Physical Processes in Contact Erosion

Abstract

The various causes of the erosion of noble metal contacts are now isolated and understood. They give adequate interpretation of the sign of erosion under various circuit conditions, and of the space distribution of the eroded metal, as well as quantitative agreement with measurements of erosion in repetitive tests. Electric arcs are the chief cause of erosion, although at breaking contacts glow discharges are important under certain circumstances, and at higher currents bridge erosion is significant also. The arcs are of two types, ``anode arcs'' which erode the anode predominantly, and ``cathode arcs'' which erode the cathode. Anode arcs occur at small contact separations, and cathode arcs at large separations. The metals studied were palladium and silver, which have different characteristics because of their different electrical conductivities and also because of the chemical reactivity of silver. The erosion of these metals in the inactive condition, by weighing the contacts before and after many repetitive operations, is of the order of 4×10⁻¹⁴ cc/erg in anode arcs and less than this by a small factor in cathode arcs, on break as well as on closure. In a glow discharge only the chemically active metal silver is appreciably eroded and its erosion is negligible unless the current density results in a glow of the abnormal type. Activation produced by organic vapors often changes the magnitude of the erosion per unit of arc energy and sometimes its sign, these effects being probably attributable to shielding one of the contacts by carbon lying on it as well as to the greater separations at which arcs occur between active surfaces.