Examination of the contours of the depression in a copper surface made by a cylindrical indenter shows that although considerable bulk deformation has occurred the asperities are relatively little deformed. When a similar groove is made by sliding with a hemispherical slider, there is sufficient surface damage to obliterate completely the surface irregularities. In the presence of a lubricant, similar effects are observed, though on a reduced scale. The tops of the asperities are wiped away, showing that there is considerable interaction between the metal surfaces through the lubricant film. A detailed examination of the surface damage produced during sliding shows that metallic junctions are formed and sheared during the sliding process. These junctions are formed even when the sliding speeds are so small that the temperature rise due to frictional heating is negligible, and it is suggested that they are produced by a cold welding of the surfaces as a result of the high localized pressures developed at the points of real contact. These junctions are often strong enough to rupture the stronger of the two sliding metals. Thus when copper slides on steel (unlubricated) there is not only a marked tearing out of copper fragments from the copper surface, but also a plucking of minute particles of steel out of the steel surface. Further, the shearing of metallic junctions and the smearing of metallic fragments over the surfaces may produce considerable work hardening below the actual points of contact. Although a lubricant reduces the amount of intimate metallic contact, the investigation shows that metallic junctions are formed through the lubricant film by an essentially similar mechanism. These observations provide graphic evidence for the view that the frictional force for both clean and lubricated surfaces is due mainly to the shearing of metallic junctions formed by cold welding at the points of intimate contact. The significance of these conclusions in the burnishing of metals and in the behaviuor of bearing alloys is discussed.
Deformation of metals in static and in sliding contact