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Advanced engineering polymers of the polyaryletherketone (PAEK) family with carbon fibre reinforcement are finding application in engineering systems as tribological bearing surfaces under severe operating conditions that cyclically move the polymer into and beyond the glass transition temperature region. To support such an application, the friction in high-speed and low-load PAEK-steel sliding contacts was investigated both unlubricated and lubricated with a trinonyl trimellitate ester, a base fluid for high temperature industrial lubricants. Four polymers in the PAEK family, PEEK, PEK, PEKEKK and PEKK, with 30 wt% of carbon fibre whiskers were tested against an AISI 4140 steel disc. When unlubricated, low friction depended upon the formation of a PAEK transfer film on the steel disc and when this became unstable in the glass transition region the friction increased to much higher levels with associated polymer surface damage. Frictional heating due to the high sliding speed dominated the differences in glass transition behaviour between the four PAEKs. When lubricated, the lubricant film controlled friction and there was no significant effect of the glass transition of any of the PAEKs. The irreversible nature of the glass transition in PAEKs in such tribological applications, due to high surface damage at high temperature, means that it is essential to ensure effective lubrication in both fluid film and boundary lubrication.

For a chromium-nitrided steel piston ring section sliding against an electroplated nickel matrix silicon carbide coated eutectic aluminium—silicon alloy automotive cylinder bore in a reciprocating tribometer, the ionic liquid (IL) imidazolium tetrafluoroborate has an excellent tribological performance as a lubricant additive in the presence of tritolyl phosphate (TTP). A low friction coefficient of 0.015 and a substantial tribofilm was generated. The performance of this base oil/additive combination was superior to the base oil alone or the separate additives in the base oil. The substantial improvement in tribological performance given by addition of an IL and TTP, 1 per cent by volume of each, to a group III base oil indicates a synergistic interaction between the IL and TTP. The need to understand the chemical and physical nature of the tribofilm is emphasized.

The use of imidazolium tetrafluoroborate, IMM+BF4−, and hexafluorophosphate, IMM+PF6−, ionic liquids as lubricants was investigated at 25 and 100 °C to show lower friction coefficients but higher wear rates than a reference hydrocarbon lubricant. The ionic liquids readily form tribofilms at the lower temperature but have difficulty in forming partial films at the higher temperature. Wear tracks for a Plint TE77 reciprocating ball-on-plate test using ionic liquids show smoother surfaces, with small pits developing, compared to the reference hydrocarbon lubricant test result. Similar ionic liquids not containing fluorine were found to be less effective as lubricants. Addition of ionic liquids to a base grease and a formulated high temperature grease gave surprisingly large increases in the weld load for the Four Ball extreme pressure test. Little difference between base grease and base grease+5 per cent ionic liquid was observed for the TE77 test. There was no clear indication of the effect of alkyl substituent chain length on the imidazolium cation for the Four Ball test wear scar diameter. Addition of 1 per cent tricresyl phosphate (TCP) to ionic liquids rapidly establishes a tribofilm and reduces the wear volume by 64 per cent compared to the same test for the neat ionic liquid or neat TCP. Addition of 1 per cent TCP and 1 per cent ionic liquid to a Group III base oil also establishes a substantial tribofilm and reduces wear volumes compared to the base oil with 1 per cent TCP alone or the base oil with 1 per cent ionic liquid alone. Ionic liquids show promise as neat liquid lubricants by establishing a tribolayer chemically adsorbed to the steel surfaces. They are not as effective as a reference hydrocarbon lubricant in reducing wear of those surfaces by tribocorrosion. The fluorine-free ionic liquids investigated were not as effective as those containing fluorine. The addition of ionic liquids to grease, base or fully formulated, gave a substantial improvement in performance, which indicates a synergistic interaction with the additives present in the formulated grease. There is also clear evidence of a strong synergistic effect between ionic liquids and TCP, both for the neat ionic liquids and for 1 per cent dilution of each respectively in a Group III base oil to give a thick tribofilm and substantially reduced wear in the TE77 ball-on-plate test. The nature of the synergy between ionic liquids and TCP requires further investigation.