CONTACT RESISTANCE VARIATIONS OF TUNGSTEN CONTACTS OPERATED IN AIR AND SILVER TUNGSTEN CARBIDE CONTACTS OPERATED IN VACUUM

Two experiments were conducted in order to determine the effects of W and oxide formation on the variations of contact resistance (R(c)) of Ag-W contacts. The first consisted of switching pure W contacts in air in a 20-A circuit for 500 operations and then observing the variations of R(c) while they passed a 20-A steady-state current. In the second experiment Ag-WC contacts were operated in vacuum and then subjected to a series of steady-state currents up to 5000 A. The pure W contacts exhibited a wide variation in R(c). The Ag-WC contacts showed a consistent steady R(c) value. These results are discussed with reference to previous data on Ag-W and Ag-WC contacts operated in air and on the formation and dissociation of W oxides and silver tungstates.