Lubricating the wheels of innovation to reduce friction

Paul Boughton

Drake Calvin takes a look at some up and coming innovations in friction reduction.

Mineral-oil derived liquid lubricants are presently the mainstay of mechanical systems - reducing friction where moving surfaces touch.

By reducing friction, they reduce waste heat, noise, and vibrations. Any improvement in friction reduction results in greater efficiencies, requiring less power to operate the system; specifically for power generation, improved friction reduction means more power out per unit of energy in. The financial gains of reducing friction are therefore quite obvious.

Liquid crystal lubricants

Andreas Kailer, head of department at the Fraunhofer Institute for Mechanics of Materials in Germany, hopes that liquid crystal lubricants - the same crystals you find in your LCD television screen - will offer the next big improvement in friction reduction.

He is trying to uncover liquid crystals that are most suited to friction reduction and under what conditions they best work.

Kailer's research has been ongoing for three years; it involves moving a clamped metal cylinder back and forth across a supporting contact surface to determine which liquid crystals require the smallest amount of energy to move the cylinder (Fig.1).

Kailer says the friction hardly changes with conventional oil but drops close to zero with liquid crystals.

The molecules, in contrast to the molecules in oils, have a certain orientation.

"Its properties are dependent on direction," says Kailer. "Viscosity is key for frictional behaviour. The crystals have very low friction co-efficients because they orientate themselves so that the direction of the lowest viscosity is perfect in the direction of frictional contact."

Kailer is working with industry partners to optimise this effect across a wide range of temperatures, contact loads, and sliding velocities. He predicts the technology will initially find application in sliding bearings. He admits that the widespread application of his technology is presently limited by contact pressure. "Currently, we are limited to ten megapascals which rules out ball bearings that typically work at one gigapascal.

"But it will be applied in any kind of sliding bearing in technical apparatus such as in turbines and technical plants."

He admits this could be a way off - as far as 20 years for large power infrastructure but Kailer predicts that his friction reduction innovation could improve efficiencies and output by 10 per cent or more. Such a gain is certainly worth the wait.

"Also, in sliding bearings the thermal effects are important," he adds. "Less heat means less thermal expansion. We could increase the range of velocities that a sliding bearing is stable at."

In the meantime Kailer believes that his liquid crystal's extremely low frictional co-efficients make them suitable for replacing the additives used to enhance mineral oil lubes.

Kailer says that in the three to five years before the product is commercialised he must find the ideal crystal and reduce the manufacturing cost.

"Manufacturers use many synthesis steps to produce high-purity crystals for electronic applications but for lubricants we don't require such purity. Later this year, we will modify the production process to reduce the steps and see how the substances compare. We hope the mixture of different molecules will work better than the pure system."

Diamond lubricants

Another researcher developing improved friction reduction is Robert Carpick, University of Pennsylvania's associate professor of mechanical engineering and applied mechanics.

Instead of liquid crystals, Carpick is working to improve ultra-nano-crystalline diamond (uncd), a multi-purpose coating marketed by Argonne National Laboratory (ANL) spin-off Advanced Diamond Technologies (Fig. 2). The coating is presently limited to friction reduction of mechanical pump seals (Fig. 3) in pumping systems in processing plants. Carpick hopes his research will diversify its use by improving understanding of how uncd works. He recently made a breakthrough using surface chemical spectroscopy. He has proved how the diamond coating works - disputing a theory that friction on the diamond coating results in crystalline graphite.

"Graphite is another form of smooth carbon with non-reactive surfaces and low friction. The idea that the diamond was turning into graphite made sense because it's thermally-stable at room temperature and can be made by heating diamond. It was a logical proposal but ultimately wrong."

Instead, where others expected graphite Carpick found hydrogen and hydroxyl groups. He has proved that the coating's friction-reducing properties work by pacifying atomic bonds via the dissociative adsorption of water.

"Imagine two surfaces rubbing against one another: one or both surfaces are coated in uncd. What happens is, as they slide and shear, you break a bond at the diamond surface resulting in a carbon atom with an unpaired electron - a dangling bond. If the opposite surface has an unpaired electron, a bond may form across the interface, which increases friction. If the bond remains as they slide apart then it may pull some of the surface away - this is the onset of wear."

Carpick has found that the dangling bond and resulting friction and wear can be pacified by adding a little water. The water splits apart, or dissociates, into a hydroxyl group and a hydrogen atom in the presence of the dangling bond. One of these then joins with the bond before it can attach to the opposite surface. Hey presto - friction reduction.

He will now take this understanding further. "We're trying to understand exactly how much water it needs and if there are other chemical species that protect the surface to extend the range of environments the coating can work in."

Much like Kailer's liquid crystal technology, the uncd coating technology has its limitations. It is currently practically-difficult to coat uncd over large areas and irregular shaped components. Also the surface being coated must withstand temperatures between 400_C to 700_C. Due to Carpick's work on the need for water, the technology is known to be impractical in arid or vacuum conditions.

Carpick is now working with Advanced Diamond Technologies with funding from the US Air Force and the National Science Foundation to address these limitations. "If we're successful, then coatings could make a big advance in tribological applications - for example these films could have a friction co-efficient ten times lower than a well lubricated automobile engine. If we could even reduce frictional losses by a factor of two, the savings would be substantial to say the least."

"We think we'll learn a lot over the coming years," he says. "We can certainly improve performance. We're optimistic they'll be advances in the next couple of years."

Both researchers know their technologies can improve on present friction reduction technologies but how far can they go? Both hinted that coatings or liquid crystals could one day replace mineral oil lubricants but both agreed that it could take decades of work to achieve this potential.

"It takes a while for laboratory breakthroughs to get adopted in industry," says Carpick. "Companies are sensibly conservative about wholesale changes in established technologies."

Both researchers will need to prove that their replacement lubricants have similar lifetimes to commercial lubricants and are just as reliable across a large range of operating conditions.

"Frictional interfaces in a mechanical system are lynch-pins," explains Carpick. "If they fail then the whole system fails. Also whatever we come up with has to cost less. If it costs one more cent per application then it's a loser. We have to show it not only works but it's cheaper too."

This may seem like an uphill struggle but the present climates, both environmental and economic, may be warming to such innovation causes.

President Obama's $787b stimulus package is looking to kickstart the economy through new infrastructure build, specifically green projects.

Other governments are expected to emulate his 'low carbon recovery' strategy. By driving innovation now, nations hope to give themselves a headstart in the race to becoming export leaders in clean energy and energy efficient technologies whilst pulling themselves out of the recession.

Obama's package puts $32b into clean energy projects and $4.5b into upgrading the country's electricity transmission system - a necessary move once more and more intermittent renewable energy comes online.

"The green energy agenda is certainly on the radar," says Carpick, "but it can take many years to develop a new source of energy or a new fuel or a method of delivering energy. The advantage that we have is that we can rapidly reduce consumption on the demand side using existing low friction materials. It would be worthy investment to support development and get more of these materials into commercial applications. For me, it hasn't been highlighted enough in the discussions about the impending energy crisis. It would be a net positive to support this research. Governments and researchers should prioritise this because it will deliver benefits - it will pay for itself."

Obama vowed during his presidential campaign that he would listen to expert advice. To some degree, Carpick's prayers could well be answered.

The National Science Foundation has now received a $6.16b pledge with instructions in the 2010 budget to improve clean technology innovation. As one of Carpick's current backers, we can only hope that the National Science Foundation pins back its ears, heeds Carpick's advice, and now lubricates the research path to increased efficiency gains.


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