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.

Understanding the behaviour of surfactant molecules on iron oxide surfaces is important for many industrial applications. Molecular dynamics (MD) simulations of such systems have been limited by the absence of a force-field (FF) which accurately describes the molecule-surface interactions. In this study, interaction energies from density functional theory (DFT) + U calculations with a van der Waals functional are used to parameterize a classical FF for MD simulations of amide surfactants on iron oxide surfaces. The Original FF, which was derived using mixing rules and surface Lennard-Jones (LJ) parameters developed for nonpolar molecules, were shown to significantly underestimate the adsorption energy and overestimate the equilibrium adsorption distance compared to DFT. Conversely, the Optimized FF showed excellent agreement with the interaction energies obtained from DFT calculations for a wide range of surface coverages and molecular conformations near to and adsorbed on α-Fe 2 O 3 (0001).

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.

<p>Phosphate esters have a wide range of industrial applications, for example in tribology where they are used as vapour phase lubricants and antiwear additives. To rationally design phosphate esters with improved tribological performance, an atomic-level understanding of their film formation mechanisms is required. One important aspect is the thermal decomposition of phosphate esters on steel surfaces, since this initiates film formation. In this study, ReaxFF molecular dynamics simulations are used to study the thermal decomposition of phosphate esters with different substituents on several ferrous surfaces. 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 is increased from 300 to 1000 K. Conversely, on hydroxylated, amorphous Fe₃O₄, most of the molecules are physisorbed, even 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 film formation. On Fe₃O₄(001), thermal decomposition proceeds mainly through C-O cleavage (to form surface alkyl and aryl groups) and C-H cleavage (to form surface hydroxyls). The onset temperature for C-O cleavage on Fe₃O₄(001) increases in the order: tertiary alkyl < secondary alkyl < primary linear alkyl ≈ primary branched alkyl < aryl. This order is in agreement with experimental observations for the thermal stability of antiwear additives with similar substituents. The results highlight surface and substituent effects on the thermal decomposition of phosphate esters which should be helpful for the design of new molecules with improved performance.</p>

Diamond and diamond-like carbon coatings are used in many applications ranging from biomedicine to tribology. A wide range of dopants have been tested to modify the hydrophilicity of these surfaces, since this is central to their biocompatibility and tribological performance in aqueous environments. Despite the large number of

<div>Understanding the behaviour of surfactant molecules on iron oxide surfaces is important for many industrial applications. Molecular dynamics (MD) simulations of such systems have been limited by the absence of a force-feild (FF) which accurately describes the molecule-surface interactions. In this study, interaction energies from density functional theory (DFT) + U calculations with a van der Waals functional are used to parameterize a classical FF for MD simulations of amide surfactants on iron oxide surfaces. The Original FF, which was derived using mixing rules and surface Lennard-Jones (LJ) parameters developed for nonpolar molecules, were shown to signi cantly underestimate the adsorption energy and overestimate the equilibrium adsorption distance compared to DFT. Conversely, the Optimized FF showed excellent agreement with the interaction energies obtained from DFT calculations for a wide range of surface coverages and molecular conformations near to and adsorbed on a-Fe2O3(0001). This was facilitated through the use of a Morse potential for strong chemisorption interactions, modi fied LJ parameters for weaker physisorption interactions, and adjusted partial charges for the electrostatic interactions. The Original FF and Optimized FF were compared in classical nonequilibrium molecular dynamics (NEMD) simulations of amide molecules con fined between iron oxide surfaces. When the Optimized FF was employed, the amide molecules were pulled closer to the surface and the orientation of the headgroups was more similar to that observed in the DFT calculations compared to the Original FF. The Optimized FF proposed here facilitates classical MD simulations of amide-iron oxide interfaces in which the interactions are representative of accurate DFT calculations.</div>

<p>Phosphate esters have a wide range of industrial applications, for example in tribology where they are used as vapour phase lubricants and antiwear additives. To rationally design phosphate esters with improved tribological performance, an atomic-level understanding of their film formation mechanisms is required. One important aspect is the thermal decomposition of phosphate esters on steel surfaces, since this initiates film formation. In this study, ReaxFF molecular dynamics simulations are used to study the thermal decomposition of phosphate esters with different substituents on several ferrous surfaces. 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 is increased from 300 to 1000 K. Conversely, on hydroxylated, amorphous Fe₃O₄, most of the molecules are physisorbed, even 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 film formation. On Fe₃O₄(001), thermal decomposition proceeds mainly through C-O cleavage (to form surface alkyl and aryl groups) and C-H cleavage (to form surface hydroxyls). The onset temperature for C-O cleavage on Fe₃O₄(001) increases in the order: tertiary alkyl < secondary alkyl < primary linear alkyl ≈ primary branched alkyl < aryl. This order is in agreement with experimental observations for the thermal stability of antiwear additives with similar substituents. The results highlight surface and substituent effects on the thermal decomposition of phosphate esters which should be helpful for the design of new molecules with improved performance.</p>

Understanding friction at diamond–rock interfaces is crucial to increase the energy efficiency of drilling operations. Harder rocks usually are usually more difficult to drill; however, poor performance is often observed for polycrystalline diamond compact (PDC) bits on soft calcite-containing rocks, such as limestone. Using macroscale tribometer experiments with a diamond tip, we show that soft limestone rock (mostly calcite) gives much higher friction coefficients compared to hard granite (mostly quartz) in both humid air and aqueous environments. To uncover the physicochemical mechanisms that lead to higher kinetic friction at the diamond–calcite interface, we employ nonequilibrium molecular dynamics simulations (NEMD) with newly developed reactive force field (ReaxFF) parameters. In the NEMD simulations, higher friction coefficients are observed for calcite than quartz when water molecules are included at the diamond–rock interface. We show that the higher friction in water-lubricated diamond–calcite than diamond–quartz contacts is due to increased interfacial bonding in the former. For diamond–calcite, the interfacial bonds mostly form through chemisorbed water molecules trapped between the tip and the substrate, while mainly direct tip–surface bonds form inside diamond–quartz contacts. For both rock types, the rate of interfacial bond formation increases exponentially with pressure, which is indicative of a stress-augmented thermally activated process. The friction force is shown to be linearly dependent on the number of interfacial bonds during steady-state sliding. The agreement between the friction behavior observed in the NEMD simulations and tribometer experiments suggests that interfacial bonding also controls diamond–rock friction at the macroscale. We anticipate that the improved fundamental understanding provided by this study will assist in the development of bit materials and coatings to minimize friction by reducing diamond–rock interfacial bonding.