Non-Amontons behavior of friction in single contacts

TL;DR

This study reveals that friction in single contacts between a glassy polymer and silica does not follow Amontons' law and is highly influenced by the molecular structure of the surface layer, affecting dissipation mechanisms.

Contribution

It demonstrates non-Amontons behavior of friction and links velocity dependence to surface layer structure, providing new insights into interfacial dissipation mechanisms.

Findings

Friction does not obey Amontons' law in single contacts.

Frictional behavior depends on the surface monolayer structure.

Dissipation mechanisms differ between disordered and self-assembled layers.

Abstract

We report on the frictional properties of a single contact between a glassy polymer lens and a flat silica substrate covered either by a disordered or by a self-assembled alkylsilane monolayer. We find that, in contrast to common belief, the Amontons proportionality between frictional and normal stresses does not hold. Besides, we observe that the velocity dependence of the sliding stress is strongly sensitive to the structure of the silane layer. Analysis of the frictional rheology observed on both disordered and self-assembled monolayers suggests that dissipation is controlled by the plasticity of a glass-like interfacial layer in the former case, and by pinning of polymer chains on the substrate in the latter one.