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Certain Polytetrafluoroethylene (PTFE)- Alumina (Al2O3)- composites have wear rates up to four orders of magnitude less than unfilled PTFE (Kcomposite~10-7-10-8 mm3/Nm, Kunfilled~10-4 mm3/Nm). This reduction in wear has been attributed to the formation of thin tribofilms. The tribochemical mechanism to form these transfer films is hypothesized by Harris et al. The mechanical stress imposed by sliding causes the C-C bond in PTFE to break. At the broken chain ends, carbon radicals will react with environmental oxygen and moisture to form carboxylic acids. These carboxylic acids then react with the surface of the metal countersample and alumina particles within the composite. These bonds reinforce the polymer surface and transfer film, which minimizes wear of both surfaces. Most tribological testing on this material system has been done using flat-on-flat contact geometry. In this study, a sphere-on-flat contact was used to determine if similar tribochemical mechanism occurs under different loading conditions. Moreover, use of this condition should minimize the amount of material needed to evaluate these tribosystems and could accelerate the development of novel solid lubricants. |
