| Nowadays,as the concepts of environmental protection and sustainable development are deeply rooted in their minds,people are paying more attention to protect the environment.Therefore,the high-efficiency and environmentally-friendly nano-based lubricant additives have become the research hotspot in the field of tribology.Carbon-based nanomaterials play an important role in nano-based lubricant additives family owing to their eco-friendly and unique self-lubricating properties.Carbon quantum dots(CDs)as a new type of carbon-based nanomaterials following the fullerene,carbon nanotube and graphene,have been widely used in many fields because of their outstanding properties including favorable physicochemical property,low toxicity,environmental friendliness,simple preparation and easy functionalization.However,the researches on the application of CDs in the field of nano-based lubricant additives have just begun.Based on the one-step pyrolysis method,we prepared a series of CDs with high yield by selecting different carbon sources and modification regents,optimizing the reaction conditions,as well as tuning the types and structures of surface functional groups of CDs.Then the tribological properties of CDs as the lubricant additives were investigated in detail.The lubrication mechanism of CDs-based additives was also discussed.The specific results are listed as follows:(1)Using lactic acid as the carbon source and 1-aminopropyl-3-methylimidazolium bromide as the reaction medium and modification reagent,the ionic liquid capped CDs were synthesized by one-pot pyrolysis method.The characterization results indicated that the CDs with an average particle size of 4.4 nm and high thermal stability were almost amorphous and covalently capped by ionic liquids and oxygen-containing groups.The CDs exhibited outstanding dispersion stability in base liquid.The tribological properties of CDs as water-based lubricant additives were investigated under four-ball mode.The effects of additive concentration and load on tribological performances of CDs were studied.The CDs showed the best friction-reducing and antiwear properties at the concentration of 0.15 wt%and load of 40 N.Specifically,the mean friction coefficient and wear volume of the base liquid at the load of 40 N were reduced by 57.5%and 64%.The results of wear scar analyses demonstrated that the CDs could form solid lubrication films on the rubbing surfaces by electrostatic adsorption,which effectively protect the rubbing surfaces from direct contact and hence greatly reduced the friction and wear of friction pairs.Then the lubrication ability of base liquid was obviously improved.(2)Based on the amidation and esterification reactions,four kinds of CDs modified by different surface groups were synthesized by one-step pyrolyzing the gluconic acid(GA),GA-ethanolamine,GA-ionic liquid and GA-polyethylene glycol(PEG)mixtures.The characterization results exhibited that all of CDs are spherical and almost amorphous with average particle sizes smaller than 30 nm.The synthesized four CDs are modified by oxygen-containing,ethanolamine,ionic liquid and PEG groups,respectively.The dispersion stability of CDs modified by ionic liquid and PEG are obviously better than that of CDs modified by oxygen-containing and ethanolamine groups.The tribological properties of all four CDs as the water-based lubricant additives were investigated under four-ball mode.The PEG modified CDs exhibited the best friction-reducing and antiwear properties at the concentration of 0.20 wt%and load of 40 N,reflecting the remarkable effects of surface groups on the tribological properties of CDs.The PEG modified CDs as additives could made the mean friction coefficient and wear volume of base liquid reduced by 83.5%and 90.0%.It is worth noting that the lubrication ability of base liquid containing PEG modified CDs was even comparable to some common lubricant oils.In addition,the PEG modified CDs as additives could largely enhanced the load-carrying ability of base liquid from 50 to at least 200 N and had a long service life under 100 N.The results of wear scar analyses demonstrated that the embedding stability of CDs in the micro convex structures of rubbing surfaces was closely linked to their surface groups.At low load,the PEG modified CDs as additives could form mixed boundary lubrication film on rubbing surfaces composed of CDs and Fe3O4.At high load,despite the Fe3O4 film was destroyed during the friction process,the film composed of CDs still play an important role in reducing friction and wear.(3)Based on the amidation,the ionic liquid modified CDs were synthesized by one-pot pyrolyzing the mixture of citric acid monohydrate(carbon source)and 1-aminopropyl-3-methylimidazolium bromide(reaction medium and modification reagent).The Br-on CDs were displaced by NTf2-by the anion exchange process.Then the water insoluble CDs possessed good dispersion stability in PEG were obtained.The tribological properties of CDs as the lubricant additives of PEG200 were investigated under four-ball mode.The effects of additive concentration,load and test time on their tribological properties were exploited.At the load of 392 N,the mean friction coefficient and wear volume of PEG200 were reduced by 70.0%and 76.6%when the additive concentration was 0.3 wt%.The wear volume reduction was further increased by 87.1%at load of 600 N.Moreover,the long-time test illustrated that the CDs as additives possessed a long service life.Based on the results of wear scar analyses,the lubrication mechanism of CDs was discussed.The excellent tribological properties of CDs were attributed to the film lubrication effect of surface groups and nanolubrication effects(such as rolling,mending and polishing effects)of carbon cores.(4)Using ethanolamine as carbon source and 1-carboxyethyl-3-methylimidazolium chlorine as reaction medium and modification reagent,the ionic liquid modified CDs were synthesized by one-pot pyrolysis method.The Cl-on CDs were displaced by NTf2-,PF6-,BScB-and oleate by the anion exchange processes.Then four kinds of water insoluble ionic liquid modified CDs with different anions and favorable dispersion stability in PEG200 were gained.The tribological properties of all four CDs as the lubricant additives of PEG200 were investigated under linear reciprocating four-ball mode.At load of 20 N,the CDs could make the mean friction coefficient and wear volume of PEG200 reduce by 40.4～52.2%and 71.7～79.5%.Thereinto,the CDs with oleate anion exhibited the best friction-reducing and antiwear properties.The results of wear scar analyses demonstrated that the boundary lubrication film was formed on the rubbing surfaces by electrostatic adsorption and deposition of CDs.The DFT dimulation results reflected that has the best absorption stability on the metal rubbing surfaces.The CDs with oleate anion as additives could form more solid and stable boundary film on the rubbing surfaces and hence displayed the best friction-reducing and antiwear properties.(5)Using citric acid as carbon source and oleylamine as reaction medium and modification reagent,four kinds of oil-soluble CDs with different particle sizes were synthesized by one-pot pyrolysis method under different reaction temperature.The four synthesized CDs,with particle size of 47.1,38.9,26.6 and 17.5 nm,are almost amorphous and capped by oleylamine groups.The CDs exhibited good dispersion stability in base oil of poly-α-olefin(PAO).Then the tribological properties of all four CDs as the lubricant additives of PAO were investigated under linear reciprocating four-ball mode.At load of 20 N,the CDs with particle size of 26.6 nm showed the best friction-reducing and antiwear properties,reflecting the tribological properties of CDs are closely related to their particle size.At concentration of 0.3 wt%,the largest friction and wear reductions of PAO were up to 31%and 62.7%.The results of wear scar analyses demonstrated that the CDs could form boundary lubrication film and nanolubrication effects.At the same concentration,the embedding stability and film-building abilities of CDs were poor when their particle size was large.The CDs could form aggregates when their particle size was small.Hence,the optimal particle size for CDs was 26.6 nm. |
