Model for the static friction coefficient of spherical contact with a soft metal coating

An elastic-plastic spherical contact with soft metallic coating under combined normal and tangential loading is studied by finite element analysis. Full-stick contact condition is assumed and sliding inception is related to vanishing tangential stiffness of the contact junction. Previously observed, both experimentally and theoretically, effects of increasing coating thickness on static friction coefficient that were published in the literature are thoroughly explained here, to the authors' best knowledge, for the first time. These effects include initial sharp drop of friction as soon as a thinnest coating film is applied, followed by a transitional behavior from decrease to increase of friction when the coating thickness is continuously increased. An intensive parametric study is performed and the effects of substrate and coating material properties on the static friction coefficient are revealed and thoroughly explained. An empirical expression for the static friction coefficient is derived along with the values for optimum coating thickness that provides the minimum friction coefficient.