Variations in Contact Resistance Resulting from Oxide Formation and Decomposition in Ag-W and Ag-WC-C Contacts Passing Steady Currents for Long Time Periods

Two silver-refractory contact structures, Ag (35 wt%)-W and Ag (85 wt%)-WC-C were used to switch a 20-A 110-V circuit. At regular intervals the contacts were left closed with 20 A passing through them for long periods. The voltage drop across the contacts and the temperature rise of the stationary contact arm were continuously measured. Four characteristic voltage traces were observed for Ag-W: type 1, a steady voltage less than 100 mV; type 2, a voltage plateau (range 150-600 mV) with frequent rapid excursions to lower voltages and rapid recoveries; type 3, rapid voltage oscillations (as much as ±100 mV) superimposed upon a slower varying mean (~400 mV); and type 4, extreme type 3 oscillations (peaks as high as 1.3 V) with sudden excursions to a lower plateau (~300 mV). The Ag-WC-C contacts only showed the type 1 and 2 characteristics. The data were analyzed in terms of the expected temperature at the contact spot, the formation and decomposition of silver tungstates, and the volatility of tungsten oxides above 1000 °C. These data showed a self-limiting effect for the contact resistance of Ag-W contacts, with the temperature rise of the contact arm never moving higher than 220°C.