Molecular Dynamics Simulation Of Mixed Alkane Nano-lubrication

With the continuous development of precision manufacturing,in various precision instruments and equipment,the requirements for friction and lubrication design are also increasing.When friction and lubrication reach the nanometer level,the description of the continuum theory on the lubricating properties of lubricating materials will no longer be fully applicable,and the molecular dynamics theory plays an important role at this time.At the nanometer scale,the lubricating properties of lubricating materials are mainly described by intermolecular forces.A large number of experiments and theoretical studies have shown that molecular dynamics theory has obvious advantages in the research of friction and lubrication,including intuitiveness,high efficiency,and It can complete the research under the extreme conditions that many actual experiments can't reach.At present,many molecular dynamics studies on friction and lubrication are mainly limited to a single alkane material,and in reality,lubricants are mixed materials,so molecular dynamics research on the lubrication characteristics of mixed alkanes can better guide the design of friction and lubrication.In this study,with the help of molecular dynamics software LAMMPS and Materials studio,an all-atom nano-lubrication model was established.Through comparison,the force field and method suitable for the study were selected and the accuracy of the selected force field and method was verified.Finally,the selected force field and method were used to perform a lubricant mixture of three alkanes,n-hexadecane(C₁₆H₃₄),cyclohexane(C₆H₁₂)and2,2,4-trimethylhexane(C₉H₂₀)under different working conditions.It also compares the advantages and disadvantages of mixed alkanes and three single alkanes under the same working conditions,and finally draws the following conclusions:(1)When mixed alkanes are subjected to pressure loads,density stratification occurs.Among different lubricating layers,the lubricating layer close to the metal wall has the highest density and appears solid.The inner lubricating layer still conforms to the continuum theory.An increase in pressure load will also cause a sharp increase in friction and viscosity.The adsorption energy of lubricant molecules at the boundary also increases with the increase in pressure,but the adsorption energy of internal molecules is basically unchanged.In addition,boundary slip occurs only when the pressure reaches 1000 MPa,but it is not obvious.(2)As the temperature rises,the density of the three lubricating layers of the lubricating film does not change significantly.The viscosity of the lubricant shows a downward trend,while the frictional force and the molecular adsorption energy at the boundary show an upward trend,and the internal molecular adsorption energy is basically unchanged.There is no slipping phenomenon.(3)The increase of wall speed load only affects friction and viscosity,but has no effect on lubricant density,adsorption energy,and wall slip.The friction force increases with the increase in speed,but the increase is smaller.The viscosity increases with the increase of the speed load,and there is no shear thinning.(4)The initial thickness of the lubricating film also has an effect on the lubrication characteristics.The larger the initial thickness,the less obvious the layering of the lubricant,and the lower the friction and viscosity under the same load,but the adsorption energy is not affected.(5)Compared with single alkanes,mixed alkane has better lubricating performance,because mixed alkane can weaken the shortcomings of single alkanes and reflect their advantages;mainly reflected in the carrying capacity of mixed alkane second only to cyclohexane;the viscosity under load is lower Single alkane;moderate friction;the lowest shear stress at the boundary,which can effectively avoid lubrication failure.