Research On Interface Slip And Tribological Properties Of Film Lubrication In Nanogap

With the development of mechanical manufacturing technology and materials science,precision parts with ultra-smooth surfaces are used more extensively and the thickness of lubricant film between friction pairs of these parts descends towards the nanoscale.As a result,some traditional theories of tribology are no longer applicable.The investigation on flow and friction properties of lubricants in nanoscale gets more important.In this paper,the friction coefficient tests of two solid iron walls with different lubricants are performed using the MMS-2A friction testing machine,which provides the basis for further investigation.Besides,this paper compares the computational accuracy of five molecular force fields.Molecular dynamics（MD）method is utilized to program and simulate the film lubrication of three types of lubricants（n-hexadecane,Squalane and PETE）with different molecular structures;to research interface slip and tribological properties of film lubrication in nanogap,mass density and velocity distribution of lubricants are investigated at different film thicknesses,shear velocities and pressure.Meanwhile,the density and velocity distributions of PETE confined in smooth iron walls and iron walls with nanoscale roughness are investigated respectively at different pressure,and then the mechanical responses of the solid walls in the x and z directions are output to explore the mechanism of film lubrication.The results show that:the adsorption energy of n-hexadecane on the crystal planes of Fe（100）is maximum so stable and high-capacity lubricant films are likely to form.With the increasing pressure,systematic height decreases significantly and the layered distribution of lubricant in nanogap appears;with the descent of film thickness,the layered distribution of lubricant in nanogap gets more obvious,the density of solidified layers gets higher,and the interface slip gets more significant.For the lubricant n-hexadecane,slip factor（S）is almost zero at low pressure,indicating the slip is not obvious,which increases severely with the increasing pressure;when the pressure is higher than 500MPa,S increases slowly as the pressure;when the pressure exceeds 1500MPa,S is approximately 1.0 and there is full slip.When the pressure is constant,with the increasing shear velocity V,systematic height Lz,the shear stressτon the lubricant and friction coefficientμincrease but the average densityρdecreases;when the shear velocity is constant,with the increasing pressure,the shear stressτon the lubricant increases but the friction coefficientμdecreases.When the initial film thickness h₀ is 62.3（?）,the friction coefficients of the smooth walls and rough walls of V-groove are 0.161 and 0.150 respectively.V-groove contributes to the reducing the friction coefficient of lubrication system;when the film thickness h₀descends to 41.1（?）,the friction coefficient of the rough walls of V-groove is 0.208,a relative large value,and the lubrication state transfers to boundary lubrication state from fluid lubrication state.For the lubricant PETE,when the initial film thickness h₀ is 62.3（?）and the pressure is zero,there is also friction in the system because of the adhesion between the solid walls and lubricant molecules,which is called Derjaguin offset.The Derjaguin offset is 24.03Kcal/（mole·（?））.MD method is adopted in this paper to anticipate quantificationally the density and velocity distributions of lubricants in nanogap and show dynamically the rules of the molecular movement and distribution.Moreover,MD method solves the problem that the density distribution and the rules of movement for lubricant molecules cannot be obtained during high-speed loading process in the experiment,which reveals the internal mechanism of interface slip.