TRIBOCHEMICAL INTERACTIONS OF A PTFE/ALPHA ALUMINA COMPOSITE AT THE SLIDING INTERFACE: A MECHANISM FOR ULTRA LOW WEAR

Sammanfattning: The wear and friction behavior of ultralow wear polytetrafluoroethylene (PTFE)/αalumina composites first described by Burris and Sawyer in 2006 has been studied intensively in the years hence. The mechanisms responsible for the remarkable improvement in wear over unfilled PTFE are not yet fully understood. The formation of tribofilms on the countersurface and the running face of the polymer is crucial to the ultra-low wear behavior of the composite on a metal countersurface. The complete chemical mechanism of transfer film formation and adhesion, and its role in the exceptional wear performance has yet to be elucidated. Some debate exists regarding the role of chemical interactions between the PTFE, the filler, and the metal countersurface. Some have concluded that chemical changes are not an important part of the ultralow wear mechanism in these materials at all. A “stripe” test allowed comprehensive spectroscopic studies of PTFE/α-alumina transfer films in various stages throughout development and led to a proposed mechanism which details the initiation and adhesion of the tribofilms formed on both surfaces of the wear pair. PTFE chains (carbon-carbon bonds) are broken mechanically during sliding and undergo a cascade of reactions to produce carboxylate chain ends that chelate to the metal surface and to the surface of e porous, friable alumina filler particles. This tribochemical process forms a robust polymeron-polymer system that protects the steel countersurface from abrasion, and the polymer surface from wear. The system is able to withstand hundreds of thousands, and possibly millions of cycles of sliding with almost no wear of the polymer composite after an initial period of high wear during run-in. A mathematical model in support of the hypothesis of mechanical scission of carboncarbon bonds in the backbone of PTFE in simple sliding contact is detailed, using the Hamaker model for van der Waals interactions between polymer fibrils and the countersurface (a cylinder and a flat surface). The proven necessity of ambient moisture and oxygen is explained in the mechanism, and model experiments using small molecules further support the assignment of reactions in the proposed mechanism to the processes at the sliding interface.