Introduction of a "modified Archard wear law" to predict the electrical contact endurance of thin plated silver coatings subjected to fretting wear

The main requirement of materials used in electrical connectors is to allow low and undisturbed electrical contact resistance. However due to engine vibrations and thermal fluctuations, fretting wear damage can occur in the interface, reducing the electrical conductivity. In this paper the electrical performance of pure silver coatings deposited on a Ni interlayer and CuSn4 substrate, submitted to various fretting sliding conditions, is studied. Five different coating thicknesses; e = 1.7 mu m, 3.9 mu m, 4.7 mu m, 5.2 mu m and 7.1 mu m, are considered. In order to estabish the influence of the Ag coating thickness on the electrical lifetime, a quantitative approach is developed. It consists of reporting the electrical endurance (i.e., when the electrical contact resistance surpasses a threshold value: Nc => Delta R> 4 m Omega) as a function of the applied sliding amplitude. These "electrical endurance charts" are defined for each coating thickness and modeled by power functions. The step-by-step degradation process of the contact interface is then investigated. It is shown that the electrical decay is related to the fretting wear of the silver layer and the successive formation of oxide debris extracted from the Ni interlayer and CuSn4 substrate. Assuming that the electrical endurance is controlled by the silver fretting wear rate, various wear models derived from the Archard law are compared to the "electrical endurance charts". The best correlation is obtained using a modified Archard wear expression developed for an adhesive wear interface. A general expression is then introduced which allows one to predict the electrical endurance of silver coatings whatever the coating thickness, sliding amplitude and the contact pressure.