Polytetrafluoroethylene (PTFE) Lubrication of Rolling Point Contacts by Double Transfer Films: Relationships between Friction and Lubricant Film Distribution Revealed by Spacer Layer Imaging and Molecular Dynamics

Solid lubricants such as polytetrafluoroethylene (PTFE) are used to lubricate rolling-element bearings (REBs) when grease or oil cannot be applied. To compensate for PTFE’s high wear, the rolling contacts can be relubricated by solid transfer films that form at the sliding contacts between the rolling elements and PTFE reservoirs in the cage pockets. Excessive PTFE relubrication causes a rapid depletion of the PTFE reservoirs, while insufficient PTFE relubrication leads to high friction and wear. Both situations can limit the REB’s lifetime. In this study, we use a ball-on-disk tribometer enhanced with a PTFE relubricating pin as a model system for sliding and rolling contacts in an REB. Using the spacer layer imaging method (SLIM), we measure the film thickness distribution in the rolling point contact for different normal loads applied to the PTFE pin. This reveals spatially and temporally inhomogeneous films. Non-reactive, all-atom molecular dynamics (MD) simulations show that the PTFE lubricating film can be described as a Herschel-Bulkley fluid with a yield stress that is caused by perturbations of the perfectly layered molecular flow. By combining the MD-derived friction laws with the experimental film thickness distributions, we can quantitatively reproduce the measured macroscopic friction coefficients. The study shows that a tradeoff between friction reduction and low lubricant consumption can be obtained with a homogenous PTFE lubricant film with a thickness of only about 20 nm.