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April 27, 2015

When I first entered graduate school, I investigated the scientific credentials of my professors. I read that one of them did "tribology" after obtaining his Ph.D., and I had no idea what tribology was. Today, you can just type the word into a search engine, or in the Wikipedia search box, and get a full and instant explanation. I knew that the "-logy" part derived from the Greek word for study (λογια). If I had studied my Greek a little harder, I would have known that "tribo-" derived from τριβω, from the verb, "to rub."

Tribology is the study of friction and wear, and their mitigation through lubrication. Tribology had a rather recent incarnation as an "-ology," receiving its name in 1966. It was named in a UK scientific study, the Jost Report, and the name stuck.[1-2] Some scientists didn't like this name; but, what other name would you give to this field? Tribology is interesting, since it's interdisciplinary between physics, chemistry, materials science and engineering.

Figure caption

The only occurrence of a form of the verb, "to rub," διατριψας, in Homer's Iliad (Book XI, ll. 846-848). In this passage, Eurypylus, wounded in the thigh with an arrow, is attended by Patroclus, who cuts out the arrow, washes the wound, and then rubs a bitter herb onto it. The herb has both an analgesic and an astringent effect. (From Project Perseus, licensed under a Creative Commons Attribution-ShareAlike 3.0 United States License)[3)]

Tribology impacts the life and bank account of every automobile owner. We require a high degree of friction between our tires and the road for efficient locomotion and adequate stopping, but we also want tires that don't wear as quickly. We likewise change the engine oil every few thousand miles to prevent the wear of internal engine parts to maintain engine efficiency. In just tire and friction brake replacement, and oil changes, tribology costs each automobile owner an average of at least a hundred dollars per year.

Since there are about quarter million passenger vehicles in the US, this sums to an expense in this area of about $25 billion. This number could probably be tripled to include the country's tribology-related cost for the trucks that bring our food to market, and the trains that carry other cargo. Since the US has a gross domestic product of about 16 trillion dollars ($16,000 billion), my estimate of tribology-related costs from transportation alone are at least a half percent of our GDP.

As you might remember from your introductory physics course, friction is classified as being of two types, static friction and dynamic friction. Static friction is characterized as the force that prevents an object from sliding down a slope, while dynamic friction is the force that impedes the motion of moving bodies. I wrote about the physics of friction in a previous article (Friction and Wear at the Atomic Level, February 6, 2013).

The static friction between objects depends on their surface finish, and also the composition of the rubbing materials. As the following table shows, the measure of static friction, called the coefficient of static friction, μs, varies greatly depending on which two materials (called a friction couple) are rubbed together.[4]

Cast ironCopper1.05
Concrete (dry)Rubber1.0

In 1699, Guillaume Amontons was the first to publish a theory of static friction. His theory was that static friction was just the force needed to raise an object above the height of surface irregularities, just as a ball won't roll out of a dimple until the surface is tilted. In 1750, Leonhard Euler developed a mathematical model for static friction with reference to the angle of a plane at which on object begins to slide. The coefficient of static friction is the tangent of this angle, known as the angle of repose.[5]

Portrait of Leonhard Euler by Joseph Frederic Auguste Darbes, 1778

A 1778 oil portrait of Leonhard Euler by Josef Friedrich August Darbes (1747-1810).

This portrait resides at the
Musée d'Art et d'Histoire, Geneva.

(Photograph by
Sailko, via (Wikimedia Commons))

Although some of the irregularity in the coefficient of static friction among materials could be explained by how well their surfaces can take, and hold, a polish, it's agreed that atomic interaction between materials has an affect. In this present day of atomic force microscopy, we now have tools to examine these interactions more closely; and, most importantly, how lubricants can be optimized to reduce this friction.

Lubricating oils of today include additives, such as zinc dialkyldithiophosphate (ZDDP), that create useful antiwear tribofilms at the sliding interface. A team of scientists from the University of Pennsylvania (Philadelphia, Pennsylvania), and ExxonMobil Research and Engineering (Annandale, New Jersey) have researched at the atomic level the hitherto unknown mechanism for the beneficial action of such additives.[6-7]

Chemical structure of a zinc dialkyldithiophosphate

A zinc dialkyldithio-

(Modified Wikimedia Commons image.)

As I've often remarked in this blog, many scientific discoveries happen by accident. ZDDP was proposed as an oil additive in the 1940s as a rust inhibitor, and it was found that it also inhibited wear.[7] The mechanism of this anti-wear property of ZDDP was at first unknown. Eventually, it was discovered that the anti-wear property arose from the decomposition of ZDDP into a tribofilm that adheres to the friction surfaces. Since the decomposition process was unknown, this additive provoked further scrutiny.[7]

Robert Carpick, a professor in Penn's Department of Mechanical Engineering and Applied Mechanics, led this research effort that included other Penn scientists, as well as scientists from the Corporate Strategic Research division of ExxonMobil Research and Engineering Company.[7] They probed these films at the nanoscale, and they were able to determine the molecular mechanisms behind the action of ZDDP.[7]

Says Nitya Gosvami, a research project manager in Carpick's laboratory,
"ZDDP has been used for more than 70 years... It's one of the most successful antiwear additives we have, but we still don't understand how it works. We do know that everything that happens during sliding is occurring on the first few atomic layers of the surfaces, so we have to use the knowledge we have from nanotechnology and apply it to understand what's going on there." [7]

The motivation for this study, aside from an understanding of ZDDP tribofilms, is the idea that other compounds might have better anti-wear properties. While ZDDP reduces wear, it also increases friction slightly and decomposes into materials that somewhat "poison" an automobile's catalytic converter. While ZDDP works well on steel, it doesn't have as much of an anti-wear effect on other engine materials. Says Carpick, "Considering the massive use of vehicles, a small gain in efficiency has a big impact in saving energy and reducing carbon emissions annually."[7]

The wear in piston engines arises from the inherent roughness of the sliding parts. Peaks in the local topography, known as asperities, rub against each other and erode the sliding surfaces because of enhanced local stress. debris from this erosion causes additional abrasion, leading to continuing wear. To study this process, the research team simulated a single asperity by the tip of an atomic force microscope (AFM). They scanned an AFM tip across an iron plate immersed in a ZDDP-containing lubricant at elevated temperatures. The iron plate simulated the mostly iron engine alloy.[6-7]

atomic force microscope and a tribofilm

The tip of an atomic force microscope (AFM) simulating friction from an asperity as it's scanned across a surface.

The AFM measured the force required to decompose an additive into a tribofilm.

(University of Pennsylvania illustration by Felice Macera.)

In looking at just a single, simulated asperity, the research team was able to gain an understanding of the affect of such parameters as contact stress and geometry on the tribofilm creation process. It was found that the tribofilm will only form under sufficient tip pressure, and the tribofilms grew faster when the AFM tip squeezed and sheared the ZDDP-containing lubricant harder.[7] The growth rate increased exponentially with either temperature or the applied compressive stress, a result that's consistent with a thermally-activated, stress-assisted reaction rate model.[6] The films grew even in the absence of iron on either the tip or substrate.[6]

It's known that the tribofilms stop growing when a certain thickness is reached, and this is beneficial, since it permits a reserve of ZDDP to remain in the lubricant for later tribofilm deposition.[7] This thickness limiting arises from the mechanical properties of the tribofilm. A thick tribofilm acts as a "cushion" that prevents compressive stress from exceeding the point at which additional tribofilm will grow.[7] These AFM results now allow a way to select and compare new anti-wear additives.[7] The research was funded by the National Science Foundation, and by the Marie Curie International Outgoing Fellowship for Career Development of the 7th European Community Framework Programme.[7]


  1. John Field, "David Tabor: 23 October 23, 1913 - November 26, 2005," Biographical Memoirs of the Fellows of the Royal Society, vol. 54 (December 12, 2008), pp. 425-459, DOI: 10.1098/rsbm.2007.0031.
  2. H. Peter Jost, "Committee on Tribology Report, 1966-67," Great Britain Ministry of Technology (H.M. Stationery Office, 1968).
  3. Homer's Iliad, Book XI, ll. 846-848, Project Perseus, Department of the Classics, Tufts University.
  4. Coefficients of Static Friction Table, Wikipedia.
  5. Leonhard Euler, "Sur le Frottement des Corps Solides," Mémoires de l'académie des sciences de Berlin, 1750, pp. 122-132.
  6. N. N. Gosvami, J. A. Bares, F. Mangolini, . R. Konicek, D. G. Yablon, and R. W. Carpick, "Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts," Science, Advance Online Publication, March 12, 2015, DOI: 10.1126/science.1258788.
  7. Penn and ExxonMobil Researchers Address Long-standing Mysteries Behind Anti-wear Motor Oil Additive, University of Pennsylvania Press Release, March 12, 2015.

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