Preparation Of Water-soluble Lanthanum Fluoride And Lanthanum Fluoride Composite Nanoparticles And Investigation Of Their Tribological Behavior

Water-soluble LaF₃nanoparticles were prepared by in-situ surface-modification method. The phasecomposition, morphology and chemical structure of as-synthesized LaF₃nanoparticles were analyzed bymeans of X-ray diffraction （XRD）, transmission electron microscopy （TEM） and Fourier transform infraredspectrometry （FTIR）. The tribological properties of surface-modified LaF₃as an additive in water wereevaluated with a four-ball machine, and the morphology and elemental composition of worn steel ballswere examined with a scanning electron microscope （SEM） and an X-ray photoelectron spectroscope（XPS）. Furthermore, the antiwear and friction-reducing mechanism of surface-capped LaF₃nanoparticleswas explored. The main contents and research results of this thesis are as follows:（1） Preparation of water-soluble LaF₃nanoparticles modified by citric acid and investigation of theirtribological propertiesWater-soluble LaF₃nanoparticles surface-modified by citric acid were synthesized in aqueous solutioncontaining citric acid as the surface-capping agent. The phase composition, chemical structure, andmicro-morphology of as-prepared LaF₃nanoparticles surface-capped with citric acid were analyzed byXRD, FTIR, and TEM. A laser granulometer was performed to determine the size distribution ofsurface-modified LaF₃nanoparticles, and a thermal analysis system （TG-DTA） was performed to evaluatetheir thermal stability. In the meantime, the tribological properties of water soluble LaF₃nanoparticles asadditives in water were evaluated with a four-ball tribometer, and the morphology and elementalcomposition of the wear scar were analyzed SEM and XPS. Results show that LaF₃nanoparticlessurface-capped with citric acid have an average diameter of about4nm and exhibit good dispersibility inwater. Besides, as-prepared LaF₃nanoparticles are able to form a boundary lubricating film mainlyconsisting of Fe2O3and La2O3thereby exhibiting good antiwear and friction-reducing performance.（2） Preparation of water-soluble LaF₃nanoparticles modified by OP derivatives and investigation oftheir tribological propertiesEmulsifiers OP-10and OP-20were used as the starting materials to prepare dialkyl polyoxyethyleneglycol thiophosphate ester （OP derivatives; denoted as DTP-10and DTP-20）. As-obtained OP derivativesDTP-10and DTP-20were then adopted as surface-capping agents to fabricate water-soluble surface-capped LaF₃nanoparticles （denoted as LaDTP-10and LaDTP-20） via a surface-modificationmethod. The phase composition, morphology and chemical structure of surface-modified LaF₃nanoparticles were analyzed by XRD, TEM, and FTIR. Moreover, the tribological properties ofas-synthesized LaF₃nanoparticles as additives in distilled water were evaluated with a four-ball friction andwear tester, and the morphology of wear scar and the chemical states of some typical elements thereonwere investigated by SEM and XPS. It has been found that as-prepared LaDTP-10and LaDTP-20nanoparticles have a size of19.6nm and8.5nm, respectively, and they have good dispersibility in distilledwater. Moreover, as-synthesized LaDTP-10and LaDTP-20nanoparticles as lubricant additives in distilledwater exhibit good friction reducing, antiwear, and extreme pressure properties as well as highload-carrying capacity. This is because LaF₃nanoparticles can be deposited on sliding steel surfaces toafford a surface protective layer, and they may also tribochemically react with rubbing steel surfaces togenerate a boundary lubricating film mainly composed of phosphate, sulfide, sulfate, La2O3, and LaF₃.（3） Preparation of water-soluble LaF₃nanoparticles via a one-step route and investigation of theirtribological propertiesWater-soluble LaF₃nanoparticles were prepared via a one-step method in water in the presence ofLa（OH）3and hydrofluoric acid as the starting materials as well as DTP-10as the surface-capping agent.The phase composition, morphology and chemical structure of LaF₃nanoparticles surface-modified byDTP-10were analyzed by XRD, TEM, and FTIR; and the tribological properties of as-synthesized LaF₃nanoparticles as additives in distilled water were evaluated with a four-ball friction and wear tester. It hasbeen found that as-prepared LaF₃nanoparticles have a small size and possess good dispersibility in distilledwater, while they exhibit good antiwear and friction-reducing performance as water-based lubricantadditives.（4） Preparation of LaF₃/Fe₃O₄composite nanoparticles surface-capped by citric acid andinvestigation of their tribological propertiesWater-soluble LaF₃/Fe₃O₄composite nanoparticles were prepared by in-situ surface modification methodwith lanthanum citrate and sodium fluoride as the starting materials as well as citric acid as the modifier.The phase composition, chemical structure, and micro-morphology of as-prepared LaF₃nanoparticlessurface-capped with citric acid were analyzed by XRD, FTIR, and TEM. The thermal stability of as-prepared LaF₃/Fe₃O₄nanocomposite was evaluated by thermogravimetric analysis, and their tribologicalproperties as additive in distilled water were investigated with a four-ball machine. Results indicate thatas-prepared LaF₃/Fe₃O₄composite nanoparticles have good dispersibility in water and exhibit goodtribological properties as well, and as water-based lubricant additives, they can significantly improve theantiwear and friction-reducing performance of distilled water. This is because LaF₃/Fe₃O₄nanocompositesurface-capped by citric acid is able to form a deposited coating mainly composed of LaF₃and Fe₃O₄onsliding steel surfaces, and such a deposited coating exhibits good synergistic friction-reducing and antiweareffect with the boundary lubricating film of the surface-modifier.