HomeWikiMechanism of Metallic Friction as described by Bowden and Tabor
January, 15 2025
Table of Contents
Kinetic friction is not merely a surface phenomenon but rather depends on the bulk properties of the interacting materials, such as their relative hardness and melting points. Experimental investigations indicate that friction primarily arises from shearing or adhesion of the softer material in contact with the harder surface. Even highly polished surfaces, which may appear smooth at the macroscopic level, possess microscopic irregularities that contribute to friction. The temperature rise at the interface is not the primary cause of friction, as friction can still occur at cold asperity contacts due to localized adhesion and welding under high pressure. However, when sliding occurs at high loads and speeds, the resulting temperature increase at the interface may give the impression that elevated temperatures directly cause higher friction. Importantly, this relationship does not imply that higher ambient (room) temperatures always lead to increased friction. Instead, the frictional behavior is influenced by temperature generated during sliding, which depends on parameters such as load and speed, rather than the initial room temperature.
The relationship between frictional force, contact area, and applied load has been extensively studied. Coulomb and Amontons formulated two key laws: (i) frictional force is independent of the apparent contact area, and (ii) it is proportional to the applied load. While these laws initially lacked a theoretical basis, studies on the real area of contact have provided explanations. With respect to the first law, various experiments revealed that the real area of contact between metals was measured using electrical conductance techniques. The findings revealed that contact occurs primarily at the summits of surface asperities, resulting in a real contact area that is significantly smaller and largely independent of the apparent contact area. This insight helps to explain why the frictional force is unaffected by changes in the apparent contact area, as the real contact area—and consequently the frictional force—remains consistent under similar loading conditions. Amontons’ second law, which states that frictional force is proportional to the applied load, initially posed challenges because it was assumed that the surfaces deformed elastically. Elastic deformation would predict the contact area, and thus the frictional force, to vary with the two-thirds power of the load, not linearly. However, conductivity measurements revealed that the surface deformation is predominantly plastic. Under plastic deformation, the material flows until the contact area becomes proportional to the applied load, resolving this inconsistency.
Under most experimental conditions the metallic surfaces are covered with thin oxide layers and contaminating films. During sliding, these films are withered, allowing some metallic contact leading to penetration of surface irregularities through them. However, the adhesion and shear strength at these junctions are lower than those of pure metal. Earlier experiments confirm this, showing that removing surface films by outgassing in a high vacuum significantly increases friction (e.g., for nickel or tungsten, the coefficient of friction rises from ~0.3 to 6). Similarly, adding lubricants reduces contact and adhesion strength, though metallic adhesion still occurs as surface irregularities tear through the film. Lubricants significantly reduce the area of metallic seizure, primarily by lowering both the coefficient of friction and the shear strength at the junctions.
It has been studied that when any moving system has significant elastic freedom, this motion may become intermittent progressing via “stick-slip” behavior. This occurs because the kinetic friction during slipping is lower than the static friction during sticking. The type of sliding is strongly influenced by the properties of the metals and lubricant films, as well as the mechanical characteristics of the system, such as natural frequency, moment of inertia, and damping. Researchers have replicated these experiments, confirming that the elastic properties of the system play a key role in determining the motion. Stick-slip behavior is common in systems with elastic freedom or surfaces capable of slight elastic deformation, provided the surface and operating conditions are conducive to such motion.
In earlier experiments, Amontons’ Law was observed because changes in the load naturally caused proportional changes in the area of contact, preventing independent variation of these factors. This limitation can be overcome by using a hard steel substrate coated with a thin layer of soft material, such as indium. When a hemispherical slider is placed on this surface and different loads are applied, the slider sinks into the indium until the load is supported by the underlying steel. Since the steel deforms minimally under further increases in load, the contact area between the slider and the indium remains largely unaffected, enabling independent control of load and contact area.
[1] Bowden, F.P. and Tabor, D., 1942. Mechanism of metallic friction. Nature, 150(3798), pp.197-199.
[2] https://physics.aps.org/articles/v17/s120
I am a postgraduate researcher at the University of Leeds. I have completed my master's degree in the Erasmus Tribos program at the University of Leeds, University of Ljubljana, and University of Coimbra and my bachelor's degree in Mechanical Engineering from VTU in NMIT, India. I am an editor and social networking manager at TriboNet. I have a YouTube channel called Tribo Geek where I upload videos on travel, research life, and topics for master's and PhD students.
