Experimental Investigations about Adhesion Component of Friction Coefficient Dependence on Road Roughness, Contact Pressure, Slide Velocity and Dry/Wet Conditions

The results of an experimental activity carried out with the aim to investigate on the adhesive behaviour of visco-elastic materials in sliding contact with road asperities are presented. Experiments are carried out using a prototype of pin on disk machine in which pin is constituted by a specimen of rubber coming from a commercial tire, while different disks are realized in glass, marble, steel and in abrasive paper of different roughness. Tests are performed in both dry and wet conditions. Roughness of the test surfaces is evaluated by a rugosimeter, while pressure is evaluated, off-line, analysing the extension of the contact patch left by the pin on a sheet of graph paper under known applied loads. Slide velocity is imposed by an inverter controlled motor driving the disk. Basing on well known theoretical hypotheses, adhesive component of friction coefficient is estimated making the specimens slide on surfaces characterized by low values of macro-roughness, in order to underline the differences in rubber behaviour respect to micro-roughness surface variations. The results confirmed adhesion dependence on pressure and sliding velocity in both cases of smooth surfaces, where the main friction mechanism is the adhesive one, and of rough surfaces, where the main friction mechanism is the hysteretic one. Analysing various surfaces roughness it is possible to notice a maximized adhesive contribution on flat surfaces; it reduces with increasing roughness, while hysteretic friction comes over instead of it because of asperities penetration into rubber sliding surface. Moreover in the case of rough surfaces the separation between static and dynamic friction coefficient is evident and the static coefficient is greater than the dynamic one. On the other hand in case of smooth surface the absence of indentation phenomena doesn’t allow to recognize, in the measured force time history, the “classical” peak usually associated to the static friction coefficient. Dry and wet tests performed on different micro-roughness profiles highlighted that friction coefficient in dry conditions is greater on smoother surfaces, while an opposite tendency is shown in wet condition, when asperities are greater enough to break the thin water layer, providing a certain degree of indentation. A proposal for a methodology able to estimate the only adhesive friction component, developed thanks to wet contact tests, is expressed in the end of the paper.