Changes

This group of contact materials contains the typically 40-65 wt-% of the veryhard and erosion wear resistant tungsten carbide and the high conductivity silver''(Fig. 2.135) (Table 2.36)''. Compared to Ag/W the Ag/WC (SIWODUR C)materials exhibit a higher resistance against contact welding ''(Table 2.37)''. Therise in contact resistance experienced with Ag/W is less pronounced in Ag/WCbecause during arcing a protective gas layer of CO is formed which limits thereaction of oxygen on the contact surface and therefore the formation of metaloxides.

Higher requirements on low temperature rise can be fulfilled by adding a smallamount of graphite which however increases the arc erosion. Silver–tungstencarbide–graphite materials with for example 27 wt% WC and3 wt% graphite or 16 wt% WC and 2 wt% graphite are manufactured using thesingle tip press-sinter-repress (PSR) process ''(Fig. 2.136)''.