CHARACTERISTICS OF AG CONTACT MORPHOLOGY IN BREAKING ARCS

The morphology of Ag contacts was observed by scanning electron micros copy (SEM) in different breaking DC current arcs in air. It was found that when an are develops into the gaseous phase the cathode surface w as usually smooth, and that the anode surface was characterized as thr ee parts (''wave-shape'' with plastic deformation in part I, smooth wi th pinholes in part II, and rough with globules in part III) from the center to the outside of the arced spot. The pinholes in part II are a bout 10 mu m in diameter, and the globular particles in part III are a s big as 32 mu m in diameter. The temperature of the base metal is con sidered to control the formation of these characteristics, and a plaus ible mechanism is proposed. This mechanism was also verified by using Pd material, which is considered to be oxide free, as well as Ag mater ial with breaking arcs in nitrogen gas with different oxygen concentra tions.