Page 191 - Handbook of Surface Improvement and Modification

186 The Coefficient of Friction

Figure 11.29. Optical microscopy images of the worn PTFE composite surfaces. [Adapted, by permission, from
Conte, M; Pinedo, B; Igartua, A, Wear, 307, 81-6, 2013.]

generated brown-colored tribofilms on both the polymer and metal surfaces, which were
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indicative of tribochemical changes.
The tribological behavior of polytetrafluoroethylene-based composite material filled
with polyimide microparticles has been investigated using pin-on-disc and thrust washer
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measurements. The data from two different methods were comparable. The presence
of polyimide microparticles in the PTFE matrix significantly decreased the friction coeffi-
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cient and enhanced the wear resistance of the fluorinated matrix.
An analytical model was developed for the dynamic sliding friction of polytetrafluo-
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roethylene on a dry glass surface as a function of the angle of inclination. The velocity
greatly increased at the beginning of sliding (acceleration by gravity force), then reached a
maximum value in the middle stage, to significantly decrease at the end (increased contact
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area and the build-up of debris).
The matrix and filler properties affect tribological behavior of polytetrafluoroethyl-
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ene composites (Figure 11.29) but the mechanism is not fully known. If the fillers are
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thermally conductive, the frictional heating is dissipated more effectively. Glass fibers
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and carbon black reduce frictional heating by ~10% as compared with the virgin PTFE.
Polyimide/epoxy resin contained polytetrafluoroethylene solid lubricant filled with
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in situ synthesized silver nanoparticles. The silver citrate precursor was decomposed to
generate Ag nanoparticles (100 nm) during the curing process of the lubricating coating. 39