粗糙壁面下同分异构基础油对硬脂酸吸附性能的影响

Effect of Isomeric Base Oil on the Adsorption Performance of Stearic Acid on Rough Wall Surface

分别建立具有4种粗糙度壁面的边界润滑模型,采用分子动力学方法定量分析了十八烷及其同分异构体的3种基础油的密度分布、润滑体系的摩擦因数、硬脂酸的吸附能、吸附时间和扩散系数,研究了受限于粗糙壁面的十八烷同分异构基础油中硬脂酸摩擦改性剂的吸附过程及其摩擦性能。结果表明:基础油的分子结构和壁面粗糙度对润滑剂的分层、吸附和摩擦性能影响较大。油膜中正十八烷分层明显,近壁面密度峰值很大;壁面粗糙度越大,分层现象越弱。当壁面均方根粗糙度R q=0.20 nm时,添加了硬脂酸的十八烷及其同分异构体所组成的3种润滑剂的吸附能都分别达到最大值且摩擦因数达到最小值。基础油在近壁面形成的高密度吸附层会抑制硬脂酸的吸附。在具有长支链的二乙基十四烷基础油中,润滑体系的摩擦因数最大、硬脂酸分子的吸附能最小、吸附时间最长、扩散系数最小。正十八烷的长直链分子结构使其吸附层致密,阻碍硬脂酸的吸附;硬脂酸在二甲基十六烷中扩散最快,而且随着壁面粗糙度增大,扩散加快。

Boundary lubrication models with four kinds of roughness walls were established respectively.The molecule dynamics method was used to quantitatively analyze the density distribution of three base oils of octadecane and its isomers,friction coefficient of lubrication systems,adsorption energy,adsorption time and diffusion coefficient of stearic acid;meanwhile,an investigation was performed on the adsorption process and tribological properties of stearic acid friction modifiers in octadecane isomeric base oils restricted by rough wall surfaces.The results show that the molecular structure of the base oil and wall roughness have significant effects on delamination,adsorption and tribological performance of the lubricants.The delamination of n-octadecane in the oil film is obvious,and the density peak near the wall surface is large.The larger the wall roughness,the weaker the delamination.When the root mean square roughness of the wall surface R q=0.20 nm,the adsorption energy of the three lubricants consisting of octadecane and its isomers after addition of stearic acid can reach the maximum value and the friction coefficient will reach the minimum value,respectively.The high-density adsorption layer formed by base oil near the wall can inhibit the adsorption of stearic acid.For the 3,5-diethyltetradecane base oil with long branched chains,the friction coefficient of the lubrication system is the largest,the adsorption energy of stearic acid molecules is the smallest,the adsorption time is the longest,and the diffusion coefficient is the smallest.The long straight chain molecular structure of n-octadecane results in the formation of dense adsorption layer,thus hindering the adsorption of stearic acid.Stearic acid diffuses fastest in 2,4-dimethylhexadecane,whose diffusion rate is accelerated with the increase of wall roughness.