Adsorption Behavior and Nanotribology of Amine-Based Friction Modifiers on Steel Surfaces

This work describes the effect of the adsorption behavior of fatty amine-based organic friction modifiers on their tribological performance. The adsorption of four different fatty amines with a varying number of amine functional groups (two vs three) and different anchoring structures (straight vs branched) on stainless steel surfaces in n-hexadecane was monitored using a quartz crystal microbalance (QCM). It is shown that the fatty amines form weakly adsorbed and disordered boundary films, and the molecules lay horizontally on the surface. A higher number of amine functional groups at the anchoring end of the fatty amines results in higher adsorbed masses; however, their larger steric hindrance results in slower adsorption kinetics. Atomic force microscopy shows that the loosely packed adlayers decrease the adhesion between the tip and the stainless steel surface and yield a linear increase in friction force with load at low loads. Comparing adsorbed masses and adsorption kinetic constants with the coefficients of friction measured by lateral force microscopy reveals that faster surface-adsorption kinetics of the additive molecules enables a more effective healing of the worn track, which dictates the friction force in the boundary lubrication regime.