Role of Displaced Metal in the Sliding of Flat Metal Surfaces

This paper describes an investigation of the interaction between pairs of flat metal surfaces sliding against one another. The experiments show that the junctions which are formed when the surfaces are first placed in contact do not break when the surfaces begin to slide. The relative motion of sliding is accommodated by plastic deformation of the metal near the contact areas. The plastic shearing takes place in a direction slightly inclined to the surfaces and causes the formation of wedges of displaced metal in much the same manner as was observed in a previous investigation with hemispherical riders sliding against cylindrical drums. With a pair of flat surfaces several wedges are formed simultaneously at various places between the surfaces. The wedges force the surfaces apart, and the separation is often as much as 0.1 to 0.4 mm. The variations in the distance of separation during sliding were measured with a dial indicator. The wedges and the gap between the surfaces were also observed visually with and without the microscope. Metallographic sections cut through the wedges were examined. Differences in the phenomena observed with various metals indicate that with some metals the plastic shearing by which the wedges are generated continues to occur during extended periods of sliding, while with others it occurs mainly during an initial period of sliding, though the wedges once formed remain between the surfaces.