O’zbekistоn respublikasi оliy va o’rta maxsus ta`lim vazirligi

7.2 Fatigue wear


Fatigue wear occurs during repeated deformation in the process of friction of microvolumes of the material of the surface layer. A characteristic sign of fatigue wear is the appearance of a network of surface and subsurface microcracks, which, when combined, contribute to the formation of wear particles. As a result, on the friction surfaces, defects are formed in the form of pits, pockmarks, shells with a diameter of tens of micrometers to several millimeters (called pitting ).
In the presence of stress concentrators, the process of fatigue wear is accelerated. Stress concentrators include surface defects in the form of scratches, dents, scratches, burns, as well as non-metallic inclusions, micropores, chemical compounds, etc.
The formation of microcracks under the action of cyclically changing contact stresses is due to the accumulation of damage. For crystalline bodies, at first, the accumulation of elastic lattice distortions and the increase in the dislocation density occur. After reaching a certain critical density of dislocations, submicroscopic cracks are formed. Along with irreversible distortions of the crystal lattice, interatomic bonds and individual microvolumes of the material are destroyed.
One of the main causes of fatigue wear of materials based on polymers, in contrast to metals, is the tribocracking of polymer molecules, which occurs due to intense mechanical and thermal effects on actual contact spots. The confirmation of the occurrence of tribocracking is the decrease in the molecular weight of the polymer in the surface layer and the amorphization of the structure of the crystallizing polymers.
Fatigue wear mainly occurs in well-lubricated mechanisms, the interfaces of which are operated in the kinematic rolling, sliding or rolling with slipping mode. Gears, rolling bearings, cam gears, etc. are most susceptible to fatigue wear.
An increase in resistance to fatigue wear is achieved by increasing the physical and mechanical characteristics of the near-surface and surface layers of the material, as well as by creating the optimal microgeometry of friction surfaces.