Thermally induced mechanical degradation of contact spots in aluminum interfaces

Deterioration of an electric contact is closely related to phenomena occurring in the microscopic contact spots in the contact interface. Contact spots in idealized aluminum-aluminum contact interfaces, that have passed heavy alternating currents, have been carefully examined by scanning electron microscopy and metallographic imaging techniques. Small cracks are observed in the contact spot regions, and have to a great extent impaired the current-carrying ability of the area. The temperature in the contact spots cycles with twice the power frequency, and in heavily stressed contacts the maximum temperature reaches several hundred degrees centigrade. The associated local thermal expansion and contraction in and around the immediate vicinity of the contact spot generates cyclic mechanical forces of substantial magnitudes. Indications that this repetitive stress causes the observed cracks are found. It is suggested that the resistance increase observed in practical aluminum connectors being subjected to short-circuit currents can have a similar origin.