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Friction

Because of the mathematical simplicity of the basic laws of friction (first put forward in the mid-seventeenth century by Leonardo da Vinci) it comes as a surprise to many people to discover that the exact mechanisms of friction on both the atomic and macroscopic scales is as yet unknown.  The study of friction, or 'Tribology' as it is known in scientific circles (from the Greek word meaning rubbing or attrition, is of increasing importance.  The economic losses due to friction and wear have been estimated at 5% of the gross national product of many countries.

Manufacturers of bearings need to know how to reduce friction and to discover materials that have low coefficients of friction in order to improve bearinglife expectancy.  But it is not just heavy engineering that benefits from a detailed knowledge of friction.  The ever-decreasing distance between the magnetic pickup and a computer disc requires a great understanding of the tribological behaviour of the materials used.

Cambridge has been at the forefront of tribology research since the discipline was founded this century, and the Physics and Chemistry of Solids Group has a long association with this field of study.  Indeed most of the key ideas about the mechanism of friction stem from work conducted in this laboratory between the late 1930's and 1970, by the late Prof. Philip Bowden and the late Prof. David Tabor.  Their work is described in detail in the two part classic text 'The Friction and Lubrication of Solids' [1].

Current research interests in the department include the frictional properties of both single crystal and polycrystalline diamond.  Diamond is unusual in that it possesses an exceptionally low coefficient of friction but the coefficient is anisotropic i.e. the friction coefficient takes a different value dependent on the direction of sliding.

This is scientifically interesting for several reasons:

Firstly, diamond is the 'ideal' material being very hard, chemically inert to most reagents, and having a high thermal conductivity (particularly useful in bearings).  These unique properties are gradually being harnessed in technologically applications due to the many advances in the production of chemically vapour deposited (CVD) diamond (see photograph below).

 

Photograph of CVD diamond dome

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Secondly, Seal [2] noted a high correlation between the frictional anisotropy and the wear anisotropy of diamond (known since polishing of gems began in India in the fourteenth century).  Thus the friction of diamond sliding on diamond can be used as a tool in investigating the polishing of this material.   Recent research suggests that while polishing in the so-called 'hard' directions may induce microcleavage, evidence from polishing in the 'soft' directions is probably due to a mechanically induced phase transition from diamond to graphite.  However, it is still not clear if diamond is removed as diamond (which then transforms to graphite), or if diamond is first transformed to graphite and then removed [3].

Other materials of interest in our friction studies are Diamond-Like Carbon (DLC) and Cubic Boron Nitride (CBN).

[1] Bowden F.P. & Tabor D., "The Friction and Lubrication of Solids", Part I (publ. 1950) & Part II (publ. 1964), Oxford University Press [2] Seal, M., 1958, Proc. R. Soc. Lond. A248 (1958) 379-393 [3] van Bouwelen F., Brown, L.M. & Field, J.E., Indust. Diamond Rev. 1997/1, pp. 21 - 25

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