Nanotribology: From lab to real world
Jean Van Rensselar, Contributing Editor | TLT Cover Story January 2009
A dozen nano-products are commercially introduced each month—and lubrication issues are at the heart of nearly all of them.
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KEY CONCEPTS
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The properties of nanotribology are vastly different from those of traditional tribology and require new ways of reducing friction.
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Until researchers find a better way to measure nanoscale friction, nanotribology’s progress will be gradual. Until nanotribology advances, nanotechnology’s progress will be gradual.
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The very properties that make nanoscale materials so appealing—particularly in the medical field—create health and environmental risks.
The coming decade is poised to explode with new applications for nanotechnologies. And experts say tribological advances and novel molecular lubrication will be crucial to the successful development of these new technologies.
The question is no longer whether nanotechnology is viable but how well the many interrelated disciplines—physics, chemistry, nanoscience and nanotribology—can coordinate their efforts toward long-term success.
As a key component of nanotechnology, nanotribology doesn’t exist in a vacuum. The field is very much driven by the progress of nanotechnology and new requirements. According to STLE-member Dr. Stephen Hsu, professor of engineering at George Washington University in Germantown, Md., tribology has always been a demand-pull technology. In other words, it exists to fill the needs of technologies and products.
Given this, the interests of forward-looking tribologists lie not just in nanotribology but in the bigger nanotechnological picture as well. Questions arise such as:
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What nanotechnological products are in development and being marketed?
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How critical is nanoscale friction and lubrication to these new products?
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How viable is friction control?
And there are other, more global, questions: What contribution will this product make to society? What impact will it have on long and short-term human and environmental health?
In many regards, nanotechnology is waiting on nanotribology, which is waiting on better tools for measuring nanoscale friction. It’s a frustrating conundrum for many tribologists. But help is on the way.
THE NANO EXPLOSION
According to the Project on Emerging Nanotechnologies (PEN), new nanotechnological products are entering the market at the rate of three to four per week—everything from life-saving medical devices to Swissdent’s Nanowhitening Toothpaste with nanoparticle calcium peroxides. In fact, there are a total of nine nanotoothpastes on the market, and the list of products currently made from nanomaterials ranges from A (Atomic Snow® Izor Skis) to Z (Zelens® Fullerene C-60 Night Cream).
There are 35 automotive products in the PEN inventory, including the Hummer H2, which GM says has a cargo bed made of nanocomposite parts for its trim, center bridge, sail panel and box rail protector.
According to PEN, nanoparticle silver, found in 143 products, is the most widely used nanomaterial. From a profit perspective, the biggest nanoparticle markets are black rubber tire filler and the silver particles used for photography. Other nanoparticle hits include carbon, zinc, titanium, silica and gold (
1).
And where does nanotribology fit into all this?
Gears, bearings and liquid lubricants can reduce friction on the macroscopic scale, but some nanomaterials and devices require unconventional solutions. Because of their enormous surface-volume ratio and limited space between surfaces (nanometers), the need for friction and wear reduction is not only different but much greater than for macromaterials.
New nanomaterials exhibiting new properties often require new ways of reducing friction. Experts say that superlubricity and thermolubricity applications will play crucial roles in bringing some nanoscale products to market. Nanotribology, as an element of multidisciplinary nanotechnology, raises the same issues such as cost vs. benefit, academic-industry cooperation, government oversight, toxicity and environmental impact and global economics.
Figure 1. NanoWax® car care products are just one of a dozen new nano-products that are entering the market at a rate of three to four per week.
BACKGROUND
One nanometer (nm) is one billionth of a meter. The comparative size of a nanometer to a meter is the same as a marble to the size of the earth. Nanotribology is the study of interfacial phenomena at the nanoscale.
Hsu believes that a consensus on the definition of nanotribology is paramount. “Most people would tell you that nanotribology is the study of nanoscale friction, lubrication and wear, but that is not quite true,” said Hsu, who defines the discipline this way: the control of interfacial properties of contacts at nanoscale (either the distance or the size of contact) to ensure functionality, reliability and durability of the system.
Hsu explains, “The nanoscale actually is two parameters—either the distance between two surfaces is below 50 nanometers or the size of the contact is below 100 nanometers. Strictly speaking, at least one of the critical dimensions needs to be small enough to induce unusual behavior. The U.S. National Nanotechnology Initiative (NNI) defines nanotechnology as having two attributes: one of the dimensions is below 100 nanometers and it creates unusual properties.”
The term “nanotribology” was first coined by STLE-member Dr. Jacqueline Krim of North Carolina State University in Raleigh, N.C. in 1991. While it’s the study of nanoscale friction, it’s much more than just a size difference.
In nanotribology, the involvement of surface forces or atomic forces determines the final behavior of the system. This difference between tribology and nanotribology is comparable to the difference between Newtonian physics and quantum physics, where radical new properties emerge at the atomic and subatomic levels. For example, Krim and others discovered that, at the nanoscale, friction forces for liquid films were about five times higher than for solid films. They also discovered that at the nanoscale, the importance of surface roughness diminishes.
Years ago researchers discovered that the introduction of a minute number of nanoparticles could completely transform the chemistry and nature of mundane materials. For example, the automotive industry is using polymers embedded with clay nanoparticles to create stronger, more resilient, lighter-weight body parts (GM’s Hummer).
A significant contribution to the field occurred in the mid-1980s when researchers adapted the atomic-force microscope (AFM) to measure nanoscale friction. The AFM can detect friction, adhesion and external loading forces to the scale of a pico, which is one trillionth of a Newton.
The AFM is now routinely used for studying nanotribology, and the door is open for many new applications.
Examples of the AFM’s applications to tribology include:
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Direct three-dimensional visualization of surface wear tracks (scars).
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Thickness measurement of solid and liquid lubricants having nanometer or even monolayer thickness.
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Measurement of frictional forces at the nanometer scale.
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Surface characterization of morphology, texture and roughness.
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Evaluation of mechanical properties such as hardness, elasticity and plastic deformation at the nanometer scale.
While the advent of the AFM marks a significant step forward in nanoscale friction measurement, experts agree that new methods and instruments are needed.
Figure 2. Scientists at Argonne National Laboratory are working to develop new materials at the nanoscale. This atomic force microscopy image shows a nanopore array used as a template to make nanowires. The template shown is only 500 nanometers wide—less than the period at the end of this sentence. (Courtesy of Argonne National Laboratory)
NANOSCALE DIFFERENTIATORS
According to Hsu, there are five basic differences between nanotribology and tribology:
1.
Scale effect due to shrinking size.
2.
The inclusion of surface forces between contacts. At the nano level, the forces attracting the surfaces become overwhelming.
3.
Unusual properties of materials at the nanoscale.
4.
Control of the interrelated contact phenomena such as adhesion, stiction, wetting, friction, hydrophobicity, etc. At the nanoscale, all of these contact phenomena become interrelated and no longer function separately.
5.
The potential change from continuum mechanics to quantum mechanics in describing atomic scale interactions.
Many physical phenomena become prominent as the size of the system decreases. These include statistical-mechanical effects as well as quantum-mechanical effects. The quantum size effect comes into play when the properties of solids are altered with nanoparticles.
Many physical (mechanical, electrical, optical) properties change. Familiar materials, when observed at the nanoscale, display startling properties. Some transmit light or electricity; others approach diamond-like hardness or turn into powerful chemical catalysts. Opaque substances, such as copper, become transparent; stable materials, such as aluminum, become combustible; and inert materials, such as gold, become catalysts. Much of this is due to the enormous increase in surface-to-volume ratio. It’s noteworthy that the catalytic activity of nanomaterials also creates potential risks as they interact with biomaterials.
Figure 3. The Hard X-Ray Nanoprobe was designed to study novel nanoscale materials and devices aimed at, for example, harvesting solar energy more efficiently, providing more efficient lighting and enabling next generation computing. The Nanotube uses X-rays with photo energies between three and 30 kiloelectron volts to produce images with initially 30 nanometer resolution—roughly the size of 100 atoms. (Courtesy of Argonne National Laboratory)
NANOSCALE FRICTION
Friction depends on both the chemical and mechanical interaction between the surfaces, and friction, at the nanoscale, is a completely different animal. For example, dry surfaces can be slicker than wet surfaces. To explain the phenomenon, researchers simulated one-atom-thick krypton films sliding on crystalline gold surfaces. The liquid krypton atoms, which are more mobile than solid krypton, got stuck more easily in the gaps between the solid gold atoms. Unlike macroscopic lubrication, where the shearing takes place at the liquid-liquid interface, the nanoscale shearing took place between a solid and liquid surface.
In a 1997
Lubrication Engineering article, Krim proposed three new nanotribological rules of friction:
1.
The friction force depends on how easily two surfaces become stuck relative to becoming unstuck: It is proportional to the degree of irreversibility of the force that squeezes the two surfaces together rather than the outright strength of the force.
2.
The friction force is proportional to the actual, rather than apparent, area of contact.
3.
The friction force is directly proportional to the sliding speed of the interface at the true contact points—as long as the surfaces are not allowed to heat up and the sliding speed remains well below the speed of sound. Near that speed the friction force levels off because the lattice vibrations cannot carry away the sound energy quickly enough.
Even where nanoscale sliding-interfaces can be perfectly characterized, no one can accurately predict the friction that will occur at the interface (although the AFM can measure changes in chemical composition that cause friction). Researchers such as Krim believe that when the precise nature between nanoscale contacts is understood, it will lead to real-world industrial innovations such as improved lubricants and wear-resistant machine parts.
“Researchers should not make the mistake of extending what we understand in macroscale tribology into nanoscale,” Hsu said. “There have been more mistakes made from this assumption than anything else. In fact, they should just study the phenomena and not assume anything.”
THE CURRENT STATE
Since March 2006 when PEN first started tracking nanotech consumer products, the number has grown from 212 to 609. In 2006 nanotechnology played a role in more than $50 billion worth of manufactured goods. By 2014, researchers estimate that $2.6 trillion in manufactured goods (15% of total global output) will incorporate nanotechnology in some way. Currently, with existing sunscreens, drug patches and cosmetics widely employing nanotechnology, other consumer goods are catching up.
The GM nanocomposite, which is essentially a malleable injection-molded polymer blend with nanoscale clay particles, is as strong as some metals but significantly cheaper. It doesn’t have to be polished, machined or grinded.
“It also reduces the weight, without compromising safety, which has an effect on fuel economy,” Hsu said. “For every 100 kilograms of body weight reduction, you increase fuel economy by one mile per gallon. Put in those terms, it’s easy to see how consumers benefit from nanomaterials.”
STLE-member Dr. Fran Lockwood, senior vice president-technology for Valvoline in Georgetown, Ky., explains that the company has had a line of NanoWax® car-appearance products on the market for several years. The merchandise incorporates nanoparticles for better scratch filling, surface smoothing and UV protection.
“There is a tribological element to those products,” Lockwood explained. “When people wax their cars, they want something that fills the scratch marks very well and leaves a very smooth surface. One of the ways to determine the products’ performance is to measure the coefficient of friction. Using nanotechnology, we’ve also been able to control for water and dirt repellency. These are really impressive benefits that we’ve been able to achieve with nanoparticles.”
IBM created disk drives incorporating nanostructured giant magnetoresistive (GMR) materials. ExxonMobil is working on zeolites with nanoscale pore sizes that could crack hydrocarbon molecules and form gasoline.
The biggest venture capitalist investment in nanotechnology right now—and this is good news for tribologists—is in the field of biomedical devices, especially implantable devices and drug-delivery systems. More than 60 drugs and drug-delivery systems based on nanotechnology, and more than 90 medical devices or diagnostic tests, are already in-use or being tested, according to
NanoBiotech News.
One device includes the use of quantum dots coated with a material that makes them attach to specific target molecules. These target molecules may be early indicators of disease.
Studies are underway for an adaptive retinal implant designed to restore partial vision in blindness caused by retinitis pigmentosa. The system includes a tiny eyeglass frame camera that transmits surrounding images to a special adaptive signal processor 2.
Nanoscale implants, capable of releasing drugs on demand and monitoring blood chemistry—think diabetes control—are currently being developed. Joint replacements with nanoscale components (a fertile field for nanotribology) are also in the works.
Five differences between tribology and nanotribology
1.
Scale effect due to shrinking size.
2.
The inclusion of surface forces between contacts.
3.
Unusual properties of materials at the nanoscale.
4.
Control of the interrelated contact phenomena.
5.
The potential change from continuum mechanics to quantum mechanics in describing atomic scale interactions.
But there’s a holdup created by research barriers such as the lack of a comprehensive instrument to measure friction at the nanoscale, and a very incomplete understanding of nanoscale properties.
According to Hsu, the most pressing nanotribological needs for development are:
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An understanding of nanoscale adhesion and stiction—the force required to cause one body in contact with another to begin to move.
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Nanolubrication (barrier films, package films, low-friction films).
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An understanding of friction or lateral loading-induced failures in devices.
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Component life evaluation (fatigue, fracture from movement or contacts).
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An understanding of the nanomechanical property measurements of nanomaterials.
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The application of nanomaterials as tribo-materials.
“An important issue is how to design complicated interlocking devices that will not get wet,” Hsu said. “This is especially critical for nanoscale devices that have silicon components. Silicon loves water, and once water get hold of silicon the device will not function.
“You need to have a very thick film to ensure that the device will function correctly in the presence of water,” Hsu adds. “Now the question is how to devise and develop such a durable molecular film.”
Figure 4. AFM image of nanoparticles on the surface of alumina wafer. (Courtesy of Dr. Lev Rapoport, Holon Academic Institute of Technology)
Nanolubrication requirements
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Friction control from slow to high-speed oscillations.
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Can withstand billions of duty cycles.
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Can withstand extreme packaging temperatures without breakdown.
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Long-term (3-25 year) storage capacity.
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Hydrophobic, adhesion preventing, stiction control.
Source: Dr. Stephen Hsu, George Washington University, professor of engineering in the department of mechanical and aerospace engineering, Germantown, Md., stephen.hsu@erols.com.
EMERGING ISSUES
No new technology is without issues. And across much newly introduced technology (plastics, genetically engineered food) the issues are the same: cost vs. benefit, toxicology and environmental impact, academic-industry cooperation and consumer acceptance. While these issues apply to nanotechnology now, they will all apply to nanotribology at some point.
Cost vs. benefit. For researchers such as Lockwood who are employed by corporations, cost vs. benefit is a key component of every new undertaking.
“I think at the end of the day, no one cares whether something uses nanotechnology or not,” she said. “It’s only about whether they see a performance benefit for a reasonable value. So all of these academic discoveries are being explored in terms of their value proposition to the real world. It takes a lot of effort to do that comprehensively, and it takes something exceptional to break through the clutter and really present a true value proposition.
“There are so many products out there that to distribute a new one you’ve got to have something that stands out, or you need a super marketing machine. One or the other—or preferably both.”
Dr. Lev Rapoport, professor and head of the Tribology Laboratory at Israel’s Holon Institute of Technology, agrees.
“Academia studies the mechanisms of friction in conditions sometimes far removed from real contact situations,” he said. “Sometimes the effect of nanoparticles doesn’t justify the cost. Good distribution of nanoparticles in oil or in different composite matrixes can significantly improve their friction and wear properties.”
Rapoport added that specific effort is needed to reduce the price of micro/nano-electro-mechanical systems (MEMS/NEMS) devices. He believes nanotribology will play a key role in that regard, especially in the areas of thin films and nanolubricants.
According to Hsu, government funding agencies are waking up to this fact as well.
“Even though there is a lot of research money out there for nanotechnology, if you submit a proposal for nanotribology you are not going to get funding,” he said. “Nobody cares. But if you are looking at commercial products, at nanomaterials as they apply to real industry issues, not only will the government fund you, but a corporation might fund you as well.”
Toxicological and environmental challenges. We already know about the damage to health caused by asbestos and coal microns, so introducing a slew of products containing nanoparticles is causing concern and spurring the federal government toward regulation. Worries grew when researchers at Southern Methodist University reported brain damage in a large-mouthed bass that had been swimming in aquarium waters infused with common carbon-based nanoparticles (buckyballs).
Of paramount concern is the catalytic activity of nanomaterials and their potential risk as they interact with biomaterials. Groups such as the Center for Responsible Nanotechnology advocate for tight government regulation, while others counter that overregulation would stifle scientific research and the development of life-saving innovations.
According to Dr. Atanu Adhvaryu, 6 Sigma Black Belt, at Caterpillar’s Advanced Materials Technology, Technology & Solutions Division in Peoria, Ill., “Particles in nanometric dimensions have significantly different physical and chemical properties compared to the bulk material.
“Until health and safety aspects of these materials are fully understood, they will be a hard sell for industrial applications,” Adhvaryu adds. “This is particularly true for products containing novel nanomaterials that come in contact or have the potential for accidental exposure to humans and the environment.”
The National Institute for Occupational Health and Safety (NIOSH) is conducting research on the occupational safety and health implications of nanotechnology, with current efforts focused in the following three areas:
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How might workers be exposed to nano-sized particles in the manufacturing or industrial use of nanomaterials?
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How do nanoparticles interact with the body’s systems?
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What effects might nanoparticles have on the body’s systems?
NIOSH believes answers to these questions are critical for safely maintaining U.S. competitiveness in the nanotechnology market.
In addition, the Centers for Disease Control and Prevention (CDC) have developed guidelines for working with nanomaterials (
see sidebar).
“We do have to worry about nanoparticles being potential health hazards,” Hsu said. “You sprinkle nanoparticles on your skin, and some of them will go right into your bloodstream. But I believe the most significant risk is found in the production process. So far the finished products appear to be safe.”
Ironically, the very properties of nanoparticles that make them hazardous make them wonders as well.
“It’s a two-edged sword,” Hsu said. “These properties also allow amazing medical advances such as the safe delivery of medicine through patches.”
The same applies to the environment. While the potential for environmental damage caused by nanoparticles exists, using the same technology researchers are building highly mobile nanodevices for detecting air pollution.
Six CDC guidelines for handling nanomaterials
Elements of a workplace nanomaterial risk management program should include:
1.
Evaluating the hazard posed by the nanomaterial based on available physical and chemical property data and toxicology or health effects data.
2.
Assessing potential worker exposure to determine the degree of risk.
3.
The education and training of workers in the proper handling of nanomaterials (e.g., good work practices).
4.
The establishment of criteria and procedures for installing and evaluating engineering controls (e.g., exhaust ventilation) at locations where exposure to nanoparticles might occur.
5.
The development of procedures for determining the need and selection of personal protective equipment (e.g., clothing, gloves, respirators).
6.
The systematic evaluation of exposures to ensure that control measures are working properly and that workers are being provided the appropriate personal protective equipment.
Consumer acceptance. In order to move forward, nano companies must squelch fears about the effects of nanoparticles in the body—in much the same way as the genetically engineered food industry is doing.
In April 2008 testimony before the Senate Commerce Committee, PEN project director David Rejeski said, “Public trust is the dark horse in nanotechnology’s future. If government and industry do not work to build public confidence in nanotechnology, consumers may reach for the No-Nano label in the future, and investors will put their money elsewhere.”
The hearing marked the start of U.S. Senate debate on the future direction of the annual $1.5 billion federal investment in nanotechnology research and development.
According to Rejeski, “Public perceptions about risks—real and perceived—can have large economic consequences. How consumers respond to these early products—in food, electronics, health care, clothing and cars—is a litmus test for broader market acceptance of nanotechnologies in the future.”
Academic-industry cooperation. STLE-member Dr. Hong Liang, associate professor of mechanical engineering at Texas A&M University in College Station, Texas, believes the academic-industry nanotechnology partnership has been an overwhelming triumph. As examples, she points to the widespread implementation of nanoparticles in manufacturing and the considerable consumer benefit of using nanolubricants to make hard disks last longer and reduce wear.
“There has already been significant manufacturing success with nanotechnology,” she says. “As far as the lubricants go, there is no disconnect between academic research and industry use. If there is a disconnect, it resulted from people trying to get into nano-research without a clear purpose. Basic research needs to have the potential to create a lasting impact on the real world. This is true for research in nanotechnology.”
Another reason for a disconnect, according to Liang, is that some technology, such as NEMS, has not yet matured to the point where it can be fully employed.
Figure 5. MoS2 nanoparticles burnished on the laser surface textured steel. (Courtesy of Dr. Lev Rapoport, Holon Academic Institute of Technology)
THE FUTURE
Liang believes there needs to be better nanotribological measurement in order for both nanotribology and nanotechnology to move forward. Once that barrier has been overcome, she believes new engine designs and lubricants will greatly increase fuel efficiency. Another area for future development is biotribology.
“As Baby Boomers age, we need to improve the wear properties of biomedical devices such as joint replacements and heart valves,” she said.
Lockwood believes nanotribology’s future lies partly in areas of improving extreme pressure performance in the context of gear lubrications.
Hsu is looking toward TVs, integrated circuits, automotive materials, biomedical devices, time-release devices, lubricant additives, self-repairing additives, surface phenomena additives and data storage devices.
“The potential for future applications of nanotechnology lie primarily in the field of advanced lubrication and novel coatings for machine components,” Adhvaryu said. “There should be significant benefits gained from using such technologies in applications involving highly loaded contacts and components that operate under starved lubrication.”
As far as long-term issues are concerned, Adhvaryu believes more research is needed in the areas of:
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The fundamental nature of friction and adhesion.
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Contact mechanics models for nanocontacts.
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Overcoming friction in self-assembled nanomaterials and NEMS.
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The use of friction and films to assemble multifunctional structures.
The National Cancer Institute has committed to a $144.3 million, five-year initiative to develop and apply nanotechnology to cancer. The institute’s former director, Andrew von Eschenbach, said that nanotechnology has the potential to radically increase options for prevention, diagnosis and cancer treatment.
Among the most promising technologies are low-friction nanoscale devices or devices with nanoscale materials and components that can detect cancer in its earliest stages. These same devices have the potential to provide real-time assessments of therapeutic and surgical efficacy. In addition, targeted devices capable of bypassing biological barriers could deliver therapeutic agents directly to cancer cells.
“As far as nanotechnology is concerned, nanotribology is one of the top issues,” Hsu said. “There are so many nanodevices on the horizon that have the potential to change people’s lives, but they will never come to market unless we learn how to control friction at the nanoscale.”
REFERENCE
1.
A list of nanotechnology merchandise—containing everything from nanotech diamonds and cooking oil, to golf clubs and iPhones—is available
here.
Editor’s Note: This month’s cover story was conceived by STLE’s Nanotribology Technical Committee. Send comments to:
mhunter@kingindustries.com.
Jean Van Rensselar heads her own communications firm, Smart PR Communications, in Naperville, Ill. You can reach her at jean@smartprcommunications.com.