ABSTRACT
Friction occurs whenever two solid bodies slide against each other. It takes place by a variety of mechanisms in and around the real area of contact between the sliding or rolling/sliding bodies. It is through frictional processes that velocity differences between the bodies are accommodated. It is also through these processes that mechanical energy is transformed into internal energy or heat, which causes the temperature of the sliding bodies to increase. The exact mechanism by which this energy transformation occurs may vary from one sliding situation to another, and the exact location of that transformation is usually not known for certain. It is known that solid friction and related frictional processes, including frictional heating, are concentrated within the real area of contact between two bodies in relative motion. Some investigators contend that these processes occur by atomic-scale interactions within the top several atomic layers on the contacting surfaces (Landman et al., 1993), while others believe that most energy dissipation occurs in the bulk solid beneath the contact region by plastic deformation processes (Rigney and Hirth, 1979). Experimental work has shown that at least 95% of the energy dissipation occurs within the top 5 µm of the contacting bodies (Kennedy, 1982). Although there may be disagreement about the exact mechanism of the energy transformation, most tribologists agree that nearly all of the energy dissipated in frictional contacts is transformed into heat (Uetz and Föhl, 1978). This energy dissipation, called
frictional heating
, is responsible for increases in the temperatures of the sliding bodies, especially within the contact region on their sliding surfaces where the temperatures are highest. For the purposes of this discussion, it will be assumed that all frictional energy is dissipated as heat which is conducted into the contacting bodies at the actual contact interface.
