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 vapor 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 parametrization was validated for a representative system by 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 C–O 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 temperatures. The onset temperature for C–O 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.