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

Lecture number 7.
The main indicators of the wear process.
7.1 Adhesive wear
Adhesive wear occurs as a result of the interaction of friction surfaces, with the formation of adhesive bonds between them. In local zones of actual contact, under the action of normal, tangential forces and temperature, lubricating layers, films of substances and oxides adsorbed on the surface are destroyed. Molecular (adhesive) interaction on local areas of clean surfaces exceeds the strength of the bond between the individual elements of the material. The resulting adhesive bond is stronger than the less durable material of the pair, and its surface layer undergoes cohesive destruction.
The result of the adhesive interaction of rubbing bodies is the destruction of the subsurface layers of materials, the frictional transfer of wear particles. At a certain intensity of adhesive interaction, jamming, the occurrence of "cold welding", is possible.
The magnitude of the adhesive interaction depends on the features of the electronic structure of the contacting bodies, the concentration of valence electrons, the density of free electrons at the interface, the number of crystal lattice defects, the presence of polar groups in polymeric materials, and other factors.
At present, there is no theory of adhesion that allows predicting the force of adhesive interaction with sufficient accuracy by calculation. Therefore, to estimate the magnitude of adhesive wear, approximate calculated dependences are used.
According to the Archard model, the value of adhesive wear V depends on the normal load N, which determines the actual contact area, which is subject to elastic stresses and deformations:
, (7.1)

where K is the coefficient of adhesive wear; H is the hardness of the softer material; S - friction path; N - normal load.

The amount of wear depends not only on the load, the hardness of the materials and the friction path, but also on the topography of the surface, the operating temperatures, the physical and mechanical properties of the near-surface layers, the composition of the lubricants and the environment.
Adhesive wear occurs when there is a lack or absence of lubricant in friction units and manifests itself in sliding or rolling bearings, gears, cam mechanisms, and during the machining of materials.
Friction units of space technology, vacuum equipment, nuclear technology and highly loaded interfaces operating in vacuum, gaseous and liquid aggressive media are most susceptible to adhesive wear.
To reduce adhesive wear, alloys based on metals with a hexagonal crystal structure and dense packing of atoms are used. Some alloying elements such as carbon and sulfur also reduce the adhesive wear of steels.