Superlubricity of Nanocomposites of Polyaniline-Functionalized Reduced Graphene Oxide with Yttrium and Vanadium-Codoped Zinc Oxide Nanoparticles

The surface modifications of reduced graphene oxide (rGO) nanosheets have been achieved by a conducting polymer, polyaniline (PANI), in an emeraldine state to yield PANI–rGO, which has hydrogen bonding, electrostatic interactions, and π–π interactions between PANI and rGO nanosheets. For the advancement of the tribological properties of PANI–rGO, its nanocomposite was prepared with yttrium, vanadium-codoped zinc oxide nanoparticles (Y–V–ZnO). The resulting nanocomposite (Y–V–ZnO/PANI–rGO) shows improved tenacity to the tribo-surfaces, enhanced dispersibility in the base oil, and enormously reduced agglomerating and restacking tendencies. The effect of these modifications can be adjudged by the optimized concentrations required for maximum antiwear efficiency. The optimized concentration (% w/v) reduces drastically for Y–V–ZnO/PANI–rGO, Y–V–ZnO (0.25), PANI–rGO (0.0125), and Y–V–ZnO/PANI–rGO (0.005). The anchored Y–V–ZnO nanoparticles on PANI–rGO have led to superlubricity, as is visibly seen by the tribological properties established from ASTM D4172 and ASTM D5183 tests on a four-ball tester. Specifically, a very small value of the coefficient of friction, 0.0087, supports superlubricity. The tribological properties of the well-characterized additives (by FTIR, p-XRD, HR-SEM, TEM, and XPS) improved in the following order: rGO < PANI < ZnO < PANI–rGO < Y–ZnO (4%Y) < V–ZnO (4%V) < Y–V–ZnO (1%Y and 3%V) < Y–V–ZnO/PANI–rGO. The morphological studies by SEM and AFM of the wear scar surface have corroborated the abovementioned order. The nanoparticles have reinforced the composite and separated the nanosheets to avoid their repiling. On the contrary, nanosheets have averted agglomeration of the nanoparticles. Doping in ZnO nanoparticles by yttrium and vanadium in different oxidation states has ameliorated the tribological properties by introducing more defects. Thus, all the components of the composite have assisted mutually in the upgradation of tribological behavior.