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a chemistry professor at the University of Wisconsin, Milwaukee, combines high-pressure tribology measurements with high-vacuum surface studies to probe the nature of lubricating films and the source of their tribological properties. He reported on studies of the roles of chlorine and sulfur additives in so-called extreme-pressure lubricants--the type used in cutting, drilling, and other machining applications. To quantify the effectiveness of lubricant additives, Tysoe's research group uses a pin-and-V-block apparatus in which a rotating steel pin is squeezed controllably between metal blocks. The device, which is known as a tribometer, is immersed in model lubricants--for example, poly-a-olefins containing low concentrations of methylene chloride, chloroform, and carbon tetrachloride. The Milwaukee group members increase the load on the blocks while measuring the torque on the pin until they reach the seizure load--the point at which the torque rises dramatically. The measurements show that very little additive can greatly enhance a lubricant's performance. For example, less than 0.5 weight % of CCl4 in oil raises the seizure load by a factor of 10, Tysoe reported. "It's a simple experiment," he acknowledged. "But the results are very reproducible." Variations of the technique, in which pressure is applied to a lubricated interface while frictional forces are measured, are widely used in tribology studies. IN THE GROOVE Sliding a pin across a 2,000-Å-thick halide film plows the material out of its path, as seen in an AFM image. Even so, the contact remains lubricated, thanks to the ultrathin halide layer left behind. U. WISCONSIN, MILWAUKEE, IMAGE IN RELATED WORK, Tysoe and coworkers analyzed the surfaces of debris particles collected from the pin-and-block interface using X-ray photoelectron spectroscopy and other surface-sensitive probes. The studies show that chlorinated lubricant additives react with steel to form a lubricating film that contains FeCl2. Other tribochemical reactions occurring at the interface produce nanometer-sized carbon particles and a thin layer of iron carbide. When sulfur-based additives are used, Tysoe noted, ferrous sulfide forms at the interface. The thickness of the lubricating film is determined by a competition between the rate of surface reaction and the rate at which the film is worn away, Tysoe explained. His research group measures film growth using microbalance techniques and film removal using the tribometer. When the competition tips too far toward film erosion, the protective layer wears away, leading to seizure. Another way to get a handle on molecular-scale tribology events is to compare the chemistry that occurs at high pressure (monitored with a microbalance) with reactions that occur under carefully controlled high-vacuum conditions. To make the comparison, Tysoe's group used molecular-beam methods, in which chlorine- and sulfur-based model lubricant-additive molecules were directed at a heated iron surface while the reaction products were monitored. SMOOTH SAILING U.S. Naval Academy researchers Gao (from left), Chateauneuf, Harrison, and Mikulski use computational methods to study the effects of friction on monolayer films. Gellman CARNEGIE MELLON PHOTO THE RESULT was complete agreement, Tysoe noted. Reaction rates, activation energies, and products measured in the high-pressure and high-vacuum experiments were found to be identical--suggesting that the reaction pathways are also the same. Based on studies of dimethyl disulfide and diethyl disulfide, for example, Tysoe proposed that on contact with the metal surface, the molecules undergo S–S bond cleavage, which leads to surface thiolate species. Ultimately, the thiolates decompose--evolving methane (from dimethyl disulfide) or ethylene and hydrogen (from diethyl disulfide)--and leave behind sulfur atoms that react with iron to form the lubricating FeS film. |
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