An Investigation On The Relationship Between Orientation And Tribological Properties Of Polytetrafluoroethylene

Orientation has an important influence on the friction and wear properties of polymers. The study on the role of orientation in the tribological properties is one of advanced fields in the polymer tribology. However, the studies on this filed are still unsystematic and insufficient.In this paper, polytetrafluoroethylene (PTFE) was selected as researching material in order to find the relationships between the condensed state structure and the tribological properties. The effects of molecular weight and drawn conditions et al on the bulk oriented structures, draw ratio and oriented direction on the tribological properties of oriented PTFE, and sliding conditions (such as applied load, environmental temperature, the mated materials' roughness) on the friction and wear properties were systematically investigated. The structure and morphology of oriented PTFE bulk, worn surface, debris and transfer film were analyzed with the aid of scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy. On the basis of the results of tribological tests and surface analyses, the effects of bulk orientation, friction-induced orientation and the oriented transfer film of PTFE on its tribological properties, as well as the mechanism of friction and wear of oriented PTFE was detailed discussed . The researching results will deepen the understanding on polymer tribology and enrich tribological theories.The followings are some innovative conclusions obtained from this study.1．The tribological properties of drawn PTFE were dependent on the orientation direction. The higher the orientation degree was, the lower the friction coefficient and wear rate were. When the draw ratio was 7, the friction coefficient and the wear rate along the draw direction was lower than that of undrawn PTFE by about 15% and 40%, respectively.2．The oriented structure formed by drawing markedly influenced friction process, which was the essential reason of the anisotropy of tribological properties. The formation of friction-induced orientation and the oriented transfer film was one of the causes for the stable friction coefficient at any sliding direction. The increase in tensile strength was an important cause for the low wear rate for drawn PTFE.3．Environmental temperature markedly influenced the oriented structure, and hence changed the tribological properties of PTFE. The fiber structure of PTFE drawn at its melting point of 327℃was effectively for decreasing the friction coefficient and wear rate.4．The crystallinity of transfer film was higher than that of PTFE bulk, which was not affected by oriented direction. The existence of the transfer film was an important condition for decreasing friction coefficient.5．Friction-induced orientation of PTFE followed the molecular movement law of macromolecules, which indicated that the lower the molecular weight was and the higher the temperature was, the lower the friction coefficient along drawn direction was.