Friction Mechanism on Steel Surface in <i>n</i> -Hexadecane Containing Stearic Acid Based on Cross-Sectional Observation Using Frequency-Modulation Atomic Force Microscopy

Understanding the relationship between interfacial molecular structures and their frictional properties is one of the key issues in analyzing boundary lubrication mechanisms. In this study, the interfacial structure and frictional behavior of stearic acid (SA) solution were investigated using frequency-modulation atomic force microscopy (FM-AFM) and lateral force microscopy (LFM). FM-AFM visualized two distinct repulsive regions on steel and self-assembled monolayer substrates corresponding to vertically adsorbed SA molecules and a solvation layer of <i>n</i>-hexadecane (HD) molecules oriented parallel to the surface. Interaction force analysis revealed that the upper solvation layer was disrupted under approximately 15.6 pN loading. LFM measurements demonstrated a transition in the friction coefficient near 123 pN, indicating a load-dependent change in the interfacial configuration. A comparison of FM-AFM and LFM contact pressures using the Derjaguin-Muller-Toporov model showed that variations in Young's modulus and Poisson's ratio had a negligible effect on the estimated contact pressure, confirming the consistency of breakthrough pressure between the two measurement methods. These findings suggest that the low-friction regime under light pressure originates from the parallel alignment of HD molecules on the vertically oriented SA film.