Molecular mechanisms of PEEK/PTFE solid lubricants and application to rolling point contacts with PTFE-like friction and reduced lubricant consumption

Polytetrafluoroethylene (PTFE) is widely used as a solid lubricant due to its exceptional friction-reducing properties, but its high wear rates reduce its applicability in highly loaded tribological contacts. This limitation can be addressed by continuously relubricating the contact area with transfer films originating from a sacrificial PTFE reservoir, or by embedding PTFE in more wear-resistant polymers such as polyether ether ketone (PEEK). Here, we combine the two approaches and present a PEEK/PTFE compound obtained by reactive extrusion with irradiated PTFE that shows PTFE-like friction in ball-on-disk tribometer tests. The amount of material required for relubrication is reduced by a factor of 350 compared to pure PTFE. Molecular dynamics simulations with a newly developed density-functional-based, non-reactive force field suggest that PEEK/PTFE can effectively reduce friction by forming multiple PTFE shear bands, over which the shear deformation is distributed. Friction reduction is independent of whether PTFE forms a single layer or multiple thin layers, provided the thickness of the total PTFE sheared remains constant. Multiple PEEK/PTFE interfaces anchor PTFE more strongly, prevent its squeeze-out, and limit PTFE removal from the contact zone. We present a framework for designing solid lubricants that enable double-transfer lubrication in rolling-point contacts with minimal lubricant consumption, suitable for high-performance, fluid-free rolling-element bearings and other highly loaded rolling/sliding contacts, such as roller tappets or radial piston motors.