Slippery solid

A new solid lubricant would work in diverse climatic conditions

Published: Friday 31 October 1997

a solid lubricant has been developed in Israel that does not lose its lubrication properties even in humid conditions. R Tenne and his colleagues, at the Department of Mechanics and Control, Centre for Technological Education, Holon and Department of Materials and Interfaces and Chemical Services Unit, Weizmann Institute, Rehovot, synthesised hollow nanoparticles of tungsten disulphide using a solid gas reaction and yielded the material.

Hollow nanoparticles are structures similar to those of nested carbon fullerenes (Carbon-60) and nanotubes. The hollow nanoparticles of tungsten disulphide had an average particle size of about 120 nanometres -- a nanometre is a billionth of a metre (Nature , Vol 387, No 6635).

The mechanism of lubrication in case of hollow nanopowder is rolling and not sliding. Because of extremely small size of nanoparticles, they can fill submicrometre-sized valleys and reduce friction between two surfaces. Their structures impart high elasticity to them and they are chemically inert in a variety of environments.

The product is easy to manufacture. Due to this, the researchers expect that the hollow nanoparticle tungsten disulphide is ideal versatile solid state lubricants. They feel that alternative methods of production of such materials may yield solid lubricants with even more fascinating properties.

Earlier, graphite (pencil lead) had been used as a lubricant in a variety of situations. At present, transition metal dihalcogenides such as molybdenum sulphide have been extensively used. Both of them work on the same principle. The crystal structure of these materials is such that there is a interplanar weakness between layers of the material that allows them to slip when subjected to a shear force. These form transfer films that act as smoothening agents between surfaces.

But there are several drawbacks for solid lubricants. For instance, in a humid environment, the metal dihalcogenide tungsten disulphide looses most of its lubricating property (because of the oxidation of tungsten) while in vacuum, graphite does not slip easily. Thus, it is difficult to find a material that will function as a lubricant in all environments. Now, with the development of the new solid lubricant, it seems that all these problems have been overcome.

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