Multiscale correlation between molecular binding, adsorption, and macroscopic lubrication for OFMs

Abstract: Organic friction modifiers (OFMs) have been researched in great details since their discovery in the early 1920’s. Various bench-top methods, most notably the high frequency reciprocating rig (HFRR) and Mini-Traction Machine, have been used to evaluate the performance and properties of OFMs at macroscopic scale. 
 

 

In this work, we present a multiscale correlation between molecular binding strength measured by Atomic Force Microscopy (AFM), adsorption kinetics quantified by Quartz Crystal Microbalance (QCM), and macroscopic friction and wear, evaluated by a Wear Tester. The nicely agreed relationship decouples the lubrication mechanisms of OFMs. There are two primary factors for OFM functionality: interaction with the target substrate and the corresponding adsorption kinetics. We found that the adsorption kinetics correlates strongly with the wear scar observed at the macroscale, indicating that it is the driving force for reducing wear of the simple acid and amine based actives. On the other hand, kinetic coefficient of friction is controlled by the ability of the OFMs to adsorb to the surface and their tailgroup to reduce the surface adhesion at the interface.
 

 

Our work unravels the fundamental lubrication mechanisms, highlighting the synergistic effects of surface interaction and adsorption kinetics, both of which are influenced by the operational environment and the target substrate.