Layer by layer assembled functionalized graphene oxide-based polymer brushes for superlubricity on steel-steel tribocontact

This study demonstrates a simple and multistep approach for a covalent functionalization of chemically-prepared graphene oxide (GO) using branched polyethylenimine (PEI) through nucleophilic addition reaction to prepare GO-PEI. Further layer-by-layer (LBL) assembly on functionalized GO-PEI with anionic polyelectrolyte, poly(acrylic acid sodium salt) (PAA) and poly(sodium 4-styrenesulfonate) (PSS) have been undertaken to fabricate polymer brushes (PB). The physicochemical structures of GO, GO-PEI and LBL assembled PB [GO-PEI-PAA and GO-PEI-PSS] have been explored using standard spectral and morphological analysis. The macrotribological results demonstrated that GO-PEI-PAA/GO-PEI-PSS (0.5 wt%) as paraffin oil dispersible additives significantly decreased the coefficient of friction (COF) and wear at different contact pressures of steel-steel tribopairs. The influence of contact pressure and load-bearing ability of the polymer-grafted GO as nanolubricants have been examined carefully. The COF of PB particles provided a reduction of 85% (low pressure, similar to 0.9 MPa) and 66.65% (high pressure, similar to 1.35 GPa) compared to lube paraffin oil and exhibited a lower specific wear rate (2.26 x 10(-8) mm(3) N-1 m(-1)) at macrotribological pin/ball-on-disc trials, revealing superior lubricity. The PB containing nanolubricants also exhibited high load-bearing ability (till similar to 1000 N load, P-m similar to 6.1 GPa) with considerably lower COF and wear, which were investigated using a four-ball tribotester. Among the functionalized polymeric GO particles, PSS polyelectrolyte containing GO-PEI-PSS showed better COF and wear reduction ability with extremely high load-bearing capacity due to the strong interfacial adhesion properties of PSS to generate strong protective synergetic lubricating tribofilm into the rubbing interfaces, which is comprehensively investigated by post-tribological analysis.

Automated summary

This study demonstrated a simple and multistep approach for the covalent functionalization of chemically-prepared graphene oxide with branched polyethylenimine to prepare polymer brushes as nanolubricants. The PB particles significantly reduced the coefficient of friction and wear in steel-steel tribopairs, while exhibiting high load-bearing ability. Among the functionalized polymeric GO particles, PSS polyelectrolyte containing GO-PEI-PSS showed the best COF and wear reduction ability with high load-bearing capacity.