Covalent
functionalization of graphene oxide (GO) was performed
by 2-aminoethyl diphenyl borate (ADB) to overcome the demerits of
plain GO/reduced GO (rGO) during lubrication such as the agglomeration
and restacking of nanosheets, poor dispersibility in the base lube,
poor adhesion to the steel surface, inadequate friction and wear-reducing
characteristics, and last abysmal load-carrying ability. ADB was deliberately
chosen to outreach the benefits of the enhanced lubrication by in
situ formed boron nitride during tribological testing due to boron–nitrogen
synergy. Nucleophilic attack of the −NH2 group of
ADB opened epoxide rings of GO, forming −NH–C–C–OH
and simultaneously reducing GO to rGO. The product, therefore, is
represented as ADB–rGO. For the betterment of lubricity, further
noncovalent functionalization was also considered using triboactive
phthalocyanine (Pc) or copper(II) phthalocyanine (CuPc). Based on
the results of molecular dynamics and tribological tests in paraffin
oil (PO), CuPc was preferred over Pc. Configurations of the adsorbate
CuPc on the surface reveal that CuPc tends to fold/unfold upon adsorption.
Thus, the CuPc nanotubes (NTs) were used for the noncovalent functionalization
of ADB–rGO. The techniques FTIR, p-XRD, SEM/HR-SEM, TEM/HR-TEM,
EDX, and XPS were applied to characterize the additives. The tribological
activity of different additives evaluated on a four-ball tester based
on ASTM D4172 and ASTM D5183 tests reveals the order for antiwear/antifriction
efficiencies and reduction in wear rates as CuPc–(ADB–rGO)
> CuPc NTs > Pc > ADB–rGO > GO > PO. Thus, CuPc–(ADB–rGO)
overpowered the demerits of GO. The morphology of the surface lubricated
with GO, ADB–rGO, was studied using SEM and AFM (contact mode).
EDX analysis of the steel surface in the presence of CuPc–(ADB–rGO)
divulges the additional elements boron, nitrogen, and copper in the
in situ formed tribofilm, highlighting their active role in improving
the tribological activity. XPS studies of the tribofilm show boron
nitride, boron oxide, iron oxides, cupric oxide, and graphitic compounds.
The synergic performance of layered structures rGO, CuPc, and in situ
formed boron nitride validated the spectacular tribological performance
of CuPc–(ADB–rGO).
