The design of lubricants to decrease friction and wear in machine components is an important way to increase the energy efficiency of mechanical systems while taking into account restrictive environmental requirements and technological advances. Lubricants are composed of base oil and additives designed for specific performance needs.
The use of first principles calculations based on density functional theory has been traditionally very scarce in tribology, especially in the field of lubricant additives. However a fully quantum-mechanical approach is very important to provide an accurate description of the interfacial properties and of the surface chemical processes involving additive molecules, since experimental probing of tribochemistry processes in real-time is still extremely challenging.
We demonstrate the crucial role of the microscopic phenomenon of metal passivation by showing that the friction reduction is connected to to the amount of elemental sulphur and phosphorus released at the iron interface by tribochemical reactions; and that sulphur especially reduces the friction of steel-on-steel by reducing the adhesion and shear strength of iron. We also provide a description of the organophosphite dissociation paths at the iron surface.