In-situ molecular-level observation of lubricant species at tribological interface using surface-enhanced Raman spectroscopy

This study directly monitored molecular structures of lubricant species during sliding using in-situ surface-enhanced Raman spectroscopy (SERS) with plasmonic sensors developed by our research group. This method, characterized by exceptional sensitivity to interfacial chemical structures and high temporal resolution (1 s), facilitated tracking changes in lubricant species. Results for pure dodecane, used as the base oil for the lubricant, revealed immediate degradation upon sliding, forming amorphous carbon with benzene rings. This methodology provides the first observation of carbon degradation with high temporal resolution. In contrast, for the lubricant containing lauric acid, a surface-active additive at the iron interface, lauric acid primarily coordinated to the iron substrate, in bidentate coordination as iron laurate before sliding, with this structure persisting during sliding. This indicates that the carboxylate ions (COO⁻) function as a lubricating film by maintaining coordination with the iron substrate. In addition, continuously measured SERS spectra exhibited dynamic changes in the region corresponding to COO⁻, indicating that the degree of molecular interaction between carboxylate ions changes over time, with the bidentate coordination of COO⁻ remaining predominant. This finding contributes to optimizing the formulation of lubricant blends with multiple additives. In conclusion, this study provided direct observation of chemical structures during sliding, which were previously only inferred owing to observational challenges. This work is expected to significantly enhance the molecular-level understanding of various base oils and additives under frictional conditions, facilitating the molecular design and formulation of more effective lubricants.