Effect of surface reactivity of lubricants on the properties of aluminum electrical contacts

Lubricants are used in electrical contacts to minimize friction and mechanical wear and protect electrical interfaces against corrosion. This paper provides an example of the importance of differentiating between the lubrication and "electrical-contact protection" properties of lubricants. The paper reports on the frictional properties and the effect on contact resistance of stearic acid boundary films on aluminum. Stearic acid is an effective lubricant for aluminum because the long-chained acid molecules are chemically attached to the native aluminum oxide surface film. In an electrical contact, mechanical disruption of this native oxide film during sliding exposes the underlying aluminum metal to the stearic acid. The ensuing chemical reaction of the acid with the exposed metal leads to chemical breakdown of the lubricant and immediate surface oxidation of the aluminum. This reaction engenders a large increase in contact resistance. These observations provide unambiguous evidence that stearic acid is unacceptable as a lubricant for aluminum electrical contacts, although it is effective as a conventional lubricant for aluminum.