# Friction Mechanism on Steel Surface in n‑Hexadecane Containing Stearic Acid Based on Cross-Sectional Observation Using Frequency-Modulation Atomic Force Microscopy

## TL;DR

This study explores how stearic acid and n-hexadecane molecules arrange on steel surfaces and how this affects friction, using advanced microscopy techniques.

## Contribution

The novel contribution is the direct observation of interfacial molecular structures and their load-dependent frictional behavior using FM-AFM and LFM.

## Key findings

- FM-AFM revealed two distinct repulsive regions corresponding to vertically adsorbed stearic acid and parallel-oriented n-hexadecane molecules.
- Friction coefficient transition at 123 pN indicates load-dependent interfacial configuration changes.
- Low friction under light pressure is attributed to parallel alignment of n-hexadecane molecules on the stearic acid film.

## Abstract

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 n-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.