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

Figure 14.1 Scheme of a set of tribotechnical materials .
1 - antifriction, 2 - friction, 3 - materials used in different units as antifriction and friction
Creation concept tribotechnical materials is based on the concepts of the molecular-mechanical theory of friction about the two components of the friction coefficient. As a result of severe plastic deformation during wear, the microstructure of these layers becomes similar to the structure of strongly deformed materials (compression with torsion, rolling, etc.), differing from the initial microstructure in the bulk of the bodies. Given this, I.V. Kragelsky proposed to consider the friction zone as an intermediate " third body ".
The deformations that occur during the friction of bodies made of tribotechnical materials should be localized in the surface layer and not affect the main volume of the bodies. The surface layers included in the "third body" should be low-modulus (with elastic contact) or plastic (with plastic contact) as compared to the materials of abraded bodies and have the property of being repeatedly deformed without noticeable damage. The totality of these requirements, which distinguish the "third body" from the base material, is reduced to the need to create a positive gradient of mechanical properties in the contact. These considerations apply to both antifriction and friction materials.
The purpose of friction materials is to provide a high coefficient of friction. To do this, their molecules must interact intensively with the counterbody. In order to fully realize this property, it is necessary to provide the maximum possible area of actual contact in the friction pair. This will increase the adhesive component of the coefficient of friction. At the same time, the compliance under load of the surface layer, which contributes to the formation of areas of actual contact, will cause an increase in the deformation component of the friction coefficient. The hysteresis loss coefficient of the friction material should be as large as possible ( 1). This corresponds to a significant energy consumption for the re-deformation of microprotrusions and a delay in restoring their shape.
Substances that weaken the molecular interaction with the counterbody are introduced into the composition of antifriction materials with a low coefficient of friction. The actual area of contact of the antifriction material with the counterbody should be small and, if possible, be formed due to the elastic deformation of microroughnesses. This condition is met if the material has sufficient hardness and a high modulus of elasticity. One should strive to reduce the relative penetration of microroughness and hysteresis losses ( 1).
The combination of materials in a friction pair must meet the compatibility criteria. Compatibility - the property of materials to ensure optimal performance of a friction pair under specified operating conditions. Lubricant compatibility - the ability of materials to mix with each other without compromising performance and storage stability. In non-lubricated friction, the main criterion for material compatibility is the prevention of entrapment. With mixed friction, compatibility is most fully manifested during the periods of running-in, start-up and stop of friction pairs. On fig. 15.2 it can be seen that the compatibility with the counterbody of lead bronze is lower than that of aluminum alloy. Running-in on the verge of seizing allows you to determine the load capacity of the friction pair. Compatibility in fluid friction is evaluated by the degree of corrosion damage of abrasive bodies in lubricants.
In recent decades, one of the leading trends in ensuring compatibility has been the application of coatings on friction surfaces. Interest in them is stimulated by the increasing requirements for wear resistance, bearing capacity of friction pairs and energy consumption during friction, as well as the need to save strategic and expensive materials.
A number of friction pairs with unique tribological characteristics were obtained due to the formation of thermodynamically unstable phases in the form of coatings. Examples are coatings obtained by laser remelting of the surface layer, created by vacuum, ion-plasma methods, etc.
Self-lubricating materials have the property of forming an extreme pressure release film. The main role in the "dry" friction in the mechanism of self-lubrication is played by the phenomenon of frictional transfer - the movement of the material during friction from one friction surface to another. The kinetics of frictional transfer film formation for most self-lubricating materials includes the following steps (Figure 14.3).