Tribological performance of graphite-filled polyimide and PTFE composites in oil-lubricated three-body abrasive conditions

Graphite filled polytetrafluoroethylene (PTFE) and polyimide (PI) composites were tested against tool steel under nominal contact pressure of 6.32 MPa and sliding speed of 1.91 m/s in clean ISO 46 lubricant, and also contaminated lubricant with 4200 ppm of silica sand. Pin-on-disk test results show higher friction and wear of the tribopair in the contaminated condition. Micromechanical properties of the bulk polymers were measured using nanoindentation technique, and softening of the material at elevated temperatures was measured. SEM analysis of the contacting surfaces before and after testing showed abrasive wear as the dominant wear mechanism, and microcutting was a less pronounced wear mechanism in the case of PI composite in contaminated test condition. EDS analysis showed that there was no film transfer from polymers to the countersurface, however it indicated higher oxidation on the wear track of the steel disk slid against PI. Particle embedding in the polymer was shown by EDS and is the reason for scratches seen on the steel disk wear tracks. Both polymers are found to be suitable materials from a tribological standpoint for bearing pad applications.