Friction-reducing and anti-wear properties of 3D hierarchical porous graphene/multi-walled carbon nanotube in castor oil under severe condition: Experimental investigation and mechanism study

Traditional lubricants fail to meet the trend of green development. Consequently, eco-friendly hybrid nano lubricants are synthesized by incorporating 0.5 wt% 3D hierarchical porous graphene (3D HPG) and 0.7 wt% multi-walled carbon nanotubes (MWCNT) nanoparticles to castor oil. The performances of nano lubricants are determined by linear reciprocating wear test, and the obtained wear marks on steel balls and plates are evaluated by depth-of-field microscopy, scanning electron microscopy, energy dispersive spectrometer and Raman spectroscopy. The results of steel balls and steel plates wear analyses are highly consistent. Characterization results for steel plates show that the 3D HPG: MWCNT (0.5:0.7) achieves a reduction in friction coefficient and wear volume up to 26% and 48% at low speed and heavy load conditions (4 Hz - 20 N, 2.7 GPa), respectively. The mechanism of friction-reducing and anti-wear at low speed and heavy load is that 3D HPG is torn into 2D graphene nanosheets due to its easy shear performance, which forms a 2D graphene nanosheets/MWCNT/2D graphene nanosheets sandwich structure to be further involved in lubrication. This rolled bearing-like synergistic effect can further reduce friction and wear. All in all, the environmentally friendly nano lubricants consisting of 3D HPG/MWCNT hybrid nanoparticles and castor oil have sufficient potential for application in the field of heavy machine parts. •A good dispersion stability and eco-friendly nanolubricant is developed.•Friction-reduction and anti-wear of nanolubricant are improved by 26% and 48%.•Synergistic mechanism of 3D HPG/MWCNT nanocomposites as oil additive is revealed.