This article discusses that the Argonne National Laboratory is beginning real-world tests of a carbon-based substance that has set records for low friction. An extremely hard ultralow-friction carbon coating, developed at Argonne National Laboratory in Argonne, IL, may offer a way to address friction and wear. Introduced about a year and a half ago, the new coating is nearly friction less under inert conditions. Argonne is working with three development partners, which have signed three-year cooperative research and development agreements to transfer this technology to industry. Two of the companies are working with engine applications; one is a commercial coater, adapting the near-frictionless material to its coating process. One of Argonne’s partners, Front Edge Technology Inc., an industrial coater in Baldwin Park, CA, is using a plasma-enhanced chemical vapor deposition process with the coating. Potential applications of the coating are in the mechanical drive portion of the engine, in which the reciprocating piston motions are converted into a rotating shaft motion.
At a time when engine designers have to sacrifice some of the lubricant qualities in fuels to help us breathe easier, the Argonne National Laboratory may have hit on just the thing to help protect parts that slide, roll, or rotate. Argonne is beginning real-world tests of a carbon-based substance that sets records for low friction. Not all the kinks have been worked out yet, but Argonne and several commercial partners believe development is far enough along to merit a try.
Engine manufacturers are facing new challenges from the move to reduce sulfur content in diesel fuels, as well as interest in cleaner-burning alternatives to gasoline, such as alcohol-based formulations.
Removing sulfur-a decidedly bad actor with regard to air quality-reduces a fuel's lubricity, and alcohol-based fuels have low lubricious qualities. At the same time, the effort to reduce fuel emissions by controlling diesel engine combustion more precisely is requiring fuel injectors to operate at much higher pressures, and with closer tolerances and greater loads than ever before. All of these trends increase the chance of scuffing and wear of fuel components.
Now, an extremely hard ultralow-friction carbon coating, developed at Argonne National Laboratory in Argonne, Ill., may offer a way to address friction and wear. Introduced about a year and a half ago, the new coating is nearly friction less under inert conditions. The lab is just now beginning to conduct real-world evaluations with selected industry partners, while it continues to conduct fundamental research into chemical and physical properties.
The new coating is an amorphous, diamond-like carbon compound, one of a class of materials available commercially. However, this new material is different from others of its kind because it demonstrates exceptionally low-friction properties when tested under inert environments, according to Ali Erdemir, an Argonne research scientist who led the team that developed the coating.
Key properties of the material are low friction, extreme hardness, and excellent wear resistance, he said. The new material has a coefficient of friction of less than 0.001 when measured in a dry nitrogen atmosphere; the number is 20 times lower than that of molybdenum disulfide, the previous record holder. When tested under the same conditions, Teflon's coefficient of friction is around 9.04. T he carbon compound has a peel resistance of 200,000 psi.
Although the new coating is similar in structure to other diamond-like compounds, it has been chemically modified to achieve its low-friction properties, according to Erdemir. The exact composition and how it is modified are the subjects of a pending patent, he said. "It's produced under a high vacuum environment, where you can establish some very supercritical conditions, under which this particular structure can be obtained. There is a narrow window at which this particular form of carbon is unique."
Ongoing fundamental research is aimed at exploring the material's chemical and physical properties that can explain its low friction coefficient.
Argonne Labs has fielded approximately 2,500 inquiries from industry about the new substance, according to George Fenske, section manager of Argonne's tribology group.
The lab has received funding from the Department of Energy to explore the coating's potential use in two areas: fuel cell system compressors and advanced diesel engine fuel injection systems. Both technologies are candidates for the Partnership for a New Generation of Vehicles, the joint federal government-industry project to develop a prototype for a "clean" car by 2004.
Argonne is working with three development partners, which have signed three-year cooperative research and development agreements to transfer this technology to industry. Two of the companies are working with engine applications; one is a commercial coater, adapting the near-frictionless material to its coating process. In addition, sa.id Fenske, the lab has about 15 other application agreements, many of which the lab is bound not to disclose.
Fenske expects applications to be fairly broad-based. Besides transportation applications, potential uses for the coating may be in compressors, where new operating fluids such as R-134A and CO2 will require new lubricant packages, he said. Other potential application areas for the carbon compound include electronics, as coating for data storage devices, and in aerospace design.
Inquiries from industry, which are being fielded through Argonne's technology transfer office, are screened for technical merit and cost before being passed on for further evaluation, according to Don Knight, a commercial development agent who is handling many of the inquiries for the coating. "My job is to focus the coating's unique technology on whatever has the most practical value," he said. In evaluating potential applications, Knight said that it's important to take into account the coating's lil11jtations as well as its potential.
For example, the material's 0.001 coefficient of friction was achieved in an inert environment of dry nitrogen. If the same tests are run in air, or if humidity is added, the coefficient of friction goes up, to a range of 0.02 to 0.07, but is still in the low range of diamond-like coatings, said Fenske. Argonne is testing the coating in a variety of other environments, especially in the presence of oil, gasoline, and alternative fuels, he related.
Knight pointed out that the coating's maximum thickness is about 3 microns. Despite the hardness of the coating, the thinness of the layer may require use of a liquid lubricant, such as oil, with the coating. "Because the film is so thin, it's probably not going to survive in high-wear applications," he said.
Another limitation is high temperature. According to Erdemir, "You cannot use this coating for applications involving temperatures in excess of 200°C." Higher temperatures will probably reduce the life of the coating and produce high wear, he said.
In Knight's view, the coating's highest potential is in inert environments or in low-friction applications with marginal lubrication. "We are trying to learn what makes the friction on the near-frictionless coating so low, and look at the effects of oxygen and air, which reduce its friction and wear capabilities," he said. "At the same time, we're evaluating a variety of applications." The lab is in the very early stage of the process on both counts, he added.
The near-frictionless coating can be deposited on a range of surfaces-including aluminum, steel, ceramics, and many plastics. One of Argonne's partners, Front Edge Technology Inc., an industrial coater in Baldwin Park, Calif., is using a plasma-enhanced chemical vapor deposition process with the coating, according to the company's president, Simon Nieh. Once it adapts the coating to its equipment, the company plans to coat parts and set up coating lines in customers' plants, said Nieh.
One challenge in making the coating economically viable is in increasing the output, according to Nieh. "It's difficult to just increase the rate, because to have this low-friction performance, the operation window is pretty narrow," he said. One limiting factor here is temperature. "This new material does not like high temperature.. When you get the deposition rates higher and higher, the parts get hot," he explained.
This also can be a problem with some plastic substrates, which may no longer be able to handle the increased temperatures at higher deposition rates, Nieh noted.
One of the companies investigating engine applications is Diesel Technology Co. of Wyonung, Mich., a supplier of fuel injection systems for heavy-duty diesel engines. The company is boosting the operating pressures of its fuel injectors to improve fuel atonuzation, resulting in better fuel economy and lower emissions, according to engineering manager Tony Wu. Diesel Technology has established a goal of boosting injection pressures to 32,000 psi or higher, from 30,000 psi.
Higher injection pressures will increase the chances of wear and seizure between the plunger and plunger bore. Going to higher pressure causes the plunger's diameter to grow, reducing the clearance between the plunger and the plunger bore from the current 5 or 6 microns to virtually zero, says Wu. In addition to the tight tolerances, the plunger operates at very high speeds, moving from low pressure to peak pressure in milliseconds. This requires very good wear resistance of the plunger and bore.
Because of the critical working environment, the plunger is constructed of very hard (HR62) tool steel, which is coated with titanium nitride. Titanium nitride coating resists seizure, but does not by itself provide lubrication. The carbon coating is attractive because it will resist seizure and provide self- lubrication from its extremely low friction coefficient, Wu said. In addition to the plunger, other fuel injection components that could be candidates for this coating are the needle valve in the spray nozzle, and the electronic control valve that meters the fuel, both of which also will be subject to increased operating pressures.
Another potential use of the coating is the plunger-follower surface, the contact area between the plunger and the rocker arm that drives it, which is subject to high stress.
The issue of lubrication is becoming more important with low-sulfur content fuels. "We now use fuel as the lubricant," Wu explained. "In the future, the fuel will have low sulfur content, which reduces lubrication capability." R educed tolerance between two moving parts leaves less room for the liquid lubricants to fill, increasing the likelihood of seizure. A layer of near-frictionless coating on the part surfaces could help to prevent the parts from seizing, even if the contact line between the surfaces allows no gap for the lubricants to come in, he said.
Another potential advantage of this coating is that it can be applied at room temperature with the chemical deposition process, he said. Titanium nitride is deposited using a physical vapor deposition process at high temperatures (800-900° F), requiring relatively long coating times and expensive tool steel substrate that must be triple tempered. The near-frictionless coating can be deposited with a chemical deposition process at room temperature, which may allow the use of lower-cost steel substrates that require only one tempering step.
Wu sees wider potential applications for this coating in the future. One area is light-duty diesel engines, which face similar challenges regarding increased operating pressures. Anoth er possibility could be diesel engine turbo blades, in which the coating could help prevent seizure during cold starts.
The issue of lubrication is becoming increasingly important with low-sulfur content fuels.
STM Corp. of Ann Arbor, Mich., another partner working on engine applications, is considering the nearfrictionless coating for internal parts of its Stirling engine. The Stirling engine is a closed-cycle, externally heated engine, in which a gaseous working fluid- usually hydrogen or helium-absorbs heat and converts it into power. The working fluid is enclosed in the machine and is passed back and forth by a series of pistons between hot and cold sections, where the working fluid is alternately expanded and compressed.
Potential applications of the coating are in the mechanical drive portion of the engine, in which the reciprocating piston motions are converted into a rotating shaft motion. According to research and development manager Benjamin Ziph, depending on the application, the coating may increase durability and decrease energy loss.
Ziph sees opportunities to use low-friction materials in the unlubricated thermodynamic section of the engine that contains the working fluid. Although the high temperature of the expansion side, which can reach 800°C, prohibits use of the coating, the compression side is at room temperature, and so is cool enough for the coating to be useful. "In the future, everywhere we have a sliding contact within the pressurized containment of the working fluid is an area we should look at for potential use of the near-frictionless coating," Ziph said.
Another potential application for the coating is a reciprocating rod seal between the thermodynanuc section of the engine, where the gas is compressed and expanded at high pressure, and the drive case, which is at ambient pressure. "Piston rods that go from one section to the other have to be sealed," Ziph said. "Those seals need to have a certain compliance, and in order to achieve that compliance, we need to support them with a low-friction device." In that application, a low-friction coating could increase durability and reduce maintenance, he said.
In addition, the company is contemplating the use of near-frictionless coating in the crossheads, which take side loads from the swashplate. "Because of the construction of this device, we do not rely on hydrodynanmi lubrication of the crossheads," Ziph said. "They would benefit considerably from these coatings to reduce mechanical losses."