Rubber friction on road surfaces: Experiment and theory for low sliding speeds B. Lorenz, Y. R. Oh, S. K. Nam, S. H. Jeon, and B. N. J. Persson Citation: The Journal of Chemical Physics 142, 194701 (2015); doi: 10.1063/1.4919221 View online: http://dx.doi.org/10.1063/1.4919221 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nanoscale wear and kinetic friction between atomically smooth surfaces sliding at high speeds Appl. Phys. Lett. 106, 081604 (2015); 10.1063/1.4913465 Effect of fluorocarbon self-assembled monolayer films on sidewall adhesion and friction of surface micromachines with impacting and sliding contact interfaces J. Appl. Phys. 113, 224505 (2013); 10.1063/1.4808099 Evaluation of sliding friction and contact mechanics of elastomers based on dynamic-mechanical analysis J. Chem. 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The road surface topogra- phies are measured using a stylus instrument and atomic force microscopy, and the surface roughness power spectra are calculated. The rubber viscoelastic modulus mastercurves are obtained from dynamic mechanical analysis measurements and the large-strain effective modulus is obtained from strain sweep data. The rubber friction is measured at different temperatures and sliding velocities, and is compared to the calculated data obtained using the Persson contact mechanics theory. We conclude that in addition to the viscoelastic deformations of the rubber surface by the road asperities, there is an important contribution to the rubber friction from shear processes in the area of contact. The analysis shows that the latter contribution may arise from rubber molecules (or patches of rubber) undergoing bonding-stretching-debonding cycles as discussed in a classic paper by Schallamach. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4919221] I. INTRODUCTION contributions to the friction from the area of real contact may result from wear processes (involving bond-breaking), or the Rubber friction is a topic of great practical importance, interaction between hard filler particles (usually carbon or e.g., for the tire-road interaction,1–25 or for the friction between silica particles) at the rubber surface with the countersurface. the rubber stopper and the barrel in syringes.26,27 There are This interaction will result in wear processes where the hard several different contributions to rubber friction, the relative filler particles scratch the countersurface (usually resulting in importance of which depends on the rubber compound and polishing of the countersurface19). countersurface properties. For hard substrates, such as road The contribution from the area of real contact to rubber surfaces, a contribution to rubber friction arises from the time- friction depends sensitively on contamination particles and dependent viscoelastic deformations of the rubber by the sub- fluids.33 Thus, on wet road surfaces, at high enough sliding (or strate asperities. That is, during sliding an asperity contact re- rolling) speed, the surfaces in the apparent contact regions will gion with linear size d will deform the rubber at a characteristic be separated by a thin fluid film, in which case the viscoelastic frequency ! ∼ v=d, where v is the sliding speed. Since real deformations of the rubber give the most important contribu- surfaces have roughness over many decades in length scales, tion to the friction (see Fig.2). Similarly, if the rubber surface there will be a wide band of perturbing frequencies, all of is contaminated by dust particles, the interaction between the which contribute to the viscoelastic rubber friction. In addition, dust particles and the countersurface may give a weakly veloc- there will be a contribution to the friction from shearing the ity independent contribution to the rubber friction coefficient area of real contact. (typically of order ∼0:1). The contribution from the area of real contact can have Rubber friction is a complex topic and any theory for several different origins. If a thin (say nanometer) confined rubber friction should first be tested for the most simple situ- fluid film, e.g., oil or wear products from the tire,orthe ation. In this paper, we will present experimental results for contamination film which prevails on almost all natural sur- the rubber friction for several rubber tread compounds and faces, exists in the contact region then shearing this film will road surfaces. We consider only very small sliding speeds, generate a frictional shear stress which will contribute to the from ∼1 µm=s to ∼1 mm=s. In this case, frictional heating rubber friction. We note that confined fluid films of nanometer results in negligible temperature increase which is important thickness may have very different rheological properties than as the viscoelastic properties of rubber materials are extremely the corresponding bulk fluids.28–30 For relative clean surfaces, sensitive to the temperature. We also present results for several direct bonding (even if mainly of the weak van der Waals type) different background temperatures. The experimental results of rubber molecules to the substrate may result in bonding- are analyzed using the Persson contact mechanics theory, and stretching-debonding cycles which result in energy dissipation we conclude that there is an important contribution to the fric- (see Fig.1). This mechanism was first considered in a pioneer- tion from the area of contact. The analysis shows that the latter ing work by Schallamach31 and has recently been extended contribution may arise from bonding-stretching-debonding cy- to real rubber materials by Persson and Volokitin.32 Other cles. In this paper, we first present the results of the road sur- a)www.MultiscaleConsulting.com. face topographies and power spectra (Sec.II). In Sec. III, the 0021-9606/2015/142(19)/194701/12/$30.00 142, 194701-1 © 2015 AIP Publishing LLC This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 134.94.122.86 On: Fri, 04 Dec 2015 06:11:09 194701-2 Lorenz et al. J. Chem. Phys. 142, 194701 (2015) FIG. 1. The classical description of a polymer chain at the rubber-block countersurface interface. During lateral motion of the rubber block, the chain stretches, detaches, relaxes, and reattaches to the surface to repeat the cycle.31 The picture is schematic and in reality no detachment in the vertical direction is expected, but only a rearrangement of molecule segments (in nanometer- sized domains) parallel to the surface from pinned (commensurate-like) do- mains to depinned (incommensurate-like) domains (see Ref. 32). rubber viscoelastic modulus mastercurves are obtained from FIG. 3. The surface roughness top power spectra CT as a function of the DMA measurements, and the large-strain effective modulus is wavevector q (log10–log10 scale) for the asphalt road surfaces a, b, and c, and a sandpaper surface. The dashed line has the slope −2 1+ H = −3:6 obtained from strain sweep data. SectionIV presents a short re- ( ) corresponding to the Hurst exponent H = 0:8 and fractal dimension Df view of the theory used for analyzing the experimental friction = 3− H = 2:2 based on 1D stylus line scan data. data. In Sec.V, we present experimental rubber friction data obtained at different temperatures and velocities, and compare the measured data to the theoretical prediction. SectionVI con- Fig.3 shows the surface roughness top power spectra tains a discussion and Sec. VII the summary and conclusion. CT as a function of the wavevector q (log10–log10 scale) for the asphalt road surfaces a, b, and c, and for the sandpaper surface. The measurements were performed on the unused II. SURFACE TOPOGRAPHIES AND SURFACE road and sandpaper surfaces, i.e., on surface areas not covered ROUGHNESS POWER SPECTRA by the sliding track of the rubber block. The surfaces appear self-affine fractal with the Hurst exponent H ≈ 0:8 (or fractal In this study, we use three asphalt road surfaces, denoted dimension D = 3 − H ≈ 2:2) which is typical for many sur- a, b, and c in what follows, and a sandpaper surface. We f faces.34 Note that the root-mean-square (rms) roughness ampli- have observed that the asphalt road surfaces start to wear and tude of the sandpaper surface is smaller than for the asphalt plastically deform strongly when a rubber block is sliding on road surfaces, but the surface roughness power spectrum at them at temperatures above 40 ◦C, and therefore the highest large wavevectors is largest for the sandpaper surface. This fact temperature used below was T ≈ 40 ◦C. manifests itself in the cumulative rms roughness and rms slope We have measured the surface topography of all surfaces curves to be presented below (see Fig.5).