Friction of clean metals and the influence of adsorbed films

Abstract

An experimental Study has been made of the frictional behaviour of thoroughly degassed metal surfaces. An apparatus is described, in which friction can be measured at any desired temperature up to 1200° C or more, either in vacuo or in a particular gas. It is found that when sufficiently clean metals are allowed to touch, even at room temperature, complete seizure occurs. Over the real area of contact the specimens adhere with the bulk strength of the metal, and this area increases greatly with continued sliding or attempted sliding. Various gases and vapours have been adsorbed on to the clean specimens, and their in­fluences on friction determined. These specific contaminants are most effective as lubricants where they can react with the metal to form a solid and chemically attached film which is several molecular layers in thickness. Otherwise the friction remains very high, although seizure is prevented. It is shown that physical adsorption may occur in addition, with a significant reduction in the friction. The results support the view that the friction of metals is due mainly to adhesion at the points of real contact, and is governed by the extent to which even the thinnest of surface films can reduce this metallic contact.