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16.4 Friction and fatigue resistance
Fatigue of a material is a change in the physical and mechanical properties of a material as a result of the action of stresses that change cyclically over time. The property of materials to resist fatigue is called endurance . Fatigue resistance is characterized by a limit endurance - the highest stress ( ) that a material can withstand without destruction for a given number (N) of cyclic impacts (Fig. 16.3).

Figure 16.3 Typical metal fatigue curve
Fatigue failure of parts usually begins with the surface layer, so its change during friction affects the endurance of parts. If friction reduces the roughness of the part and hardens the surface layer (hardening), the endurance of the part increases. Friction may be accompanied by scratching surfaces, seizing and tearing , i.e. rejection of irregularly shaped material conglomerates from the surface layer. This leads to the appearance of stress concentrators on the friction surface and to a decrease in the endurance of parts.
The Rehbinder effect is a decrease in the strength of solids in an adsorption-active medium due to physical and chemical interactions in the surface layer. P. A. Rebinder (1898-1972) discovered it in 1928. The Rehbinder effect significantly changes the nature of the influence of friction on the endurance of parts. Adsorption-active media contain surface-active substances (surfactants). They are intensively adsorbed on the surface of a solid, reducing its surface energy. Many lubricants contain organic surfactants - fatty acids, alcohols, soaps. When they interact with the friction surface, the surface layer is weakened at a depth of up to 0.1 µm. It is caused by the adsorption of surfactants on the areas of the freshly formed ( juvenile ) surface that appear during friction . As a result, the microhardness and yield strength of the material of the surface layer decrease, which affects the wear and endurance of abraded parts.