Study on the characterisation of the PTFE transfer film and the dimensional designing of surface texturing in a dry-lubricated bearing system

The ability of polytetrafluoroethylene (PTFE) to form tribo-film in dry sliding condition makes it a popular liner material for self-maintenance bearings, such as those in the helicopter main and tail rotors. Nowadays, hard fillers or reinforcing-fibres are added to form the PTFE composite with enhanced strength and wear resistance; however, this also leads to increasing possibility of wear tracks on counter-surface and therefore more difficulty in observing and characterising the transfer film. In this paper, a surface analysis method based on using SEM analysis, supplemented with a Time-of-flight Secondary-ion mass spectrometry (Tof-SIMS) techniques for calibration was developed to give a comprehensive view of the transfer film formation on bearing steel surfaces. Previous challenges in transfer film identification and characterisation were also pinpointed. A series of laser surface textures (dimples), designed for improving the tribological performance of the bearing were tested, and the effects on transfer film formation have been evaluated by the surface analysis method. It is found that the depth of the dimples should match with the thickness of the transfer film to achieve a benefiting effect of the film formation, while the diameter and the coverage of the textures needs to be relatively small to avoid a high level of abrasion.