Contributed by the Tribology Division for publication in the ASME JOURNAL OF TRIBOLOGY. Manuscript received by the Tribology Division July 27, 2000; revised manuscript received March 15, 2001. Associate Editor: G. G. Adams.

When two nominally plane and parallel surfaces are brought together gently, contact will initially occur only at a few points. As the normal load is increased the surfaces move closer together and a larger number of the taller asperities on the two surfaces come into contact. The total area of these point contacts forms a real area of contact, $Ar,$ which, as a rule, does not exceed 5–10 percent of the nominal (geometric) area, $An.$ Theoretical predictions and experimental measurements of $Ar$ and the micro-displacement normal to the surface and their relationship to the external loads are necessary for an understanding of many problems related to tribology.

Abbott and Firestone 1...