Tribological Properties Of Magnetic And Clay Nanoparticles In Lubricating Oil

As lubricating additives, inorganic nanoparticles (NPs) have superior anti-wear andanti-friction properties due to their distinctive physical and chemical characterizations (i.e.,great specific surface area, superior diffusivity, low sintering temperature, low fusion and highhardness, etc.). The multilayer films with a lower shearing are formed on the friction pairsurfaces in the presence of NPs in lubricating oil, resulting in the reduction of the averagefriction coefficient, and the filling and reparation of the friction pair surfaces. This dissertationwas thus to investigate the tribological properties of the magnetic Fe3O4NPs, kaolin clay NPsand Iillite/Smectite (I/S) clay NPs in#40lubricating oil as lubricating additives.Firstly, the anti-friction properties of various NPs (i.e., magnetic Fe3O4NPs, kaolin clayNPs and I/S clay NPs) were evaluated by average friction coefficient (c f), wear scar diameter(WSD), wear weight and maximum non-seizure load (PB) of lubricating oil, etc., which wereobtained in a four-ball tribo-tester, respectively. The effects of micro-morphologies of Fe3O4NPs (i.e., hexagonal, octahedral and irregular morphologies), NPs concentration in thelubricating oil and friction duration on the anti-friction properties in#40lubricating oil werealso investigated. The results show that all the NPs additives (i.e., magnetic Fe3O4NPs, kaolinclay NPs and I/S clay NPs) have superior anti-friction properties. The optimum concentrationsfor Fe3O4NPs, kaolin clay NPs and I/S clay NPs as an additive in lubricating oil are1.5,1.0and2.0wt%, respectively. The low values of averagec fand WSD of the friction pairsurfaces are obtained at each optimum concentration. Compared to the pure lubricating oil,the WSD was reduced by13.87,11.17, and10.18%and thec fwas reduced by58.16,47.96,and34.69%in the case of the oils containing Fe3O4NPs with hexagonal, octahedral, andirregular morphologies, respectively, at the concentration of1.5wt%. In the case of the oilscontaining kaolin clay NPs at the concentration of1.0wt%, the WSD andc fwere reducedby16.43%and24.21%, respectively. In the case of the oils containing I/S clay NPs at theconcentration of2.0wt%, thec fwas reduced by60%. The friction pair surfaces appear smooth and the furrows are shallow after the friction in the oils with NPs. In addition, thehexagonal morphology of Fe3O4NPs has a positive effect on the anti-friction properties ratherthan the octahedral and irregular morphologies due to its sliding friction mechanism, resultingin the more whole coverage for the friction pair surfaces and the more uniform protectivefilm.Secondly, the tribochemical behaviors of the NPs on the friction surfaces in thelubricating oil were investigated via the analysis of the properties of NPs (i.e., magnetic Fe3O4NPs with hexagonal morphology, kaolin clay NPs and I/S clay NPs), such as phasecomposition, chemical elemental composition, non-crystallizing degree, interplanar spacing,average grain size, magnetic characterization and transition temperature of oxidation, etc..The tribochemical properties of the lubricating oil and the friction pair surfaces related to theNPs were also analyzed. Moreover, the anti-friction mechanism of various NPs in lubricatingoil as additives were discussed, particularly for the formation, chemical composition andanti-friction properties of the dynamic self-repairing film, and the tribological properties ofvarious NPs (i.e., magnetic Fe3O4NPs with hexagonal morphology, kaolin clay NPs and I/Sclay NPs). The results show that, compared to the properties of the original NPs (beforefriction), the non-crystallizing degree and transition temperature of oxidation are reduced forall the NPs after friction. The phase composition and chemical composition on the frictionpair surfaces lubricated with the lubricating oil containing Fe3O4NPs with hexagonalmorphology at a concentration of1.5wt%are most abundant among all the NPs. Theproperties of the lubricating oil containing Fe3O4NPs with hexagonal morphology after48-dfriction does not change. There exists crystal water in the lubricating oil when kaolin clay NPsand I/S NPs are used. It is indicated that the adhesion between the NPs (i.e., Fe3O4NPs withhexagonal morphology, kaolin clay NPs and I/S clay NPs) and the friction pair surfaces canbe enhanced due to the presence of van der Waals force, organic chain introduced by surfacemodification, and magnetic force from the remanent magnetization (Mr) of Fe3O4NPs. Thetribochemical reactions between the NPs and the friction pairs surface occur in the drasticfriction, finally resulting in the formation of a mono-or multi-layer self-repairing film with various phases containing the elements Fe, C, O and the characteristic elements (i.e., elementFe for Fe3O4NPs and elements Al and Si for kaolin clay NPs and I/S clay NPs, respectively.)on the friction surfaces. The film could prevent the direct contact between friction pairsurfaces and decrease the friction and wear for the improvement of the tribological propertiesof the lubricating oil.In addition, compared to the anti-friction properties of various NPs (i.e., Fe3O4NPs withhexagonal morphology, kaolin clay NPs and I/S clay NPs) in lubricating oil, all the NPsadditives have superior tribological properties, and the adhesion and coverage of various NPson the friction pair surfaces are enhanced due to the layered structure, resulting in theformation of the uniform and whole coverage self-repairing film on the friction pair surfaces.It is indicated that the chemical composition of the lubricating oil containing various NPsafter48-h friction does not change. However，the physical and chemical properties of the NPsafter friction (i.e., phase composition, crystallizing degree, interplanar spacing, average grainsize, magnetic properties and transition temperature of oxidation, etc.) do vary. Besides thesize effect, van der Waals force, organic chain introduced by surface modification, theadhesion between Fe3O4NPs with hexagonal morphology and the friction pair surfaces can beenhanced due to the presence of magnetic force from the remanent magnetization (Mr). Thefriction pair surfaces can be entirely coverd by kaolin clay NPs and I/S clay NPs due to theirlarge surface areas and the unsaturated bonds (like Si-O、Si-O-Si、-OH). The release of O2from the clay NPs during friction results in the tribochemical reactions and the formation ofthe self-repairing film on the friction pair surface to improve the anti-friction properties of thefriction pairs.Thirdly, the thickness and pressure distribution of the oil film without and with Fe3O4NPs with hexagonal morphology at a concertration of1.5wt%between the crankshaft andconnecting rod of a gasoline engine were analyzed by a numerical method. The results showthat, compared to those of the lubricating oil, Fe3O4NPs with hexagonal morphology have aslight impact on the thickness of the oil film, even less in some positions on the journalbearing. However, the load carrying capacity of the oil film can be improved, which is in accordance with the PBresults of lubricating oil containing Fe3O4NPs.Finally, the effect of Fe3O4NPs with hexagonal morphology in#40lubricating oil on theanti-friction performance in a gasoline engine was evaluated. The results show that theconnecting rod surface appears smooth, the furrows are shallow and the chemical elementsdistribution on friction surface are uniform after friction in the lubricating oils containingFe3O4NPs with hexagonal morphology. The surface roughness and the average gasolineconsumption are reduced by approximately25%and10%, respectively. Moreover, thethickness of self-repairing film on the friction surface determined by X-ray photoelectronspectroscopy (XPS) is40～50nm.