Contact resistance variations of tungsten contacts operated in air and silver-tungsten carbide contacts operated in vacuum

Previous research on W-Ag contacts has shown the moderating effect that silver has on the formation of high-resistance contact spots. The effects of the two other components in the formation of the complex oxides are explored: the role of tungsten and oxygen (with no silver) and the role of silver and tungsten in the absence of oxygen. Two experiments were conducted in order to determine the effects of tungsten and oxide formation on the variations of contact resistance (R _c) of Ag-W contacts. The first consisted of switching pure tungsten 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 tungsten 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 the formation and dissociation of tungsten oxides and silver tungstates