Evaluation of tribological behavior of paraffin oil based lithium greases with graphene oxide and reduced graphene oxide nano-sheets as an additive

The present study deals with the tribological evaluation of graphene oxide (GO) and reduced graphene oxide (rGO) in paraffin oil based lithium grease. GO nano-sheets were synthesized by harsh oxidation and subsequent exfoliation of graphite powder. Further hydrazlne hydrate was used as a reducing agent to develop rGO nano-sheets. In the formulation of paraffin grease, the paraffin oil was used as a base oil and 12-lithium hydroxyl stearate metal soap as a thickener agent. The concentration of metal soap was fixed at 14%w/w. The concentration of GO and rGO nano-sheets in the formulated grease was varied between 0.01-0.05% w/w. The anitwear property of GO and rGO in paraffin grease was evaluated as per ASTM standards using four-ball tester. The antiwear test results show that maximum reduction in friction by GO and rGO nano-sheets were ~34% and ~50% respectively at a concentration of 0.01% w/w. The reduction in the wear scar diameter was found to be about ~37% in both greases at a concentration of 0.01% w/w. The synthesized GO and rGO nano-sheets had a flake like structure and they get deposited on the worn surfaces and this has formed a surface protective tribo-film at the contacting surfaces. The various analytical tools such as X-ray powder diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM) techniques were used for the characterization GO and rGO nano-sheets. Worn surface morphology was scanned by scanning electron microscopy (SEM) and elemental characterization of the worn surface was done by energy dispersive spectroscopy (EDS). (author)