Systematic investigation of organosiloxane derived surface modifications in tribochemical processes

Abstract

By introducing low-viscosity engine and gear oils, the energy efficiency of oil lubricated drive trains and transmissions can be increased. However, by reducing the viscosity of lubricating oils, transmission components run for longer periods in component damaging conditions. In order to ensure protection by the lubricant in such severe conditions, surface-active additives are added. The present work deals with surface modifications caused by organosilanes / -siloxanes in rubbing contacts. The investigations focus on the chemical changes and formed structures, as well as their influences on the lubricating behavior. For this purpose, vinyltrimethoxysilanes / siloxanes were added to a mineral oil and investigated in ball disc tribometers to investigate their influence on friction changes and lubricant film thicknesses. By varying test conditions, reactivity, chemical changes and influences on the lubrication behavior of the silane / siloxane oil additives were investigated. In addition, sol gel coatings were applied and also subjected to tribological tests and chemical characterization. It turns out that the used organosilanes and siloxanes deposit a multi-layered structure when exposed to tribological stress. While the top layer consists of weakly adhesive polysiloxanes with viscous properties, the bottom layer has an elastic character and is stronger bonded to the steel substrate. The different properties are based on the degree of crosslinking, which increases with increasing proximity to the substrate surface. The adhesive deposition is rich in SiO2, which can be explained by a polymer into glass / ceramic conversion. The polymerization proceeds via condensation reactions and shows a strong dependence on the temperature. The adhesive layer results from the decomposition and further crosslinking of the polymers by the severe tribological stress. This transformation requires a certain contact pressure or shear stress. In addition, it has been shown that the formation of iron oxide is an important factor for the conversion and is as well part of the generated tribofilm. With the knowledge of chemical analysis, a lubrication model for the multi-layer system is derived, which shows that both layers have opposing influences on the lubricating behavior of the lubricant. The in-situ generated polymers form thick polymer-rich boundary films which effectively increase the lubricating film thickness, especially at low entrainment speeds. This can reduce friction by up to 40 %. On the other hand, the adhesive tribofilm creates resistance to the oil drag in the lubrication gap and thereby increases the friction compared to the polished steel surfaces. Overall, a wear reduction and corrosion protection is also detected, however the present results only indicate tendencies from which layer the protective effects arise. Furthermore, investigations show that big differences between the film formation and thereby tribological influences are found between the use of oligomeric and monomeric vinylmethoxysilanes / siloxanes. The monomeric precursor forms much lower polymeric film thicknesses but exhibits a more homogeneous and smoother tribofilm deposition. As a result, no reduction in friction and wear is achieved.