Preparation of Nickel-Based Nanolubricants via a Facile In Situ One-Step Route and Investigation of Their Tribological Properties

Nickel-based nanolubricants containing size-tunable monodispersed nickel nanoparticles were in situ synthesized in poly-alpha-olefin (denoted as PAO6) via a simple one-step thermal decomposition method with Ni(HCOO)₂·2H₂O as the Ni source, PAO6 as base oil, as well as oleylamine and oleic acid as the surface-capping agents. The size-dependent tribological properties of as-synthesized Ni-based nanolubricants were evaluated with a four-ball friction and wear tester. The morphology of wear scar and chemical states of some typical elements on worn steel surfaces were analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy. The morphology and structure of Ni nanoparticles separated from as-synthesized nanolubricants were characterized by means of transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectrometry. It has been found that, when the volume of PAO6 in the reaction system is adjusted as 80, 40 and 20 mL, respectively, the size (average diameter) of Ni nanoparticles can be tuned from 7.5 to 13.5 and 28.5 nm. Ni nanoparticles separated from as-synthesized nanolubricants consist of face-centered cubic nanocrystal and have good dispersibility in a variety of organic solvents and good thermal stability as well. Moreover, as-synthesized Nickel-based nanolubricants exhibit good antiwear behavior even at a low Ni concentration of 0.05 % (mass fraction). This is because surface-capped Ni nanoparticles in as-prepared nanolubricants are able to release highly active Ni nanocores and O- and N-containing organic modifying agents that can readily form boundary lubricating film on sliding steel surfaces. In the meantime, Ni nanoparticles with a smaller size are of high surface activity and can be readily deposited on sliding steel surfaces to form a stable and continuous protective layer thereon. As a result, both the boundary lubricating film and the chemically adsorbed and deposited Ni layer contribute to greatly improve the antiwear behavior and load-carrying capacity of PAO6 base stock.