Ab Initio Insights into Zinc Dialkyldithiophosphate Linkage Isomers and Oxidative Degradation: Implications for Tribology

Zinc dialkyldithiophosphates (ZDDPs) are widely used as antiwear additives in lubricants, forming protective tribofilms that reduce wear and friction in metallic contacts. However, the atomic-scale mechanisms governing their performance remain poorly understood. A key factor is the role of linkage isomers─alternative molecular forms in which sulfur (S) and oxygen (O) atoms exchange positions within the ligand structure. These isomers arise through alkyl group transfers and may significantly influence tribofilm formation. Using density functional theory (DFT), we systematically characterize ZDDP linkage isomers by analyzing their stability, vibrational spectra, and dissociation pathways. Our results show that linkage isomers are more stable than standard ZDDP forms due to the greater strength of Zn–O bonds and primarily dissociate through Zn–S(O) bond cleavage under a 1 GPa load at ferrous interfaces. Additionally, we explore oxidative degradation pathways, where S atoms are replaced by O, altering the molecular stoichiometry. We find that oxidation is favorable in the gas phase, and it is exothermic when mediated by a ferrous substrate. To support experimental validation, we provide vibrational spectra for these isomers, enabling direct comparison with spectroscopic measurements. Bond strength analysis via static fragmentation and Integrated Crystal Orbital Overlap Population (ICOOP) further elucidates their structural stability.