As a rule, in low-current electrical contacts pressures are light and the current produces minor effect on the durability. Therefore, if no severe electrical erosion occurs, the provision of a stable and low transition resistance is the main operation requirement.
Taking into account the presence and properties of surface layers and their considerable effects exerted on the operation of low-current contacts, the following requirements for the contact materials can be put together:
- Physical properties: high electric and thermal conductivities; low thermal resistance and thermoelectric coefficients; high melting and evaporation temperatures; high specific heat of fusion, specific heat of evaporation, and specific heat; high voltage and current of arcing; no phase transitions in operation.
- Chemical properties: composition constancy; a high corrosion resistance; a low catalytic reactivity; resistance to various contaminations; capability of forming a thin surface film to act as a lubricant.
- Mechanical properties: a high elastic modulus, hardness, strength, and elastic limit, along with ductility, i.e., the absence of brittle fracture; a low friction coefficient; no adhesive wear; a high wear resistance.
Many of these requirements are contradictory; therefore, the selection of suitable contact materials for low-current contacts is an extremely arduous task. The best contact materials contain from four to seven components. The problem should be solved together with the development of an optimal design of the contact unit, taking into account a variety of factors controlling contact operation.
For example, sliding low-current contacts in complex devices are often grouped into a unit comprising tens of single rings or tracks. If voltages, currents, and loads and loading modes in different circuits differ, the contact materials should be different and provide reliable operation of the contacts. Since from a technological viewpoint this is inconvenient, the same contact pairs are often used both for commutating microcurrents (10-6
A) and in circuits with currents 1–4 A. In this case, the contact quality, life, and reliability in different circuits may differ.