Molecular Simulation of Lubrication Behavior of Stearic Acid as an Organic Friction Modifier under Electric Field

The increasingly severe operating conditions of friction and contact components place higher demands on lubrication systems. In the boundary lubrication regime, organic friction modifiers (OFMs) in lubricants play an important role in separating the contact surfaces and thereby reducing friction and wear. However, the lubricating effect of OFMs is mainly controlled by the interaction between the OFM molecules and the contact surface, which is usually influenced by many factors, such as temperature, pressure, shear rate, surface humidity, etc. In recent years, the electric field has been used to regulate the structure of liquid lubricant molecules to achieve excellent lubrication performance. This method can overcome the limitations of the physicochemical properties of the frictional surface and other influencing factors, and thus achieve significant improvement in boundary lubrication performance. In this work, molecular dynamics simulation was used to study the lubrication behavior of stearic acid as an organic friction modifier under the action of an electric field, and to reveal the adsorption behavior and tribological properties of stearic acid on SiO₂ surfaces at the molecular scale. Poly-alpha-olefin (PAO4) was used as the base oil. The electric field intensity was reflected by the surface charge density based on its relevance. The microscale configurations, density distribution and the probability profiles of both molecular adsorption angle and tilt degree (ΔZ) were analysed to represent the adsorption structure of OFM film. The results indicated that the positive surface charges promoted the formation of a dense and regular adsorption film of stearic acid, while the negative charges showed an opposite effect. The adsorption film was disordered on the surface without charges. This means that the strength and direction of the electric field determined the interactions between the polar head group of stearic acid molecule and the friction surface. The shear configurations and velocity of lubricants were extracted to show the friction process. The velocity of PAO4 base oil showed a linear gradient distribution, while the velocity of OFM adsorption layer was the same as that of SiO₂ slab. The friction coefficient decreased significantly as the charge density increased from negative to positive, ignoring the case of zero charge. The different friction coefficients were mainly controlled by two interactions of OFM-SiO₂ and OFM-PAO4 interfaces. At positive electric field, the OFM film could not slip on SiO₂ surface and also had a clear interface with PAO4 layer, resulting in very weak interfacial shear. Conversely, at negative electric field, the OFM film had a relatively weak attraction to the surface, and the shear action existed simultaneously at the OFM-SiO₂ and OFM-PAO4 interfaces, increasing the friction. In the case of no electric field, OFM and PAO4 molecules became entangled with each other, and their mixture could easily slide on SiO₂ surface to significantly reduce the interfacial shear. This showed that the presence of an electric filed influenced the interaction of adsorption film with friction surface and base oil, as well as the adsorption behaviors, so that the reduction of friction coefficient and the improvement of lubrication performance could be achieved by regulating the electric field. From a microscopic point of view, the insight into the adsorption and tribological performances of stearic acid would be helpful to understand the microscopic mechanism of boundary lubrication and further guide the regulation of interfacial friction.