Influence of contact geometry and current on effective erosion of Cu-Cr, Ag-WC and Ag-Cr vacuum contact materials

Effective ac erosion rates were measured for Cu-Cr, Ag-WC and Ag-Cr contacts in vacuum contactors. Half-cycle are currents of 450-600 A rms were used, with the full gap set to 10%-25% of the contact diameter. The contacts parted during the rising current, and the are stopped at the next current zero. The polarity changed for each operation to ensure uniform effects for both contacts. The effective linear volume erosion rate [cm(3)/C] was determined by monitoring the axial thickness of the contacts versus the number of operations. When this was converted to an effective mass erosion rate [mu g/C], it was significantly smaller than the reported absolute cathode erosion rate based on measured loss of cathode mass. Both the dependence of the effective erosion on the gap and the distribution of metal droplets on the are shield were studied. Droplets were a significant cathode material loss which left the gap close to the plane of the cathode, while a large fraction of the ionized vapor leaving the cathode spots was deposited onto the anode. The effective erosion rate increased when spiral-slotted contacts were used.