Effects of graphite-based material size on the mechanical and tribological performance of polyimides under drying sliding conditions

The present investigation evaluates the effect of graphite-based filler (GBF) size on the mechanical and tribological performance of polyimide (PI) solid lubricant composites. During dry sliding tribological experiments, micro-, nano-graphite (MG, NG), and ultrathin graphene (GN) additives were considered. The results revealed that atomic-thickness GN outperformed micro- and nano-sized GBF filler in terms of mechanical and tribological performance when added into PI matrix. It was inferred that the GN was able to generate sufficient lubricating phases at the frictional interface due to their small size and ultrathin morphology, which provided them with enhanced tribological properties in comparison with the micro and nano-sized GBF fillers. At the friction interface, GN was oxidized by reciprocating shear force and friction heat and formed a lubricating layer of graphene oxide (GO), as evidenced by Fourier transform infrared (FTIR) and Raman spectroscopy. Atomistic modeling techniques were also used to elucidate the surface/interface lubrication mechanism, where GO was tightly adsorbed at frictional interfaces by van der Waals. The results facilitate better understanding of the size effect on the wear mechanism of solid lubricants.