Study On The Synergetic Lubrication Properties Of Graphene And 5CB Liquid Crystal On Textured Surface Of Bearing Steel

Lubrication technology and surface texture technology are two important means of improving friction conditions.In this thesis,GCr15 bearing steel balls/rings with high hardenability,high hardness and high wear resistance are selected as friction pairs.Pentyl biphenyl cyanide(5CB)with a flat structured rigid chain end is selected as the base lubricant.Graphene with good mechanical properties,large specific surface area and ultra-thin sheet structure is selected as a lubrication additive.The synergistic lubrication effect of graphene/5CB suspension on the surface of the weave was investigated by machining a square array distribution of dimples texture on the friction disc surface by laser processing technique.The main research of this thesis is as follows:The formation of the lubricating oil film on the friction surface is affected by the wetting and rheology of the lubricating oil.In this thesis,the effect of graphene addition concentration on the wetting performance was investigated by measuring the contact angle of 5CB and different concentrations of graphene/5CB suspensions.The contact angle of 5CB at room temperature is 19.519°.When the graphene concentration was0.15 wt%,the contact angle was the smallest,10.080°.The effect of rheology on lubricating properties was investigated by measuring the viscosity,shear stress,viscoelasticity,adhesion and thixotropy of 5CB and different concentrations of graphene/5CB suspensions.The experimental results show that the rheological properties of graphene/5CB suspension have been enhanced after the addition of appropriate amount of graphene.It was demonstrated that graphene significantly improved the rheological properties of base lubricants when used as a lubricant additive.A series of surface textures with different geometric parameters were processed on the surface of GCr15 steel rings using a pulsed nanosecond laser.The influence of parameters such as area density,diameter and depth on lubrication performance under the condition of starved lubrication was studied,and the optimal texture parameters(texture depth 10 μm,area density 8%,texture diameter 100 μm)were determined.The coefficient of friction under the optimal texture parameter is 0.031,which is 32.6%lower than the coefficient of friction on smooth surfaces(0.046).This is because the texture can store lubricating oil and play a secondary lubrication role in the friction process.The texture prevents the suspension being moved out of the friction interface by the centrifugal force in friction process from.The suspension is sufficient to form a continuous lubricant film,which provides a large enough load-bearing capacity,so the wear resistance is better.The friction reduction and wear resistance of smooth and texture surfaces were investigated at different concentrations,loads and speeds.The optimum graphene addition concentration of 0.15 wt%,a load of 3 N and a speed of 250 r/min were determined,which has the best lubrication performance.At this time,the friction coefficient of the smooth surface is 0.029,and the friction coefficient of the textured surface is 0.018.Study of its lubrication mechanism found that 5CB in the sliding shear direction,behaves as a low-viscosity fluid,effectively reducing the coefficient of friction.Meanwhile,SEM demonstrated that the graphene in the suspension entered the friction interface to participate in the friction reaction,and subjected the friction shear force,thereby preventing direct contact of the friction interface.The XPS analysis proves that the suspension reacts with the friction surface during the friction process,generating new compounds,and the friction chemical reaction film formed protects the friction surface and plays the role of friction reduction and wear resistance.