Design,Preparation And Tribological Performances Of Lubricating Additives Based On Two-Dimensional Materials

Two-dimensional materials have a typical layered structure and weak interlayer van der Waals force,which make them easy to slide under external shear force,therefore showing good lubricating properties.Two-dimensional materials with high specific surface area and nano-thickness are able to enter contact areas and adsorb on the surface of the friction interfaces to form a uniform and dense tribofilm,which can greatly improve the tribological properties of the lubricant.In addition,during the sliding process,the two-dimensional materials show excellent friction reduction and anti-wear properties because they can be filled into the grooves on the surface of the friction pair and further reduce the surface roughness and repair the wear.Based on the above advantages of two-dimensional materials,in this thesis,two-dimensional cobalt hydroxide（Co（OH）₂）nanosheets,MXene/hydroxylsalt（MXene/HS）nanocomposite,and magnesium-aluminum hydrotalcite/silica（Mg Al-LDH/Si O₂）composites were prepared as lubricating additives.With good dispersibility and tribological properties,these additives can effectively improve the anti-friction and anti-wear properties of base oils.The main research contents and results are summarized as follows:（1）Co（OH）₂ nanosheets can act as lubricant additives because of their regular morphology,thin sheet thickness,stable physical and chemical properties,simple synthesis process,low cost,and environmental friendliness,et al.In this study,a precipitation method was developed to synthesize Co（OH）₂ nanosheets with a regular hexagonal sheet structure and size of～700 nm.Additionally,FT-IR and TG tests have been employed to probe surface functional groups and thermal stability of Co（OH）₂nanosheets.PAO-4 with various contents of Co（OH）₂ nanosheets formed homogeneous oil samples after sonication,and the properties were evaluated in terms of coefficient of friction（COF）and wear volume.Tribological tests indicated that the average friction coefficient（ACF）and wear volume of PAO-4 containing 2 wt%Co（OH）₂ nanosheets were reduced by 58%and 90%as compared with that of pure PAO-4.The outstanding tribological properties of Co（OH）₂ nanosheets are mainly attributed to the friction film formed by adsorption and tribochemical reactions.（2）Owing to good thermal stability,excellent mechanical properties,high specific surface area,easy shearing properties,and strong bonding ability,MXenea are promising candidates for tribological applications.Hydroxysalt（HS）are composed of metal ions,hydroxide（OH^-）and other anions(CO₃^2-,SO₄^2-,NO₃^-,etc.).Merits of the controllable structure and composition endow HS wide applications in adsorption,catalysis,and electrochemistry.However,the applications in the field of tribology have not yet been explored.In this study,a new MXene/HS heterostructure nanocomposite was prepared by in-situ growth,aiming to decrease the stack of MXene and avoid the aggregation of HS.MXene acts as a support can anchor Ni²⁺and Co²⁺on its surface.The formation of HS on MXene enlarges the interlayer spacing of MXene,creating a uniform and stable dispersion system in the base oil.Tribological tests showed that the addition of MXene/HS imparted PAO-10 with excellent anti-wear and anti-friction properties and the lubrication performance is always stable under different loads（200 N,250 N,300 N,350 N）.The COF,wear volume,wear track width and wear scar diameter of PAO-10 containing 1.0 wt%MXene/HS additive were reduced by 24%,82.0%,49.1%,49.3%as compared with that of pure PAO-10.The excellent tribological behavior could be attributed to the synergistic lubrication effect of MXene and HS.（3）Layered double hydroxides（LDH）have attracted extensive interest due to their low cost,facile preparation,and environmental friendliness.However,insufficient wear resistance and poor dispersibility of single LDH remains an insurmountable challenge.Studies have shown that growing oxide nanoparticles on two-dimensional layered material substrates as integrated composites would produce synergistic lubrication effect.It is an attractive strategy to optimize the lubricating properties of the base oil.In this work,Mg Al-LDH/Si O₂composites were synthesized by hydrothermal reaction.The Si O₂ nanospheres can embed in the LDH nanosheet layer to restrict their self-aggregation.Therefore,Mg Al-LDH/Si O₂ exhibited good self-dispersing ability in PAO-10.Tribological studies showed that the ACF,wear volume and wear track width of PAO-10 containing 1.0 wt%Mg Al-LDH/Si O₂composites were reduced by 30.5%,84.1%,and 36.4%,respectively,compared with pure PAO-10.Remarkably,the introduction of Mg Al-LDH/Si O₂ endowed PAO-10with more excellent lubricating properties.The lubrication mechanism can be attributed to two points:Si O₂ can act as a miniature ball bearing,transforming the sliding into rolling in the sliding process;the easy-to-shear properties of Mg Al-LDH nanosheets shared part of the friction force.