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

Abstract

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.

Recent Development in Modeling of Coated Spherical Contact

“…Similar effect of coating thickness on surface friction has been reported in experimental studies for indium coated tool steel surface [46] and silver-coated hardened M2 steel surface [47]. The µ behavior as a function of t/R in [44,45] is explained in [45] by the competition of two mechanisms. One is that a thicker soft coating decreases the tangential stiffness of the contact junction leading to decreasing µ.…”

“…In the figure, z, d and  are the asperity height measured from the mean of the It was demonstrated in [34] that soft coatings exhibit a mirror image behavior of hard coatings in terms of the yield resistance. For the static friction coefficient, Chen [44] and Zhang et al [45] showed that the soft coatings also exhibit a mirror image behavior of hard coating (Figure 6). Hence, opposite to Figure 6, in the case of soft coatings, there exists a (t/R) m corresponding to the lowest µ For t/R < (t/R) m , µ decreases with increasing t/R.…”

“…Hence, the assumption that the contact interface maintains full stick seems to be valid and greatly simplifies the problem. Therefore, the modeling work on the coated spherical contact under combined loading presented in [43][44][45] considered elastic-plastic deformation and adopted the full stick assumption.…”

“…Chen and Etsion [43] investigated this contact problem using a coated sphere in contact with a rigid flat (see Figure 1) for the case of hard coatings Y co /Y su > 1 (assuming E co /E su > 1). Chen [44] and Zhang et al [45] investigated the case of soft coatings Y co /Y su < 1 (assuming E co /E su < 1). In [43][44][45], a normal load P was first applied so that the coated sphere deforms elastic-plastically.…”

“…Chen [44] and Zhang et al [45] investigated the case of soft coatings Y co /Y su < 1 (assuming E co /E su < 1). In [43][44][45], a normal load P was first applied so that the coated sphere deforms elastic-plastically. Subsequently, a tangential displacement u x was applied and increased incrementally.…”