Chemical Origin of Sodium Phosphate Interactions on Iron and Iron Oxide Surfaces by First Principle Calculations

Density functional theory calculations of chemical interactions of lubricant additives sodium pyrophosphate (Na₄P₂O₇) and orthophosphate (Na₃PO₄) on nascent iron Fe(110) and iron oxide Fe₂O₃(0001) surfaces have been carried out. Comparisons of adsorption behaviors of the two lubricant additives on different surfaces have been implemented on the basis of the thermodynamics of adsorption and electronic structure analyses. The results indicate that sodium phosphates chemically adsorb on iron and iron oxide surfaces by forming Fe–O bonds and stick on the surfaces through Fe–O–P linkages. The stronger binding of Na₃PO₄ than that of Na₄P₂O₇ on both Fe(110) and Fe₂O₃(0001) surfaces is consistent with its better antiwear performance observed by the experiments. It is found that Fe–O bonds formed during the phosphate adsorption are stable covalent bonds that are as strong as P–O bonds of the Fe–O–P linkages. However, the binding of Fe–O–P on Fe surface is caused by a donor–acceptor mechanism; in contrast, the donation/back-donation interaction mechanism on the Fe₂O₃ surface. This study provides an in-depth understanding of the early stage of the tribochemical reaction between polyphosphates and metal/oxide surfaces.