Just Rolling Along


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Electron microscope image of an inorganic fullerene-like molecule
A smooth, friction-free future may be in the offing - for machinery, that is. Prof. Reshef Tenne and his team in the Materials and Interfaces Department have created a new kind of lubricant that promises to cut friction in half. The synthetic material is made of inert, round molecules of tungsten disulfide. Says Tenne: "They just roll against each other and against the machinery parts, and don't stick to anything, like Teflon."

The synthetic molecule has a structure similar to the soccerball-like clusters of carbon atoms called fullerenes, or buckyballs, named after R. Buckminster Fuller, architect of the geodesic dome. Fullerenes were discovered in the last decade when a U.S.-British team of scientists noted that, under certain conditions, carbon atoms will cluster together to form a stable, hollow sphere. The discovery won the researchers the 1996 Nobel Prize in Chemistry.

Initially, it was believed that only carbon, or molecules containing carbon, exhibit this behavior. But in 1992, Tenne and his Institute colleagues succeeded in producing inorganic fullerene-like molecules from tungsten disulfide. Since then, several other inorganic buckyball compounds have been synthesized at the Institute and elsewhere. To Tenne, the properties of the new, inert molecules seemed to have great potential for the development of a new generation of solid lubricants.

Why solid? Liquid lubricants, it turns out, are not appropriate for all environments. They freeze in the extreme cold of outer space and lose their effectiveness in a heated engine and in heavy-load transmission systems. Currently available solid lubricants, even ones made of tungsten or molybdenum disulfides, have drawbacks too.

"Existing solid lubricants contain crystallites, which are shaped like flat platelets and have chemically reactive edges," says Tenne. "In working conditions, they stick to machinery parts and undergo chemical reactions that lead them to decompose and rub off." The parts are then subject to grinding, substantially shortening the lifespan of the machinery.

The Weizmann tungsten disulfide buckyballs get "around" this problem. Being round and inert, they have no edges where the chemical reactions that make other lubricants stick can take place. Since machine parts just roll over them, they make reliable chemical ball-bearings. They wear better, too, because they are made up of many layers, like an onion. If the top layer wears off, the underlying layer continues the lubricating action. These balls are also larger than the carbon fullerenes, thus keeping the metal parts further separated and giving more bounce to resist mechanical pressure.
Prof. Reshef Tenne. spheres
Tenne's next challenge was to produce the new material in the laboratory and test it under conditions simulating those prevailing in industry. The results that rolled in proved that this was definitely the right stuff. The new lubricant outperformed all existing solid lubricants, including normal tungsten disulfide and molybdenum disulfide. The synthetic buckyballs caused half the friction and only one-sixth as much wear.

The potential market for this new substance is tremendous. The automobile industry faces ever stricter environmental regulations that require it to reduce pollution and make engines and transmission systems more efficient. In general, earthbound enterprises are looking for ways to conserve resources and cut costs by making machinery last longer. In microelectronics, where minuscule transistors are produced under sterile conditions, solid lubricants are preferred over liquid ones because they cause no contamination of the electric circuitry. And in space, where commercial projects are proliferating, more and more equipment that can function in extreme temperatures will be required.

Currently, the Weizmann Institute laboratory can synthesize about a gram a day of inorganic buckyballs. To get this enterprise moving, it will be necessary to scale up the synthesis to at least a couple of hundred grams daily, a matter for smart engineering. Then a homogenous and stable emulsion of the solid particles in oil and cooling fluids must be formulated. And finally, extensive field tests have to be carried out to ascertain the stability of the lubricant in various environments. Yeda Research and Development Co. Ltd., the Institute's technology transfer arm, has filed patent applications for the new material. Interest in it is being expressed by industrial companies around the world.

Tenne's team was made up of doctoral students Yishay Feldman and Moshe Homyonfer, Dr. Sidney Cohen of the Institute's Chemical Services Unit and Dr. Lev Rapoport and other researchers from the Center for Technological Education in Holon.

This research was funded in part by Yeda Research and Development Co. Ltd; the Israel Ministry of Science; the U.K.-Israel Science and Technology Foundation; the Minerva Foundation, Germany; the NEED International Projects, Japan; and the Petroleum Research Foundation of the American Chemical Society.