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

TL;DR

This work addresses how phosphorus-based anti-wear additives interact with iron and iron oxide surfaces under boundary lubrication. It combines ab initio density functional theory (DFT) with ab initio molecular dynamics (AIMD) and experimental QCM/XPS measurements to quantify adsorption energies, dissociation pathways, and surface chemistry for ANAP, DBHP, and OAP. Key findings show ANAP has the strongest adsorption on Fe, DBHP dissociates readily under tribological stress forming P-based surface deposits, and OAP interacts more with hematite via hydrogen-mediated oxide reduction; temperature modulates adsorption and deposition as evidenced by QCM. The results provide atomic-level design rules for phosphorus-containing lubricants to optimize tribofilm formation and scuffing resistance under extreme conditions.

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.

Figures (8)

• Figure 1: The three molecules studied in this work. From now on, white atoms represent H, black C, yellow P, red O and blue N, respectively.
• Figure 2: Adsorption of ANAP (a), DBHP (b), and OAP (c) on Fe (upper panel) and of their simplified versions, denoted with *, on hematite (lower panel). The adsorption energy values are reported in the picture.
• Figure 3: Comparison of the adsorption energy per molecule for OAP at low (panel a) and high (panel b) coverages. The energy values take into account the VdW corrections.
• Figure 4: AIMDs results for the three systems investigated: *DBHP (panel a), *OAP (b), and *ANAP (c). On the left (right), the initial (reported simulation time) configurations are shown.
• Figure 5: In the upper panel (a to c), alkoxy chain removal energy costs for DBHP (a), amine neutralized acid phosphate (b) and OAP (c). In the lower panel (d to f), alkyl chain removal energy costs for the same molecules.