Impact of contact size and geometry on the lifetime of a solid lubricant

The selection of low friction coatings is of great interest to industrial applications. Nevertheless, regarding the lifetime of the coatings, the selection criteria often depend on the experimental apparatus and contact configuration and then cannot be applied to real cases. In this study, we use a model based on the local dissipated energy due to friction under gross slip conditions in fretting wear. Indeed, the maximum value of the local dissipated energy is a unique parameter that takes into account the two major variables in fretting wear experiments: the normal force and the sliding amplitude. Two kinds of contact geometry are tested to assess the stability of the proposed approach: a cylinder on plane contact and a plane on plane contact (which consists of a circular contact surface with rounded edges to remove the discontinuity). This approach is applied to a commercially available polymer bonded MoS2 solid lubricant film used in aeronautical applications to protect titanium surfaces in contact. The effects of normal force and displacement amplitude are investigated. The results show that, even if there is a great difference in contact size and pressure distribution between the two types of contact, the lifetime is the same if the maximum local dissipated energy is the same. Thus, the local variable used is representative of the wear kinetics and the "lifetime versus local dissipated energy" curve is independent of the contact configuration: the proposed approach can then be used to select coatings whatever the contact size. (C) 2003 Elsevier Science B.V. All rights reserved.