Packing of simulated friction modifier additives under confinement

Area of Science:

• Tribology
• Materials Science
• Computational Chemistry

Background:

• Friction modifiers (FM) are crucial lubricant additives that reduce friction and wear.
• Understanding the molecular structure and packing of FM on surfaces under shear is vital for designing effective lubricants.

Purpose of the Study:

• To investigate the molecular structure and packing of friction modifier (FM) chains adsorbed on confining surfaces under sliding conditions using molecular dynamics simulations.
• To analyze the stability of these adsorbed FM structures under varying normal pressures and sliding velocities.

Main Methods:

• Molecular dynamics simulations were employed to model FM chains adsorbed between two surfaces.
• Analysis included density profiles, density fluctuations, and parallel position correlation functions to characterize FM structure and packing.
• Simulations covered sliding speeds from 0 to 7.5 m/s and a range of normal pressures.

Main Results:

• Heterogeneous structures of adsorbed FM chains were observed, forming semi-ordered monolayers perpendicular to the surface.
• More fluid interlayers with higher fluctuations were formed by unoriented FM chains parallel to the surface.
• Correlations in segment positions parallel to the surface extended up to 30 Å.
• The observed packings remained stable for over 4 ns across tested sliding conditions and normal pressures.

Conclusions:

• Adsorbed friction modifier chains form distinct semi-ordered and fluid layers under sliding conditions.
• The stability of these structures under varying pressures and sliding speeds suggests robust performance in lubricating environments.
• The findings provide molecular-level insights into the tribological behavior of friction modifiers.