Some Fundamental Properties of Aluminum-Aluminum Electrical Contacts

The results of a fundamental Investigation of the properties of stationary aluminum-aluminum (AA1350) electrical contacts at room temperature both in an ultrahigh vacuum and in an oxygen atmosphere are reported. The contact couples consist of two polished spherical surfaces held together under constant load over a small area; the work was carried out in an environmental chamber which allows manipulation of the contact samples and external adjustment of contact loads. The loads required to rupture the oxide films on the contact surfaces have been measured and yield estimates of fracture strain for aluminum oxide films in compression. Once the insulating oxide films are ruptured, the area of metallic contact is generally found to grow with time. In a vacuum the growth rate is consistent with the rates predicted from sintering theory; the experimental evidence suggests that sintering is driven by capillarity forces and controlled largely by dislocation creep. Growth in oxygen is slower but can be also understood qualitatively in terms of creep-controlled sintering. Although growth in electrical contacts has been observed by earlier investigators, this is the first time that the phenomenon has been correlated with sintering. These results suggest that stationary aluminum-aluminum electrical contacts are characterized by "self-heal" properties which should be conducive to exceptionally stable performance in a vacuum or in an Inert gas environment.