Molecular ball bearings lube metal parts

Most people are familiar with oil's ability to grease moving parts, but some of the best lubricants used in industry today are solids, such as tungsten disulfide. As powders, these solids can be used to coat surfaces, where their slippery layers of molecules allow metal parts to slide past each other easily.

 Now, a team of scientists has demonstrated that tiny, round particles of tungsten disulfide, typically 120 nanometers in diameter, make an even better lubricant than the powdered version. The particles act as miniature ball bearings, rolling to reduce friction between surfaces, says Reshef Tenne of the Weizmann Institute of Science in Rehovot, Israel. He and his colleagues at the Center for Technological Education in Holon report their findings in the June 19 Nature.

 Tenne calls the particles "inorganic buckyballs," since they take the shape of fullerene molecules but don't contain any carbon. The researchers performed standard wear tests with the material, comparing it to tungsten and molybdenum disulfide powders.

 Irwin L. Singer of the Naval Research Laboratory in Washington, D.C., says that the modest improvement in wear is "kind of interesting" but not large enough to convince him that the material will replace existing lubricants anytime soon. Powdered tungsten and molybdenum disulfides already work so well that "to beat them is really hard."

 Round particles would seem to make ideal ball bearings. However, Jacob N. Israelachvili of the University of California, Santa Barbara says that "just because something rolls, it doesn't mean that it's going to be good at lowering friction." The carbon buckyballs, for example, don't reduce friction well on their own, but as Israelachvili and his colleagues recently demonstrated, they show some promise as additives to conventional liquid lubricants.

 Tungsten disulfide might work better than buckyballs, Tenne says, because the larger ball size increases the distance between the metal surfaces. Also, because the particles are built like onions, with up to 20 concentric layers, the particles retain their roundness even when some material wears off.

 The chemical structure of the particles may also give them an advantage over powders. Unlike the powders, which take the form of flat platelets about 500 nm across, the round particles have no "dangling bonds" at the edges that can catch on metal surfaces, Tenne says. Singer notes, however, that nobody has shown whether these incomplete bonds really affect friction.

 Whether the material turns out to be a practical lubricant "remains to be seen," says Israelachvili. The particles' slow wearing away suggests "a finite lifetime for these things." Lubricants for auto engines -- the application targeted by Tenne's group -- must last months.

 The group hasn't yet done the wide spectrum of tests that would indicate whether the inorganic material will work in engines, Singer says. According to Tenne, the biggest problem the team now faces is being able to make particles in large enough quantities to perform the tests. Currently, they can only synthesize about 1 gram per day.