Influence of Hardness and Fatigue on the Lifetime of a Modified Silver Coating in Fretting Wear and Corrosion Tests

The reliability and lifetime of electrical contacts is an important aspect in system reliability and is influenced by numerous factors. Micro motions as well as vibrations lead to fretting wear, which can result in wear through of the protective coating. If this layer is worn through, the non-noble layer underneath is exposed, resulting in the occurrence of fretting corrosion with further relative motion. This leads to an increased electrical contact resistance (ECR) and can cause the contact to fail. Increasing the hardness of the coating material can reduce the wear and in turn increase contacts’ lifetime. The micro hardness, wear and lifetime of contacts with modified hard silver coatings are investigated in fretting wear and corrosion tests and the results compared to a conventional silver coating. Since one of the modifications shows a significant reduction in wear and hence improvement in lifetime, further analysis with SEM and FIB is conducted in order to identify the key mechanisms leading to this improvement. With a further increase in lifetime however, fatigue as well as delamination of the coating are revealed to be of high relevance. Both can be main causes of electrical contact failure under fretting load. In general, at lower number of cycles, increased micro hardness has the greatest effect on lifetime and wear while at the higher number of cycles, fatigue is observed to be the dominant failure mechanism.