This is a very unique discovery, and one that was a little unexpected. 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.

—Ali Erdemir, Argonne Distinguished Fellow and team leader

Moving mechanical interfaces are commonly lubricated and separated by a combination of fluid films and solid tribofilms; together, these reduce friction, ensuring easy slippage and longer wear life.

The efficacy of the fluid film is governed by the viscosity of the base oil in the lubricant; the efficacy of the solid tribofilm, which is produced as a result of sliding contact between moving parts, relies upon the effectiveness of the lubricant’s anti-wear additive (typically zinc dialkyldithiophosphate). Minimizing friction and wear continues to be a challenge, and recent efforts have focused on enhancing the anti-friction and anti-wear properties of lubricants by incorporating inorganic nanoparticles and ionic liquids.

Here, we describe the in operando formation of carbon-based tribofilms via dissociative extraction from base-oil molecules on catalytically active, sliding nanometre-scale crystalline surfaces, enabling base oils to provide not only the fluid but also the solid tribofilm. … Structurally, the resulting tribofilms are similar to diamond-like carbon.

Ball-on-disk tests at contact pressures of 1.3 gigapascals reveal that these tribofilms nearly eliminate wear, and provide lower friction than tribofilms formed with zinc dialkyldithiophosphate. Reactive and ab initio molecular-dynamics simulations show that the catalytic action of the coatings facilitates dehydrogenation of linear olefins in the lubricating oil and random scission of their carbon–carbon backbones; the products recombine to nucleate and grow a compact, amorphous lubricating tribofilm.

—Erdemir et al.

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.

Atomistic mechanism of tribofilm formation by MoNx–Cu, deduced from ab initio and reactive molecular-dynamics simulations. a–c, AIMD simulations illustrate the catalytic action of copper, dehydrogenating and breaking linear olefins into shorter-chain hydrocarbons. d–f, A similar reaction pathway is predicted by reactive molecular-dynamics simulations. The end result is that dehydrogenated short-chain hydrocarbons recombine to nucleate, and grow into, a compact amorphous carbon tribofilm. Simulations suggest that this tribofilm mechanism is suppressed on surfaces where carbide formation is thermodynamically favored. g, h, Snapshots from reactive molecular-dynamics simulations show that two carbide-forming surfaces (vanadium and molybdenum) do not, to all intents and purposes, form tribofilms. Although a sliding molybdenum surface does (like copper) show olefin degradation, the catalytic activity at 1,000 K is much lower and the resulting kinetics of tribofilm formation is very sluggish. Detailed first-principles calculations show that MoN and VN have much reduced catalytic activity compared with copper and nickel. Erdemir et al. Click to enlarge.

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, 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.

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 with better performance attributes.

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.

To provide the theoretical understanding of what the tribology team was seeing in its experiments, they turned to Sankaranarayanan and Narayanan, who used the 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. This new field of tribocatalysis has the potential to change the way we look at lubrication.

—Subramanian Sankaranarayanan

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. This opens up new pathways for scientists to use extreme conditions to enhance catalytic activity.

—Badri Narayanan

In addition to its other benefits, 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.

The research was funded by DOE’s Office of Energy Efficiency & Renewable Energy. The team also includes microscopy expert Yifeng Liao and computational scientist Ganesh Kamath.

Resources

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