Scanning Electron Microscopy of Contact Surfaces Before and After Arcing

An apparatus has been constructed which allows the manipulation of electrical contacts inside a scanning electron microscope. The contact members can be positioned so that an image of their edge profiles can be obtained at magnifications as high as 10,000X. The microgeometry of both the anode and cathode surfaces is clearly imaged. The contacts can be brought together to make electrical contact and then separated to a desired distance. The interelectrode spacing and both contact members can be viewed on the cathode ray tube of the scanning electron microscope. With this technique, contact gaps of less than one micron can be set and measured. Both contacts are connected to coaxial cable circuitry external to the microscope. A pulse of predetermined potential and duration can be transmitted to one of the contact members. If breakdown occurs, an oscillographic record of the event is obtained as well as an immediate image of the damage on the anode and cathode profiles. One of the contacts can then be rotated to obtain a front view for measurements of the arc damage. Using this technique the phenomenon of electrode bridging from an eruption of molten metal from the anode has been observed. Electric fields required for the initiation cf short arcs have been measured as have the diameters of anode craters and their rim heights