Mechanochemistry of phosphate esters confined between sliding iron surfaces

Carlos Ayestarán Latorre,Hugh A Spikes,James P Ewen,Joseph E Remias,Daniele Dini,Joshua D Moore

doi:10.1038/s42004-021-00615-x

Abstract

The molecular structure of lubricant additives controls not only their adsorption and dissociation behaviour at the nanoscale, but also their ability to reduce friction and wear at the macroscale. Here, we show using nonequilibrium molecular dynamics simulations with a reactive force field that tri(s-butyl)phosphate dissociates much faster than tri(n-butyl)phosphate when heated and compressed between sliding iron surfaces. For both molecules, dissociative chemisorption proceeds through cleavage of carbon−oxygen bonds. The dissociation rate increases exponentially with temperature and stress. When the rate−temperature−stress data are fitted with the Bell model, both molecules have similar activation energies and activation volumes and the higher reactivity of tri(s-butyl)phosphate is due to a larger pre-exponential factor. These observations are consistent with experiments using the antiwear additive zinc dialkyldithiophosphate. This study represents a crucial step towards the virtual screening of lubricant additives with different substituents to optimise tribological performance.

Highlights

The molecular structure of lubricant additives controls their adsorption and dissociation behaviour at the nanoscale, and their ability to reduce friction and wear at the macroscale
The steady-state shear stress values shown in Fig. 1 were used in the Bell model[15] (Eq 1) to calculate the activation energy, activation volume, and preexponential factor of TNBP and TSBP dissociation[13]
We have used nonequilibrium molecular dynamics (NEMD) simulations with ReaxFF to compare the mechanochemical responses of primary (TNBP) and secondary (TSBP) trialkylphosphates heated and compressed between sliding iron surfaces

Summary

Introduction

The molecular structure of lubricant additives controls their adsorption and dissociation behaviour at the nanoscale, and their ability to reduce friction and wear at the macroscale. When the rate−temperature−stress data are fitted with the Bell model, both molecules have similar activation energies and activation volumes and the higher reactivity of tri(s-butyl)phosphate is due to a larger pre-exponential factor These observations are consistent with experiments using the antiwear additive zinc dialkyldithiophosphate. An improved understanding of the atomic-scale behaviour of antiwear additives inside tribological contacts is required to rationally design new molecules with improved performance and reduced environmental impact[9] This information is challenging to obtain from macroscale tribometer experiments in the mixed/ boundary lubrication regime[10]. Gosvami et al.[12] showed using atomic force microscopy (AFM) that the tribofilm formation rate of ZDDP is exponentially dependent on both temperature, T, and normal stress, σzz. Along with accompanying quartz crystal microbalance (QCM) experiments, these observations suggested that strong surface adsorption is required to form a tribofilm and that the rate of tribofilm formation was faster on harder surfaces, which led to higher contact stresses[19]

Methods

Results

Conclusion