On the frictional behavior of an incipient-sliding contact of sphere-on-flat with high normal loads

Levinson et al [1] present experimental results that show a steep reduction in the static coefficient of friction of a dry sphere-on-flat contact as the normal load increases. The experiments cover a range of loading conditions from elastic contact to contact with significant plastic deformation. A theoretical analysis is carried out in this study using a contact model of an elastic-plastic sphere on a rigid flat. The shear strength of the contact interface is assumed to be proportional to the contact pressure until it reaches a limiting value that is below the bulk shear strength of the sphere. The theory predicts a friction behavior that is consistent with that from the experimental results in [1] in all key aspects. The analysis reveals that a relatively low limiting shear strength in the contact interface is likely the key factor leading to the dramatic reduction in the friction coefficient and the negligible junction growth obtained in the experiments.