Molecular Simulation Of The Tribological Properties Of Nano-magnesium Hydroxysilicate Lubricating Additives

Magnesium hydroxysilicate(Pal),as an ideal component of palygorskite mineral,is a natural nanomaterial with a special layered chain-like structure.It has good adsorption and purification properties,and has broad applications in friction materials,environmental protection,antibacterial and other fields.Studies have shown that Pal as a lubricant additive has metal wear properties of auto-restoration,but the microscopic interaction mechanism between base oil and Pal is not very clear.Therefore,understanding the microscopic interaction mechanism between Pal and mineral base oil molecules is the basis for revealing the friction reducing and anti-wear mechanism of nano-magnesium hydroxysilicate lubricating additives.Molecular dynamics simulation can study intermolecular interactions from the molecular scale,thereby revealing the rules of intermolecular and intramolecular interactions has playing an increasingly important role in the study of the microstructure and properties of materials.To this end,this article has carried out the following research on nano-magnesium hydroxysilicate lubricating additives:(1)Molecular simulation of the interaction between nanometer Pal and linear alkanes.This article has established molecular models for nano Pal and four typical mineral base oils,which are dodecane(C₁₂),tetradecane(C₁₄),hexadecane(C₁₆)and octadecane(C₁₈),and carries out molecular simulation on the interaction systems of Pal and linear alkanes.The changes of structure and energy of the systems,the binding energy,the mean square displacement,the self-diffusion coefficient of alkane molecules as well as the interaction mechanism between Pal surface and base oil molecules were studied systematically.The results show that C₁₂ molecules have higher binding energy and stronger diffusion ability on the Pal surface,so it is easy to form a uniformly distributed lubricating oil film.(2)Molecular simulation of lipophilic modification of nano-Pal.In this paper,by establishing the Pal molecular model of esterification modification with different organic acid molecules,which was dodecanoic acid(HC₁₂),myristic acid(HC₁₄),hexadecanoic acid(HC₁₆)and octadecanoic acid(HC₁₈),the interfacial interaction laws between Pal and C₁₂ alkanes are studied.The results show that C₁₂ molecules and the Pal surface modified by HC₁₆ have higher adsorption strength and form a uniform oil-filled film.The vertical arrangement of C₁₂ molecules and the grafted molecular chains increases the thickness of the oil film.Under the synergistic effect of the filling oil film and grafted molecular chains,C₁₂ molecules can form a thicker and firmer adsorption film on the Pal surface.(3)Molecular simulation of shear friction characteristics of nano-Pal.In this paper,the four kind of molecular simulation systems of iron matrix-base oil-iron matrix(Fe/C₁₂/Fe),magnesium hydroxysilicate-base oil-iron matrix(Pal/C₁₂/Fe),magnesium hydroxysilicate-base oil-magnesium hydroxysilicate(Pal/C₁₂/Pal)and iron matrix-base oil-modified magnesium hydroxysilicate(Fe/C₁₂/Pal-HC₁₆)are established,then the microstructure changes of the friction system and the interaction law of the base oil-matrix interface system are simulated dynamically,which reveals the friction-reducing and anti-wear molecular mechanism of nano-Pal in the shear friction pair.The results show that the binding energy in the Fe/C₁₂/Fe and Pal/C₁₂/Pal systems comes from van der Waals energy and electrostatic energy,respectively,and a symmetrical interlayer shear slip occurs.In the Pal/C₁₂/Fe system,the binding energy between C₁₂ molecules and iron surface comes from van der Waals energy,while the binding energy between C₁₂ and Pal surface mainly comes from electrostatic energy,and the strength of electrostatic energy is much greater than van der Waals energy,which results in asymmetric interlayer shear slip.In the Fe/C₁₂/Pal-HC₁₆ system,the synergistic effect between organic molecular chains grafted on the surface of Pal and C₁₂ molecules form a thicker molecular film.At the same time,a C₁₂ molecular layer with a concentration gradient is formed on the surface of Fe.The strength of the interaction between the C₁₂ sliding layers is low,which makes the sliding between the layers more likely to occur,so that has a better friction reducing and anti-wear effect.(4)Tribological performance research of nano palygorskite lubricant additive.In this paper,the surface hydroxylation of nano palygorskite particles was carried out,and organic acids with different chain length were grafted by esterification reaction.Then,the experimental oil samples were prepared by adding the modified nano palygorskite to the base oil at the mass fraction concentration of 0.5%.The tribological properties of the additives were studied.The results show that the nano palygorskite additive modified by HC₁₆ has better friction reducing and anti-wear properties.Finally,the results of molecular simulation and tribological experiments were combined to explain the mechanism of friction reducing and wear resistance of the modified palygorskite.The above research provides a new insight into the internal interaction of linear alkane/magnesium hydroxysilicate composite lubrication system and the dispersion behavior of magnesium hydroxysilicate in lubricating oil can provide a reference for the design of solid-liquid composite lubricants.