Rubber friction on ice: investigation of frictional heating and melt water film thickness
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Friction on ice is important for many different fields such as winter sports and vehicle traction. In vehicle handling, maximising the friction coefficient between tyres and the ice surface is key to safety. The friction coefficient between tyre rubber and ice has been observed to be as high as unity at low temperatures and as little as 0.05 close to the ice melting temperature. The observed low friction is due to thin water films generated through frictional heating. Little is known about the formation and behaviour of this fluid film and its thickness has been difficult to measure. Previous attempts included techniques such as capacitance, conductivity and fluorescence spectroscopy, however results have been inconsistent. The primary aim of this study was to develop a measurement technique for this lubricating layer, establishing its thickness and the conditions under which its presence results in low friction. This was done by designing a micro-scale linear tribometer (microtribometer) to measure the friction coefficient (μ) on ice under a microscope. Clear ice, and ice with fluorescent particles of various sizes were created. The two ice types were then joined and tested on. During a friction test on the microtribometer, the particles displaced in the direction of sliding due to melt water presence. Images of the ice surface were taken before and after a friction measurement was made, and the amount of particle movement was assessed. The size and displacement of particles were correlated to the range of melt water thickness for a given sliding condition. This study is the first direct measurement method for the melt water layer on ice. Frictional heating is generated through the physical contact of rubber and ice asperities. By conducting friction measurements with rubbers of varying shear modulus (G*) and surface roughness (Ra) on both the microtribometer and on our large–scale tribometer, FRIMA, analysis of the contact can be made over a range of length scales. Further insight into the interfacial effects was provided by surface roughness measurements of both ice and rubber before and ice after microtribometer tests and in FRIMA. Finally, the use of differential interference contrast in reflected light microscopy with ice friction measurements provided visual evidence of the melt water presence. It was found that the melt water thickness based on particle displacement was between 0.1 to 2μm, for a velocity of 4.3 ⇥ 10−4 ms−1 and 0.5MPa nominal load, between –6#C to 0#C. A decreasing film thickness was found with decreasing temperature and the friction coefficient increased with decreasing temperature. At higher temperatures, the differences between the rubbers on both FRIMA and the microtribometer were found to be negligible because of the lubricating layer. Increasing the rubber surface roughness was found to increase at high and decrease friction significantly at low temperatures. These phenomena can be explained by the decreasing amount of solid-solid contact. A simple analytical approach to interpret the results suggests that the rubber compresses significantly and the surface roughness is smoothened upon contact with ice. Furthermore, simple hydrodynamic shear calculations show that pure liquid shear is insufficient to account for the measured low μ values close to melting temperatures. The combined effect of the increasing melt water presence reducing the real contact area and the significant deformation of the rubber asperities failing to compensate for this reduced contact area can account for the results. It is anticipated that these results provide direction for finding ways to improve solid-solid contact between rubber and ice, as there is some viscoelastic dissipation even at conditions close to the melting temperature. Some topics for example are: improving the thermal conductivity of tyre compounds or further investigating the surface roughness of rubber and ice separately and with the thin liquid film in between.