Predicting frictional aging from bulk relaxation measurements

Area of Science:

• Tribology
• Materials Science
• Surface Physics

Background:

• Static friction increases with contact time (frictional aging), impacting static vs. dynamic friction.
• Existing theories attribute aging to atomic contact area expansion, which is hard to quantify due to surface roughness.
• Friction is not always directly proportional to contact area.

Purpose of the Study:

• To understand the underlying mechanism of frictional aging in rough surfaces.
• To establish a predictive model for frictional aging based on material properties.

Main Methods:

• Investigated normalized stress relaxation of surface asperities during frictional contact.
• Compared stress relaxation of asperities with that of bulk materials.
• Utilized two polymers, polypropylene and polytetrafluoroethylene, for experimental validation.

Main Results:

• Normalized stress relaxation of surface asperities matches bulk material relaxation, irrespective of asperity size or compression.
• Demonstrated a consistent relationship between bulk material properties and frictional aging behavior.
• Successfully predicted frictional aging for rough interfaces.

Conclusions:

• Frictional aging can be accurately predicted using bulk material properties.
• The study provides a new mechanistic understanding of frictional aging in rough surfaces.
• This work has implications for designing materials with controlled friction properties.