Zinc oxide (ZnO) and magnesium-doped zinc oxide nanoparticles, Zn 0.88 Mg 0.12 O (ZMO), were prepared by autocombustion method. Further, nanocomposites of the asprepared nanoparticles with microwave-synthesized reduced graphene oxide (rGO) nanosheets, ZnO−rGO and ZMO− rGO, have also been prepared with a view to see the effect of doping of magnesium in zinc oxide on the tribological properties of the nanocomposite. Morphologies of nanoparticles/nanosheets and their nanohybrids have been studied by employing scanning electron microscopy (SEM)/highresolution (HR) SEM with energy-dispersive X-ray (EDX), transmission electron microscopy (TEM)/HR-TEM, X-ray diffraction, Fourier transform infrared, UV−visible, Raman, and Xray photoelectron spectroscopy (XPS) techniques. Triboactivity of the additives in paraffin oil has been interpreted considering the parameters mean wear scar diameter, coefficient of friction, load-carrying capacity, and wear rates obtained from ASTM D4172 and ASTM D5183 tests using a four-ball lubricant tester at optimized concentration (0.125% w/v). The performance of base lube and its admixtures has been found to lie in the order ZMO−rGO > ZnO−rGO > ZMO > ZnO > rGO > paraffin oil. Outstanding enhancement in triboactivity of nanocomposites, particularly that of ZMO−rGO indicates that nanoparticles are irrefutably instrumental in reinforcement of rGO, and on the other hand, rGO is associated with abatement of agglomeration of the nanoparticles. Thus, interactions between rGO and nanoparticles are vehemently synergic in nature. It is noteworthy that the best results were obtained with the following optimized concentrations: ZnO/ZMO 0.25%; rGO 0.15% and composites 0.125% w/v. Morphological studies of the wear track lubricated with different additives have been performed using SEM and contact mode atomic force microscopy. Results are in conformity with the order given above. The EDX analysis of ZMO−rGO exhibits the presence of zinc and magnesium on the worn surface, supporting their role in the formation of in situ tribofilm. Their role is further corroborated by XPS studies. Owing to their excellent tribological behavior, these sulfur-and phosphorusfree composites may be recommended as potential wear and friction modifiers.

Zirconia and 10%, 20%, and 30% cerium-doped zirconia nanoparticles (ZCO), ZCO-1, ZCO-2, and ZCO-3, respectively, were prepared using auto-combustion method. Binary nanohybrids, ZrO2@rGO and ZCO-2@rGO (rGO = reduced graphene oxide), and ternary nanohybrids, ZrO2@rGO@MoS2 and ZCO-2@rGO@MoS2, have been prepared with an anticipation of a fruitful synergic effect of rGO, MoS2, and cerium-doped zirconia on the tribo-activity. Tribo-activity of these additives in paraffin oil (PO) has been assessed by a four-ball lubricant tester at the optimized concentration, 0.125% w/v. The tribo-performance follows the order: ZCO-2@rGO@MoS2 > ZrO2@rGO@MoS2 > ZCO-2@rGO > ZrO2@rGO > MoS2 > ZrO2 > rGO > PO. The nanoparticles acting as spacers control restacking of the nanosheets provided structural augmentation while nanosheets, in turn, prevent agglomeration of the nanoparticles. Doped nanoparticles upgraded the activity by forming defects. Thus, the results acknowledge the synergic effect of cerium-doped zirconia and lamellar nanosheets of rGO and MoS2. There is noncovalent interaction among all the individuals. Analysis of the morphological features of wear-track carried out by scanning electron microscopy (SEM) and atomic force microscopy (AFM) in PO and its formulations with various additives is consistent with the above sequence. The energy dispersive X-ray (EDX) spectrum of ZCO-2@rGO@MoS2 indicates the existence of zirconium, cerium, molybdenum, and sulfur on the wear-track, confirming, thereby, the active role played by these elements during tribofilm formation. The X-ray photoelectron spectroscopy (XPS) studies of worn surface reveal that the tribofilm is made up of rGO, zirconia, ceria, and MoS2 along with Fe2O3, MoO3, and SO42− as the outcome of the tribo-chemical reaction.

For enhancement of the tribological activity of nanolamellar graphene oxide (GO), its nucleophilic substitution was performed by methionine to yield methionine-functionalized reduced graphene oxide (M-rGO). Further, noncovalent functionalization of another tribo active material, nanolamellar MoS2, was accomplished by lanthanum (7%)-doped yttria nanoparticles (NPs), resulting in the formation of a nanocomposite, (La-Y2O3)-MoS2. The doped NPs were deliberately chosen for this purpose because there was a clear increase in the wear/friction-reducing tendencies of yttria after doping with lanthanum. For further advancement of the tribological activity, a ternary nanocomposite (La-Y2O3)-MoS2-(M-rGO) was synthesized containing lanthanum-doped yttria NPs, M-rGO, and MoS2 nanosheets. The NPs, nanosheets, and composites have been characterized by powder X-ray diffraction, high-resolution scanning electron microscopy (HR-SEM), transmission electron microscopy, and Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) was employed to study the chemical states of different elements in (La-Y2O3)-MoS2-(M-rGO). The tribological properties of well-characterized composites were evaluated in paraffin oil (PO) using a four-ball tester according to ASTM D4172 and ASTM D5183 standards at the optimized concentration, 0.20% (w/v). There was incremental evolution in the tribological properties from plain PO through Y2O3, MoS2, M-rGO, La-Y2O3, and (La-Y2O3)-MoS2 and finally to (La-Y2O3)-MoS2-(M-rGO). Here functionalization of GO has invigorated its structure. Both nanosheets coordinated to control agglomeration of the NPs. The NPs prevented the nanosheets from restacking. SEM and atomic force microscopy images of the wear scar validated the results of tribological tests. The presence of yttrium, lanthanum, sulfur, and molybdenum besides carbon, nitrogen, and oxygen in the energy-dispersive X-ray spectrum of the worn surface in the presence of PO blended with (La-Y2O3)-MoS2-(M-rGO) is indicative of its strong adsorption on the surface. On the basis of XPS studies of the wear track, the constituents of the tribofilm could be identified as adsorbed rGO, yttria, lanthanum oxide, and MoS2 in addition to tribochemically produced Fe2O3, MoO3, and SO4 2–. The mutualistic approach of the constituents has yielded splendid results.

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).

The tribological activity of the synthesized nanosheets of a polymeric graphitic carbon nitride, g-C3N4, was upgraded by introducing hydrothermally synthesized N-doped zinc oxide nanorods. The high-resolution scanning electron microscopy, TEM, and HR-TEM studies of the hybrid (N–ZnO/g-C3N4) reveal that N-doped ZnO nanorods were spread over g-C3N4 nanosheets. The tribological activity of well-characterized nanomaterials was graded in paraffin oil (PO) at an optimized concentration, 0.20% w/v, on a four-ball tester conducting ASTM D4172 and ASTM D5183 tests. According to the observed tribological data, mean wear scar diameter, friction coefficient (COF), and seizure load, nanorods performed much better than the nanosheets. The hybrid exhibited highly advanced activity because of the synergy between noncovalently interacting nanorods and nanosheets. The SEM and AFM analyses of the wear pathway corroborated the tribological results. The energy-dispersive X-ray and X-ray photoelectron spectroscopy studies of the tribofilm confirmed the elemental composition and chemical states of all the elements, respectively. A possible mechanism of lubrication has been presented.

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