Friction of Polyaromatic Thiol Monolayers in Adhesive and Nonadhesive Contacts

We have used friction force microscopy to study the effects of adhesion on the boundary friction of self-assembled monolayers of the aromatic compounds thiophenol, p-phenylthiophenol, p-terphenyl thiol, 2-naphthalenethiol, and benzyl mercaptan on gold. To control the adhesion between the monolayer-covered tip and substrate, the friction measurements were made in dry N-2 gas or in ethanol. At low loads, low adhesion (in ethanol) resulted in a linear dependence of the friction force on load (i.e., F = mu L) whereas higher adhesion between the same monolayers (in N-2) gave an apparent area-dependent friction, The friction in the adhesive systems was well described by F = S(c)A with the contact area, A, calculated for a thin, linearly elastic film confined between rigid substrates using the thin-coating contact mechanics (TCCM) model in a transition regime between its DMT- and JKR-like limits. With increasing packing density of the monolayers, a systematic decrease was found in the friction coefficient (mu) obtained in ethanol and the critical shear stress (S-c) obtained in N-2. To describe these aromatic monolayers with the extended TCCM model, a higher Young's modulus was neeeded than for fatty acid monolayers of similar packing density.