Powertrain system engineers know that of the energy consumed in transportation, 10% to 15% is lost due to parasitics in engines and drivelines. Researchers at Argonne National Laboratory have developed a new breed of nanocomposite coatings, which are made of the nitrides of transition metals and metal catalysts. These coatings provide a catalytically active, hard, and slick surface on metal components. They could have a major impact on improving the efficiency of automotive engines and gearboxes.
Transportation vehicles account for about 19% of annual world energy consumption and approximately 23% of total greenhouse gas emissions. With the global vehicle parc steadily growing, these numbers will likewise swell and present serious challenges for a sustainable mobility future. The new nanocomposite coatings also can work in concert with engine start-stop, downspeeding and cylinder-deactivation systems to further reduce vehicle fuel consumption.
In automobiles, tribological inefficiencies due to friction and wear in machine components are some of the greatest sources of energy and material losses. This has perhaps been recognized since man first fit a wheel on a wooden axle. By the 16th century, the great Leonardo da Vinci—considered the “father” of the modern study of friction and lubrication—had invented a self-oiling system for axle ends. Today, friction in a vehicle’s engine, transmission, brakes and other moving parts consumes nearly one-third of the fuel’s energy. Also, progressive wear that takes place between moving parts eventually causes component breakdown and eventually costly repair and/or replacement.
Argonne’s smart, slick technology
Current lubricants are made of two key parts. One is a base oil, which accounts for nearly 80% of the total volume. The other is an additive package, which is literally a “soup” of various types of chemicals—many of which, unfortunately, are now considered environmentally harmful. These additives also poison the catalyst in exhaust systems. Without the additive package, however, the base oil will be useless. In a car’s engine, neat oils will cause instant engine failure due to sever wear and scuffing.
The Argonne development team has designed “smart” catalytically-active surface layers, which can crack long-chain hydrocarbon molecules of base oils and turn them into diamond-like carbon “tribofilms” and other forms of carbon nanostructures on rubbing surfaces. The result is an ultra-low-friction carbon film on metal components that is slick and highly protective. When worn away, the film self-heals by a catalytic reaction with the lubricant.
This invention might even one day eliminate the need to lubricate an engine or gearbox more than once during a vehicle’s entire life cycle. The Argonne team’s discovery may redefine the whole lubrication field by dramatically cutting down friction and wear losses of moving parts and by potentially enabling fill-for-life lubrication possibilities.
See this Argonne video on the technology: https://youtu.be/iSOidHlMsmc.
The 10-hour test and simulations
Argonne scientists Giovanni Ramirez and Osman Eryilmaz determined the friction and wear behavior of test pairs with a ball-on-disk test rig. In this set up, a stationary ball was pressed against a rotating disk of the test material lubricated by a poly-alpha-olefin (PAO) oil for 10 hours (see figure). The catalyst test pair consisted of a molybdenum nitride and copper coating on stainless steel, and the results were compared with a test pair of uncoated bearing steel. In these tests, the catalyst coating in pure (“unadditized”) PAO oil reduced friction by about 50% and essentially eliminated wear thanks to the formation of a blackish diamond-like carbon tribofilm in and around the rubbing surface.
By contrast, during the same 10-hour test, an uncoated baseline steel wore out catastrophically when tested with the unadditized oil. When a fully formulated, state-of-the-art synthetic engine oil was used with full additive package (Mobil 1), some wear of the uncoated steel still occurred.
After testing, Ramirez and Yifeng Liao characterized the tested materials down to the nanoscale. High resolution microscopy of the tribofilm revealed the presence of a large amorphous area with some nanocrystals and onion-like carbon. Spectroscopic analysis indicated the film is made of mainly graphitic carbon.
To understand the mechanism of tribofilm formation at the atomic scale, Argonne team scientists Subramanian Sankaranarayanan, Badri Narayanan, and Ganesh Kamath performed extensive computer simulations. They started from an initial configuration consisting of olefin lubricant chains sandwiched between sliding tribological interfaces (see figure). In these simulations, they observed that on the non-carbide forming surfaces such as copper, the olefins catalytically convert to tribofilms reminiscent of an amorphous carbon combined with hydrogen.
At the start, the olefin molecules are uniformly distributed. Under the sliding action at the copper/olefin interface, the olefin molecules degrade via two competing steps: (1) splitting of carbon-hydrogen bonds near the copper surface leading to carbon chains with the hydrogen removed and (2) random breaking of the carbon-carbon bonds in the coating backbone to form shorter hydrocarbon fragments.
Subsequently, the short-chain hydrocarbons recombine to form the graphitic tribofilm. This knowledge about the catalytic mechanism will allow Argonne scientists to further improve their technology.
Benefits and applications
The co-principal investigator, Osman Eryilmaz, summarizes the many benefits of this new technology: “The discovery can make automotive engines and gearboxes more energy efficient, reliable, and ‘green’ in the sense that it may eliminate the need for phosphorus- and sulfur-bearing additives to the lubricant, which interfere with catalytic converters and after-treatment devices,” he noted. “Because of its self-healing effect on rubbing surface, it might also eliminate the need for changing the lubricant during the vehicle’s entire life span.”
As the above experimental and computational results suggest, the technology is fully demonstrated and will require minimal tweaking or tailoring to apply to a potential product. This discovery is not limited to automotive applications. For example, it could be used in wind turbine gearboxes. Moreover, it has been shown that graphitic tribofilms can even form in vivo on the rubbing surfaces of metal-on-metal hip replacements, which are made of cobalt, chrome, and molybdenum.
Proteins are suspected to be the main carbon source in tribofilms forming due to the catalytic nature of the cobalt and molybdenum in the metal-on-metal alloy. In this case, the formation of a tribofilm from olefin molecules is due to the catalytic nature of the composite coating.
Work at Argonne National Laboratory was supported by the U.S. Department of Energy under Contract DE-AC02-06CH11357.
Ali Erdemir is a Distinguished Fellow and Senior Scientist at Argonne National Laboratory working in the fields of materials science, surface engineering and tribology. His discoveries of nearly frictionless carbon and superhard nanocomposite coatings, as well as a range of novel nanolubricants and lubrication additives, have been hailed as major achievements in the field.