Solid lubricants such as polytetrafluoroethylene (PTFE) are used in rolling-element bearings (REBs) when conventional lubrication (i.e. by fluids or greases) cannot be applied owing to extreme operating conditions (e.g. high temperatures or vacuum). Often a double transfer film mechanism is used with a cage acting as a lubricant reservoir resupplying the REB with solid lubricant by cage wear. An increase in service life of such bearings requires a better understanding of the transfer processes and friction mechanisms in the sliding and rolling contacts.

In model experiments [1], the effect of PTFE resupply on friction and lubricant film formation in steel/steel and steel/glass rolling contacts is investigated. A ball-on-disk tribometer is enhanced by a pin-on-disk sliding contact that transfers PTFE to the steel or glass disk. Increasing the pin load results in an increased PTFE film thickness in the rolling contact and a significant decrease in friction.

To elucidate the observed film transfer and friction mechanism, we perform classical molecular dynamics sliding simulations with a newly developed density-functional-based, non-reactive force field for PTFE-lubricated iron oxide contacts. A strong adhesion of PTFE chains to iron oxide drives transfer film formation, while shear-induced chain alignment within PTFE results in reduced friction. The simulations reveal an anti-correlation between PTFE film thickness and friction coefficient – in agreement with the experiments. These investigations are a first step towards methods to control PTFE transfer film formation in REBs.

References

[1] S. von Goeldel; T. Reichenbach; F. König; L. Mayrhofer; G. Moras; G. Jacobs, M Moseler; Tribology Letters, 2021, 69, 136, 1-16.