The tribological behaviour of glass filled polytetrafluoroethylene

Polymers and polymer composites are steadily gaining ground over metals in the field of engineering applications in tribology. Laboratory wear tests were carried out under ambient temperatures with no lubricant as well as in distilled water at an average sliding velocity of 0.2 m/s and contact pressures of 2.6-6.4 MPa. Three forms of glass viz. glass fibres, glass beads and glass flakes, each with a content of 25% weight were used in this study. Both hollow and solid glass beads were used. The sliding wear of the different glass filled PTFE composites was dependent on their ability to form transfer films on the counterface. The glass bead filled PTFE showed comparatively thicker films and higher wear rates than other forms of glass filled grades. The glass fibres and solid glass beads showed the lowest wear whilst hollow beads showed the highest under both low and high pressures due to crumbling and crushing of the beads during the sliding process. The glass flake filled PTFE showed relatively high but stable wear results up to 4.5 MPa above which the wear rate increased dramatically. A marginal increase in wear was achieved by using high aspect ratio glass fibres to the PTFE matrix. No correlation between the size of glass reinforcement and wear rate was established. The addition of a lamellar solid lubricant to the glass fibres reduced both the wear and friction of PTFE. The study of the transfer film growth by means of an optical microscope revealed that it was due to the mechanical interlocking of the polymer fragments into the metal asperity valleys. The compositional changes in the transfer film were studied by XPS which, among other things, showed presence of metal fluoride on the metal counterface.