Alkyl chain length effect on tribological behavior of alkanethiol self-assembled monolayers on Au

We have investigated the frictional properties of alkanethiol self-assembled monolayers (SAMs) by pin-on-plate methods. SAMs made from octadecanethiol. (C18) with a longer alkylchain exhibited lower friction coefficients (approximately 0.2), while the friction coefficients were higher (approximately 0.4) in the case of the SAMs made from other alkanethiols with short alkylchains. The reasons for the increase in friction were i) the distortion of SAM structures because of the penetration of pins into SAMs and ii) oxidation of molecules as a result of X-ray photoelectron spectroscopy (XPS) measurements. Further friction tests on C18 SAMs for 600 cycles of sliding (I h) caused the higher friction coefficients (0.6-0.7) and oxidation of alkylchains. It was considered that repeating of slides led to the distortion of SAMs, the oxidation of alkylchains, or adsorption of water or oxygen. The total XPS peak ratios of C(Is)/Au(4f) hardly changed after 1800 cycles of sliding Q h), indicating that C18 SAM remained even. after many repetitions of sliding.