Investigating the Structural Chemistry at the Interface Formed between Zinc Dialkyldithiophosphate (ZDDP) and Mild Steel Using Ultra-soft X-ray Absorption Spectroscopy

Ultra-soft X-ray absorption spectroscopy at the oxygen K-, iron L- and zinc L-edges has been used to probe the structural chemistry taking place at the interface between polished mild steel in contact with lubricating oil mediated by the presence of ZDDP, an anti-wear additive. The data provides evidence for cationic exchange between Fe 2+ in the substrate and Zn 2+ in the ZDDP film surface adsorbed at 60°C. After decomposition at 135°C the surface layer appears zinc deficient with a chemical composition consistent with a mixture of oxide and phosphate phases.