Third-bodies in tribology

In the last decades, Tribology has moved, from the Tribology of Volumes, which attempted to produce friction and wear laws for different material combinations, through the Tribology of Surfaces, which rests strongly on surface science, to the Tribology of Interfaces which focusses on the role of the interface on friction and wear. Interfaces, or third-bodies can be defined in a material sense, as a zone which exhibits a marked change in composition from that of the rubbing specimens or in a kinematic sense, as the thickness across which the difference in velocity between solids is accommodated. The third-body or interface approach is useful as the third-body model presents all of Tribology, from thick film lubrication to dry friction, as a single science centered on the notion of flow. It also insists on the difference between wear and particle detachment mechanisms which are shown not to be equivalent. Interface kinematics in dry friction is presented in the light of “velocity accommodation sites and modes”. The effect of sites and modes on friction and wear are also discussed.