Substituent Effects on the Thermal Decomposition of Phosphate Esters on Ferrous Surfaces

Abstract

Phosphate esters have a wide range of industrial applications, for example in tribology where they are used as vapour phase lubricants and antiwear additives. An atomic-level understanding of phosphate ester tribofilm formation mechanisms is required to improve their tribological performance. A process of particular interest is the thermal decomposition of phosphate esters on steel surfaces, since this initiates polyphosphate film formation. In this study, reactive force field (ReaxFF) molecular dynamics (MD) simulations are used to study the thermal decomposition of phosphate esters with different substituents on several ferrous surfaces. The ReaxFF parameterisation was validated for a representative system using density functional theory (DFT) calculations. During the MD simulations on Fe3O4(001) and α-Fe(110), chemisorption interactions between the phosphate esters and the surfaces occur even at room temperature, and the number of molecule-surface bonds increases as the temperature increases from 300 to 1000 K. Conversely, on hydroxylated, amorphous Fe3O4, most of the molecules are physisorbed and some desorption occurs at high temperature. Thermal decomposition rates were much higher on Fe3O4(001) and particularly α-Fe(110) compared to hydroxylated, amorphous Fe3O4. This suggests that water passivates ferrous surfaces and inhibits phosphate ester chemisorption, decomposition, and ultimately polyphosphate film formation. For the alkyl phosphates, thermal decomposition proceeds mainly through CO and C-H cleavage on Fe3O4(001). Aryl phosphates show much higher thermal stability, and decomposition on Fe3O4(001) only occurs through P-O and C-H cleavage, which require very high temperature. The onset temperature for CO cleavage on Fe3O4(001) increases as: tertiary alkyl < secondary alkyl < primary linear alkyl ≈ primary branched alkyl < aryl. This order is consistent with experimental observations for the thermal stability of antiwear additives with similar substituents. The simulation results clarify a range of surface and substituent effects on the thermal decomposition of phosphate esters on steel that should be helpful for the design of new molecules with improved tribological performance.

Thermal Decomposition of Tricresyl Phosphate on Ferrous Surfaces

“…Atomic interactions were modeled using the ReaxFF 33 force field with a parameter set previously developed for Fe/P/O/C/H. This parameter set has been shown to accurately reproduce P and PO adsorption energies on Fe(001) 19 as well as the adsorption and dissociation energies of intact and dissociated tri(n-butyl)phosphate on α-Fe(100) 18 by comparison to energies calculated using DFT. All simulations were run using the large-scale atomic/molecular massively parallel simulator (LAMMPS) 34 with a time step of 0.25 fs.…”

“…This result, which was consistent with the experimental observation of more reactivity on iron oxide, 8 was explained by the need for oxygen atoms on the surface to enable P−O scission via nucleophilic substitution. 18 Other reactive MD simulations of thermal decomposition of TCP on Fe 3 O 4 −OH showed that the dominant reaction pathway involved Fe−C bonding. 19 These reactions were then shown to be accelerated by the presence of nanodiamonds on the surface in both simulations and quartz crystal microbalance experiments.…”

“…However, the P−O cleavage rate was faster for the primary alkyl than the other molecules, and this was explained by steric hindrance that inhibited nucleophilic attack by the surface oxygen. 18 As the above review demonstrates, the rates and pathways of decomposition reactions leading to film formation depend on the reactant species and surfaces. However, trends are inconsistent across the different studies, possibly because most studies used just one experimental or simulation technique to quantify reactivity.…”

Interfacial Bonding Controls Friction in Diamond–Rock Contacts

“…Chemical bonding information was output every 1.0 ps, using a bond order cutoff of 0.3 to identify covalent bonds . The choice of bond order cutoff only affects the postprocessing analysis and does not influence the ReaxFF energy or force calculations. , …”

Computational Tribochemistry: A Review from Classical and Quantum Mechanics Studies

“…Under the load of 1 and 2 GPa, tribochemical reaction occurs by forming multiple C–O S chemical bonds between the alcohol and the SiO 2 surface, making the molecule distorted . Ewen et al investigated the thermal decomposition of phosphate esters on ferrous surfaces. The result indicated that thermal decomposition rates were much higher on Fe 3 O 4 (001) and α-Fe(110) than on the hydroxylated, amorphous Fe 3 O 4 surface.…”