Digging into the friction reduction mechanism of organic friction modifiers on steel surfaces: Chains packing vs. molecule–metal interactions

We present a study of five commercially available Organic Friction Modifiers (OFMs) that combines micro-Infrared reflection absorption spectroscopy (IRRAS), classical MD, and DFT/DFT-MD computational methods. We investigate the relation between the molecular structure of the OFMs and their temperature-dependent friction performance. Classical dynamics simulations show that at low temperatures, the OFM molecules, arranged in self-assembled monolayers, undergo collective torsional motion during a friction event. This becomes progressively easier as temperature increases, resulting in lower friction coefficients, until a temperature where these collective motions cannot be maintained anymore, leading to an increase of the friction coefficient. Static DFT calculations indicate that the strength of the packing between OFM molecules determines the friction performance. The temperature at which the collective behavior fails coincides with the temperature at which the packing-free energy is found to diminish.