Rubber friction on ice and snow surfaces
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The friction of rubber on ice and snow surfaces is complex. Deeper scientific understanding is important for optimising performance of tyres in winter. Rubber, ice and snow systems exhibit frictional behaviour which depends on their material properties. The viscoelastic nature of rubber results in a higher real contact area compared to most other solids. At temperatures close or below the glass transition temperature, the frictional behaviour of rubber changes and its hardness increases. Thus, the real area of contact decreases, while the dissipation in the bulk of the rubber increases. Sliding of rubber on ice or snow leads to a temperature increase at the interface because of frictional heating, this can cause the surface to melt which decreases friction significantly. In this study we measured the friction of rubber on ice and snow and related the behaviour to mechanisms that occur. Key parameters affecting friction were examined and quantified. For this work a cold room and a new linear tribometer were specially designed and constructed. The rubber samples were made from various compounds and had different geometries. Typically they were the size of a “tread block element”. The geometries were chosen systematically to investigate the effects of surface area, sharp/rounded edges and sipes (small slits in the tread block that are used on snow tyres). A significant part of the work was developing consistent and reproducible ice and snow surfaces. New protocols were devised for these. The ice surfaces were made of de-ionised water, tap water and de-ionised water with salt. For the snow surface production: artificial snow was made and then compacted in a specially manufactured press, resulting in hard packed snow tracks for testing. Static and dynamic friction were investigated. Both were affected by speed, load, temperature and ice composition. The dynamic friction behaviour on ice was explained in terms of melt-water formation and the real area of contact of the rubber. The static friction was significantly affected by the losses inside the rubber bulk, the adhesive forces at the interface, and the time of stationary contact before the test. The investigation of rubber sliding on snow showed some similarities with sliding on ice; the surface of the rubber block slides over snow particles resulting in similar mechanisms as are seen on ice. However with snow there can also be a “ploughing” effect, where snow is cut by the leading edge of a sharp tread block. This effect contributes to friction. Experiments were made with simple rounded edged samples to avoid ploughing; the results showed the same trends as seen on ice, i.e. lower friction with increased speed, load and temperature. Investigations of siped tread blocks showed the same friction at low speeds as tread blocks without sipes. At higher speeds siped blocks exhibited less, or no, decrease in friction; more sipes gave less friction decrease. Our industrial collaborator, Michelin, made vehicle tests on snow using whole tyres with similar tread blocks. The trends they found were identical to our tests despite the dynamics of the system being more complex. This indicates how powerful the approach of using simple systematic experiments is for generating deeper understanding of the processes involved in sliding on ice and snow.