Friction of fatty acids in nanometer-sized contacts of different adhesive strength

The effects of adhesion, contact area, and pressure on the lubricating properties of self-assembled monolayers on steel have been investigated with friction force microscopy. The adsorbed molecules were fatty acids with varying degrees of unsaturation (0-2 double bonds; stearic, oleic, and linoleic acid) and a rosin acid (dehydroabietic acid), adsorbed from n-hexadecane solution. The friction of these loose-packed monolayers was studied in dry N-2 gas and in ethanol. Low adhesion (in ethanol) resulted in a linear increase in friction force at low loads, that is, F = mu L, whereas higher adhesion (in N-2 gas) gave an apparent area-dependence at low loads of the form F = S(c)A, where S-c is the critical shear stress. A recent model for the contact mechanics of a compliant elastic film confined between stiffer substrates was applied to the data obtained in dry N-2. Using this approach, we obtained interfacial energies of the compliant monolayers in good agreement with van der Waals-Lifshitz theory. With a low monolayer elastic modulus of E-1' = 0.2 GPa, we obtained a slightly higher value of S, for stearic acid than that established for more close-packed stearic acid monolayers. An increase of p and Sc was found with increasing degree of unsaturation of the fatty acid.