Paper 25S: Sliding Noble Metal Electric Contacts

Abstract

Sliding electric contacts have the unique requirement of low and stable resistance without high friction and wear. Electrical breakdown of surface films, which can establish metallic contact between touching members, occurs only with appreciable circuit voltages. Consequently, noble metals alone are suitable for contacts that serve the low energy circuits of modern computer, communications and aerospace equipment. Approaches used by the engineer to reduce the friction and wear of noble contact metals include: design with low normal loads, careful selection of contact materials and lubrication. Gold coatings, generally as thin electrodeposits, have become the most widely used contact material for separable connector contacts, instrument slip rings and similar devices. The ‘prow formation’ wear mechanism dominates the dry sliding of most noble contact metals, particularly during the early stages of sliding where break-in phenomena occur. Recent work shows that there is a critical load-dependent roughness for prow formation. The wear mechanisms of gold electrodeposits may differ from those of the wrought metal because of their characteristic topographic and crystallographic orientations. Lubricants are often used for sliding noble metal contacts. Solid lubricants are effective only on rough surfaces. When contacts become burnished through use the lubricant reservoirs disappear and high wear commences. Thin films of lubricants having a fluid component, however, do not have this limitation, for they demonstrate hydrodynamic lift and self-healing qualities. Above a certain velocity, characteristic of the system, wear may be negligible with fluid-containing lubricants. Their ability to function on smooth surfaces is important, for the as-plated porosity of electrodeposited metals (which leads to deleterious contact corrosion) is directly related to roughness. Finally, coatings for thin gold-plated contacts have been developed which combine corrosion inhibitors and lubricants.