Tribology of metal coatings for electrical contacts

Noble metals are required for sliding low voltage low current electrical contacts such as those in electronic connectors, instrument slip rings and switches. This is to ensure that the contact resistance will be low and will remain stable. Gold, palladium and their alloys are the most commonly used materials and are employed primarily as electrodeposits and clad coatings.The major contact wear processes are adhesion, abrasion and fretting. Adhesive wear can operate in mild or severe regimes. Prow formation is the dominant wear process in the severe regime and is characterized by transfer from the member with the larger surface involved in sliding to that with the smaller surface, after which loose debris is formed. On repeat-pass movement, a transition to rider wear occurs in which the direction of metal transfer reverses. Unique gold electrodeposits have been developed that are relatively resistant to severe adhesive wear. These deposits are brittle, which lowers the tendency of adherent asperities at the mating surfaces to grow to large prows during sliding.Abrasive wear, in contrast, is accentuated when the coatings have low ductility, but as with other wear processes may be controlled if the contact materials are hard.Fretting wear of noble metal coatings leads to high contact resistance when they wear through to their base substrates. Frictional polymerization is the formation on the contact of insulating organic layers, which originate in organic air pollutants in the environment, and occurs during sliding and fretting of platinum group metals. Hard substrates and underplates are desirable because they reduce adhesive, abrasive and fretting wear. Smooth surfaces are superior to rough surfaces when adhesive and abrasive wear occur.