces but also help clarify the constrained multiscale mechanism of wear. [ABSTRACT FROM AUTHOR]

Modeling the real contact area plays a key role in every tribological process, such as friction, adhesion, and wear. Contact between two solids does not necessarily occur everywhere within the apparent contact area. Considering the multiscale nature of roughness, Persson proposed a theory of contact mechanics for a soft and smooth solid in contact with a rigid rough surface. In this theory, he assumed that the vertical displacement on the soft surface could be approximated by the height profile of the substrate surface. Although this assumption gives an accurate pressure distribution at the interface for complete contact, when no gap exists between two surfaces, it results in an overestimation of elastic energy stored in the material for partial contact, which typically occurs in many practical applications. This issue was later addressed by Persson by including a correction factor obtained from the comparison of the theoretical results with molecular dynamics simulation. This paper proposes a different approach to correct the overestimation of vertical displacement in Persson's contact theory for rough surfaces with self-affine fractal properties. The results are compared with the correction factor proposed by Persson. The main advantage of the proposed method is that it uses physical parameters such as the surface roughness characteristics, material properties, sliding velocity, and normal load to correct the model. This method is also implemented in the theory of rubber friction. The results of the corrected friction model are compared with experiments. The results confirm that the modified model predicts the friction coefficient as a function of sliding velocity more accurately than the original model. [ABSTRACT FROM AUTHOR]

Here, the authors propose an energy method to obtain the friction coefficient of a rubber block sliding on self-affine substrates via simulation. Due to the multi-scale property, the rough surfaces are modelled with four harmonic rigid frictionless roads. As the rubber block is a viscoelastic solid, energy dissipation in the contact area results in friction. The authors propose a new simulation method to obtain friction coefficient through energy dissipation. With different loads and sliding velocities, the change law of friction coefficient is calculated. The authors design a test instrument to verify the friction coefficients of rubber on some certain surface. The findings of this research constitute a theoretical basis for safe tire design and tire grip improvements.

Here, the authors propose an energy method to obtain the friction coefficient of a rubber block sliding on self-affine substrates via simulation. Due to the multi-scale property, the rough surfaces are modelled with four harmonic rigid frictionless roads. As the rubber block is a viscoelastic solid, energy dissipation in the contact area results in friction. The authors propose a new simulation method to obtain friction coefficient through energy dissipation. With different loads and sliding velocities, the change law of friction coefficient is calculated. The authors design a test instrument to verify the friction coefficients of rubber on some certain surface. The findings of this research constitute a theoretical basis for safe tire design and tire grip improvements. [ABSTRACT FROM AUTHOR]

Tribological behavior of Styrene-Butadiene Rubber (SBR) compound reinforced by silica and carbon-black fillers on two different asphalt-like surfaces is studied using theoretical and experimental approaches. Linear sliding friction and wear tests were performed on two different roofing asphalt papers. The surface roughness power spectra of asphalts and the master curves of the large-strain viscoelastic modulus of SBR compound were used to model the contact area and friction coefficient theoretically. The theoretical results are in good agreement with experiment providing the tribochemical degradation is not the dominant wear mechanism. When the formation of a tribo-modified layer prevails the mechanical abrasion, the wear rate, in terms of mass loss, does not necessarily increase with the increase of frictional work as expected theoretically. • Tribological behavior of Styrene-Butadiene Rubber on asphalt papers is studied. • Theoretical approaches are used to calculate the contact area and rubber friction. • Rubber friction and wear rate are obtained experimentally and compared with theory. • A tribo-modified layer is formed on the rubber surface at high sliding velocities. • The tribo-modified layer significantly affects the tribological behavior of rubber. [ABSTRACT FROM AUTHOR]

The coefficient of friction due to bulk viscoelastic losses corresponding to multiscale roughness can be computed with Persson's theory. In the search for a more complete understanding of the parametric dependence of the friction coefficient, we show asymptotic results at low or large speed for a generalized Maxwell viscoelastic material, or for a material showing power law storage and loss factors at low frequencies. The ascending branch of friction coefficient at low speeds highly depends on the rms slope of the surface roughness (and hence on the large wave vector cutoff), and on the ratio of imaginary and absolute value of the modulus at the corresponding frequency, as noticed earlier by Popov. However, the precise multiplicative coefficient in this simplified equation depends in general on the form of the viscoelastic modulus. Vice versa, the descending (unstable) branch at high speed mainly on the amplitude of roughness, and this has apparently not been noticed before. Hence, for very broad spectrum of roughness, friction would remain high for quite few decades in sliding velocity. Unfortunately, friction coefficient does not depend on viscoelastic losses only, and moreover there are great uncertainties in the choice of the large wave vector cutoff, which affect friction coefficient by orders of magnitudes, so at present these theories do not have much predictive capability. [ABSTRACT FROM AUTHOR]

We discuss on the rough contact mechanics in presence of a graded viscoelastic solid. In particular, we derive the effective surface admittance of a stepwise or continuously-graded half-space, which is then embedded in a Fourier-based residuals molecular dynamics formulation of the contact mechanics. The viscoelastic multilayer theory, which is of general applicability, is then applied to discuss on the role of small-scale wavelengths on polymer friction and contact area. We demonstrate that the rough contact mechanics exhibits effective interface properties which converge to asymptotes upon increase of the small-scale roughness content, when a more realistic rheology of the confinement is taken into account. [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]

One of the most important factors that affect tire friction is surface roughness, which determines the size of the real contact area, real pressure distribution on the contact interface, and scales of mechanical engagement between viscoelastic rubber and a rough substrate. The need to predict coefficient of friction (COF) for rubber on rough surfaces for applications such as traction of tires on the road surfaces led to some physical models such as Heinrich-Kluppel’s model. The current study examines the applicability of the Heinrich-Kluppel model, using different viscoelastic representations, in numerical simulations of COF for rubber, and its agreement with the experimental results. For this purpose, roughness characteristics of the surfaces and viscoelastic properties of rubber were measured by fractal analysis and dynamic-mechanical-thermal analysis (DMTA), respectively. These data were employed in the numerical code to simulate COF for a rubber sample. The model was also modified by replacing the Zener viscoelastic representation in the original model with the generalized Maxwell viscoelastic representation. On the other hand, COF for rubber was measured on the same rough surface (different sand-papers) by an in-house friction tester, and results were compared with the numerical results. It was shown that computer simulation could predict the load and speed dependence of rubber friction very well. The application of the generalized Maxwell model improved agreement between the numerical and experimental results for high sliding speeds where the Zener viscoelastic model failed to predict the right trend in variation of COF with speed. This speed range was matched with the sliding velocities in the footprint of tire under rolling conditions.

Fluid leakage out of mechanical equipment such as gearboxes, hydraulic systems, or fuel tanks could cause serious problems and thus should be avoided. Seals are extremely useful devices for preventing such fluid leakages. We have developed a theoretical approach for calculation of the leak rate of stationary rubber seals and the friction force for dynamic seals. The theory is based on a recently developed theory of contact mechanics, which we briefly review. To test the theory, we have performed both simple model experiments and experiments on engineering seal systems. We have found good agreement between the calculated and measured results, and hence our theory has the potential to improve the future design of efficient seals. We briefly review the processes that determine rubber friction on lubricated smooth and rough substrate surfaces. We present experimental friction results for lubricated surfaces, obtained using a simple Leonardo da Vinci setup. The data is analyzed using the Persson rubber friction and contact mechanics theory. [ABSTRACT FROM AUTHOR]

We discuss the influence of geometrical and rheological non-linearities on the prediction of rubber friction and true contact area for rough sliding interactions. In particular, we compare the results of a linearly-viscoelastic linear-contact model, formulated in the Fourier space, with those obtained from non-linear finite element calculations. A sinusoidal rigid profile indenting a rubber block is here considered for simplicity, whereas the effects of non-linearity are evaluated by varying the aspect ratio, loading conditions and sliding speed of the contact interface. It is found that accurate friction predictions can be obtained through the linear viscoelastic model, provided that the roughness under investigation features moderate values of root mean square slopes, whereas non-linear finite element computations should be adopted for large root mean square slopes. [ABSTRACT FROM AUTHOR]

We present the numerical results for the viscoelastic and adhesive contribution to rubber friction for a tread rubber sliding on a hard solid with a randomly rough surface. In particular, the effect of the high- and low-frequency roughness power spectrum cut-off is investigated. The numerical results are then compared to the predictions of an analytical theory of rubber friction. We show that the friction coefficient for large load is given exactly by the theory while some difference between theory and simulations occur for small loads, due to a finite sample-size effects, whereas the contact area is almost unaffected by the low frequency cut-off. Finally, the role of a finite rubber thickness on viscoelastic friction and contact area is introduced and critically discussed. Interestingly, we show that classical rough contact mechanics scaling rules do not apply for this case. [ABSTRACT FROM AUTHOR]

In this study, the work of Mofidi et al., 2008 [6] is cited and the focus is put on the finding that the friction reported for lubricated apparently smooth surfaces (sealing applications) may be dominated by micro- or surface roughness generated hysteresis. Contrary to the very high coefficients of friction measured, Mofidi et al. did not analyze their test results and test configuration in depth and did not compare their results to experimental observations taken from the literature. In this study, the author makes an attempt to meet these needs and reanalyze/reinterpret the results of Mofidi et al. [ABSTRACT FROM AUTHOR]

The most important contribution to rubber friction on rough surfaces is the so-called hysteresis. The excitation of the viscoelastic material is induced by the surface asperities. Hysteresis friction originates from the internal energy dissipation. Since the roughness occurs over a wide range of length scales, in this work a multiscale FEM approach is developed to solve the problem with acceptable computational costs. A split of the surface in macroscopic and microscopic parts is carried out via a decomposition of the power spectral density. The coupling between the scales is performed by passing of homogenized, velocity- and pressure-dependent coefficients of friction. The values are gained from frictionless calculations on the micro scale. The proposed multiscale FEM approach is analyzed and proven on an artificial sinusoidal surface. Finally, the approach is applied to a measured rough surface profile showing good agreement with experimental data for different rubber materials. [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]