Arc erosion patterns of pure Pd contacts tested with switching inductive loads at 20 V, 1.0/spl sim/4.0 A were determined by means of SEM and material transfer was measured with an electronic balance. Three types of erosion patterns were observed at the different load current regions. Firstly, distributed craters occurred on the anode and scattered redeposited mounds surrounded by sprayed particles on the cathode when the current was less than about 2.5 A, which we called a critical transition current because it demonstrates the change of metal transfer direction; secondly, if the current was larger than the critical current value, pips occurred on the anode and craters on the cathode; thirdly, when the current was around the critical current value, both pips and craters co-existed either on the cathode or on the anode. The behaviours of electrical contact resistance and arc duration in the metallic phase and the gaseous phase at different load currents are reported. The particle sputtering-depositing model is presented and used to explain the mechanism of material transfer processes and the formation of erosion patterns. The material transfer direction is affected by the arc duration of the metallic phase as well as the gaseous phase. The electrical contact resistance not only depends on the surface films and surface morphology, but is also influenced by the particle sputtering.
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