Formation, stability and degradation of transfer films formed by polyphenylene sulfide (PPS) and its composites in dry sliding against steel

Abstract

Transfer films are known to be important determinants of friction and wear, specifically in dry sliding of polymers on metals. Still, the kinetics of transfer film formation, the stability of transfer films, the in-situ correlation of friction and the extent of transfer film formation, are not yet fully understood. The main reason for this is the lack of time-resolved quantitative data on transfer film formation. Therefore, Transfer Film Luminance Analysis (TLA) was applied to block-on-ring sliding wear tests of polyphenylene sulfide (PPS) and selected PPS composites containing graphite, polytetrafluoroethylene (PTFE) and short carbon fibers (SCF) against steel. Though fundamental in nature, the tests’ parameters were chosen to closely match the operating conditions of dry-running bushings. While PPS filled with graphite or PTFE formed laterally homogeneous and temporally stable transfer films, SCF-containing composites exhibited recurrent spontaneous partial degradation of the transfer film. Significant fluctuations of the coefficient of friction (COF) were found to be the result these. Notably, when the extent of transfer film formation is reduced by such degradations, friction increased. Regarding material composition, mixed fiber/solid lubricant grades still exhibited transfer film instabilities and COF fluctuations, albeit at a much lower level. At the same time, no impact of partial transfer film degradation on wear was found.