Meet the Presenter
This article is based on a webinar presented by Auburn University on March 2, 2021. The webinar series, titled The Beard Tribology Webinar Series, is named after the late Ralph Beard, who pioneered the study of tribology at Auburn. Vanishing Friction: How Close are We? is available here.

STLE Life Member Dr. Ali Erdemir is professor and Halliburton chair in mechanical engineering and materials science and engineering at Texas A&M University, who also rose to prominence as a scientist and distinguished Fellow at Argonne National Laboratory. He has authored more than 300 publications, which have been cited over 25,000 times. He is a member of the National Academy of Engineering, current president of the International Tribology Council and STLE Past President. He is a Fellow of STLE, American Society of Mechanical Engineers, American Association for the Advancement of Science, America Vacuum Society and American Society for Metals, or ASM International.

You can reach Erdemir at aerdemir@tamu.edu.

Dr. Ali Erdemir 

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KEY CONCEPTS
• Research into the eventual elimination of friction has been exploding in recent years.
• 2D lubricating materials, such as graphene, have shown great promise.
• Other materials such as diamond-like carbons and vinyl-like carbons that come in different forms also may lead to near zero fiction.

Vanishing friction also is known as superlubricity. There is a very strong parallel between friction, energy and the environment. Friction reduction, and ultimately the elimination of friction, has and will continue to have a strong impact on energy security and environmental sustainability.

Vibration, heat, sound, static electricity, light, etc., all relate to friction. While friction and friction reduction have their roots at an atomic scale, their consequences are felt at engineering and societal levels.

Friction and wear almost consume a quarter of global energy output and are a major source of lost energy. Given that most of the energy used to overcome friction is produced in power plants that run on fossil fuels, the effort to reduce friction ends up also helping to reduce a significant amount of CO₂ along with other hazardous chemicals.

Over time, these emissions have been accumulating in the upper atmosphere creating weather/environmental-related issues. If these emissions could be reduced or even “vanished,” it would lead to a very sustainable, very clean, very healthy environment into perpetuity.

STLE Life Member Dr. Ali Erdemir, professor and Halliburton chair in mechanical engineering and materials science and engineering at Texas A&M University, says, “But unfortunately we are very much dependent on non-renewable energy resources. And I think this will continue well into the second half of this century. While there is a lot of positive activity regarding clean energy, these developments, unfortunately, are happening at a very slow pace.”

This article is based on a webinar, titled Vanishing Friction: How Close are We?, and presented by Erdemir and Auburn University. See Meet the Presenter for more information.

In the U.S. alone, there are almost 300 million transportation vehicles in operation. Sacrificing nearly two-thirds of the fuel’s energy to thermal losses, these vehicles are already extremely inefficient in terms of energy.

While at Argonne, Erdemir conducted a focused study with Dr. Kenneth Homberg of Finland to see how much of the total energy in a typical gas tank actually moves the car. “It turns out that if you consider all these thermal energy losses, the air drag and everything else, only about 20% of the total energy that you recover out of the liquid fuel is used to move the car,” Erdemir concludes. “This is really unsustainable and has to change.”

With the current interest in electric vehicles (EVs), Erdemir hopes that by the middle of this century, half of the vehicles on the road will be powered by electricity. Erdemir conducted another study with Holmberg, concluding that the switch to battery powered EVs could improve energy efficiency by more than 70%.

All this said, in terms of the carbon footprint, EVs may not make much difference if the reliance on fossil fuel power plants remains the same. “Then we are just shifting the carbon footprint to another source,” Erdemir says. “But if you can somehow switch or develop technologies that will enhance or reduce frictional losses—that will have a huge impact on the environment and also energy security.”

Atomistic origins of vanishing friction
The earliest scientific studies on friction go back to the middle ages. Leonardo da Vinci was an early pioneer, developing sound scientific principles that are still relevant—the same experiments could be replicated today and the results would pretty much be the same. Since then, there has been remarkable progress with atomic scale models ushering in a new age in the study of friction.

In the first half of the 20th century, researchers developed realistic models in which vanishing friction actually became a possibility. M. Hirano and Jeffrey Sokoloff were two pioneers of these early studies that developed commensurability versus incommensurability—states of contact that make an impactful difference in the frictional properties of two contacting surfaces.

With the understanding of electronic and phononic dissipation and developing leading-edge materials based on that, it is now possible to achieve superlubricity (nearly zero friction) at the interfaces of some sliding surfaces.

Rubbing steel against itself or aluminum against itself produces high friction. However, if PTFE, MoS₂ or graphite is sprinkled with boron nitride, it reduces friction substantially; these layered or otherwise very slippery solids make that entire face very, very slick.

Since early 2000, there has been an explosion in studies that lead to perfecting 2D materials in a more controllable manner; under the right conditions, these materials providing levels of friction down to 0.001, “This is quite remarkable,” Erdemir says. “I think that shows us how sliding interfaces can be engineered or lubricated to achieve extremely low friction coefficients. From a more practical point of view, liquid superlubricity will, of course, be far more desirable.”

2D materials
2D lubricating materials were first introduced about a decade ago with the discovery of graphene in 2004 by Professor Andre Geim and Professor Konstantin Novoselov at The University of Manchester for which both were awarded the Nobel prize. Graphene, as one of the strongest materials, possibly 200 times stronger than steel, also had very unique friction and wear properties that have had a hugely positive impact on tribology.

“Argonne was one of the very first research institutions that started rubbing and scrubbing graphene,” Erdemir says. “We published some very good papers, a number of which showed that even one monolayer of graphene would provide very long life despite having very high contact pressure. And there also was the possibility of further functionalization of a graphene sheet to achieve nearly zero friction. 

“We were not the only researchers to get these kinds of results. Several other laboratories started working on these things. Under the right conditions, these surfaces could actually provide an extremely low friction coefficient. One of these conditions actually relates to the commensurate or non-commensurate state of contact. In the case of commensurate contact, you have two surfaces that could be made out of the same material or dissimilar material, and their atoms on the surface would be in full registry. Because of this full registry, you have strong atomic scale interactions and, thereby, you have atomic locking. This will produce very significant interfacial adhesion, which will make these surfaces move with great difficulty.”

In the case of incommensurate contact, research shows that if the surface atoms are twisted and they are brought out of the full registry, the result would be a state of incommensurability that would produce or give rise to weak inter-atomic potential. The energy loss and gain between these atoms will cancel itself out and, in this particular case, produce nearly zero friction.

“It’s fascinating because the nature of these sliding interfaces is physically and structurally very unique,” Erdemir adds. 

One of the earliest verifications of superlubricity was produced by a group of scientists from the Netherlands. They were able to test a model to see if zero friction can be achieved by using a nanoscale tribometer. They attached a layer of graphite or graphene to the surface of the tip, and they rubbed this against the highly oriented graphite. By twisting these two surfaces, they were able to see very high stick-slip or adhesive interaction and show that after reaching a certain twist angle, friction would nearly vanish.

This effect was not limited to very conformal surfaces; it also could be achieved with graphene applied to the microsphere and also the microsphere being rubbed against either graphene or graphite.

For example, a silicon dioxide microsphere against a high-purity graphite actually behaves, with regard to increasing load, at a very high frictional force. But if there is a graphite coating on the microsphere rubbing against the same kind of high-purity graphite surface, the friction almost vanishes.

Argonne has conducted many studies in this particular area. “We have seen extreme resistance of graphene to wear, and these graphenes could actually be produced using the chemical vapor deposition (CVD) method,” Erdemir explains. “And even one layer of graphene under some reasonably high contact pressure could have nearly 6,000 sliding cycles before wearing off. This type of lifetime out of one-atom-thick graphene sheets, if you build a few additional layers, could extend incredibly—to almost 50,000 cycles.”

The study was performed in a hydrogen environment. Hydrogen seems to make the broken surfaces of graphene or crystal under sliding stabilized or neutralized by hydrogen so that they don’t necessarily further disintegrate or participate in the stick-slip type of behavior and trigger very high friction.

In further studies, Erdemir and his colleagues also have seen that superlubricity can be achieved with discrete graphene flakes sprinkled on a surface. They combined these flakes with diamond nanoparticles typically in the range of three to five nanometers, and rubbed them against a diamond-like carbon (DLC) coated ball so that they had carbon material in different forms. These combined carbon structures were able to bring the friction coefficient down to the 0.004 level. A very high resolution transmission electron microscopy of these surfaces revealed scroll-like formations with diamond nanoparticles in the center.

“This gave us the impression that these nano flakes actually were attaching to the surface of the nanodiamond material,” Erdemir says. “This makes perfect sense. Both the edges and the surface of the diamond particles will have sigma bonds or covalent bonding to edges of graphene flakes. Once that happens, then the rest of the graphene would actually just scroll to form these rolls.”



There are many other studies that attest to the importance of this structural superlubricity in one form or another, and graphene has been studied quite extensively. Other colleagues looked at MoS₂ and other 2D material. It also was found that, even in nanotubes, a similar effect could be achieved by twisting the inner and outer shells of the tube and bringing them to this non-incommensurable state of contact. 

“There seems to be a lot of potential in terms of industrial applications in the near future and the fact that these can be functionalized by some polar molecules or additives and added to a lubricant,” Erdemir concludes. “So this sounds very exciting in terms of large scale industrial applications.”

Coatings (DLCs and so on)
Another class of material that exhibits superlubricity is DLC in different forms or chemistries. By and large, they are amorphous in nature, but depending on the structure and the chemical state, they could have, for example, no hydrogen and be mostly made of sp³ bonded carbons.

This class of DLCs is similar to diamond in terms of friction and wear properties. There also can be another type of DLC that is mostly made of sp² bonded carbon, with tribological properties similar to those of graphite. Yet another class of DLC may contain lots of hydrogen. These are mostly produced in a hydrogen-containing plasma environment where hydrogen actually incorporates into the structure. Finally, there are many hydrogenated carbon variations along with metals and elements that can be doped into DLCs to achieve other types of physical and mechanical properties. Metal-doped DLCs also are very important in terms of their industrial promise.

“We started our studies on DLC toward the end of the 1980s, and we published one of the very first papers on DLC back in 1991,” Erdemir says. “What we had seen since then was a very unique correlation between the amount of hydrogen that we put in the DLC versus the friction coefficient, and we further optimized these to achieve a very unique chemical composition that resulted from 25% methane and 75% hydrogen in the plasma environment. The DLC film grown in this kind of plasma environment gave a friction coefficient down to a level of 0.001.

“So this was quite remarkable. Based on our analytical studies, we came up with a model in which we said that, since we put a lot of hydrogen into these films, not only their bulk, but also the surfaces are fully hydrogen saturated.”

Erdemir and his colleagues collaborated on studies that looked into using initial calculations and density functional theory (DFT) calculations. To further prove their point, they started with a hydrogen-free DLC, which can be directly sputtered out of a carbon target in the absence of hydrogen. They were primarily sp², sp³ bonded carbon.

Using an ion bombardment technique and with these hydrogen atoms ionized and bombarding the hydrogen-free carbon surfaces, the highly and artificially hydrogenated carbon surfaces also should yield superlubricity. “We did the experiment, and it was no surprise that a hydrogen-free or non-hydrogenated surface would fail after about 5-10 meters and with friction typically going all the way up to 0.85,” Erdemir says. “So the hydrogen termination of the carbon surfaces of these DLCs turned out to be a very important mechanism to achieve superlubricity and, of course, this also reflected very well on the wear of these materials.”

In the case of non-hydrogenated or hydrogen free surfaces, the DLC film wore out very quickly. In the other case, with the plasma hydrogen-treated surface, the places where the sliding had taken place could hardly be seen.

Similar things could be achieved with highly polar oil additives such as oleic acid. This is a highly polar molecule, and, if added to a base oil on the surface of the DLC—it could be either hydrogenated or hydroxylated—tends to attach to the surface. With that attachment, the surface again terminated with something more benign and less chemically active.

So, in the case of DLC, it looked as if the superlubricity had a lot to do with the hydrogen termination of the top sliding surfaces. The surfaces also can be terminated with hydroxide or oxygen.

Erdemir also conducted a test running in a pure oxygen or moist environment. The friction coefficient was low, but not as low as hydrogen, mainly because even these oxygen or hydroxyl-terminate surfaces would have some polar activity and attraction and, thereby, their friction would be higher.

Conclusions
“We are still working very hard,” Erdemir concludes. “In fact, studies related to superlubricity quite recently skyrocketed. In a given year, there will now be hundreds of papers published on the subject. With all these developments, I’m very hopeful that in the future—in another 10-20 years—we might actually see some real mechanical systems utilizing some of these technologies, achieving near zero friction and, with that, also consuming the least amount of energy. This will be especially valuable if you can somehow tie this into EVs—consuming very little energy due to frictional dissipation or traditional energy losses. This is even better if that energy is coming from the sun or wind or something renewable. That will make these vehicles totally green.”

Erdemir and Jean-Michel Martin published a book in 2007 titled “Superlubricity” and recently updated it to a second edition. A summary is available here.

Jeanna Van Rensselar heads her own communication/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at jeanna@smartprcommunications.com.