Molecular dynamics simulation of adsorption and lubrication of hydrocarbons and aqueous copolymer lubricants on iron and iron oxide surfaces

A classical molecular dynamics (MD) simulation has been used to investigate the adsorption and tribological performance of hydrocarbon lubricant between different iron and iron oxide surfaces. A realistic all-atom model of alkane was employed using the COMPASS force field (FF) while the relaxed surfaces and an effective force field for interactions between surface and lubricant were obtained from ab-initio calculations. A comparative analysis of adsorption of six n-alkanes (CnH2n+2, n = 4, 6, 8, 10, 12, 16) on Fe(110), FeO(110), and Fe2O3(0001) and thin film lubrication of hexadecane between Fe(100), Fe(110), Fe(111), FeO(100), FeO(110), FeO(111), Fe2O3(001), and Fe2O3(012) surfaces has been carried out. A quantitative surface parameterization was introduced to investigate the influence of surface properties such as crystalline structure, surface corrugation, and crystal plane on the structure, rheological properties, and tribological performance of the n-alkanes. The effects of working conditions such as loading pressure, shear rate, and temperature are also considered. The results show that alkane molecules orient randomly on Fe(110) and Fe2O3(0001) surfaces but they preferentially orient in (010) direction on FeO(110) at low temperature. Additionally, alkanes adsorb physically on iron oxides, in the following decreasing order Fe(110) > FeO(110) > Fe2O3(0001). The adsorption energies per saturated carbon site decrease with an increase of molecular chain length and this propensity is similar for different surfaces. In contrast, the saturated carbon density is insensitive to the surface potentials and shows an increasing trend for short alkane chains but it remains steady for longer chains. Although the wallfluid attraction of hexadecane on pure iron surfaces is significantly stronger than its oxides, there is a considerable reduction of shear stress of confined n-hexadecane film between Fe(100), and Fe(110) surfaces compared with FeO(110), FeO(111), Fe2O3(001), and Fe2O3(012). It was found that, in thin film lubrication of hexadecane between smooth iron and iron oxide surfaces, the atomic roughness plays a role more important than the wall-fluid adhesion strength.