Ab Initio Molecular Dynamics Simulation of Tribochemical Reactions Involving Phosphorus Additives at Sliding Iron Interfaces

Sophie Loehlé,Maria Righi

doi:10.3390/lubricants6020031

Abstract

We performed, for the first time to our knowledge, fully ab initio molecular dynamics simulations of additive tribochemistry in boundary lubrication conditions. We consider an organophosphourus additive that has been experimentally shown to reduce friction in steel-on-steel sliding contacts thanks to the tribologically-induced formation of an iron phosphide tribofilm. The simulations allow us to observe in real time the molecular dissociation at the sliding iron interface under pressure and to understand the mechanism of iron phosphide formation. We discuss the role played by the mechanical stress by comparing the activation times for molecular dissociation observed in the tribological simulations at different applied loads with that expected on the basis of the dissociation barrier.

Highlights

The technologies nowadays available to reduce friction and wear are based on materials, an important class of which is represented by lubricant additives included in motor oils
That in boundary lubrication conditions, elemental O and C atoms are not released from TMPi because these two atoms remain bounded in the CO molecule, which does not dissociate on iron [38] and can desorb from the interface
The potential of ab initio molecular dynamics in the research on lubricant materials has been highlighted through the study of the tribochemistry of a model organophosphorus additive

Summary

Introduction

The technologies nowadays available to reduce friction and wear are based on materials, an important class of which is represented by lubricant additives included in motor oils. The design of lubricant additives to decrease friction and wear in machine components is an important way to increase the energy efficiency of car engines while taking into account restrictive environmental requirements and technological advances [1]. Lubricants are formulated products composed of base oils and a package of additives designed for specific performance needs. The additives can be classified as chemically active, i.e., designed to chemically interact with the surface and form protective layers; or chemically inert, i.e., with the function of improving the physical properties of the base oil. We consider extreme-pressure (EP) additives that operate in boundary lubrication conditions, where the thickness of the oil film becomes too thin to prevent direct contact between the metal asperities. It is highly desirable to understand how these protective films are formed by tribochemical reactions

Methods

Discussion

Conclusion