Boundary Friction of Aromatic Silane Self-Assembled Monolayers Measured with the Surface Forces Apparatus and Friction Force Microscopy

The boundary friction of two different aromatic silane monolayers was measured in a single-asperity contact with the surface forces apparatus (SFA) and friction force microscopy (FFM). The measurements were done in ethanol to significantly reduce the adhesion, which allowed a direct, quantitative comparison of load-dependent friction at the molecular level between surfaces of very different radii. A linear dependence of the friction force on load was obtained, and good agreement was found between friction coefficients measured with the two techniques despite the large differences in contact areas, applied loads, and pressures. The friction forces were lower in systems where both contacting surfaces were covered with a monolayer, and chemically bound monolayers were found to better protect the substrates from wear than physisorbed ones.