Friction modifier additive technologies play a crucial role in controlling friction and wear of lubricated tribological systems. Novel additives are usually evaluated using formulations of varying concentrations. It can be very difficult to understand the underlying mechanisms in those laboratory tests because of the interaction of base oil with the additives. It thus can be insightful to perform model experiments in a controllable atmosphere. This can be achieved for instance by integrating a tribometer into a vacuum system comprising in-situ surface analytical methods.
In this work, a nitrogen containing organic friction modifier is adsorbed from the gas phase onto a Fe2O3 surface. Different coating thicknesses are prepared by varying the duration of the vapor deposition, so that the influence of the coating thickness on the friction behavior can be investigated. The chemical composition of the coated surfaces is also analyzed by coupling to an XPS photoelectron spectrometer.
Contrary to the assumption that layers are formed, this friction modifier accumulates in droplets on the Fe2O3 surface. The number of droplets as well as the radii of droplets increase with evaporation time. The chemical composition of the additive does not change as a result of the gas phase deposition. In the friction tests, the smallest friction values are found for a very low coverage of droplets. For larger droplets, friction increases due to a capillary neck of additive that forms between the sliding surfaces and is dragged along during the friction test.
Using gas phase adsorption of a nitrogen containing organic friction modifier it was possible to establish a correlation between droplet morphology and the friction behavior.