Tactile friction and perception of UV-curable coatings and their relation to physical surface parameters and contact mechanic simulation

In this paper, the results of new investigations on the relationship between physical surface parameters of polymer-based coatings, skin hydration levels and tactile friction are presented. For this purpose, the chemical composition of the investigated coatings was varied on the basis of the curing and binding agent and also with respect to the addition of selected fillers. This allowed the adjustment of various surface parameters such as hardness, surface wettability and roughness over a wide range. Tribological tests were carried out in which the coefficient of friction between the finger pad and the respective coating was measured for different skin hydration levels. A specially developed setup was used that minimized changing skin hydration levels during the experiments that would impede reproducible friction measurements. While for dry skin the perceived friction correlates well to the coefficient of friction and surface hardness, this is not the case for moist finger skin. The results are explained under the assumption of adhesion dominated friction and the application of the Hertz contact theory. This finding was further investigated through contact mechanic simulations using the finite element method (FEM). To this end, the contact area formation dependence on the reaction force was studied for surfaces of different hardness and skin with different mechanical properties reflecting various hydration levels. The results qualitatively confirmed the experimental findings and the analysis based on the Hertz contact theory.