The Switching Performance of Refractory Carbide-Silver Contacts

Eight refractory carbide-silver contacts have been evaluated for switching performance: WC-Ag (WC 40 volume percent); WC-AG-Co ((a)WC 45 volume percent, Co 7 volume percent, (b) WC 35 volume percent, Co 7 volume percent); TiC-Ag ((a)TiC 80 volume percent, b) TiC 60 volume percent, c) TiC 48 volume percent); and NbC-Ag ((a)NbC 72 volume percent b) NbC 48 volume percent). The contacts were placed in an experimental switch and were operated to make and break a 20-A 110-V resistive circuit. The contact resistance was monitored at 0, 1000, 2000, and 3000 switching operations by measuring the temperature rise at the fixed contact terminal after 20 A had been flowing for ½ h. The contact surfaces were examined using a scanning electron microscope. The performance of the WC-Ag was similar to that which was previously observed with W-Ag contacts; high contact resistances were measured after only 1000 operations. The WC-Ag-Co contacts showed consistently lower contact resistance values than were observed for the WC-Ag contacts. The TiC-Ag and NbC-Ag contacts performed about as well as the WC-Ag-Co contacts did. These data showed that the presence of carbon in the WC-Ag contacts did little to alleviate the problem of high contact resistance values resulting from insulating oxides of W and Ag forming on the contact surfaces. The presence of Co, on the other hand, resulted in conditions which were not favorable to the formation of the silver tungstate. The TiC-Ag data showed that Ag was always present on the contact surfaces together with TiC and with oxides of titanium; there was no evidence of the formation of silver titanate. The NbC-Ag data were similar to the TiC-Ag data. After continued switching, however, the TiC and NbC contacts began to form a surface skin of refractory material on the contact surface. The relative merits of these refractory carbide-silver contacts are discussed.