Effect of the Normal Force upon the Friction Coefficient between two Solid Surfaces

Abstract

The results of the research related to the dependency of the frictioncoefficient, µ, between two solid surfaces, on the normal force, N, ispresented. A review of models and experimental results in this area ismade, and results of friction experiments with steel wire bristles for streetsweepers are referred. Many theoretical and experimental works haveshown that µ may increase or decrease when N increases. However, theliterature indicates that mostly µ reduces as N is increased. This mainlyoccurs for small forces and very smooth surfaces. It is concluded thatthis phenomenon may be caused by several factors. Adhesion forces,which affect very smooth surfaces, tend to be significant when N is smalland have the effect of increasing µ. The differences in the contact radii ofthe asperities may also cause this phenomenon. When N is small, a greatnumber of asperities in contact have small radii, and large shear frictionstresses may be developed. In contrast, for large forces, the larger contactradii dominate friction; friction stresses, which are smaller than the shearones, appear producing small friction coefficients. It is also concluded thatthe change from elastic contact, for small forces, to elastic-plastic contact,for larger ones, may reduce µ when N increases.