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Tribology
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.
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]
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]
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]
2.png)
A zinc dialkyldithio-
phosphate.
(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]
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]
References:
- 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.
- H. Peter Jost, "Committee on Tribology Report, 1966-67," Great Britain Ministry of Technology (H.M. Stationery Office, 1968).
- Homer's Iliad, Book XI, ll. 846-848, Project Perseus, Department of the Classics, Tufts University.
- Coefficients of Static Friction Table, Wikipedia.
- Leonhard Euler, "Sur le Frottement des Corps Solides," Mémoires de l'académie des sciences de Berlin, 1750, pp. 122-132.
- 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.
- 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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