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.