The Decrease of Static Friction Coefficient with Interface Growth from Single to Multiasperity Contact

TL;DR

This study investigates how the static friction coefficient decreases as contact transitions from single asperity to multiasperity, revealing load-dependent friction behavior through experiments and modeling.

Contribution

It provides experimental evidence and a simple model explaining the reduction of static friction coefficient with increasing normal load in multiasperity contacts.

Findings

Friction drop decreases with higher normal loads.

Presence of presliding and subcritical contact points reduces static friction.

Model bridges microscopic and macroscopic friction behaviors.

Abstract

The key parameter for describing frictional strength at the onset of sliding is the static friction coefficient. Yet, how the static friction coefficient emerges at the macroscale from contacting asperities at the microscale is still an open problem. Here, we present friction experiments in which the normal load was varied over more than 3 orders of magnitude, so that a transition from a single asperity contact at low loads to multiasperity contacts at high loads was achieved. We find a remarkable reduction in the friction drop (the ratio of the static friction force to the dynamic friction force) with increasing normal load. Using a simple stick-slip transition model we identify the presence of presliding and subcritical contact points as the cause of smaller static friction coefficient at increased normal loads. Our measurements and model bridge the gap between friction behavior commonly observed in atomic force microscopy experiments at microscopic forces, and industrially relevant multiasperity contact interfaces loaded with macroscopic forces.