Erosion behavior and structure of silver-intermetallic compound electrical contact materials

Abstract

The action of an arc discharge on a contact made of a silver-intermetallic compound composite material is accompanied by selective erosion in the affected zone, which is a result of the anchoring of the arc discharge to particles of the intermetallic compound — a phase of comparatively low thermal conductivity. A characteristic feature of the selective erosion is a nonuniform structure of the affected zones of contacts, the nonuniformity increasing in intensity with rising volume concentration and Tm of the intermetallic compound. On contacts of hypoinversion compositions the character of structural nonuniformity in the affected zone on the cathode is different from that on the anode, but on contacts of hyperinversion compositions the affected zones on the cathode and anode exhibit the same type structural nonuniformity. There is a tendency for the functions of the cathode and anode to become equalized in the supply of eroded material to the arc gap. Increasing the current shifts the composition boundary between the ranges in which different types of cathode erosion are observed and hence the inversion range toward lower intermetallic compound contents. The structural nonuniformity of affected zones gives rise to“spike formation” during the excitation of electrical discharges between stationary contacts. The greatest propensity for“spike formation” is exhibited by refractory intermetallic compound-containing composites of hyperinversion composition. During erosion testing the rate of erosion varies, being a maximum at the beginning of testing. The fall in and subsequent stabilization of erosion rate are brought about by accommodation of the surface layers of contacts and by the formation in these layers of a structure exhibiting optimum resistance to erosion.