Friction between unlubricated metals: a theoretical analysis of the junction model

Abstract

According to Bowden & Tabor (1950), friction between unlubricated metals is due to shearing of junctions formed by adhesion between minute asperities on the sliding surfaces. The theory is elaborated in this paper to give a clearer picture of how the stresses and deformations in junctions are determined. First, it is shown that the distinctive feature of steady sliding is that, on the average, the surfaces move parallel to each other. Under this condition the deformation of individual junctions, from their formation to fracture, has been studied using Plasticine models. Theoretical plastic solutions for junctions with simple shapes (Green 1954a) provide rough estimates of the stresses involved and a qualitative picture of a junction’s ‘life cycle’. The effects of ductility and hardness are discussed. Mechanisms are suggested to account for high friction or seizure between outgassed surfaces in vacuo. Finally, certain limits of validity of the theory are pointed out and related to the breakdown of Amontons’s laws.