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CAER Seminars


Dr. Kathryn J. Wahl
Naval Research Laboratory
Washington, DC

Thursday, January 6, 2000 3:00 pm
Ben Bandy Conference Center
Center for Applied Energy Research


Friction is usually treated as a two-body problem, in which two counterfaces move against each other and a "magical" parameter - the friction coefficient - comes into being. However, at all scales -- from atomically thin surface films to chunks of wear particles -- "third bodies" play an important role in friction. They might be introduced intentionally by a tribologist to lubricate one or both counterfaces, or they might arise simply from atmospheric gases. Consideration of these third bodies helps remove some of the mystery surrounding friction and related processes. In any case, third bodies play a far more important role in friction and wear than one would gather from the literature.

In this presentation, I address the issue of sliding behavior of low friction, low wear coatings in concentrated contacts. The focus will be on "how" third body films form on the stationary counterface, "what" is their composition, and "why" they provide low friction and prevent wear. Examples of several coating systems will be given, including MoS2 and diamond-like carbon (DLC) based coatings. Specialized reciprocating sliding tests have been developed and are used in combination with optical and surface analytical techniques to monitor material buildup and loss, as well as chemistry and morphology of the worn surfaces. The role of third bodies in tribological performance, and the influence of the underlying original surfaces and environment, will be discussed.