Tungsten and tungsten carbide based contact materials used in low voltage vacuum contactors

By means of a single phase vacuum model switch with a current range from 50 A (for current chopping) to 1500 A (for AC4 contact erosion) contact materials of the group of tungsten carbide/silver, tungsten carbide/copper and tungsten/copper are evaluated. The properties focused on are current chopping in new state as well as during service life, contact erosion due to AC4 life due to cracking and reignition behavior. It is found that in regard to current chopping best results are obtained with WC/Ag and WC/Cu, while W/Cu and even an optimized W/Cu having antimony additives show substantially higher chopping currents. In regard to AC4 life and especially to reignitions the copper containing materials were found to be superior to the silver containing materials. In the case of WC/Cu there are indications of poor contact resistance during service life which is attributed to the evaporation of the copper from the arc treated area in combination with a poor wetting behavior of the molten copper to the WC particles.