Research On Lubricating Oil Degeneration Process Based On Fluorescent Carbon Dots

Lubricating oil can effectively reduce the friction and wear of mechanical equipment,ensure the safe,reliable and long-lasting operation of mechanical equipment,and is widely used in aerospace,petroleum,automobile manufacturing,transportation,shipping,metallurgy,machinery,chemical industry and other fields.However,in the actual application process,due to friction heat,stress,catalysis and other effects,lubricating oil will oxidize to form alcohols,aldehydes,ketones,carboxylic acids and other oxygen-containing hydrocarbons,causing its lubricating performance to deteriorate and affecting the normal operation of mechanical equipment,and even causing serious safety accidents.Therefore,lubricating oil condition monitoring is crucial to prevent the equipment failures.Fluorescence technology has the advantages of fast,sensitive and easy operation,which shows potential application in lubricant condition analysis.In this work,the chemical sensing properties of nitrogen-containing fluorescent carbon dots（CDs）were used to study the hydrocarbon oxidation products generated by the oxidative degradation process of lubricating oil.The specific research contents are as follows:1.Nitrogen-containing fluorescent CDs were synthesized by one-step solvothermal method using p-phenylenediamine and diphenyl ether as raw materials,which could be dispersed in a variety of organic solvents.The size,composition and microstructure of nitrogen-containing fluorescent CDs were characterized by transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）and Fourier transform infrared spectroscopy（FTIR）,respectively.The surface of nitrogen-containing fluorescent CDs contained-NH₂ functional groups,and the particle size distribution was2-4.5 nm.Based on the characteristics of nitrogen-containing fluorescent CDs,the fluorescence response of the CDs to different oxygenated hydrocarbons was studied.It was found that carboxylic acid could cause fluorescence quenching of nitrogen-containing fluorescent CDs,nitrogen-containing fluorescent CDs had fluorescence response to-COOH functional groups.The fluorescence response behavior and sensitivity of nitrogen-containing fluorescent CDs to carboxylic acids were studied by introducing them into different kinds and concentrations of carboxylic acid solutions.Based on this,a series of lubricant samples with different degrees of degradation were simulated and prepared using high-temperature oxidized trimethylolpropane ester（TMP108A）base oil and waste automotive engine oil,and the prepared lubricating oil samples were characterized by viscosity,acid value,molecular structure,and fluorescence spectra.The responsiveness of nitrogen-containing fluorescent CDs to-COOH in degraded lubricating oil samples were further studied,and the correlation between the change of fluorescence signal of nitrogen-containing fluorescent CDs and oxidation time,acid value and viscosity was established.The results showed that the carboxylic acid oxidation products were produced in the process of lubricating oil degeneration,so that the fluorescence emission intensity of nitrogen-containing fluorescent CDs decreased gradually with the increase of lubricating oil degeneration degrees,and the change of fluorescence signal was positively correlated with the lubricating acid value.This means that nitrogen-containing fluorescent CDs are expected to be an effective tool for monitoring the degeneration process of lubricating oil.2.In view of the overlapping problem between the intrinsic fluorescence characteristics of lubricating oil and CDs fluorescence,the fluorescence analysis of lubricating oil degeneration by CDs is interfered to a certain extent.Considering that the prepared fluorescence of nitrogen-containing CDs has solvent response properties,the fluorescence characteristics of nitrogen-containing fluorescence CDs can be adjusted by changing the solvent to avoid interference from the intrinsic fluorescence of lubricating oil,and improving the accuracy and reliability of nitrogen-containing fluorescence CDs in response to lubricating oil degradation.The fluorescence properties of nitrogen-containing fluorescent CDs were studied in different solvents,indicating that their fluorescence emission undergoes a redshift with increasing solvent polarity.According to the response characteristics of nitrogen-containing fluorescence CDs to carboxylic acids in different solvents,acetone/toluene（V/V,10/90）and ethanol/toluene（V/V,10/90）with the optimal fluorescence emission wavelengths of 540 nm and 593 nm were selected as the analytical solvents,and the fluorescence response sensitivity of nitrogen-containing fluorescence CDs to different degrees of degradation in TMP 108A base oil and waste automotive engine oil was compared in different solvents,the correlation between the change of fluorescence signal of nitrogen-containing fluorescent CDs in different solvents and the degree of degeneration,acid value and viscosity of lubricating oil was established.The results showed that compared with toluene solvent,acetone/toluene solvent and ethanol/toluene solvent could improve the fluorescence response sensitivity of nitrogen-containing fluorescence CDs to waste automobile engine oil with different degrees of degradation,and acetone/toluene solvent showed the best response characteristics,there was a good correlation between the variation of fluorescence signal of nitrogen-containing CDs in different solvents and the lubricating oleic acid value.This indicates that optimizing the analytical solvents can further improve the accuracy and sensitivity of nitrogen-containing fluorescent CDs for analyzing the degree of lubrication degradation.