Combined ab initio and experimental study of phosphorus-based anti-wear additives interacting with iron and iron oxide

TL;DR

This study combines computational and experimental methods to analyze how phosphorus-based additives interact with iron surfaces, revealing key factors influencing their stability and effectiveness in lubrication.

Contribution

It provides a detailed atomic-level understanding of additive adsorption and stability, integrating ab initio calculations with experimental validation, which is novel in this context.

Findings

ANAP shows strongest adsorption on iron; DBHP dissociates more readily under tribological conditions; DBHP forms stable deposits confirmed by XPS.

Hydrogen loss enhances OAP chemisorption on hematite; additive molecular structure critically influences tribofilm formation; Experimental results align with simulation predictions.

Abstract

The performance of phosphorus-based lubricant additives is governed by their adsorption, stability, and reactivity at the metal interface. In this study, we investigate the adsorption behavior and tribochemical stability of three additives: Octyl Acid Phosphate (OAP), Dibutyl Hydrogen Phosphite (DBHP), and Amine Neutralized Acid Phosphate (ANAP). These additives are studied on iron and hematite surfaces using both ab initio calculations and experimental analyses on steel. Simulations revealed that ANAP exhibited the strongest adsorption on iron, followed by DBHP, while OAP showed weaker interactions, though its chemisorption was enhanced on hematite via hydrogen loss. Under tribological conditions, the DBHP phosphite dissociated more readily than the other two phosphates molecules due to its lower phosphorus coordination, as confirmed by bond order analysis. Quartz crystal microbalance (QCM) measurements indicated significant differences in adsorption behavior across temperatures, with DBHP forming stable deposits, while ANAP exhibited poor retention, in agreement with ab initio molecular dynamics simulations. X-ray photoelectron spectroscopy (XPS) confirmed DBHP's strong chemisorption and molecular dissociation, leading to increased phosphorus deposition. OAP, despite forming a phosphorus-based layer, caused a reduction in Fe oxide, consistent with its hydrogen release mechanism observed in simulations. These findings highlight the critical role of molecular structure and oxidation state in tribofilm formation and stability. Understanding these interactions at the atomic level provides valuable insights for designing high-performance lubricant additives for extreme operating conditions.