Some fundamental aspects of static friction

The present report summarizes the results of an experimental study of static friction. Stick-slip type vibration was produced in a pin on disk friction system in order to determine the effect of various parameters on the static friction force. In the first part of the study it was found that two distinct types of sliding behaviour could be produced by controlling the preparation of the surfaces prior to sliding. If the surfaces were freshly polished and kept relatively free of organic contaminants then the sliding behaviour was characterized by high friction coefficients and extensive surface damage. Under lubricated conditions however, stick-slip vibration was possible at low driven surface velocities. These results show the importance of surface history with regard to friction phenomena and aid in the interpretation of subsequent experimental results. By measuring the electrical resistance of the slider-disk interface it was possible to monitor changes in the area of contact during stick-slip. It was observed that there was metallic contact throughout the complete cycle although the amount of metallic contact varied. The stick portion of the cycle was found to be a region of increasing metallic contact. The amount of metallic contact area at slip was a function of the rate at which the tangential force was applied. During slip, the area in metallic contact decreased rapidly and there was some evidence that the normal load on the slider was partially supported by a lubricant film in this part of the cycle. The amount of contact during slip was determined by the relative velocity during slip. By monitoring the resistance of the interface during a "delayed" stick cycle it was possible to show that the area in contact did not increase with time under a stationary loading condition. Instead the area of contact at slip and also the static friction force were found to be dependent on the rate of application of the tangential force. These findings contradict earlier work on time-dependent creep models for static friction and provide additional evidence in support of the visco-elastic model proposed by Johannes [27].Applied Science, Faculty ofMechanical Engineering, Department ofGraduat