Ali Erdemir
Fighting Friction
February / March 2005
Volume 3 Number 1
Friction may have its advantages, but it's primarily a mechanical nuisance that has cost the world billions of dollars in lost labor, down-time and cost of replacement parts. Mechanical challenges like dealing with friction have sparked the new field known as tribology—€”a branch of engineering devoted to studies of friction, wear and lubrication. Recently, a joint effort between tribologists at the Department of Energy's Argonne National Laboratory and materials scientists at the University of Illinois at Chicago and Drexel University in Philadelphia has resulted in the development of a carbon-based coating that can reduce mechanical friction by up to 75 percent. This coating, known as nanostructured Carbide Derived Carbon (CDC), was named one of the top 100 inventions of the year by R&D magazine in 2003.
Key to the coating's success is the presence of carbon nanostructure—€”tiny particles spanning no more than a few nanometers across (a nanometer is a billionth of a meter). According to Argonne scientist Ali Erdemir, who developed the technology in collaboration with scientists from the University of Illinois at Chicago and the Drexel Nanotechnology Institute, these nanostructures can be arranged in at least five different forms, ranging from flat sheets of graphite to cylindrical structures known as nanotubes.
"There are many coatings on the market that are based on only one phase of carbon, such as graphite or diamond," says Erdemir. "However, the nanostructures we found in CDC coating represent almost all of the major chemical phases of carbon. This combination makes our coating particularly unique."
Laboratory tests show that CDC is more wear-resistant than coatings made from single-phase graphite or glassy carbon. However, the main benefit of CDC technology is its versatility. Since each phase of carbon has different characteristics, scientists will eventually be able to apply the technology to a wide variety of materials and applications simply by adjusting the ratios of the chemical phases of nanostructures within the coating.
Friction, a force that occurs when two surfaces rub against each other, has been one of mankind's worst enemies when it comes to creating long-lasting, efficient machines. Even with two objects that look smooth, microscopic rough edges can be found along their surfaces that generate resistance when one slides across another. In macroscopic terms, this resistance translates into material wear-and-tear and wasted energy in the form of heat.
"It is estimated that anywhere from one-third to one-half of the world's energy production is used to combat friction and wear," says Erdemir, commenting on the importance of energy efficiency. "Therefore, even very small improvements in efficiency and durability within mechanical systems can save billions of dollars." For these improvements to happen, however, scientists have to find ways to decrease resistance between mechanical surfaces while maintaining durability. The solution to this problem, making the surfaces smoother, may seem simple. Yet, tribologists are still looking for slicker and more durable coatings that will make surfaces as close to friction-free as possible.
Carbon: the secret weapon
Following on the heels of widely used friction reduction approaches such as WD-40 and Teflon, carbon-based coatings seem to be the next step in the search for the ideal surface treatment. As the sixth most common element on earth, carbon is known for its ability to bond in many different ways. This ability allows carbon to be used for high-friction applications, such as in aircraft brakes, as well as for low-friction coatings.
Erdemir, for his part, is well-versed in the many ways that carbon can help tribologists. In 1991, he began work on a near frictionless carbon coating. The success of this coating, which also had the highest wear resistance of any solid material under laboratory conditions, inspired Erdemir to conduct further investigation into other ways to synthesize carbon lubricants.
"My main interest was in finding ways to synthesize diamonds, a form of carbon that's known for its excellent hardness and low friction properties. If we could find ways to synthesize diamond films at atmospheric pressure, it could be a very economical way to protect machines against the deleterious effects of friction," says Erdemir.
While searching for diamond synthesis methods in the mid-1990s, Erdemir came across a group of researchers from the University of Illinois at Chicago and the Drexel Nanotechnology Institute who had developed a way to make carbon films from metal carbide, a compound of carbon with one or more chemically bound metallic elements. Their film, which was made by taking metal carbide and exposing it to high-temperature chlorine gas at atmospheric pressure, appealed to Erdemir because of its simplicity. He began to collaborate with the researchers in 1997, using his expertise to adapt the Carbide Derived Carbon for use as a low-friction coating.
A harmony of nanostructures
Closer examination of the CDC film revealed just what Erdemir was looking for: a thin layer of diamond nanocrystals. The presence of that layer marked the first time anyone had ever synthesized diamonds at atmospheric pressure. However, his discovery also came amidst a treasure trove of other nanostructures that no one expected to find.
"It's amazing. The film is like a repository for all different kinds of carbon structures," Erdemir recalls.
The CDC coating formed in this way turned out to be comprised of five or six different layers, each of which is dominated by the presence of a particular nanostructure. Graphite, diamond, polyhedral nanotubes and "nano-onions"—€”small carbon spheres with concentric rings—€”are just some of the structures unearthed by Erdemir and his team. These layers exhibit different characteristics of lubrication and vary between two and ten nanometers in thickness.
"Having these layers of nanostructures co-exist with each other is crucial for the success of CDC coating," said Erdemir. "Not only does their overall effect allow for the coating to be an excellent low friction surface lubricant, but it also allows us to —€˜tune' or adjust the coating for use in many different applications."
Erdemir expects this flexibility to be one of the main advantages of CDC coatings. Indeed, just increasing the width of the nano-onion layer could allow the coating to be used for hydrogen storage, while increasing the width of the graphite and diamond layers could allow it to be used on gears and other mechanical sliding devices.
After examining the chemistry and structure of the coating, Erdemir turned to his Argonne-based team of researchers to find out the coating's properties related to friction and wear. Using tools such as wear machines and controlled atmosphere gas chambers, the researchers constructed a variety of "torture tests" that would expose the CDC film to harsh environments similar to those found in real-world applications. Along the way, Erdemir, in collaboration with the researchers at the University of Illinois and at Drexel, improved the chemistry and resulting performance of the coating.
Stress tests on mechanical seals offered conclusive proof of CDC's usefulness. For example, water pump seals that were treated with the special coating lasted over seven times longer in a dry-run test than those that were untreated.
Erdemir and his collaborators are now working toward transferring the CDC technology to the equipment industry, seeking industrial partners to exploit CDC properties in specific applications requiring durability and low friction characteristics. In the meantime, he continues to envision many non-tribological applications for the coating such as filters and adsorbents for biohazardous agents as well as for use in prolonging the life of automotive engines.
For example, its use with water pump seals would have immediate benefits. Water pumps are important in numerous situations, ranging from automotive to building protection to municipal pumps that supply water to homes. Current high-end seals are primarily protected by mechanically processed silicon carbide surfaces, which do not tolerate use under dry conditions. Competing products are produced as discrete coatings that suffer from poor adhesion, large internal stress, degradation and high temperatures and limited thickness that can be usefully applied. These coatings range in cost per square inch applied from 25 cents to $5; Carbon Derived Carbide can be applied for about 30 cents per square inch, which makes it competitive in the marketplace. However, CDC economics are even more attractive when viewed as a total process. CDC can be formed by the reaction of the substrate carbide seal surfaces with chlorine, resulting in the conversion of part of all of the carbide to CDC carbon, an inherent film. CDC coatings are self-adjusting—€”that is, they conform during run-in. This eliminates the need for expensive lapping and polishing of the hard carbide surfaces, steps which cost approximately 60 cents per square inch. If CDC polishing is needed, it is cheap compared to carbide polishing, making the total cost of the seal with the CDC film the same or less than that of the silicon carbide seal, with far better performance.
"It's the harmony of the nanostructures within CDC that makes it so versatile.
We are still working to understand the material at the nanoscale level. An understanding of the nanoscience behind CDC coating would definitely shed light on more efficient ways to reduce friction and save energy in mechanical systems," says Erdemir.
In addition to the work of Erdemir and the Energy Technology division at Argonne, other collaborators on the CDC coating project include Michael J. MacNallan, professor at the University of Illinois at Chicago, Bart Prorok, professor at Auburn University, Yury Gogotsi, director of the A.J. Drexel Nanotechnology Institute; and Sascha Welz and Daniel Ersoy, both Ph.D. students at the University of Illinois at Chicago.
Information regarding intellectual property rights to Argonne inventions is available through Argonne's Office of Technology Transfer. Project manager Jim Gleeson can be reached at (630) 252-6055 or Gleeson@anl.gov.
Catherine Foster is manager of media and publications at Argonne National Laboratory.
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