Evolution of ZDDP-derived reaction layer morphology with rubbing time

Area of Science:

• Tribology and Lubrication Science
• Materials Science and Engineering

Background:

• Functional additives, especially extreme pressure and antiwear types, compete for adsorption in tribological contacts.
• Poly-alpha-olefin (PAO) base oil blended with zinc dialkyl dithiophosphates (ZDDP) is a common lubricating formulation.

Purpose of the Study:

• To investigate the formation and evolution of ZDDP-derived reaction layers in a PAO base oil.
• To correlate the properties of these layers with tribological performance under mixed rolling-sliding conditions.

Main Methods:

• Ball-on-disk tribometry at 90°C under boundary lubrication regime.
• In situ interferometry to monitor reaction layer formation.
• Atomic Force Microscopy (AFM) to study topography and mechanical properties evolution.

Main Results:

• Reaction layer thickness reached a limiting value of approximately 70 nm.
• Atomic Force Microscopy revealed continuous roughening and hardening of the ZDDP-derived layer with increased rubbing time.
• Observed changes in layer properties correlate with varying tribological performance.

Conclusions:

• ZDDP additives form a protective tribological layer whose properties evolve with operational time.
• The dynamic evolution of layer topography and hardness significantly influences the overall tribological behavior and wear protection.