Changes

Ag/W (SIWODUR) contact materials combine the high electrical and thermalconductivity of silver with the high arc erosion resistance of the high meltingtungsten metal ''(Table 2.36)''. The manufacturing of materials with typically50-80 wt% tungsten is performed by the powder metallurgical processes ofliquid phase sintering or by infiltration. Particle size and shape of the startingpowders are determining the micro structure and the contact specific propertiesof this material group ''(Figs. 2.134 and 2.135) (Table 2.37)''.

During repeated switching under arcing loads tungsten oxides and mixedoxides (silver tungstates – Ag₂ WO₄ ) are formed on the Ag/W surface creating 2 4poorly conducting layers which increase the contact resistance and by this thetemperature rise during current carrying. Because of this fact the Ag/W is pairedin many applications with Ag/C contact parts.

Silver–tungsten contact tips are used in a variety of shapes and are produced forthe ease of attachment with a fine silver backing layer and quite often anadditional thin layer of a brazing alloy. The attachment to contact carriers isusually done by brazing, but also by direct resistance welding for smaller tips.

Ag/W materials are mostly used as the arcing contacts in disconnect switchesfor higher loads and as the main contacts in small and medium duty powerswitches and industrial circuit breakers ''(Table 2.38)''. In north and south americathey are also used in large volumes in miniature circuit breakers of small tomedium current ratings in domestic wiring as well as for commercial powerdistribution.