Contact hysteresis and friction of alkanethiol self-assembled monolayers on gold

Nanoindentation has been combined with nanometer-scale friction measurement s to identify dissipative mechanisms responsible for friction in hexadecane thiol self-assembled monolayers on Au. We have demonstrated that friction i s primarily due to viscoelastic relaxations within the films, which give ri se to contact hysteresis when deformation rates are within the ranges of 5 and 200 Angstrom/s. We observe that this contact hysteresis increases with exposure to air such that the friction coefficient increases from 0.004 to 0.075 when films are exposed to air for 40 days. Both hysteresis and fricti on increase with probe speed, and we present a model of friction that chara cterizes this speed dependence and that also predicts a linear dependence o f friction on normal force in thin organic films. Finally, we identify seve ral short-term wear regimes and identify that wear changes dramatically whe n films age.