How phosphorus and sulfur additives cooperate under pressure

Lubricant additives protect moving metal parts by forming thin reaction films during sliding. Phosphorus additives are known for antiwear protection, while sulfur additives are often used for extreme-pressure performance, but the molecular reasons behind this division of labor are difficult to observe directly.

In this combined experimental and theoretical study, researchers from the University of Hyogo and Idemitsu corp connect friction tests and surface analysis with AMS-based reactive simulations. The experiments showed that phosphate additives form harder, more persistent films that reduce friction and wear, while sulfur additives form softer films that become especially useful under severe loading where preventing metal adhesion is critical.

Reactive Force Field (ReaxFF) simulations in AMS helped explain how these films form at the molecular level. Phosphate species adsorbed stably on iron oxide, supporting low-friction surface protection. Sulfur species reacted more readily with exposed iron, forming sulfur-containing films under harsher conditions. The authors also used ParAMS, an AMS module for training and reparametrizing ReaxFF, DFTB, and ML models, to extend ReaxFF for phosphorus-sulfur interactions. This allowed both additive types to be modeled together at the reactive interface.

The simulations revealed a cooperative effect: phosphate and sulfide species can interact in a way that promotes S-S bond cleavage, helping sulfur react with the iron surface. This provides an atomistic explanation for why combining phosphorus and sulfur additives can broaden lubricant protection across different load regimes.

For lubricant formulation, the work shows how AMS, ReaxFF, and ParAMS can complement surface experiments by revealing not just which tribofilm forms, but how competing additives adsorb, react, and cooperate during sliding.