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

18.5 Technological methods
Technological methods for increasing wear resistance are aimed at creating friction surfaces, the structure and properties of which ensure minimal wear and the impossibility of seizing friction pairs.
The choice of the type of processing and the roughness of the friction surface is made with the following in mind. Regardless of the initial roughness of the friction surface, by the end of the running-in, an equilibrium long-term roughness is established, which is characteristic of a given friction pair. If it were possible to immediately process the friction surface to the equilibrium roughness, the running-in would not be needed. However, this is almost impossible to do. Therefore, the finishing operation of processing is assigned so that the roughness of the friction surface is close to equilibrium. If it differs from the equilibrium by at least two classes, regardless of whether it is cleaner or coarser, the wear resistance of the pair is significantly reduced. Coarsely rough friction surfaces are in contact on small areas where high pressures are localized. This increases wear and the likelihood of seizing. Smooth surfaces tend to absorb grease poorly and are prone to seizing.
The operational properties of friction pairs are affected by the direction of the traces of processing of friction surfaces . The following regularities have been established for boundary friction. The least wear occurs for surfaces with low roughness - when the directions of the traces of processing and movement are mutually perpendicular, for coarsely rough surfaces - when these directions coincide. The smallest coefficient of friction is observed if the traces of processing on the friction surfaces are located at an angle to each other, the largest - when they are parallel and directed perpendicular to the movement.
The effect of waviness on the performance of a friction pair is ambiguous. First, the contact of wavy surfaces is characterized by the concentration of stresses on small areas. Therefore, unlubricated wavy friction surfaces are more prone to seizing than flat ones. Secondly, on wavy surfaces in the depressions between the waves, lubricants are better retained. In addition, waviness impairs the performance of rolling units, increasing vibration. When sliding, lubricated wavy friction surfaces work better than smooth ones.
Special technologies for processing friction surfaces provide for plastic deformation of parts without chip removal. These are processing with shot, rolling with a roller, diamond tools, etc. The result is: 1) smoothing of microroughnesses, 2) plastic deformation and hardening of the surface layer, 3) “healing” of microcracks and other surface defects that initiate the destruction of the friction surface.
Coating - a thin layer of material adhesively bonded to the surface of the body, different from the material of the body. The application of coatings on friction surfaces makes it possible to solve two groups of problems: to realize a positive gradient of mechanical properties in a friction pair and to increase the reliability of pairs under severe operating conditions.
In tribotechnics, the method of frictional copper plating, brassing and bronzing of steel parts is used. It is based on the phenomenon of copper transfer during friction. Previously, the rotaprint lubrication method was mentioned, with the help of which a small amount of solid lubricant is fed into the friction zone, forming a film of graphite, fluoroplast, MoS _{2 on the friction surface} .
Chemical-thermal treatment of friction surfaces of metal parts consists in heat treatment of parts in chemically active media at elevated temperatures. It is carried out in order to change the chemical composition and properties of the surface layer. The main types of chemical-thermal treatment are oxidation, phosphating, carburizing, nitriding, diffusion metallization processes.
Oxidation is the artificial formation of an oxide film on the surface of metal parts.
Phosphating is a chemical process of formation of a thin layer of phosphate salts on the surface of steel, aluminum and zinc products.
With the help of carburizing, cyanidation, nitriding, diffusion metallization, it is possible to increase the strength of the surface layer, increasing the wear resistance of pairs when working in abrasive environments.