A_review_of_the_friction_of_solids

with the larger movements of crossed cylinders. a rigorous experimental
and with the bre area of contact
in a single, small region, by examining of the effect of small displacement interferometer,
plastic, the area of contact
ll’/+, where fi is the “yield pressure” so that the coefficient over-simplified of friction
of the softer solid. If the adhesion in the contact
(KHAIKIN,LISSOVSK~ANDSALOMONOVICH JOHNSON~~) and static experiments. VENS~~,using a two-beam using a capacity micrometer, interferometer
14115,S~~~~~~~~~~~25,M~~~~~~~W~~~~1T, have been made by STEby KANKIN~~, have also been to measure the displacements,
by VERKYOVKSII~~ and COCKS', who both used optical ,PARKEKANDHATCH~~.COCKS,PARKER lo COCKS was concerned films on the the on of protective
levers, and by COURTNE>--PRATT*. A number of isolated observations recorded,forexample, AND HATCH examined mainly surfaces. contact ERNSTANDMERCH.~NT the area of contact during the experiment.
VOL. 1 @957/58)
WEAR
333
A REVIEW
OF THE FRICTION F. P. BOWDEN
OF SOLIDS*
Research Laboratory for the Physics and Chemistry of Solids, Cambridge
(England)
INTRODUCTION It is now commonly accepted that when two supposedly “flat” surfaces are placed together they touch only locally and that the frictional behaviour is dominated by the properties of these small regions of contact. This is true of most metal and solid surfaces, even when they have been carefully polished and smoothed by advanced techniques. An interesting exception to this can be provided if a solid is cleaved so that the surface is free from cleavage steps and is flat to molecular dimensions. Some elegant experiments with mica cleaved in this way are being made by Dr. ANITABAILEY(COURTNETPRATTANDBAILEY~,and BAILEY,unpublished) **, using sensitive optical techniques to ensure that the surfaces are in molecular contact over the whole region. Under these conditions strong adhesion occurs which resists movement in a direction either normal to, or tangential to the surface; the behaviour is quite different from that of ordinarily flat surfaces, and the usual laws of friction break down. In general, however, the contact will be local and it is important to our understanding of the friction and wear that we should have a clear picture of the physics of the formation, deformation and subsequent breaking of these contact regions. Clearly we are interested in the way in which the high spots come into contact, their number, size, and distribution, the nature of their deformation (how far it is elastic or plastic), the extent of the adhesion at the interface between the solids, the effect of tangential displace-ments on these properties, and finally, in the way in which the junctions are broken and reformed. We are fortunate in having at this session a number of papers dealing with many of these problems. It is clear, of course, that we cannot be dogmatic about these processes, nor argue that the conclusions for one system of solids will necessarily apply to another. In particular, we must bear in mind that the act of sliding will change the conditions: it will, for example, raise the temperature and hence profoundly affect the mechanical properties of the contact region. * Preprintpresentedon October znd, 1957, at the Conference on Lubrication and Wear arranged by the Institution of Mechanical Engineers, London. Reprinted in Wear by permission of that Institution. The full text of this review will be published in the Official Proceedings of the Conference. ** Normal references are numbered. The Appendix gives an alphabetical list of authors of papers read at this Conference.
small to cause sliding; an interesting 76). Earlier experiments
paper on this is contributed
on this fall into two classes, those done with oscillatory The static experiments
complex than this and we cannot expect (nor indeed do we find) that the shear strength and yield pressure of the material material region will be the same as in the bulk (BowI~EN AND T.4BoK30). FRICTION AT VERY SMALL DISPLACEMENTS Some understanding measuring the tangential may be gained of the deformation force when the displacement of the contact region by (Paper motion of the solid is very small, too by SHCHEDROV
region is strong, the friction 1; -~ As, where s is the shear strength of the weaker metal, /L -- F/11- -- s/p. This picture is, of course, highly of the junction is necessarily more and crude. The mode of deformation in the contact