Rolling Friction of Single Balls in a Flat-Ball-Flat-Contact as a Function of Surface Roughness

This paper reports on the dependence of rolling resistance on surface roughness of single balls in a flat-ball-flat configuration at sub-Newton loads using an specially designed rolling resistance tribometer. The tribometer setup features an ultra-stable drive and a two-dimensional force transducer with a high ratio between load and friction force sensitivity. Using this setup, the rolling resistance was examined for different materials as well as a self-mated sapphire contact. Results indicate a very strong dependence of rolling resistance on surface roughness for the tested loads. Rolling resistance was found to decrease monotonically with decreasing roughness. A comparison was also attempted between rolling resistance and sliding friction. Sliding friction measurements show that decreased roughness does not necessarily decrease the coefficient of friction. In fact, a friction minimum is reached at a certain roughness. A further decrease of roughness increases the friction. This indicates that sliding friction measurements do not seem to be transferable to rolling resistance in every case, especially when the average surface roughness is in the range of a few nanometres.