Development of water-based lubricants

© Fraunhofer IWM
With the newly developed in situ tribometer, wear and friction values of slide bearings can be measured directly during operation.

Until now, machine bearings have mainly been lubricated with oil-based mineral lubricants. Water-based lubricants can effectively reduce friction and material wear and, depending on the additives used, represent a long term environmentally friendly alternative - after all, water is less viscous than oil. The energy consumption of the machines and the frequency of maintenance could thus be reduced. However, the disadvantages of water are: the limited operating temperature range and the corrosion of the bearing metal parts. The latter being the main reason it has not previously been used as a lubricant. The Fraunhofer IWM has used additives to modify water so that it could actually be used as a lubricant in the future. The corrosion of the metal components is prevented by galvanic coupling, providing an electric charge from within, which alleviates the need for electricity needing to be externally applied. 

For this reason, the group Wear Protection and Advanced Ceramics has developed an in situ tribometer that measures wear and friction values during the operation of many slide bearings. The measurements enable continuous monitoring of bearings and can be carried out quickly to avoid frequent rebuilds.

© Faunhofer IWM
In the galvanically controlled friction contact, the ionic fluids accumulate on the metal surface and thus improve the tribological properties of friction and wear.

The Fraunhofer IWM solves the corrosion problem of water-based lubricants by establishing an electrochemical field produced by the bearing itself. The new method has been successfully developed and demonstrated on sintered slide bearings. The lubricant itself consists of water mixed with ionic liquids - liquid salts containing anions and cations. Lyotropic sugar-based liquid crystals were investigated as additives as well. In addition, the slide bearing contains a so-called sacrificial anode made of aluminum, which, like the steel of the slide bearing components, is exposed to the lubricant. The slide bearing ring thus consists of several layers: teflon surrounds the bearing for electrical insulation on the outside, followed by an aluminum sleeve, and the bearing inside is made of sintered iron, which surrounds the shaft. This sintered inner iron layer is traversed by a small channel so that the ionic liquid can penetrate to the aluminium. The aluminium is positively charged and the steel is negatively charged thus protecting against corrosion. This battery-like method makes it unnecessary to apply corrosion-preventing electrical current from the outside, which from a manufacturing point of view would be very complicated. In this charge gradient, the ions are aligned and deposited on the inside of the sintered metal ring so that that their ends face the rotating shaft. In this way, they form a galvanically produced protective layer on which the shaft can slide.

© Fraunhofer IWM
A new slide bearing design with water lubrication and galvanic coupling that prevents tribocorrosion without additional electrical current.

There are many possible applications for water-based lubricants: Sintered slide bearings, for example, can be found almost everywhere where self-lubricating endurance runners are required: in the small electric motors of fans, audio and video games, medical and control technology, windscreen wipers and jacks, power tools and sports equipment. But precision engineering can also benefit from this development. In addition, the technology can be transferred to other slide bearing systems.

Publications

 

Chen, W.; Amann, T.; Kailer, A.; Rühe, J., Thin-film lubrication in the water/octyl β-d-glucopyranoside system: Macroscopic and nanoscopic tribological behavior, Langmuir 35/22 (2019) 7136-7145 Link

Amann, T.; Gatti, F.; Oberle, N.; Kailer, A.; Rühe, J.; Galvanically induced potentials to enable minimal tribochemical wear of stainless steel lubricated with sodium chloride and ionic liquid aqueous solution, Friction 6/2 (2018) 230-242 Link