Tribological Properties of Phosphate Ester Confined between Iron-Based Surfaces

Recent studies have revealed that phosphate ester lubricant additives undergo decomposition and polymerization when confined between two iron-based surfaces, forming tribofilms that play a crucial role in antiwear and friction reduction. However, the behaviors of decomposition and polymerization, as well as the mechanisms behind the antiwear and friction-reducing properties of these tribofilms, remain largely unexplored. To address these gaps, we employed reactive force field molecular dynamics (ReaxFF-MD) to investigate the tribochemical reactions and tribological properties of tricresyl phosphate (TCP) and tri-n-butyl phosphate (TNBP) confined between three types of iron-based substrates with varying oxygen content: α-Fe⟨110⟩, amorphous FeO (a-FeO), and amorphous Fe₂O₃ (a-Fe₂O₃) surfaces. Our findings indicate that TCP and TNBP molecules primarily undergo dissociation of P-O and C-O bonds, a process influenced by both the oxygen content of the substrates and the molecular structure of the additives. Following the dissociation of these bonds, the released carbon-hydrogen groups adsorb onto the substrates, leading to the formation of adsorbed carbon films on the surfaces. Simultaneously, the dissociation of P-O and C-O bonds triggers polymerization reactions, resulting in the creation of organic polyphosphate iron groups confined between the surfaces coated with adsorbed carbon films. Due to the difference in the contact state and shearing behavior between the carbon films and the organic polyphosphate iron groups, substrates with higher oxygen content exhibit relatively higher friction forces for both TCP and TNBP molecules. Additionally, TCP molecules demonstrate lower friction forces compared to those of TNBP molecules on all three iron-based substrates.