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

Nanoindentation has been combined with nanometer-scale friction measurements to identify dissipative mechanisms responsible for friction in hexadecanethiol self-assembled monolayers on Au. We have demonstrated that friction is primarily due to viscoelastic relaxations within the films, which give rise 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 friction increase with probe speed, and we present a model of friction that characterizes this speed dependence and that also predicts a linear dependence of friction on normal force in thin organic films. Finally, we identify several short-term wear regimes and identify that wear changes dramatically when films age.