Effect of Loading and Shear Rate on Tribological Behaviour of Dodecanethiol Self Assembled Monolayer on Au(111): A Molecular Dynamic Simulation Study

Molecular dynamic simulation of nanoscale self assembled monolayers (SAMs) is used to predict the tribological properties of thiol based monolayers attached on Au(111) surfaces. Different loading conditions between 10MPa to 7GPa have been applied to dodecanethiol SHCH₂(CH₂)₁₀CH₃ monolayers to study the molecular structure and physical properties at rest, under loading and under shear. At loads lower than 700MPa, we observe smaller fluctuations in tilt and orientation angles due to a lower level of film compression. Above this critical load, the monolayers backbone starts to compress and a dramatic increase of energy is observed. Film thickness also reduces by 87% as the loading is increased. At loads higher than 5GPa, the SAM monolayer undergoes progressive disorganization resulting in a crushed and disordered system. We have investigated the tribology of the SAM films under various loads and shear rates in the range of 10⁸-10¹¹(s⁻¹). We have found that the friction coefficient increases by increasing the shear rate and decrease by increasing the loading.