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

9.3 Basic laws of friction and wear mechanism of polymers
Polymer-based materials are more sensitive than metals to numerous factors caused by friction and the influence of the external environment.
Extensive studies of changes in the coefficient of friction and the nature of sliding during friction of plastics on steel and plastics of the same name without lubrication and lubricated with polar and non-polar oils show that a lubricating medium (boundary lubrication), for example, a non-polar liquid (diffusion oil), reduces the total friction force due to a decrease in area direct contact of rubbing surfaces. However, as a result of the introduction under the influence of the load of the irregularities of the steel surface into the polymer material and the corresponding deformation of its surface layers during sliding in certain areas, the lubricant is displaced. In this case, if the plastic has high adhesion to steel, at the moment of formation of adhesion bridges, the adhesion force can reach high values, which leads to an abrupt change in the friction force.
If the polymer material has low adhesion to steel, then the non-polar liquid reduces the total value of the friction force, mainly due to the facilitation of the process of plastic deformation of the surface layer of the polymer by the microroughness of the steel surface. In this case, the friction process proceeds more stably. When lubricated with oil with a polar additive, which forms strong oriented boundary layers on the steel surface, direct contact between the plastic and steel becomes more difficult, and friction occurs between the boundary layer of oil on the steel and the plastic surface. Possible single ruptures of the boundary layer with the formation of adhesive bridges do not have a significant effect on the nature of friction.
For polymers, due to their specific properties, the effect of lubricant on the coefficient of friction is less effective than for metals, and the traditional use of lubricant often does not lead to the expected results.
Frictional interaction is accompanied by wear - a characteristic surface destruction of materials or a residual change in the shape of the body surface, occurring due to mechanical contact action.
The external manifestations of the wear of polymeric materials are largely similar to the surface phenomena that occur during the wear of metals. This served as the basis for many researchers to apply theories and classifications known from the experience of studying the wear resistance of metals to polymers. Typically, the wear of polymeric materials occurs due to strong adhesive interaction, fatigue, macroshear, abrasive, thermal and thermal-oxidative effects, corrosion, cavitation, etc.
The mechanical, physical and chemical interactions of the contacting surfaces, the destruction and recombination of molecules and structural formations occurring in the process of frictional interaction lead to a change in the properties of the boundary layers and their destruction.
Consider the main types of wear of polymeric materials. adhesive wear. Under certain combinations of materials and operating conditions, wear of polymeric materials is observed, the mechanism of which is based on frictional transfer. In this case, in the local zones of actual contact, intense molecular (adhesive) interaction occurs, the forces of which exceed the strength of the bond between the individual elements of supramolecular formations or polymer molecules located in the stress-strain friction zone. In the process of a limited number of acts of frictional interaction, surface destruction of the material occurs, the products of which form a "third body" - more or less stable sections of the film (individual fragments of materials, molecules, etc.), which in most cases, as a result of further frictional action, are dispersed and are carried away from the friction zone. This process can be repeated many times.
Friction transfer has a favorable effect on the wear rate of crystallizing polymers with a linear structure that are prone to plastic deformation. Rigid amorphous polymers form transfer layers poorly and wear intensively under friction without lubrication.