Conference Proceedings of NordTrib2000, Porvo, Finland, 11-14 June 2000. INVESTIGATION OF THIRD BODY PROCESSES BY IN VIVO RAMAN TRIBOMETRY Irwin L. Singer 1 , S. David Dvorak 2 , Kathryn J. Wahl 1 1 US Naval Research Laboratory, Code 6176 Washington, D.C. 20375-5342 USA 2 Dept. of Mechanical Engineering Technology University of Maine, Orono, ME 04469-5711 USA ABSTRACT A Raman tribometer has been used to study third body processes and friction during sliding against two low friction coatings: annealed boron carbide and Mo-S-Pb, a MoS 2 -based coating. Reciprocating sliding tests were performed in either dry or humid air with transparent hemispheres (glass or sapphire) loaded against the coatings. Videos and Raman spectra of the sliding contact were recorded during the tests. For annealed boron carbide, friction was controlled by a mix of H 3 BO 3 and carbon; for amorphous Mo-S-Pb, friction was controlled by MoS 2 generated by sliding. Friction changes in the former were correlated to the relative amount of the two materials; in the latter, the rise in friction was ascribed to a change in interfacial shear strength of MoS 2 , inferred from the deformation of transferred debris particles. For both coatings, interfacial sliding was the dominant mode of velocity accommodation in the sliding interface. 1. INTRODUCTION Friction is often treated as a two-body problem, in which the two counterfaces move against each other and a “magical” parameter – the friction coefficient – comes into being. Not so. At some scale, from atomically thin surface films to chunks of wear particles, third bodies play an important role in friction [1]. These “third bodies” are often born in the sliding contact, detach from one surface, transfer to the counterface and eventually agglomerate as macroscopically visible debris particles. When ejected from the contact, they are recognized as wear particles and written about extensively in friction and wear literature. When entrapped in the contact, they strongly influence the way the counterfaces accommodate sliding motion, the ‘velocity accommodation mode,’ but often go unnoticed because they are buried at the sliding interface. One of the reasons that friction processes are not better understood is that the buried interface – where all the action takes place – is difficult to access experimentally. To study this interface, tribologists have traditionally had to separate the contacts before analyzing