Issue Date: October 11, 2010
In November 2008, the space shuttle Endeavor visited the International Space Station to do some important repair work. A critical part of the mission was to service the station’s two rotary joints, which allow its solar-cell arrays to track the sun. Wear and damage because of friction had rendered the joints inoperable. The fix required an hours-long spacewalk and a delicate technical procedure.
But the basic idea was simple. “We sent astronauts up with a grease gun and got it moving again,” recalls Christopher DellaCorte, a senior research engineer in the Tribology & Mechanical Components Branch at NASA’s Glenn Research Center. The joints were originally installed with a gold-film solid lubricant, “but it wasn’t adequate,” he explains. “There was more rubbing in one bearing than the other, and that bearing failed. It was not the right approach.”
The space station repair was successful, but for upcoming missions DellaCorte and his colleagues are looking for materials and lubricants that can last indefinitely. “In space, we don’t have the ability to go and change the oil,” DellaCorte points out.
On Earth, friction is a physical force, like gravity, that people take for granted. But friction is incredibly costly. In machines, friction quietly steals power by wasting kinetic energy in the form of heat. What’s worse, whenever surfaces rub, friction eventually causes the materials to wear down, ending an object’s useful life.
Researchers who study friction, part of a field of mechanical engineering called tribology, say that new performance materials hold the promise of vastly reducing the effect of friction on mechanical parts, including car engines and wind turbine gears.
On the other hand, they also point out that solving the friction problem is an unending process. Each engineering or design innovation—such as high-performance engines that increase fuel efficiency—creates a new set of challenges at material interfaces. Chemists and materials makers work continuously to develop specialty lubricants, additives, polymers, and coatings to keep anything that slips, slides, or rolls moving smoothly.
Calculating what losses due to friction cost society is difficult. One example comes from the transportation industry: At Argonne National Laboratory, the tribology group studies how energy losses and wear from friction gobble up fossil fuels.
According to Argonne Section Leader George Fenske, engine friction consumes 10% of a car’s or truck’s fuel. In the U.S., that works out to about 1.4 million barrels of oil wasted per day. At $60 per bbl, that is almost $31 billion worth of petroleum lost in automobile engines every year. “We call it the 10% incentive. If you can reduce the friction losses, you can get significant savings in fuel consumption,” Fenske explains.
“We look at any place where parts are moving, metal to metal, or with a film of oil between them,” he elaborates. The biggest friction culprits are piston rings and cylinder liners, valve trains, bearings, seals, and some auxiliary components. In all, Fenske identifies 16 areas of friction in engines alone. A car’s transmission and axle waste another 5% of the fuel.
Of course, it’s not possible to reduce friction to zero, but Fenske says it is possible to improve fuel economy by 3–5% with low-friction materials and new engine lubricants.
Although consumers understand that their car engines need oil, most people don’t really understand the interaction of surfaces and lubricants, NASA’s DellaCorte points out.
“You can achieve low friction when you have a relatively easy-to-shear material caught between two hard surfaces,” DellaCorte says. “If you want to slide material A over material B you need a soft interface in between, but to avoid wear, A and B have to be very hard so you don’t gouge out material.” Easy-to-shear materials, such as oil or polytetrafluoroethylene (PTFE), also known as Teflon, lower the amount of force on a surface when another object moves against it in a parallel or tangential direction.
The effect of friction occurs on a microscopic—even atomic—scale, DellaCorte explains. “In our understanding of tribology, there is no such thing as two surfaces that rub against each other—like a chunk of graphite rubbing against steel,” he says. “It’s actually three surfaces. There is a thin layer of water from the air that gets into the upper layer of the graphite and forms a low-shear-strength region.”
When engineers design moving parts, friction is hard to avoid, unless they use a spring or something else that bends. The next-best option is to design surfaces that roll, as in a ball bearing. Sliding surfaces generate the most friction, and that’s where lubrication is needed most. But even where contact between surfaces involves rolling, a little bit of sliding occurs because no surface is perfectly smooth. As in the case of the solar rotor, even rolling surfaces need lubrication.
When it comes to lubricants, DellaCorte says, “we find that liquid lubricants are much better than solid ones because they flow back into contact with and replenish the surface.” When a liquid lubricant won’t work—for example, at ultra-low temperatures liquids get too stiff—engineers turn to solids. When choosing a lubricant, “you can’t separate the chemistry from the engineering,” DellaCorte observes.
At Valvoline, a maker of high-performance engine oil and other automotive fluids, lubrication engineers work directly with automotive engineers at manufacturing firms, says Fran Lockwood, senior vice president for R&D. “From our point of view, we’re at the cusp, or tipping point, of technology where we’re looking for minimum friction without getting into any type of durability issue,” she reports.
In particular, Valvoline is working to make engine oil with lower viscosity than is now available, a change expected to have a large impact on friction and to reduce power use. Machine components would then have to be designed to work with a thinner layer of oil, Lockwood says.
The company recently introduced Premium Blue Extreme, an engine oil designed to boost fuel economy. The company was able to influence fuel consumption by controlling lubricant viscosity under a range of shear rates and temperature conditions.
To create new products, Valvoline chooses a base oil—either petroleum derived or synthetic and blends it with additives, in this case, viscosity-index improvers. The two most popular viscosity additives are olefin copolymers and poly(methyl methacrylates), Lockwood says. Then, friction-modifying additives based on molybdenum are traditionally added, although Lockwood also reports success with new organic friction modifiers.
As automotive technology evolves, she says, Valvoline’s additive choices also have to change. For example, current emissions systems for passenger cars rely on three-way oxidation catalysis. But one of the favored lubricant additives shortens the life span of those catalysts. In 2004, the company introduced an engine oil with a new additive from specialty chemical maker Lubrizol. Made with a low-volatility zinc diacetyl dithiol phosphate, it helps protect the emissions systems.
Additives will also need to be adjusted to ensure that cars running on alternative fuels perform well. Research at Argonne shows that, in all vehicles, some fuel gets diluted into the lubricant system. “Since gasoline is volatile, not much gets in,” Fenske reports. “But ethanol and biodiesel are not as volatile, and a higher level of fuel dissolves in the engine oil, which may affect the performance of a lubricant.”
Fenske and Lockwood emphasize chemistry’s importance to their work. All the additives have to work well together, even as they compete for access to the surfaces they are supposed to modify. In addition to friction modifiers, the friction-reducing arsenal includes antiwear additives, which create and protect a hard surface, as well as corrosion modifiers. Too much of any of them, and the others fail to work properly.
Another difficulty is that the lubricant as a whole has to function and persevere in today’s more challenging engine environments. Smaller, more efficient cars with higher horsepower engines have fuel injectors and combustion systems that run at higher temperatures, Lockwood says. “You’ve got a lot of waste heat that’s got to go somewhere,” she says.
Lubricant additives protect more than just automobile engines. According to Lubrizol, a fast-growing market is now in wind turbines, where its additives are in gearbox lubricants. “Estimates for growth in the industry are as high as 20% per year, but growth can vary depending on government incentives,” says Robert Profilet, the firm’s global commercial manager for industrial oils.
Lubricants for wind turbines have to perform in an extreme environment, Profilet says. “Fluids must be formulated to contain extreme pressure additives,” he says. “They must also be antiwear, provide resistance to micropitting, protect bearings, and last a long time in the gearbox. Wind turbines are often in remote locations where it is not convenient or cost-effective to change the oil between scheduled maintanance cycles.”
At Dow Chemical, scientists have developed a synthetic lubricant called UCON GL-320 specifically for wind turbine gearboxes. The base oil is polyalkylene glycol (PAG), and the product is designed to compete with older lubricants made with mineral oil or poly(α-olefins).
“In wind turbines the main problem the industry is facing is pitting of the gears due to wear. You’ll see some of the steel has come out,” explains Govind Khemchandani, an R&D applications leader at Dow. “When that starts to happen, you get stalling, which is a dangerous situation.”
Khemchandani says his team traced the problem to insolubility of the additives in the commonly used mineral oil lubricants. Most additives are polar compounds, and they mineralize and oxidize in nonpolar base oils, he notes. In contrast, PAG is polar. In Dow’s product, he explains, even when additive molecules oxidize, the oxidation by-products remain soluble. They don’t separate and create surface deposits.
Dow reports that a version of its PAG-based lubricant has had four years of successful use in gas-powered turbines with no varnish or other deposits forming.
In addition to lowering friction and protecting parts from wear, performance lubricants such as Dow’s boast attributes that help mechanical systems work effectively, the company says. Formulators work to ensure that viscosity stays low even at cold temperatures and that lubricants efficiently transfer heat out of the system to prevent hot spots from forming.
Improving energy efficiency through reduced friction, however, is small potatoes compared with the opportunity to reduce wear, says W. Gregory Sawyer, a professor of engineering at the University of Florida. “Reducing wear is far more important than reducing friction. It’s what we all should be working on,” he asserts.
“If you could have materials that have ultra-low wear, it would have a profound impact on the world,” Sawyer says. “Your hip replacement would last forever; satellites could operate for billions of cycles. Imagine wearing one pair of shoes and never having to buy another.” Friction is one cause of wear, he acknowledges, but wear has many other causes, including contact pressure, frictional temperature, shear rate, and oxidation.
To make ultra-low-wear materials, Sawyer first provides as much mechanical toughness as possible to the materials themselves. Then he adds ingredients that act as a frictional modifier when shear stress is applied. This adds a lubricant phase to the protective material.
The goal, Sawyer says, “is to balance the rate of expression of the low-friction phase such that just enough is there to allow surfaces to slide against each other without generating high shear forces that bring deformation and stress.”
Nanotechnology is one way to achieve that balance. Sawyer’s lab is studying how to distribute nanomaterials across surfaces while making use of the ambient environment to help them stay put. He says it’s possible to “play with reactivity to keep the nanostructures on the surface or get them to attach to other surfaces.”
“What’s nice about ‘nano’ is that you can compound it,” Sawyer enthuses. “It’s opened up new opportunities for reactivity to play a role, giving us a chance to blend functionality and maximize performance without losing other properties.”
It’s possible to achieve low friction without solving the problem of wear, Sawyer points out. For example, a solid lubricant such as PTFE reduces friction between surfaces because it has low shear strength. But it also quickly wears away. “Teflon experiences some of the highest rates of wear of any of the polymers that I’m aware of,” he reports.
To increase the durability of PTFE, Sawyer’s team has had success adding a surface treatment of alumina nanoparticles. This summer, the group began testing how various nanocomposites protect against wear in extreme conditions by attaching surface-friction-measuring devices called tribometers to the outside of the International Space Station. In space, the materials have to withstand atomic oxygen, ultrahigh vacuum, ultraviolet radiation, and temperatures ranging from –40 to 60 °C. In addition to the doped PTFE, Sawyer’s group is testing combinations of molybdenum disulfide with materials such as antimony trioxide, yttrium oxide-stabilized zirconium, gold, and carbon.
Increasing durability was the initial benefit of a wear-resistant material DuPont launched almost four decades ago. Back then, the company introduced the early members of a family of manufactured parts called Vespel. The products were based on a polyimide resin with a particularly alluring quality: It has no melting point. Consequently, the resin quickly found a market in applications that typically require engineers to fight high temperatures, such as in jet engines, oil- and gas-handling machines, and chemical processing plants.
Such an inert material was bound to have other uses. “It had very high wear and loading capabilities because it didn’t melt when something was rubbing against it,” explains David J. Ritchey, the global transportation segment leader for Vespel. “Usually, if you get a material hot enough, it melts, and the part fails. That was the ‘Aha! moment.’ ”
Reducing friction was the next logical step in the resin’s development. “We started trying to listen to what our auto transportation customers were talking about and what engineers were being asked to do,” Ritchey recalls. They were being asked to improve efficiency, reduce fuel consumption, and reduce emissions. “The common thread was friction,” he says. “Whether it was the transmission, axle, or an engine component, things were spinning and moving.”
Vespel car parts have replaced large, heavy-metal components such as needle bearings or roller bearings to save space and weight. They’ve also replaced brass, bronze, and even other polymers in places where new designs required materials that could handle higher loads, according to Ritchey.
It’s not always possible to make a part entirely out of a low-friction material. So Argonne researchers are developing specialty coatings that can be applied in layers thinner even than human hair. Argonne senior scientist Ali Erdemir began making so-called near-frictionless carbon coatings in the late 1990s. The coatings, which are 40 times as friction-reducing as Teflon, are only 1–2 μm thick and are applied in a plasma deposition chamber.
The materials’ atoms “bond together to produce a specific carbon-hydrogen network to produce the lowest friction of any known material,” Erdemir says. Since the coatings were developed, Argonne has looked into applications including automotive and airplane parts, air conditioners, vacuum devices, and even medical implants.
Recently, Erdemir proposed the coatings’ use on computer hard-drive disks to enable contact recording. “A direct-contact recording between head and slider would increase storage density from three to 10 times,” he claims. “We are getting rid of the thin lubricant film, so the disks should have much higher storage density and storage feeds.”
Erdemir has also studied the role of nanoparticles in lubricant additives. In next-generation engine oils, slippery reactive nanomaterials can attach themselves to places of high friction. He reports that several boron-based nanoparticles look promising, including those made of boric acid, boron nitrate, borax, and boron oxide.
Improved hard drives are bound to excite computer manufacturers, but it’s generally difficult to generate much enthusiasm for research into lubricants. “Not too many people get thrilled when we show them dark, dirty oil,” Argonne’s Fenske says. “Trying to get the interest of funders can be a challenge, compared with research targeted to developing alternative fuels.”
In the future, however, as engineers redesign machine parts for energy efficiency, materials suppliers with a good grasp of friction expect to make many inroads.
“The reason we’re excited is, if you look at major trends in transport—trying to reduce fuel consumption and going to smaller engines, turbochargers, emission control devices, and new transmissions—everything will be redesigned,” DuPont’s Ritchey says. For the first time in decades, he says, “manufacturers and their suppliers have a window of opportunity to try new designs and choose better materials.”
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