Ionic liquids: lightning conductor in e-mobility

We’ll be seeing this topic gaining increased importance in time, as the use of frequency converters in modern power transmission systems for continuous speed control of motors and generators is increasing and will lead to more damage caused by electric erosion. Also, the current trends towards using higher voltages, e.g. in automotive vehicle power systems, will result in higher energy discharges in the future.

For more than 20 years, the tribology expert Klüber Lubrication has been researching into the use of ionic liquids for innovative lubricant concepts especially in this segment. 

This modern concept turns the lubricant into a ‘lightning conductor’ to prevent harmful potentials from forming. In addition to the vehicle-relevant data like speed, smooth operation and extended service life, this new generation of speciality lubricants also addresses the issue of electric conductivity.

What are the damages caused by electro erosion?

The rolling bearings of electric motors and other electric components can develop electro erosion caused by an undesirable passage of current through the contact zone between the rolling element and the raceway. Therefore, the heat that’s generated locally melts the surface. From that, several micro-metre sized pittings are formed. Both types, tend to damage the lead to the generation of undesirable bearing noise at first, which finally leads to premature failure.

The problem can be solved by several design approaches, e.g. by insulating the inner and outer rings or using ceramic rolling elements. The problem with these measures is that they don’t always bring success, despite the high costs they incur. A simpler yet more promising approach is to use a lubricant of low impedance, which is a design element of the component anyway, to dissipate the currents generated.

Another property of lubricants is that they’re insulators. How? Because they block the passage of electric currents that they increase a voltage build-up in bearings components made of metal. Traditional solutions involve the use of black lubricants based on graphite and carbon, which, on the one hand, can also prevent parasitic currents. On the other hand, they contain black, solid graphite particles which impede smooth running. As such particles are displaced from the friction point over time and are subject to change by the mechanical load, electroconductivity of these greases decreases continuously.

Promising lubricant innovations

We are seeing clear advantages of making the lubricant conductive by adding ionic liquids to dissipate, which can see it replacing ‘black lubricants’. Especially high-frequency EDM (Electric Discharge Machining) currents can be neutralised by suitable lubricant designs.

This solution is offered by Klüber currently, they own three patents for using ionic liquids in lubricant formulations. One of these patents implies their use as a base oil, the other two patents refer to the additivation.

The background of Ionic Liquids

In layman terms, Ionic liquids are basically liquid salts. A stable crystal structure does not appear within them because it is prevented by charge delocalisation and steric effects. By definition, the melting point of these salts is below 100 °C and in many cases even far below this point, with the result that the salts are already liquid at room temperature. The properties of ionic liquids can be varied according to requirements by modifying the cations or anions. Thus, they are predestined for use in many areas of chemical process engineering.

Ionic liquids, along with their properties have existed and know about more than hundred years but have received very little attention and not used in products or processes. Owing to their electric conductivity they reduce undesirable passage of current, which make them an interesting solution for bearings in electric motors.

Klüberlectric BQ 72-72, EDM currents measured in A as a function of speed and temperature (TU Darmstadt)

Ionic liquids also have an adequate number of cations and anions, which offer excellent resistance to oxidation. hey are thermally stable and can be used at temperatures as high as 150 °C, are hardly inflammable, very soluble and non-toxic. Their extremely low vapour pressure is another advantage. Unlike speciality lubricants containing conductive particles, ionic liquids do not have a negative effect on noise. Several greases already proved their suitability for high speeds. They currently achieve speed factors exceeding 1 million n x dm. Klüber Lubrication aims at speed factors > 2 million n x dm.

The use of Ionic liquids can increase the service life of bearings and therefore contribute to sustainability. As the energy impact of undesirable electric currents provokes not only damage to the bearings but also affects the lubricant's performance and decomposes the lubricant due to the extremely high peak temperatures, another positive effect is that they help extend grease lifetime.

Conducting comprehensive test rig examinations along with tribological analyses can provide information on the condition of the used and the bearing too. n important feature in this context is the interplay of results relating to the bearing and those relating to the lubricant. Due to the detailed examination of each component, any possible damage can be identified with great accuracy.

From the use of ionic liquids, the test rig results showed that bearing and lubricant damage arising from electric currents are significantly reduced.

The outlook of Ionic liquids

Colliding with strong partners within the industry, Klüber Lubrication are currently working on further optimising the conductivity of lubricants while still maintaining their tribological performance by means of ionic liquids. In case of a very high level of energy passage, it is not yet possible at this stage to solve the problem of undesirable current passage merely by means of the lubricant.  This, however, would be highly desirable for the industry, because it could reduce the size of bearings. An interesting approach is the combination of insulating bearings and additional discharge elements containing electroconductive lubricants.

Conclusion

With them being a design element, lubricants are a crucial factor when it comes to efficiency and reliability of mechanical components. Experts like Klüber Lubrication research on continuous development as lubricants are key in extending the service life of components and increasing both the energy efficiency and sustainability.