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

• Published in: Soft matter 2021-07, Vol.17 (29), p.714-731
• Main Authors: Samanta, Suprakash, Sahoo, Rashmi R
• Format: Article
• Language: English
• Subjects: Acrylic acid; Additives; Adhesive strength; Bearing capacity; Brushes; Coefficient of friction; Contact pressure; Graphene; High pressure; Interfaces; Low pressure; Lubricity; Paraffin; Paraffins; Polyelectrolytes; Polyethyleneimine; Polymers; Pressure; Reduction; Rubbing; Sodium; Sodium salts; Steel; Tribology; Wear rate
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/d1sm00690h

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, ∼0.9 MPa) and 66.65% (high pressure, ∼1.35 GPa) compared to lube paraffin oil and exhibited a lower specific wear rate (2.26 × 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 ∼1000 N load, P m ∼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. Tribological behavior of graphene-based polymer brushes (Gr-PB) in lube oil is investigated. Gr-PB reduce COF and the wear of steel-steel tribocontact in variable pressure conditions aided by the formation of robust tribofilm in sliding interface.