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

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 Fe 3 O 4 (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 Fe 3 O 4 , most of the molecules are physisorbed and some desorption occurs at high temperature. Thermal decomposition rates were much higher on Fe 3 O 4 (001) and particularly α-Fe(110) compared to hydroxylated, amorphous Fe 3 O 4. 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 Fe 3 O 4 (001). Aryl phosphates show much higher thermal stability, and decomposition on Fe 3 O 4 (001) only occurs through P-O and C-H cleavage, which require very high temperature. The onset temperature for CO cleavage on Fe 3 O 4 (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.