50 Years since the Greenwood & Williamson (GW66) Paper.

J A Greenwood

In the 1950s, the Bowden & Tabor theory of contact reigned supreme: because surfaces are rough, contact occurs only at asperities, and these are crushed down until the contact pressure reaches a limiting value related to the bulk hardness. Then the real area of contact is found as  , and hence Amontons’ laws of friction. Surface roughness did not need to be measured: it merely had to exist.
Then came Archard’s challenge: that plastic deformation cannot be indefinitely repeated as would be necessary for this mechanism to apply to real machinery. In practical applications, contact must be elastic. Archard’s explanation of how the proportionality between contact area and load could be elastic still only needed the qualitative property of roughness; along with his prescient version of the fractal concept of surface features.
Williamson believed that surface roughness needed to be measured, and quantified: and so it all began! This presentation will describe both the rather simplistic method which led to GW66 and the reason for the special importance of this paper in the field of surface roughness characterization, particularly in view of earlier important works by Ling (1958), Tallian et al (1964) and others.  The simple theory expounded in GW66  triggered advances and applications: ellipsoidal asperities, elastic-plastic asperities, adhesive contacts, contacts through a lubricant film; and to Nayak’s theory of surface roughness predicting the peak and summit height distributions. But then also followed the Archard-Whitehouse work, showing that roughness parameters depend crucially on the sampling interval, so throwing doubt on the rationality of characterizing surface roughness digitally!
This presentation will commemorate Brian Williamson’s contribution not only to GW66 but also to other important areas of contact resistance.  In particular I shall refer to out joint paper of 1958 (GW58) relating to the temperature of an ohmic-heated contact spot and how Holm’s melting voltage concept sometimes does not apply. I will also describe how the Greenwood-Williamson partnership began.