SCIENCE & TECHNOLOGY
TRACK RECORD Rubbing silicon surfaces together gently in a micromachine designed to study tribology leads to nanometer-scale gouging and displacement of material (a wear track), as revealed in this atomic force microscope image.
MOLECULAR-SCALE WEAR AND TEAR The chemical underpinnings of friction, wear, and lubrication are discussed at conference MITCH JACOBY, C&EN CHICAGO
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RIVERS, START YOUR ENGINES!
To motorsport fans, that famous call brings to mind the roar of powerful automobiles and the start of high-speed competition. To some engineers and surface scientists, however, the words mark the onset of tribochemical reactions. As drivers rev their engines, machine components such as valves and gearwheels undergo furious revolutions and other types of motions that cause engine parts to slide past one another at high speed. Even in welllubricated engines, the relative motions of the machine parts cause friction and wear and tear and stimulate chemical changes at the interface between the sliding surfaces and a lubricant film that's less than a micrometer thick. Tribology—the study of friction, wear, and lubrication—has clear applications to race car engines and other topics related to classical mechanical engineering. But while today's tribologists continue to study traditional engineering subjects, some of the
field's practitioners focus on applications in developing areas. Among other specialties, the list includes tribochemical reaction mechanisms, tribological properties of nanoscale materials, and tribology of micromachines and biological systems. More than 1,000 scientists from roughly 50 countries gathered in Washington, D.C., last month to discuss those topics and others at the 3rd World Tribology Congress. Sponsored by the American Society of Mechanical Engineers and the Society of Tribologists & Lubrication Engineers, the conference, which is held every four years, gave attendees an opportunity to catch up on the latest research in a wide variety of areas. "IN TRIBOLOGY, expertise runs from A to Z," remarked Stephen M. Hsu, nanotribology group leader at the National Institute of Standards & Technology (NIST), Gaithersburg, Md. Hsu, who was one of the meeting's general organizers, explained that the field is made up of physicists, en-
gineers, chemists, and materials scientists. It also includes people with expertise in manufacturing bearings and engines and in formulating lubricants. "The groups are rather distinct, and they don't usually interact very much," Hsu acknowledged. "But interdisciplinary collaborations are essential, and they're on the rise." One of the principal aims of the congress, according to Hsu, was "to foster cross-pollination—to encourage interaction among tribologists with distinct backgrounds." In one of the symposia devoted to tribochemistry, Hsu presented an overview of the connections between tribochemistry and lubrication that included some of his group's results and those of other researchers. In an opening comment, he noted that, at the interface between surfaces in sliding contact, tribological processes initiate chemical reactions that alter the applied lubricant, transforming it into a protective film. To be an effective lubricant, the reaction product must be a strong and durable film that minimizes friction and wear between the surfaces. "We want the protective film to form as rapidly as possible and then be used as a sacrificial lamb that's worn away during contact," Hsu said. The question is, what stimulates the onset of tribochemical reactions? The N I S T researcher offered several possibilities. For example, depending on surface roughness, forces associated with rubbing motions can induce spikes in the temperature and pressure in the vicinity of microscopic bumps (asperities), which lead
From powerful automobile engines to nimble micromachines and animal joints, tribological properties control the mobility of a vast number of moving parts. WWW.CEN-0NLINE.ORG
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SCIENCE & TECHNOLOGY to chemical reactions. Or rubbing metal surfaces can scrape surface films, thereby exposing fresh metal sites that may react with a lubricant directly or catalyze lubricant reactions. Another possibility is that mechanical forces disrupt surface bonds, leading to charged-particle emission and formation of dangling bonds (unsatisfied valencies),
gano-iron compounds. Later, through the use of gel permeation chromatography and atomic absorption spectroscopy methods, Hsu, Richard S. Gates, and coworkers at NIST determined that the wear processes form organometallic polymers with molecular weights ranging from 1,000 to 100,000 atomic mass units. In follow-up studies, scientists at Penn-
According to Hsu, the conjugated species can grow via polymerization reactions until they reach a molecular weight of about 100,000 amu. Molecules that large precipitate from the oil solution in the form of a brown sludge that is often observed near wear spots. Interestingly, Hsu pointed out that in wear tests in which no high-molecular-weight products were detected, lubrica-
THERE'S THE RUB Designed to measure friction between silicon surfaces at the micrometer scale, this micromachine (MEMS device) uses comb-shaped actuators (left) to rub a mobile beam against a bathtub-shaped stationary post (closeup view, right). which in turn may stimulate chemical reactions. Sorting out the roles played by various tribological processes in lubricant chemistry and the exact nature of the reaction products has been challenging. But some pieces of the puzzle have fallen into place. More than 20 years ago, for example, Hsu and coworkers examined reaction products from paraffin lubricants used in wear tests on steel parts and identified oil-soluble or-
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sylvania State University, in collaboration with the N I S T team, proposed that the high-molecular-weight species—so-called friction polymers—are formed via a multistep mechanism that begins with oxidation of the hydrocarbon lubricant in the contact area to form carboxylic acids. On steel surfaces, the organic acids react with iron oxides, which function as hydrogenabstraction catalysts, leading to formation of conjugated molecules.
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tion was generally found to be ineffective. To gauge the importance of thermal chemistry on the lubrication reactions, the team oxidized thin films of lubricants under static conditions (no rubbing) at 225-300 °C—the range associated with temperature spikes during sliding contact. They found similar products under static and dynamic conditions and concluded that the tribochemical reactions that formed organometallic "friction polymers" were ini-
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