The friction of sliding metal surfaces is often attributed to the formation and shearing of welded junctions between the surfaces. This paper describes a more detailed investigation of the phenomena which occur in the junctions. Experiments were performed with a hemispherically ended copper rider sliding against the surface of a large cylindrical copper drum. Observation of the sliding contact through the microscope shows that most of the time during sliding, the drum and rider are held apart by a wedge of displaced metal from the drum, which becomes trapped between them. Whenever a wedge escapes from between the drum and rider, another wedge forms rapidly to replace it. The wedges are also observed with several other combinations of metals, and with pairs of flat copper surfaces. Sections cut through the wedges and adjacent copper contact members were examined. It is found that the shearing of the metal by which the sliding proceeds occurs in a direction slightly inclined to the drum surface. This causes the formation and growth of the wedges. The shearing is primarily a continuous plastic deformation process somewhat similar to the shearing process by which the chip is generated in the cutting of ductile metals with machine tools. It is possible that the adhesion between the rider and wedge may often be weaker than the junctions usually assumed to be responsible for friction. Wear debris is observed to be produced in a number of different ways.
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.
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