In Situ Measurements of Boundary Film Formation Pathways and Kinetics: Dimethyl and Diethyl Disulfide on Copper

The reaction pathways and shear-induced kinetics of methyl thiolate and ethyl thiolate species on copper are measured using in situ and ex situ techniques in ultrahigh vacuum. The in situ techniques consist of measuring the gas-phase products using a mass spectrometer placed in-line-of-sight of the rubbing interface while monitoring the variation in friction coefficient of an alkyl thiolate-covered surface as a function of the number of times it is rubbed (referred to in the paper as “number of scans”). The rubbed surfaces are analyzed using Auger spectroscopy as a function of the number of scans. The experiments are carried out for a tungsten carbide ball covered by a copper transfer film on copper surface at a normal load of 0.44 N and a sliding speed of 4 mm/s. The shear-induced reaction occurs as RS_(ads) → S_(ads) → S_(subsurface), where RS_(ads) is an adsorbed alkyl thiolate species, S_(ads) is adsorbed atomic sulfur, and S_(subsurface) is subsurface sulfur formed by shear-induced surface-to-bulk transport. The rate constants for the sequential reaction steps are found by fitting an analytical kinetic model to the yield of gas-phase products and Auger signals as a function of the number of scans over the surface. The validity of the kinetic parameters is confirmed by comparison with the variation in friction coefficient as a function of the number of scans. The analysis reveals that both ethyl and methyl thiolate species decompose under shear at approximately the same rate and that the rate of surface-to-bulk transport is higher than for thiolate decomposition.