Tag Archives: lubrication

Self-healing diamond-like carbon from the Argonne Lab (US)

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Argonne researchers, from left, Subramanian Sankaranarayanan, Badri Narayanan, Ali Erdemir, Giovanni Ramirez and Osman Levent Eryilmaz show off metal engine parts that have been treated with a diamond-like carbon coating similar to one developed and explored by the team. The catalytic coating interacts with engine oil to create a self-healing diamond-like film that could have profound implications for the efficiency and durability of future engines. (photo by Wes Agresta)

Argonne researchers, from left, Subramanian Sankaranarayanan, Badri Narayanan, Ali Erdemir, Giovanni Ramirez and Osman Levent Eryilmaz show off metal engine parts that have been treated with a diamond-like carbon coating similar to one developed and explored by the team. The catalytic coating interacts with engine oil to create a self-healing diamond-like film that could have profound implications for the efficiency and durability of future engines. (photo by Wes Agresta)

An Aug. 5, 2016 news item on ScienceDaily makes the announcement,

Fans of Superman surely recall how the Man of Steel used immense heat and pressure generated by his bare hands to form a diamond out of a lump of coal.

The tribologists — scientists who study friction, wear, and lubrication — and computational materials scientists at the U.S. Department of Energy’s (DOE’s) Argonne National Laboratory will probably never be mistaken for superheroes. However, they recently applied the same principles and discovered a revolutionary diamond-like film of their own that is generated by the heat and pressure of an automotive engine.

An Aug. 5, 2016 Argonne National Laboratory news release (also on EurekAlert) by Greg Cunningham, which originated the news item, explains further,

The discovery of this ultra-durable, self-lubricating tribofilm – a film that forms between moving surfaces — was first reported yesterday in the journal Nature. It could have profound implications for the efficiency and durability of future engines and other moving metal parts that can be made to develop self-healing, diamond-like carbon (DLC) tribofilms.

“This is a very unique discovery, and one that was a little unexpected,” said Ali Erdemir, the Argonne Distinguished Fellow who leads the team. “We have developed many types of diamond-like carbon coatings of our own, but we’ve never found one that generates itself by breaking down the molecules of the lubricating oil and can actually regenerate the tribofilm as it is worn away.”

The phenomenon was first discovered several years ago by Erdemir and his colleague Osman Levent Eryilmaz in the Tribology and Thermal-Mechanics Department in Argonne’s Center for Transportation Research. But it took theoretical insight enhanced by the massive computing resources available at Argonne to fully understand what was happening at the molecular level in the experiments. The theoretical understanding was provided by lead theoretical researcher Subramanian Sankaranarayanan and postdoctoral researcher Badri Narayanan from the Center for Nanoscale Materials (CNM), while the computing power was provided by the Argonne Leadership Computing Facility (ALCF) and the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory. CNM, ALCF and NERSC are all DOE Office of Science User Facilities.

The original discovery occurred when Erdemir and Eryilmaz decided to see what would happen when a small steel ring was coated with a catalytically active nanocoating – tiny molecules of metals that promote chemical reactions to break down other materials – then subjected to high pressure and heat using a base oil without the complex additives of modern lubricants. When they looked at the ring after the endurance test, they didn’t see the expected rust and surface damage, but an intact ring with an odd blackish deposit on the contact area.

“This test creates extreme contact pressure and temperatures, which are supposed to cause the ring to wear and eventually seize,” said Eryilmaz. “But this ring didn’t significantly wear and this blackish deposit was visible. We said, ‘This material is strange. Maybe this is what is causing this unusual effect.'”

Looking at the deposit using high-powered optical and laser Raman microscopes, the experimentalists realized the deposit was a tribofilm of diamond-like carbon, similar to several other DLCs developed at Argonne in the past. But it worked even better. Tests revealed the DLC tribofilm reduced friction by 25 to 40 percent and that wear was reduced to unmeasurable values.

Further experiments, led by postdoctoral researcher Giovanni Ramirez, revealed that multiple types of catalytic coatings can yield DLC tribofilms. The experiments showed the coatings interact with the oil molecules to create the DLC film, which adheres to the metal surfaces. When the tribofilm is worn away, the catalyst in the coating is re-exposed to the oil, causing the catalysis to restart and develop new layers of tribofilm. The process is self-regulating, keeping the film at consistent thickness. The scientists realized the film was developing spontaneously between the sliding surfaces and was replenishing itself, but they needed to understand why and how.

To provide the theoretical understanding of what the tribology team was seeing in its experiments, they turned to Sankaranarayanan and Narayanan, who used the immense computing power of ALCF’s 10-petaflop supercomputer, Mira. They ran large-scale simulations to understand what was happening at the atomic level, and determined that the catalyst metals in the nanocomposite coatings were stripping hydrogen atoms from the hydrocarbon chains of the lubricating oil, then breaking the chains down into smaller segments. The smaller chains joined together under pressure to create the highly durable DLC tribofilm.

“This is an example of catalysis under extreme conditions created by friction. It is opening up a new field where you are merging catalysis and tribology, which has never been done before,” said Sankaranarayanan. “This new field of tribocatalysis has the potential to change the way we look at lubrication.”

The theorists explored the origins of the catalytic activity to understand how catalysis operates under the extreme heat and pressure in an engine. By gaining this understanding, they were able to predict which catalysts would work, and which would create the most advantageous tribofilms.

“Interestingly, we found several metals or composites that we didn’t think would be catalytically active, but under these circumstances, they performed quite well,” said Narayanan. “This opens up new pathways for scientists to use extreme conditions to enhance catalytic activity.”

The implications of the new tribofilm for efficiency and reliability of engines are huge. Manufacturers already use many different types of coatings — some developed at Argonne — for metal parts in engines and other applications. The problem is those coatings are expensive and difficult to apply, and once they are in use, they only last until the coating wears through. The new catalyst allows the tribofilm to be continually renewed during operation.

Additionally, because the tribofilm develops in the presence of base oil, it could allow manufacturers to reduce, or possibly eliminate, some of the modern anti-friction and anti-wear additives in oil. These additives can decrease the efficiency of vehicle catalytic converters and can be harmful to the environment because of their heavy metal content.

Here’s a link to and a citation for the paper,

Carbon-based tribofilms from lubricating oils by Ali Erdemir, Giovanni Ramirez, Osman L. Eryilmaz, Badri Narayanan, Yifeng Liao, Ganesh Kamath, & Subramanian K. R. S. Sankaranarayanan. Nature 536, 67–71 (04 August 2016) doi:10.1038/nature18948 Published online 03 August 2016

This paper is behind a paywall.

Improve car performance with graphene balls

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Lubrication is vital for car engines and it can be expensive when you get it wrong or when it’s not as effective as it could be. A Jan. 25, 2016 news item on Nanowerk highlights some research focused on improving the quality of engine lubrication,

When an automobile’s engine is improperly lubricated, it can be a major hit to the pocketbook and the environment.

For the average car, 15 percent of the fuel consumption is spent overcoming friction in the engine and transmission. When friction is high, gears have to work harder to move. This means the car burns more fuel and emits more carbon dioxide into the atmosphere.

“Every year, millions of tons of fuel are wasted because of friction,” said Northwestern Engineering’s Jiaxing Huang, associate professor of materials science and engineering. “It’s a serious problem.”

While oil helps reduce this friction, people have long searched for additives that enhance oil’s performance. Huang and his collaborators discovered that crumpled graphene balls are an extremely promising lubricant additive. In a series of tests, oil modified with crumpled graphene balls outperformed some commercial lubricants by 15 percent, both in terms of reducing friction and the degree of wear on steel surfaces.

A Jan. 25, 2015 McCormick School of Engineering at Northwestern University news release, which originated the news item, provides more information about the team’s work,

About five years ago, Huang discovered crumpled graphene balls — a novel type of ultrafine particles that resemble crumpled paper balls. The particles are made by drying tiny water droplets with graphene-based sheets inside. “Capillary force generated by the evaporation of water crumples the sheets into miniaturized paper balls,” Huang said. “Just like how we crumple a piece of paper with our hands.”

Shortly after making this discovery, Huang explained it to Chung [Yip-Wah Chung, professor of materials science and engineering] during a lunch in Hong Kong by crumpling a napkin and juggling it. “When the ball landed on the table, it rolled,” Chung recalled. “It reminded me of ball bearings that roll between surfaces to reduce friction.”

That “a-ha!” moment led to a collaboration among the two professors and Wang, who was in the middle of editing a new Encyclopedia of Tribology with Chung.

Nanoparticles, particularly carbon nanoparticles, previously have been studied to help increase the lubrication of oil. The particles, however, do not disperse well in oil and instead tend to clump together, which makes them less effective for lubrication. The particles may jam between the gear’s surfaces causing severe aggregation that increases friction and wear. To overcome this problem, past researchers have modified the particles with extra chemicals, called surfactants, to make them disperse. But this still doesn’t entirely solve the problem.

“Under friction, the surfactant molecules can rub off and decompose,” Chung said. “When that happens, the particles clump up again.”

Because of their unique shape, crumpled graphene balls self-disperse without needing surfactants that are attracted to oil. With their pointy surfaces, they are unable to make close contact with the other graphene balls. Even when they are squeezed together, they easily separate again when disturbed.

Huang and his team also found that performance of crumpled graphene balls is not sensitive to their concentrations in the oil. “A few are already sufficient, and if you increase the concentration by 10 times, performance is about the same,” Huang said. “For all other carbon additives, such performance is very sensitive to concentration. You have to find the sweet spot.”

“The problem with finding a sweet spot is that, during operation, the local concentration of particles near the surfaces under lubrication could fluctuate,” Wang [Q. Jane Wang, professor of mechanical engineering] added. “This leads to unstable performance for most other additive particles.”

Next, the team plans to explore the additional benefit of using crumpled graphene balls in oil: they can also be used as carriers. Because the ball-like particles have high surface area and open spaces, they are good carriers for materials with other functions, such as corrosion inhibition.

Here’s a link to and a citation for the paper,

Self-dispersed crumpled graphene balls in oil for friction and wear reduction by Xuan Dou, Andrew R. Koltonow, Xingliang He, Hee Dong Jang, Qian Wang, Yip-Wah Chung, and Jiaxing Huang. PNAS 2016 doi:10 .1038/srep03863 Published ahead of print January 25, 2016

This paper is behind a paywall.

One final comment, it’s a bit unusual to see the term ‘carbon nanoparticle’. Generally speaking, carbon nanoparticles seem to have their own names, graphene, carbon nanotubes, and buckminsterfullerenes come to mind.

Reducing friction with snakeskin-inspired surface

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A June 30, 2015 Institute of Physics press release (also on EurekAlert) explains how snakeskin may inspire a whole new generation of robots bound for outer space along with other more earth-bound applications,

Snakeskin-inspired surfaces smash records, providing an astonishing 40% friction reduction in tests of high performance materials.

These new surfaces could improve the reliability of mechanical components in machines such as high performance cars and add grist to the mill of engineers designing a new generation of space exploration robots.

…

The skin of many snakes and lizards has been studied by biologists and has long been known to provide friction reduction to the animal as it moves. It is also resistant to wear, particularly in environments that are dry and dusty or sandy.

Dr Greiner and his team used a laser to etch the surface of a steel pin so that it closely resembled the texture of snakeskin. They then tested the friction created when the pin moved against another surface.

In dry conditions, i.e. with no oil or other lubricant, the scale-like surface created far less friction – 40% less – than its smooth counterpart.

Lead researcher Dr Christian Greiner said: “If we’d managed just a 1% reduction in friction, our engineering colleagues would have been delighted; 40% really is a leap forward and everyone is very excited.”

Applications are likely to be in mechanical devices that are made to a micro or nano scale. Familiar examples include the sensors in car anti-lock braking systems, computer hard disk drives, and accelerometers in mobile phones, which enable the device to determine for example whether it’s in portrait or landscape mode.

“Our new surface texture will mainly come into its own when engineers are really looking to push the envelope,” Dr Greiner said.

The snakeskin surface could be used in very high-end automotive engineering, such as Formula 1 racing cars. It could also be used in highly sensitive scientific equipment, including sensors installed in synchrotrons such as the Diamond Light Source in the UK or the Large Hadron Collider in Switzerland, and anywhere the engineering challenge is to further miniaturise moving parts.

There is interest in snakeskin-inspired materials from the robotics sector, too, which is designing robots inspired by snakes, which could aid exploration of very dusty environments, including those in space. This raises a new challenge for Dr Greiner’s team: to make a material that decreases friction in only one direction.

Anyone who has felt snakeskin will know that the scales all lie in the same direction and are articulated to aid the snake in its forward motion, while resisting backwards motion. The steel pins tested in this research mimic only the overall surface texture of snakeskin and reduce friction in at least two directions. Dr Greiner has made some progress with polymers that even more closely mimic snakeskin to reduce friction in only one direction. It is, he says, early days and this later work is not yet scheduled for publication.

The only caution is that this new surface doesn’t work well in an environment where oil or another lubricant is present. In fact, the snakeskin effect created three times more friction with lubricant than an equivalent smooth surface.

“This wasn’t a huge surprise,” Dr Greiner explained, “since we were looking to nature for inspiration and the species we mimicked – the royal python and a lizard called a sandfish skink – live in very dry environments and don’t secrete oils or other liquids onto their skin.”

Here’s a link to and citation for the paper,

Bio-inspired scale-like surface textures and their tribological properties by Christian Greiner and Michael Schäfer. Bioinspir. Biomim. 10 044001 doi:10.1088/1748-3190/10/4/044001 Published 30 June 2015

This paper is open access.