METALLURGICAL ASPECTS OF SILVER-BASED CONTACT MATERIALS FOR AIR-BREAK SWITCHING DEVICES FOR POWER ENGINEERING

The properties of the contact materials Ag/CdO, Ag/SnO2, Ag/Ni, Ag/C, and Ag/W are discussed on the basis of the metallurgical processes occurring due to the interaction of an electric arc with the contact material. In the case of Ag/CdO and Ag/SnO2, the arc erosion behavior is characterized by the interplay between decomposition of the oxide component (energy consumption) and viscosity effects due to the oxide particles in suspension in the silver melt. With Ag/Ni the process of dissolution and reprecipitation of the Ni appears to be the dominant factor. The switching behavior of Ag/C is largely governed by the combustion reaction between the graphite particles and the oxygen of the atmosphere. The erosion resistance of Ag/W and Ag/WC, respectively, is mainly brought about by the high melting point of the W or WC component. By means of selected examples taken from the authors' research work and field experience, it is pointed out how the contact properties are influenced by the metallurgical mechanisms involved. It is shown that every material with a special advantage in one particular property also has a pronounced disadvantage in some other properties.