Junction growth in metallic friction: the role of combined stresses and surface contamination

This paper extends earlier work on the adhesion mechanism of friction and considers in particular the growth in area of contact as the tangential force is increased to the point at which gross sliding occurs. The earlier studies assumed that the area of true contact A is the same as that produced under static loading so that A = W / p 0 where W is the normal load and p 0 the plastic yield pressure of the metal. If the junctions have a specific shear strength s , the friction F , that is the force to shear them, will be F = As and the coefficient of friction becomes μ = s / p 0 (Bowden & Tabor 1954). Recent studies, however, show that as the tangential stress is applied the area of true contact increases according to a relation of the type p 2 + αs 2 = p 2 0 where p is the normal and s the tangential stress in the contact region and α an appropriate constant. With thoroughly outgassed metals, junction growth generally proceeds until practically the whole of the geometric area is in contact and coefficients of friction of the order of 50 or more are observed (Bowden & Young 1951). If the interface is contaminated, the stresses transmitted through it cannot exceed the critical shear stress of the interface. The new point developed in this paper based on the work of Courtney-Pratt & Eisner (1957), is that until the shear stress reaches this value junction growth occurs as for clean metals. Beyond this point, however, further junction growth is impossible and gross sliding occurs within the interfacial layer itself. The analysis given here shows that if the interface is only 5% weaker than the bulk metal, junction growth ceases and gross sliding occurs when the coefficient of friction is of the order of unity. This corresponds to the experimental observation that minute amounts of oxygen or air reduce the friction of thoroughly clean metals from extremely high values to values of about 1. In the presence of a lubricant film the transmissible stresses are so small that little junction growth can occur before sliding takes place. The expression for the coefficient of friction now reduces to a form resembling that given by the earlier simpler theory, namely μ = s i / p 0 , where s i is the critical shear stress of the lubricant layer. The present treatment thus incorporates the effect of combined stresses and surface contamination into a more general theory of metallic friction.