Slipping Through Tribology’s Past, And A Pinch Of The Future

The study of friction—tribology—is all the more important in our era of machination and automation.

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AsianScientist (Aug. 28, 2015) – The circus show is about to start, and the Big Top is filled to the brim with an expectant audience. A spotlight shines on the round sand stage as a clown enters the ring, stumbling and bumbling his way past the circle of light—realizing he’s missed it, he runs back, only to (you guessed it) slip on a strategically placed banana peel. Cue laughter from the audience, and the clown’s mock pain from the fall!

We’ve probably experienced something like that in real life—on an ice-skating rink, a wet bathroom floor, or wearing socks on a smooth marble surface; the presence of friction and the lack thereof is what allows us to push against the floor to move forward, or failing which, nosedive into the ground like the clown.

Tribology in Ancient Egypt. Scores of slaves pull a plate carrying a large sculpture, while another pours a liquid as a lubricant to reduce the friction between the plate and the ground. Credit: Layard, A.G. 1853. Discoveries in the Ruins of Nineveh and Babylon, I and II.
Tribology in Ancient Egypt. Scores of slaves pull a plate carrying a large sculpture, while another pours a liquid as a lubricant to reduce the friction between the plate and the ground. Credit: Layard, A.G. 1853. Discoveries in the Ruins of Nineveh and Babylon, I and II.

The study of friction—known as tribology, which stems from the Greek word “τρίβος” (“tribos”), meaning “to rub”—is not an entirely new discipline; ancient Egyptian hieroglyphs show large carts of heavy blocks being pulled, and a liquid poured at the front of the cart. Whether the liquid is oil gleaned from animal fat, or is merely river water to liquefy the sand beneath the blades of the cart (which, by nature of their shape, also reduce friction between the sand and cart), is still argued about by historians and seems to depend on location. The instructions written in their own ancient language are, however, reasonably clear: the liquid must be poured at the front of the cart as it moves, to reduce the resistance to movement.

The Egyptians were not the only early culture to utilize such knowledge—the early Chinese had a vast collection of lubricants, studied the complex interactions and forces involved in gears, and utilized the same principles in their chariots as the Japanese did in their sledges. This knowledge is thought to have sparked mechanical exploration in those regions.

The advent of the Industrial Age brought about an even greater need for such skills—with increased machination and automation, mankind revisited Da Vinci’s principles of friction, and used them to their advantage. The relevance of tribology and the underlying sciences become even more important today—Dr. Lars Pleth Nielsen, manager of the Tribology Center, Danish Technological Institute, and president of the European Network for Industrial Wear Prevention, mentioned in a 2010 interview that “between two and four percent of an industrialized country’s GDP is lost through friction and the wearing out of mechanical parts.”

Researchers from the Massachusetts Institute of Technology (MIT) has recently observed that friction-related losses to add up to over six percent of America’s gross national product, making their the country’s military budget seem puny by comparison. Going by these figures, a industrialized country such as China could well forfeit between US$165 and 331 quadrillion a year, simply from industrial wear and tear; the same could be said for a number of Asian countries, with a great deal of them involved in manufacturing industries or the like.

But tribology is not as simple as sloshing oil onto surfaces to make them slippery. Very often, some amount of friction is required to achieve mechanical force transfer; a theoretically frictionless surface would not interact with any other surface around it! One often needs to consider an optimal range of tribological properties for effective function, while preventing excessive friction from inducing unwanted effects such as decomposition, corrosion, or most commonly, wear of the surface.

Furthermore, as surface dimensions become smaller, more complex, or are manufactured from different materials, tribologists need to consider various factors that may affect functionality—the use of solid state, thin-film, or monolayer coating lubricants are a good example, and have been well investigated for computer hard drive applications.

These demand durability, and must take into consideration interactions between the lubricant and the surfaces, both in terms of the chemical interaction, physical lubricant transfer, decomposition, electrical contacts and reading of the hard disc surface, as well as the spreading behavior of any bulk liquid lubricant. Tribology then appears to be far more complex than one might imagine, encompassing a number of sciences to support its study!

Recent advances in technology have also spurred tribological developments—the current fad of miniaturization brings new elements (figuratively and literally) into lubricating these surfaces; instead of issues regarding interaction between liquid and surface, we now face even more fundamental problems such as how to place the lubricant in such a confined space, how to maintain an extremely thin lubricant layer for durability, and how to mitigate any effects caused by an molecularly uneven or textured surface.

Vapor-phase lubrication has been one viable alternative over the past few years, but requires sealed containment for it to be effective, lest the vapor leak. Moreover, since vapor is far harder to control than liquid, researchers at the National University of Singapore have since developed a method of locally depositing a fixed amount of lubricant onto a Micro-Electro-Mechanical System (MEMS) device’s surface, allowing for control of both position and quantity of the lubricant.

Other studies in this area include the use of compound or multi-layered films, the utilization of surface textures to induce hydrophobicity/oleophobicity and to mitigate friction, and the study of hydrodynamic friction and viscous films.

Two recent studies have attempteded to control friction at the micro- or nano-scale, commonly addressed as nano-tribology—the first is the use of multiply-alkylated cyclopentanes (MACs) as an alternative to the more commonly-used perfluoropolyether (PFPE) lubricant. The latter has been found to corrode surfaces and decompose into hazardous substances, while the former is currently used extensively in space applications.

Professor Nobuyoshi Ohno (Saga University, Japan) and co-workers have also studied the friction and wear characteristics of a number of space lubricants, and found that MACs provide similar friction coefficients as PFPE in a standard four ball vacuum tribometer, but resulted in a smaller wear scar on the ball surface, implying longer wear life and therefore more prolonged performance.

This implies that the replacement of PFPE lubricants with MAC lubricants, if successful, may result in higher durability standards for most, if not all applications. However, the viscosity of the MAC lubricant which provides the boundary film leading to such properties is also a bane at higher sliding speeds, which then need to be mitigated.

In addressing this, and in investigating friction under varying conditions on a sliding micro-surface, the behaviour of additives into a bulk lubricant was studied at the hydrodynamic region, using a micro-textured disc, as a joint project between the National University of Singapore and Imperial College London.

The study found that introducing MACs into a common liquid lubricant for MEMs reduced the friction caused by viscous drag at high speeds, and was first thought to be due to heating effects. Upon repeated experiments with various liquids and additives, the heating effect was found to be negligible—instead, it was the MAC additive specifically which induced a dewetting effect, similar to hydrophobic and oleophobic surfaces, creating slip between the liquid lubricant and the surface. At an optimum concentration of MACs in hexadecane, the viscous drag at higher speeds is reduced to an extent that would make most micro-devices workable.

With increasing use of machines, technology and automation, one might imagine that tribology would become ever so pertinent, and especially so when dealing with specific, unique cases that demand peculiar solutions. Over the course of history, technological and mechanical advancements have evolved, and have birthed new solutions to emerging issues—some, like the steam engine, were solved creatively by changing the single stroke piston to a dual stroke; others required a more theoretical approach and involved a great deal of empirical testing.

Tribologists have a thankless job at times, only called upon when friction becomes a problem, and remain unnoticed when everything runs smoothly—pun intended. Even as automation becomes more commonplace and machines are set to task in increasingly complex environments, tribological solutions have evolved to deal with the issues.

In some developments, however, we have circumvented the problem completely—the prevalence of solid state drives (SSDs) over conventional hard drives now removes the need for any moving parts in computer hard drives and thus avoids the problems of scratching between the slider and drive surface, and wear on the surface arising from friction.

Certainly technological developments and advances in data storage as well as electronic components have allowed for this to be of less concern, but not all cases can be dealt with in such a manner—mechanical transfer of force or motion is still the most effective mode of transfer today, which requires contact (and therefore friction) between surfaces.

Perhaps for now, we’ll leave the banana peels to the clowns.



This article won second place in the 2015 Asian Scientist Writing Prize.

Click here to read an interview with the top three winners. See the photos or watch the video highlights of the prize presentation ceremony held on July 27, 2015.
Also, look out for the other winning entries to be published in The Best of Science Writing from Asia 2015 coming out later this year.

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Copyright: Asian Scientist Magazine; Photo: Shutterstock.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Jonathan Leong graduated from the NUS-Imperial College Joint PhD Programme at the National University of Singapore. He is interested in all things related to friction, but particularly at the micro- or nano-scale. He is a lecturer at SIM University.

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