Study On Preparation Of Graphite Carbon Nitride And Its Lubrication Performance

Graphite carbon nitride（g-C₃N₄）is a kind of graphite-like layered material.The interlayer of g-C₃N₄is combined by weak van der Waals force,so g-C₃N₄is easy to slide between layers,which endows it excellent properties in lubricating field.Compared with traditional oil-soluble additives which contain S and P,g-C₃N₄is more environmentally friendly and has better high-temperature stability and chemical stability.In addition,low-cost melamine,urea,etc.can be used as raw materials to synthesize g-C₃N₄with mass production.Therefore,g-C₃N₄is a very potential lubricant additive.However,the poor dispersion stability of g-C₃N₄in lubricating oils severely limits its practical application in the field of lubrication.In this paper,g-C₃N₄nanosheets were prepared and modified with oleic acid and stearic acid to improve their dispersion stability in lubricating oils,and the tribological properties of modified g-C₃N₄as lubricant additives were researched.The conclusions are as follows:（1）A smaller size g-C₃N₄（OCN）were obtained by thermal oxidation etching of bulk g-C₃N₄at 550℃,using bulk g-C₃N₄as raw material.Oleic acid-modified OCN（OCN-OA）was obtained by amidation reaction of OCN and oleic acid in N,N-dimethylformamide（DMF）solvent.The results showed that a few oleic acid molecules were coated to the surface of OCN,which improved the dispersion stability of g-C₃N₄in lubricating oil.However,the improvement of dispersion stability was not as much as expected because the active NH₂groups in g-C₃N₄were oxidized a lot during the thermal oxidation etching process,which inhibited amidation reaction.（2）Melamine was used as the raw material,and carbon fiber was used as the microwave absorber.G-C₃N₄（MCN）nanosheets were directly prepared by the high-energy microwave heating method,and then further etched with nitric acid to obtain smaller g-C₃N₄nanosheets（MCNS）.Finally,oleic acid and stearic acid were used to modify MCNS to obtain oleic acid-modified g-C₃N₄nanosheets（MCNS-OA）or stearic acid-modified g-C₃N₄nanosheets（MCNS-SA）.The results showed that the surface of MCNS-OA was coated with a large number of oleic acid molecules,and the dispersion stability of MCNS-OA and MCNS-SA in lubricating oil was greatly improved.Nearly no sedimentation occurred in three days.（3）Hydrothermal treatment of melamine with hydrochloric acid was used to obtain supramolecular precursors.The precursors were thermally polycondensed at550℃to obtain g-C₃N₄（HCN）.After etching by nitric acid and modification by oleic acid,oleic acid-modified g-C₃N₄was obtained（HCNS-OA）.The friction performance test of OCN-OA,HCNS-OA,MCNS-OA showed that MCNS-OA had the best anti-friction and anti-wear performance,while OCN-OA had the worst anti-friction and anti-wear performance.When the additive concentration of MCNS-OA in the lubricating oil was 0.02 wt.%,it showed the best anti-friction performance,which was improved by 39.7%compared to the pure base oil.When the concentration is 0.04wt.%,it showed the best antifriction property.The wear resistance was improved by29.1%compared to pure base oil.In addition,MCNS-OA showed the best antifriction property under a load of 490 N,which was 44.9%higher than that of pure base oil;it showed the best anti-wear performance under a load of 196 N,which was 39.4%higher than that of pure base oil.