In-Situ Synthesis And Tribological Properties Of Calcium Borate/Polydopamine/Cellulose Ester Nanocomposites

In the field of tribology,the researches on lubricant have always been the exploration subject of modern energy-saving and environmental protection.As a kind of lubricant additive,nano-borates have excellent extreme pressure,antifriction,and antiwear properties,asciribing to those borates are deposited on the surface of friction metal during the friction process,which can play a role of ball bearings.Thus,a protective film with low shear and low melting point is formed,which can effectively reduce or prevent the wear of metal surface.However,there are still some problems in the preparation and application of nano-borates,such as complex synthesis process,poor dispersion of particles,and poor adsorption of boron on metal surface.The nano-long chain cellulose esters developed by our research group in previous studies exhibited good oil solubility,dispersibility,friction reduction,and antiwear performances,but they could only play the role of friction reduction and antiwear under low or medium load,speed,and temperature.To overcome the defects of nano-borates and cellulose esters and give full play to their advantages,this paper focused on the study of the synthesis of calcium borate/cellulose ester nanocomposites with high lubrication properties using cellulose ester as carrier.Calcium borate/cellulose ester and calcium borate/polydopamine/cellulose ester nanocomposites were synthesized in-situ by low temperature hydrothermal method.The preparation mechanism,tribological properties,and synergistic antifriction mechanism of these nanocomposites as lubricant additives were studied.The contents and results of this study are as follows:（1）With mechanical activation（MA）-pretreated microcrystalline cellulose as raw material,cellulose acetate-laurate（CAL）was synthesized by liquid-phase co-reactant method.Micro-nano CAL was prepared by high-pressure homogenization and freeze-drying.Nano calcium borate（CB）and CB/CAL nanocomposite were synthesized in-situ by low temperature hydrothermal method.The structure and properties of CAL,CB,and CB/CAL nanocomposite were characterized by ¹H-NMR,FTIR,SEM,XRD and XPS.The results show that CB nanparticles with a size of 70～80 nm were uniformly distributed on the surface of CAL,and CB/CAL nanocomposite was successsfully prepared.（2）CAL,CB,and CB/CAL nanocomposite were added to PAO base oil as lubricant additives to study their tribological properties.The results show that CAL,CB,and CB/CAL with the concentration of 0.6 wt%exhibited the minimum friction coefficient（COF）and wear scar diameter（WSD）.The oil sample added with 0.6 wt%of CB/CAL showed the best extreme pressure,antifrction and antiwear performances than pure PAO base oil and the addition of CAL or CB,and the WSD and COF decreased by 25.9%and 48.7%compared with pure PAO base oil,respectively.The oil sample added with 0.6wt%of CB/CAL had the best extreme pressure performance,with the maximum non-seizure load of 880 N,which was 79.6%higher than PAO base oil.SEM-EDS and XPS were used to characterize the worn surface of the steel balls after friction test to study the tribological mechanism.The results show that the groove of the wear scar was the least as the addition of CB/CAL,and the friction surface was the most flat and smooth.This was mainly due to the lubricating film formed by CB/CAL during the friction process,and a complex tribochemical reaction occurred to form a dense composite friction film mainly containing Fe₃O₄,Fe₂O₃,Fe O,B₂O₃,Fe OOH,and Ca O metal oxides.（3）The CAL was surface modified by dopamine to form a thin and dense polydopamine（PDA）film,with its active functional groups as the sites for adhering and anchoring Ca²⁺,and the CB/PDA/CAL nanocomposite was synthesized in-situ by low temperature hydrothermal method to make the CB nanoparticles grow more evenly and firmly on the surface of CAL.FTIR,SEM,XRD,XPS were used to characterize its structure and properties and study the formation mechanism.The results show that CB/PDA/CAL was mainly amorphous,and the CB nanoparticles with diameter of 70～80 nm were uniformly distributed on the surface of CAL.The combination of CB nanoparticles and CAL was uniform and firm,showing that the introduction of PDA could improve the distribution uniformity and stability of CB nanoparticles.（4）CB/PDA/CAL was added to rapeseed oil base oil as lubricant additive to study its tribological properties.The active functional group sites of PDA layer could form hydrogen bond and Van der Waals force interaction with rapeseed oil,contributed to the significant improvement of the dispersion stability of nanocomposites in rapeseed oil,which was conducive to improving the tribological performance of base oil.The friction reduction and antiwear properties of CB/PDA/CAL were better than those of CAL,CB,and CB/CAL with the concentration of 0.6 wt%.The steel balls tested in the rapeseed oil with0.6 wt%of CB/PDA/CAL showed the minimum WSD,COF,and wear volume.SEM and XPS were used to characterize the friction surface to study the lubricating mechanism.The results show that the grooves on the friction surface of the steel balls tested in the oil sample with adding CB/PDA/CAL were smoother than those tested in oil samples with adding CAL,CB,or CB/CAL,and the number of grooves significantly reduced,with shallower and thiner grooves and scratches on the wear surface.This was due to the formation of a friction chemical reaction film on the friction surface,which was mainly composed of Fe₃O₄,Fe₂O₃,Fe O,B₂O₃,Fe OOH,Ca O,and BN compounds.CB and CAL had a synergistic anti-wear effect during the friction process to avoid the direct contact between metal and metal surfaces,which playd a good role in reducing friction and antiwear.