Researchers Discover Self-Lubricating, Ultra-Durable Tribofilm

Superman fans often remember how the Man of Steel produced a diamond out of a coal lump by using extreme heat and pressure generated by his bare hands.

Tribologists - scientists studying lubrication, wear and friction - and computational materials scientists at the U.S. Department of Energy's (DOE's) Argonne National Laboratory have recently used the same principles and detected a revolutionary diamond-like film that is obtained by pressure and heat of an automotive engine.

This self-lubricating, ultra-durable tribofilm develops between moving surfaces, and this discovery was initially reported in the recent issue of the journal Nature.

The discovery could have intense implications for the durability and efficiency of future engines and other moving metal parts that are capable of delivering self-healing, diamond-like carbon (DLC) tribofilms.

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, Distinguished Fellow, Argonne National Laboratory

This discovery was made by Erdemir and his colleague Osman Levent Eryilmaz in the Tribology and Thermal-Mechanics Department in Argonne's Center for Transportation Research several years ago. Theoretical insight was obtained from the computing resources available at Argonne in order to completely understand the workings at the molecular level in the experiments.

This theoretical understanding was offered by leading theoretical researcher Subramanian Sankaranarayanan and postdoctoral researcher Badri Narayanan from the Center for Nanoscale Materials (CNM). 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. NERSC, ALCF and CNM are DOE Office of Science User Facilities.

The very first discovery was made when Erdemir and Eryilmaz planned to see the outcome when a small steel ring was coated with a catalytically active nanocoating. This nanocoating is made up of very small molecules of metals that help chemical reactions to break down other materials.

This observation was made when the catalytically active nanocoating was subjected to high pressure and heat using a base oil without the complex additives of modern lubricants. The researchers observed the ring after the endurance test and discovered an intact ring comprising an odd blackish deposit on the contact area. The expected surface damage and rust was not present.

This test creates extreme contact pressure and temperatures, which are supposed to cause the ring to wear and eventually seize. 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.'

Osman Levent Eryilmaz, Argonne National Laboratory

The researchers observed the deposit using high-powered optical and laser Raman microscopes, and discovered that the deposit was a tribofilm of diamond-like carbon very much like several other DLCs created at Argonne in the past. However, it worked even better. Test results showed that the DLC tribofilm reduced friction by 25 to 40 percent and that wear was brought down to unmeasurable values.

Experiments headed by postdoctoral researcher Giovanni Ramirez highlighted that multiple types of catalytic coatings can generate DLC tribofilms. The experiments revealed that the coatings work together with the oil molecules to produce the DLC film, which attaches to the metal surfaces.

Damage of the tribofilm results in the coating being re-exposed to the oil, causing the catalysis to restart and produce new tribofilm layers. The process is self-regulating, and it maintains the consistent thickness of the film.

The scientists discovered that the film was developing spontaneously between the sliding surfaces and was replenishing itself, but they still needed to comprehend how and why.

To obtain the theoretical understanding of what the tribology team was observing in its experiments, they focused on Sankaranarayanan and Narayanan, who used the strong computing power of Mira, ALCF's 10-petaflop supercomputer. Large-scale simulations were performed to comprehend the happenings at the atomic level.

Determination was made that the catalyst metals in the nanocomposite coatings were stripping hydrogen atoms from the hydrocarbon chains of the lubricating oil. This was then followed by breaking of the chains into smaller segments. These chains attached themselves together under pressure in order to develop DLC tribofilm that is highly durable.

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, Theoretical Researcher, Argonne National Laboratory

The origins of the catalytic activity were explored by the theorists to understand the working of catalysis under the excessive heat and pressure in an engine. This information allowed the theorists to predict which catalysts would work and which would develop extremely beneficial tribofilms.

The new tribofilm has huge implications as far as reliability and efficiency of engines are concerned. A wide variety of coatings are already being used - a few have been developed at Argonne - for metal parts in engines and various other applications.

The issue here is that these coatings are costly and difficult to apply, and while being used they last only till the coating wears through. The new catalyst permits the tribofilm to be consistently renewed during operation.

The tribofilm is developed in the midst of base oil and can thus allow manufacturers to eliminate or reduce some of the modern anti-wear and anti-friction additives in oil. These additives are capable of reducing competence of vehicle catalytic converters and can also be dangerous to the environment because of its heavy metal content.

The results are featured in Nature in a study titled "Carbon-based Tribofilms from Lubricating Oils." The research received financial aid from DOE's Office of Energy Efficiency & Renewable Energy.

The team also includes microscopy expert Yifeng Liao and computational scientist Ganesh Kamath.

Video Source: Argonne National Laboratory/Youtube.com