Covalently grafting of self-assembled functionalized graphene oxide multilayer films on Si substrate for solid film lubrication

Graphene-based polymer brush containing multilayer film has been fabricated on silicon wafer (SW) via self-assembled multistep approach based on covalent insertion of 3-aminopropyltrimethoxysilane as a chemical linker on graphene oxide (GO) followed by covalently anchored with polyethyleneimine (PEI). A multilayer polymer brush film was then constructed by electrostatic layer-by-layer (LBL) self-assembly technique between positively charged outer layer PEI grafted GO and anionic polyelectrolyte, poly (sodium 4-styrenesulfonate) (PSS). The LBL formation and microstructure of as prepared films were characterized by Raman spectroscopy, atomic force microscopy, field emission scanning electron microscopy and elemental electron dispersive spectroscopy analysis. The changes in wettability of films during grafting of chemical components on silicon substrate were determined by water contact angle measurement. Microtribological performances of the films were investigated using ball-on-disc contact geometry in ambient condition. The influence of contact pressure and sliding velocity were also studied under reciprocating tribo contact to evaluate the load bearing ability of the films. The multilayer films exhibited low (similar to 0.15-0.17) and steady coefficient of friction (COF) at 1 N load (contact pressure similar to 0.54 GPa) compared to bare SW (similar to 0.6) with remarkable wear-resistivity. With increasing contact pressure (4 N, similar to 0.86 GPa), the multilayer films provided minimal COF and low wear depth with improved sliding durability among the films. The improved friction and wear resistivity of the multi-layer films are attributed to densely packed polymeric graphene lamella contributing low-resistance to shear during sliding and the presence of sulphur on PSS might assists in superior adhesion of delaminated film to steel counterface ball during shearing, which was investigated by microscopic and Raman spectral analysis.