Friction fighter

2 min read

German collaboration hopes lubricants made from liquid crystals will mean smoother operation for bearings and gears. Siobhan Wagner reports.

Lubricants made from liquid crystals similar to those used in flat-screen monitors could eliminate the effects of friction for many bearing and gear units, researchers claim.

A team at the

Fraunhofer Institute for Mechanics of Materials

in Germany is studying the lubricant, and says it could be on the market within three years.

Unlike traditional lubricants, liquid crystals exhibit a phase matter between liquid and solid crystal. A liquid crystal may be able to flow like a liquid but it is made up of organic molecules oriented in a crystal-like way.

Most liquid crystal molecules are roughly cigar shaped and line up parallel to one another, pointing in the same direction. When applied, these molecules reduce the points of contact between rubbing parts.

This application is a friction-reducing technique similar to the one ancient Egyptians are believed to have used to move the huge blocks of stone when building the Great Pyramids. By using logs as rollers under the stones, the points of contact between the granite and the ground were limited, thus making the movement of large objects easier.

The liquid crystal molecules replicate this on the nanoscale. Andreas Kailer and his team at Fraunhofer demonstrated that when liquid crystals are used on a slide bearing, such as in a car engine, the orientation of the crystals makes them 'more slippery' than any refined lubricant on the market.

German liquid crystal specialist Nematel has been working with the Fraunhofer team to investigate which liquid crystals are most suitable for use as lubricants, and under what conditions. Its testing unit exerts a known force on a clamped metal cylinder that is moved back and forth over a supporting contact surface.

While the friction coefficient hardly changed when using conventional oil, it dropped to almost zero when liquid crystals were used. The time this takes depends primarily on the pressure with which the moving cylinder is pressed against the contact surface.

Kailer said to date the team has not found liquid crystals suitable as a lubricant for ball bearings because the contact pressure is too high and the friction does not drop far enough. However, the team claims to have proved that liquid crystals are the ideal solution for others such as slide bearings.

The team has also found that the crystals only work better than other lubricants within a certain temperature range. 'We have shown that the liquid crystals work better between room temperature and 140ºC,' he said. 'If the temperature rises above that their effect breaks up and the friction coefficient is the same as other lubricants.'

Kailer said that while between room temperature and 140ºC is ideal for many industrial applications, it is unsuitable for the automotive sector.

'In this industry you need a minimum temperature of -40ºC,' he said. 'If, for example, you are in Greenland and need to start your car in the morning it has to work at these temperatures. The problem is that the liquid crystals are in a solid state at that temperature and we need them to be liquid.'

Kailer and his team are now working on ways to extend the liquid crystals' temperature range. One idea they have had, he said, would be to blend the liquid crystals with base oils.

One major challenge on the road to commercialisation will be the cost of the liquid crystal lubricant technology. Since the crystals have, until now, been produced mainly for monitors, they have to be ultra-pure, which makes them very expensive.

Kailer said Fraunhofer and Nematel are working to simplify the synthesis process because less-pure substances are also suitable as lubricants.

'We hope to get this synthesis in one step so we have an easy scale-up process that can cheaply produce high amounts of liquid crystals for tribological applications,' he said.