Tribological Investigations of Nano and Micro-sized Graphite Particles as an Additive in Lithium-Based Grease

Graphite is a well-known solid lubricant (SL) additive widely used both as a standalone SL and also as additive in lubricating oils, greases, composites, etc. Size of the additive particles especially nano-particles (NPs) is a major attribute to the performance properties of a composite, oil, grease, etc. This paper highlights the influence of graphite particles of four sizes viz. 50 nm, 450 nm, 4 microns and 10 microns incorporated in the grease in identical amount (4 wt. %) on the anti-friction (AF), anti-wear (AW) and extreme-pressure (EP) performance. The results indicated that all sizes proved beneficial for all the selected properties. Higher the size of particles, lower was the improvement in performance. The particles were most effective as anti-friction additive (AFA) followed by anti-wear additive (AWA) and then extreme-pressure additive (EPA). The NPs exhibited highest improvement as AFA (57%), AWA (41%) and EPA (25%). Raman Spectroscopy proved the formation of exfoliated graphitic layer on the worn surface of balls. Furthermore, SEM micrograph with elemental mapping and XPS spectroscopy analysis proved supportive in comprehending the mechanisms responsible for improved tribo-performance.