Effect of a Fatty Acid Additive on the Kinetic Friction and Stiction of Confined Liquid Lubricants

The kinetic friction and stiction properties of the dilute solution of palmitic acid (PA, friction modifier additive) in poly(a-olefin) (PAO) confined between molecularly smooth mica surfaces were investigated using the surface forces apparatus (SFA). The results were contrasted with those of confined PAO system; the effects of PA additive on the confined film structure and friction/stiction properties were discussed. The hard-wall thickness of the PA/PAO system was 4.2 +/- 0.1 nm, and that of the PAO system was below 1.4 nm. Considering the molecular size, the PA/PAO system has a monomolecular layer of PAO confined between PA adsorbed monolayers on mica surfaces. The kinetic friction of the PA/PAO system decreased with the increase of sliding velocity V, whereas the kinetic friction of the PAO system increased with the increase of V. The stiction spike height of the PA/PAO system increased with the increase of applied load L (pressure P), whereas the spike height of the PAO system decreased with the increase of L (P). These results imply totally different confined structures/sliding mechanisms for the two systems: solid-like slippage of PAO monomolecular layer between PA adsorbed monolayers for the PA/PAO system; and the extreme viscosity increase of PAO and resulting glass-like transition for the PAO system. The kinetic friction and stiction were larger for the PA/PAO system, which comes from the extreme confinement of PAO (only monomolecular layer) and strong van der Waals adhesion between the solid-like interfaces. The SFA results for smooth surfaces obtained here seem inconsistent with the observations in macroscopic tribology; palmitic acid dissolved in PAO reduces kinetic friction and stiction of engineering (rough) surfaces. The discrepancy of the friction behaviors between smooth and rough surfaces is discussed, which gives insights into designing low-friction surfaces in the oil-based lubrication of macroscopic tribology.