Preparation Of Nickel-based Nanolubricants And Investigation Of Their Tribological Behavior

Nickel nanoparticles as lubricant additives can significantly improve the tribological properties oflubricating base stock. However, few are currently available about their tribological action mechanism aswell as the correlation between their tribological properties and grain size plus morphology. Therefore,in-situ surface-modification technique is adopted in the present research to fabricate a series of Ninanoparticles as well as Cu@Ni nanoparticles with different size and morphology. Furthermore, thecorrelation among the composition, structure, size, and morphology and tribological properties ofas-synthesized additives were explored, and the friction-reducing, antiwear, and worn surface self-healingmechanisms of the additives were discussed. In the meantime, attempts were made in relation to thetribological basic study of diamond-like carbon （DLC）-Ni solid-liquid mixed lubricating system. The maincontents and research results of this thesis are as follows:（1） Preparation of Nickel-based Nanolubricants via a Facile In-situ One-step Route andInvestigation of Their Tribological PropertiesNickel-based nanolubricants containing size-tunable monodispersed nickel nanoparticles were in-situsynthesized in poly-alpha olefin （denoted as PAO6） via a simple one-step thermal decomposition methodwith Ni（HCOO）₂·2H₂O as the Ni source, PAO6as base oil, as well as oleylamine and oleic acid as thesurface-capping agents. The phase composition, micro-morphology and size, and chemical structure ofas-prepared Ni nanoparticles were characterized by means of X-ray diffraction （XRD）, transmissionelectron microscopy （TEM）, and Fourier transform infrared spectrometry （FTIR）. The size-dependenttribological properties of as-synthesized Ni-based nanoparticles as lubricant additive in PAO6wereevaluated with a four-ball friction and wear tester. The morphology of wear scar and chemical states ofsome typical elements on worn steel surfaces were analyzed by scanning electron microscopy （SEM） andX-ray photoelectron spectroscopy （XPS）; and the tribological action mechanism of the additives wasdiscussed. It has been found that the antiwear ability of as-synthesized Ni nanolubricant is closely related toits size and concentration in the base stock. Namely, the smaller the size of Ni nanolubricant, the better itsantiwear ability; and PAO6containing0.05%（mass fraction） Ni nanolubricant possesses much better antiwear ability that the base stock. This is because surface-capped Ni nanoparticles in as-preparednanolubricant are able to release highly active Ni nanocores and O-and N-containing organic modifyingagents that can readily form boundary lubricating film on sliding steel surfaces. In the meantime, Ninanoparticles with a smaller size are of high surface activity and can be readily deposited on sliding steelsurfaces to form a stable and continuous protective layer thereon. As a result, both the boundary lubricatingfilm and the chemically adsorbed and deposited Ni layer contribute to greatly improve the antiwearbehavior and load-carrying capacity of PAO6base stock.（2） Preparation of Cu@Ni Nanolubricants via a Facile In-situ One-step Route and Investigationof Their Tribological PropertiesCu@Ni nanolubricants containing different content of Cu and possessing improved dispersibility plusstability were in-situ synthesized in PAO6via a simple one-step thermal decomposition method withNi（HCOO）₂·2H₂O and Cu（HCOO）₂·4H₂O as the Ni and Cu source, PAO6as base oil, as well as oleylamineand oleic acid as the surface-capping agents. The phase composition, micro-morphology and size, andchemical structure of as-prepared Cu@Ni nanolubricants were characterized by XRD, TEM, and FTIR.The tribological properties of as-prepared Cu@Ni as additive in PAO6were evaluated with a four-ballfriction and wear tester; and their tribological action mechanism was examined based on SEM and XPSanalyses of worn steel surfaces. It has been found that, as compared to Ni-based nanolubricants,as-synthesized Cu@Ni nanolubricants exhibit better friction-reducing, anwear, and extreme pressureproperties. This is because highly active Ni nanocores and O-and N-containing organic modifying agentscan readily form boundary lubricating film on sliding steel surfaces, while Cu nanocores can be readilydeposited on sliding steel surfaces to form a stable and continuous protective layer （self-healing lubricationfilm） thereon.（3） Investigation of Tribological Properties of DLC-Ni solid-liquid composite lubrication systemThe friction and wear behavior of steel/DLC film sliding pair under unlubricated condition andoil-lubricated condition （diisooctyl sebacate base stock and the same base stock containing Ninanolubricant） was separately evaluzted, and the effects of lubricants, load, and sliding velocity on thefriction and wear behavior of DLS film were comparatively investigated. Besides, the Stribeck curves ofthe solid-liquid composite lubrication system were established. It has been found that there exists synergistic friction-reducing effect among DLC film and the two kinds of liquid lubricants; and inparticular, DLC film and Ni nanolubricant exhibit excellent synergistic friction-reducing effect. This isbecause Ni nanoparticles with high surface activity can be readily adsorbed on sliding steel surfaces toform a boundary lubricating protective layer thereby reducing the friction resistance of the sliding interfaceand decreasing wear. Moreoevr, as to DLC-Ni composite lubricating system, the bottom of correspondingStribeck curve is wide, corresponding to widened boundary lubrication regime and mixed lubricationregime.（4） Preparation of Shape-controlled Ni Nanoparticles and Investigation of Their TribologicalPropertiesShape-tunable monodispersed nickel nanoparticles were synthesized via a simple one-step thermaldecomposition method with Ni（HCOO）₂·2H₂O as the Ni source as well as oleylamine and sodiumdodecanesulphonate as the surface-capping agents. The phase composition, micro-morphology and size,and chemical structure of as-prepared Ni nanoparticles were characterized by XRD, TEM, and FTIR. Theshape-dependent tribological properties of as-synthesized Ni nanoparticles as additives in PAO6wereevaluated with a four-ball friction and wear tester; and their tribological action mechanism was examined inrelation to SEM and XPS analyses of worn steel surfaces. It has been found that Ni nanoparticles withdifferent morphology exhibit different lubricating performance; and in particular, triangular plate-like Ninanoparticles exhibit better lubricating performance than sphere-like and triangular rod-like Ninanoparticles. This is possibly because triangular plate-like Ni nanoparticles with a larger surface contactarea on sliding steel surface can be readily adsorbed and deposited to form a stable and continuousprotective layer thereon thereby avoiding the direct contact between the sliding steel interfaces.