Enhancing The Tribological Properties Of Graphene Oxide-Based Lubrication Additives Through Composition Control

From 2020,the sulfur content of marine fuels worldwide shall not exceed 0.5%.However,the use of low-sulfur fuel is causing a decrease in extreme pressure anti-wear performance and thermal stability of low alkaline lubricating oil,leading to severe contamination of piston ring grooves and cylinder liner inner wall coatings.How to reduce abnormal wear on cylinder liner surfaces and improve the anti-friction and anti-wear performance of marine lubricating oil under current emission standards has become a pressing issue in the shipping industry.Due to the coupling effect of lubricating oil and cylinder liner-piston ring friction pair under low-sulfur conditions,the frictional system exhibits a strong nonlinear evolution.In this study,we address the harsh operating conditions of the cylinder liner-piston ring assembly,including high temperature,high pressure,boundary lubrication,and corrosion wear.By designing lubricating oil additives with different compositions and concentrations,we provide new insights into the key issue of surface wear failure on cylinder liners in low-sulfur conditions.The research work and achievements of this study are summarized as follows:（1）Through molecular dynamics simulation and surface modification techniques such as dielectric barrier discharge plasma（DBDP）-assisted ball milling,the surface modification mechanisms of different modifiers（Oleic acid;Cetyl Trimethyl Ammonium Bromide;Polyethylene glycol）on graphene oxide（GO）with different ball milling times（2.5,5,7.5,and 10h）were investigated,aiming to achieve component regulation of the surface modifiers on GO.The results showed that OA molecules mainly adsorbed on the surface of GO through van der Waals forces.The hydrogen atoms in the polar group-COOH located at the tail of oleic acid were the main positive charge regions.Under the action of electrostatic force,the polar group molecular chains-COOH of OA molecules intertwined with the hydroxyl molecules on the surface of GO,forming stable adsorption sites.The ordered degree of the OA-modified layer was high,without obvious stratification phenomenon.CTAB molecules relied on their long-chain alkane surface and methyl groups at the tail to enhance the electrostatic interaction between GO and CTAB,adjusting their adsorption configuration on the surface of GO.However,during the adsorption process of CTAB molecules on the GO surface,varying degrees of stratification occurred.The adhesion force of PEG-400 molecules on the GO surface primarily relied on van der Waals forces,and serious stratification was observed,which was not conducive to the exfoliation of GO layers.In addition,the entanglement and stacking of PEG-400 molecules severely affected their adsorption on GO.（2）Based on the results of component regulation guided by molecular dynamics,the extreme pressure anti-wear performance of surface-modified GO and core-shell structure Ti O2nanolubricants,as well as the anti-friction and anti-wear effects under high-temperature boundary lubrication conditions,were evaluated.The results showed that the composite oil containing OA-GO nanolubricant exhibited the best extreme pressure anti-wear performance.The maximum non-seizure load（P_B）value of 0.1wt.%OA-GO composite oil increased by 51.6%compared to the base oil,and the average friction coefficient and wear scar diameter decreased by 39.1%and 50.9%respectively.Under boundary lubrication conditions,the central oil film thickness of 0.1wt.%OA-GO composite oil increased by 48.2%compared to the base oil,and the average friction coefficient and wear rate decreased by 51%and 50.4%respectively.The interlayer slip of OA-GO nanolubricant reduced the frictional force on the contact surface,and the carbon deposition film formed on the friction surface transformed the abrasive wear between the friction surface and the transfer film,improving the anti-wear ability of the friction surface.The core-shell Ti O₂nanolubricant reduced the contact pressure under heavy load through deformation buffering and the superior carrying capacity of the core-shell structure,thus reducing the abrasive wear of the friction pairs.At the same time,the frictional heat and deposition of nanometer particles also promoted the formation of friction transfer film,avoiding direct contact of the friction pairs.The PB value of composite oil with 0.1wt.%Ti O₂@OA increased by 79.37%compared to the base oil PB value,and the average friction coefficient and wear scar diameter decreased by 40.6%and 52%respectively.Under boundary lubrication conditions,the central oil film thickness of 0.1wt.%Ti O₂@OA composite oil increased by 41.3%compared to the base oil,and the average friction coefficient and wear volume decreased by 76.1%and 73%respectively.（3）Based on the study of frictional performance with single component,a surface-modified Ti O₂-enhanced multilayer graphene oxide composite nanolubricant additive（MGTC）was prepared through component regulation.Friction experiments were conducted on MGTC composite oils with different concentrations,and the results showed that the addition of MGTC composite nanolubricant significantly improved the viscosity and film thickness of the base oil,resulting in better extreme pressure anti-wear performance.The P_B value of the composite oil with1.0wt.%MGTC was 921N,which was 93.9%higher than that of the base oil.The wear scar diameter decreased by 47.81%compared to the base oil,and the oil film thickness increased by167%compared to the base oil.The slip of the multilayer graphene oxide sheet structure and the sliding shear between the nanoscale Ti O₂ and graphene oxide sheets in the MGTC additive surface underwent friction-induced rapid recombination,forming a third-body structure of Ti O₂+graphene oxide+abrasive particles.Therefore,the sliding friction between the friction pairs transformed into rolling friction,achieving optimal frictional performance.The MGTC adhered to the friction pair surface undergoes oxidation reaction with the base material in the complex environment of high temperature and high pressure,forming a friction transfer film on the friction pair surface to repair the worn surface.（4）The mechanism of inhibiting corrosion and wear on the surface of diesel engine cylinder liners by the surface-modified Ti O₂-enhanced multilayer graphene oxide composite nanolubricant additive（MGTC）prepared through component regulation was studied using molecular dynamics simulation and electrochemical corrosion tests.The results showed that adding 0.1wt.%MGTC significantly improved the anti-friction and anti-wear performance of the solution system compared to the original solution system.The average friction coefficient and wear volume decreased by 64.9%and 41.7%respectively compared to the oil-water mixture without additives.The polarization resistance increased by 100%and the corrosion rate decreased by 43.5%.The transfer film on the surface of the friction pair still had good corrosion resistance and anti-wear effects under the combined action of friction and corrosion.The MGTC filled in the generated cracks and gaps can repair the friction pair surface to a certain extent,while further avoiding the further contact between the cylinder liner material and the corrosive medium,thus greatly reducing abrasive wear and corrosion wear.Molecular dynamics simulation results also showed that adding MGTC nanolubricant additive increased the density of the composite oil by 11%compared to the base oil,and reduced the free volume fraction by 13.6%.Therefore,the composite oil containing MGTC nanolubricant additive can effectively reduce the diffusion of water molecules and inhibit corrosion of the cylinder liner material.