Preparation,Characterization And Tribological Performances Of Nitrogen-doped Carbon Quantum Dots As Multi-functional Lubricant Additives

With the emphasis on ecological issues and the increasingly stringent environmental regulations in the world,green and high-efficienct lubricant additive has become a research hotspot in the field of lubrication.Due to their wide source of raw materials,low toxicity and unique lubrication properties,carbon nanomaterials have draw extensive research attention.However,most researches have shown that carbon nanomaterials have poor compatibility and dispersion stability in base oils.Even after complicated chemical or physical modifications,carbon nanomaterials are still easy to accumulate and settle in base oils,which makes impossible to achieve their full potential.Carbon quantum dots are new carbon nanomaterials with very small size?<10 nm?,spherical structure and abundant surface active groups,which have effectively improved the compatibility and long-term stability in base oils,and thus exhibit excellent lubricity.However,study of carbon quantum dots as a lubricant additive in base oils is still in its infancy with problems such as mono-functionality and limited base oil types,which limits its further application in the field of lubrication.In order to address these problems,this thesis focused on the design and synthesis of novel nitrogen-doped carbon quantum dots(CQDs_-N)with multi-functionalities via its surface-controllable properties in order to explore their applications in different base oils.The antioxidant and tribological properties in different base oils were systematically investigated,and the lubrication mechanism were further explored.Detailed contents are outlined in the following 3 aspects:1.CQDs_-N with diphenylamine functional groups on the surface was prepared by in-situ co-carbonization with citric acid and N-phenyl-p-phenylenediamine as raw materials.CQDs_-N is a carbon quantum dot with an average particle size of 2 nm and stable luminescence,exhibiting long-term dispersion stability?>5 months?in polyethylene glycol?PEG?base oil.High pressure differential scanning calorimetry?PDSC?test indicated that,thanks to its surface diphenylamine functional group,CQDs_-N exhibited good antioxidant performance thus slowing down the oxidation of PEG base oil.Four-ball friction test showed that CQDs_-N exhibited best friction-reduction and anti-wear properties at 392 N load and 1 wt%concentration.Compared with PEG,the mean friction coefficient and the wear scar diameter were reduced by 72%and 42%,respectively.Under a higher load of 588 N,it still showed excellent lubricity.The worn surface analysis showed that CQDs_-N acted as the ball bearing and distributed on the friction surface unevenly to form the carbon-based protective film,thereby exhibiting excellent friction-reduction and anti-wear properties.2.CQDs_-N,MoS₂ nanosheets and h-BN nanosheets were compared and added to castor oil?CO?at a concentration of 0.05 wt%.Through natural sedimentation experiments,it was observed that the sample of CQDs_-N in the CO was clear,transparent and stably dispersed for more than 50 days.PDSC tests showed that CQDs_-N exhibited antioxidant property as an additive to CO.Four-ball friction tests indicated that the h-BN exhibited the best friction-reduction performance,and CQDs_-N exhibited the best anti-wear performance,but MoS₂ did exhibit lubrication properties in CO.Under an optimum concentration of CQDs_-N at 0.2 wt%,the wear scar diameter was reduced by 44.7%.The worn surface analysis showed that the protective film composed of carbon,nitrogen and oxygen elements was formed on the worn surface when CQDs_-N was used as the lubricant additive in CO.3.In order to further tune the oil solubility of CQDs_-N,olefin-modified nitrogen-doped carbon quantum dots(O-CQDs_-N)were prepared by covalent grafting method to adjust the polarity of CQDs_-N with small organic molecules.UV test showed that O-CQDs_-N had good compatibility and long-term stability in non-polar PAO base oils.PDSC tests showed that O-CQDs_-N could increase the oxidation stability of PAO.Four-ball friction tests proved that 0.5 wt%O-CQDs_-N had good anti-wear performance and could improve the loading capacity of PAO under high loads,which was due to the synery effect between the combination of oleylamine molecules and CQDs_-N.