Spectroscopic analysis of the tribological behavior of a model boundary layer lubricant

Highly ordered alkanethiol self-assembled monolayers (SAMs) on gold substra tes are suitable models of boundary layer lubricants and may be used in act ual nanoscale device applications. Here, such monolayers were studied by sp ectroscopic methods as a function of tribological wear (rubbing) using a pi n-on-disk microtribometer. The coefficient of friction (COF) (ratio of the frictional force to the load) was measured with the tribometer, and reflect ance infrared spectra and X-ray photoelectron spectra were obtained as the monolayer film failed and the COF changed. The results show that it is poss ible to correlate disorder in the monolayer film with tribological failure of the film, and that continued rubbing produces a chemical change in the m onolayer film. Disorder in the monolayer is distinct from the influence of wear in the underlying gold substrate. Aged SAMs, having sulfonate rather t han thiol headgroups and initially less well ordered, behave differently to the well-ordered freshly prepared SAMs. Interestingly, they show a lower C OF over many more cycles of exposure to the rubbing pin. The impact of the mechanism of film failure in boundary layer lubrication is discussed.