Testing of the thermal-stress-cracking characteristics of silver-refractory contacts

Thermal stress cracking is an important mode of contact material degradation, especially in composite silver-refractory contact materials for circuit breaker applications. The formation of cracks can affect thermal conduction, arc motion, and the restrike potential even without causing excessive weight loss. The very transient thermal and material conditions during arc interruption make effective mathematical analysis difficult. A test system which allows a regulated supply voltage to be applied to a test device which then interrupts the current in a reproducible manner is described. This test allows the cracking behavior of contact materials to be empirically evaluated and compared. The cracks which formed during the interruption of elevated currents generally had one of two orientations, perpendicular or parallel to the face of the contact. When these two types of cracks intersect, large sections of a contact´s structure can be separated and lost. Surface structures which could initiate crack formation are described. The effects of cracks on heat flow through the contact are discussed. It is shown that cracks, after forming and propagating, can be infiltrated to a significant extent with silver.