Introduction of an exponential formulation to quantify the electrical endurance of micro-contacts enduring fretting wear: Application to Sn, Ag and Au coatings

The massive use of electronic control in automotive, plane and energy systems requires the extensive application of in board low current electrical connectors. Subjected to vibrations, the electrical contacts endure severe fretting wear damage (i.e. wear of contacts induced by oscillating small amplitude displacements). One consequence of this fretting wear damage is that oxide debris are formed and maintained within the interface, decaying the electrical transmission of information. To prevent this critical damage, various soft and thin electrolytic coatings like Sn, Ag and Au layers are usually applied. In order to better understand the damage processes and provide a quantitative strategy to select the coating palliatives, an original approach coupling representative experiments and analytical models is introduced. Using a representative micro-fretting system, the electrical resistance evolution is shown to be mainly controlled by the applied displacement and sliding condition. It is confirmed that below a threshold displacement amplitude, related to a stabilised partial slip condition, the electrical endurance is infinite. Above this threshold, when full gross slip conditions are running, a finite endurance behaviour is observed. The electrical endurance is then controlled by the interfacial oxide debris layer formation and the nature of the coating. For non-noble coatings, like Sn, the electrical failure comes about as soon as an oxide debris layer, the so-called “third body” is trapped within the interface, while the endurance of noble and semi-noble materials (i.e. Au, Ag), which is significantly greater, is controlled by the progressive decrease of noble element concentrations in the third body layer and the related increase of its current resistivity. Finally, simple and basic exponential formulations are introduced to rationalise the global electrical endurance of “connector like” micro-contacts subjected to fretting wear. Using this formulation, an endurance ratio is developed to compare the endurance performance of the different coating palliatives.