The Effect of Surface Segregation of Mg on the Contact Resistance of Al-0.5-Percent Mg

Silver and Ag-W (10 weight percent W) were operated in a 5.I-A (peak) 308-V (peak) full-wave rectified dc circuit. The experimental switching system was operated 1.5 times per minute; the opening and closing velocities were 4 x 10-2 and 3 x 10-4 cm/s, respectively; the fully open contact spacing was 200 µm; and the contacts were closed for 3 s. The opening and closing of the contacts were controlled by a heater-bimetal combination attached to the moving contact. The following contact pairs were used: Ag versus Ag, Ag-W versus Ag-W and Ag versus Ag-W. All combinations of contact material and contact polarity were used. The experimental results showed that there was always a net transfer of contact material from the cathode to the anode. The volume of material transferred was strongly dependent upon whether the anode was the moving (hotter) or the fixed (cooler) contact: the greater erosion occurring when the anode was the hotter contact. For Ag versus Ag contacts thc erosion pattern resulted inone pip on the anode and one crater on the cathode. For Ag-W versus AgW contacts the presence of the small percentage of W resulted in multiple pip and crater formations. The experiments with the Ag versus Ag-W showed that the cathode electrode material determined the erosion pattern observed, i.e., when the Ag-W was the cathode, multiple pip and crater'formations occurred, but when Ag was the cathode, only a single pip and crater structure was observed. The presence of the W either in the cathode or in the anode decreased the observed erosion. These experimental results are discussed in terms of how the presence of W affects the emission characteristics of the cathode region and also affects the movement of the arc roots.