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 N2 gas and in ethanol. Low adhesion (in ethanol) resulted in a linear increase in friction force at low loads, that is, F = muL, whereas higher adhesion (in N2 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 N2. 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 Sc for stearic acid than that established for more close-packed stearic acid monolayers. An increase of mu and S(c) was found with increasing degree of unsaturation of the fatty acid.