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Fig.3. Typical types of wear of sliding surfaces:
a - chipping, b - peeling, c - corrosion, d - erosion, e - scratches, e - scuffing, g - sticking, h - deep tearing of the material and its transfer from another friction surface of the rial, leading to the formation of pits on the friction surface (Fig. 3, a). The destruction of the working surfaces of the teeth due to intense chipping (Fig. 3, b) is often called flaking (there is a separation from the friction surface of the material in the form of scales).
On fig. 8.3, c shows a surface destroyed by corrosion. The surface of the cast iron powder ring (Fig. 8.3, d) is damaged due to erosion wear, which occurs when the piston moves in the cylinder relative to the liquid; gas bubbles in the liquid burst close to the piston surface, which creates a local increase in pressure or temperature and causes wear on parts. On the surface of the brake drum (Fig. 8.3, e) shows the risks that appear when a solid body or solid particles are exposed to a rotating drum. Seizures (Fig. 8.3, e) are formed as a result of seizing surfaces during friction due to the action of molecular forces between them. On fig. 8.3, g shows the working surface of the part with foreign particles adhering to it, and in Fig.8. 3, h - the surface of the part with wear during jamming as a result of setting - deep tearing of the material and its transfer from another friction surface.


Lecture number 9.
Wear of non-metallic surfaces.
9.1 Basic provisions of the theory of friction of polymeric materials
Polymers are natural and synthetic compounds, the molecules of which, as their name implies (poly - many, measure - part), consist of a large number of repeating atomic groups of the same or different structure, interconnected by chemical or coordination bonds into long linear or branched chains.
A group of atoms that can be used to describe the structure of a polymer is called a compound unit. A compound link that repeats many times is called a repeating compound link, and groups at the ends of the chain are called end groups. A polymer molecule consisting of repeating units and end groups is called a macromolecule.
Modern composite materials based on polymers make it possible to increase the service life and reliability of machines, while providing significant material benefits and economic benefits.
Until recently, the widespread use of polymeric materials in mechanical engineering was hindered by two seemingly generally recognized shortcomings of polymers: their low (compared to grade steels) strength and low heat resistance. The boundary of the strength properties of polymeric materials was overcome by the transition to composite materials, mainly glass and carbon fiber.
Polymeric materials in their pure form are practically not used for the manufacture of parts of friction units. This is due to their high coefficient of surface friction of parts made of them, insufficient thermal and heat resistance, and low wear resistance. To improve the performance of polymeric materials, new binders with the required characteristics are being developed, materials produced in large tonnages are modified with functional additives, and friction surfaces are treated by special methods.
The composition of the polymer composite depends on specific economic, design, technological and operational requirements. For example, for polymer plain bearings in automobiles and agricultural machines, economic (cost, availability of raw materials) and technological (methods of processing into products, the possibility of regenerating technological defects) aspects come to the fore. When using polymer structures in single samples of equipment, especially those operated in extreme conditions, of course, operational and design requirements are of greater importance - given physical and mechanical properties, thermal and heat resistance, etc.
The study of the mechanism of friction and wear of polymers showed that the nature of the contacting materials, load-speed and thermal modes of friction, lubrication conditions, and topography of friction surfaces have the most significant effect on the friction characteristics. The operation of the friction unit largely depends on the temperature and composition of the environment, the presence of an abrasive, and the impact of aggressive and corrosive media.
To reduce the coefficient of friction and increase the wear resistance of the material, from 0.1 to 40% of dry lubricants - graphite, metal sulfides, salts of higher acids, talc, mica, etc. are usually introduced into the composition of the binder. Such substances have the ability to form easily moving layers on friction surfaces. In recent years, the method of increasing the frictional properties of polymeric materials by introducing liquid-phase lubricants and lubricating oils into their composition has become widespread .
An important point in the design of new polymers is the study of the influence of the monomer effect and the chemical structure of the polymer on such properties as "adhesion" (cohesion), "bonding stress", water permeability, dielectric and optical characteristics. Subsequent surface treatments are necessary to ensure good adhesion between the components, and lubricants, "lubricants", may be used to facilitate the processing of the polymer. They are substances which, when added in small amounts, provide a significant reduction in the movement of polymer chains or segments.
Let's give some examples. Replacing an aluminum alloy with graphite plastic in the manufacture of an aircraft slat makes it possible to reduce the number of parts from 47 to 14, fasteners from 1464 to 8 bolts, reduce weight by 22%, and cost by 25%. At the same time, the safety margin of the product is 178%.
According to the world's leading machine builders, almost all functional parts of brake systems for cars and about 45% for railway rolling stock are made from synthetic press materials. Approximately 50% of rotating parts and gears are made from durable engineering polymers. In the latter case, two distinct trends can be noted. On the one hand, there are more and more reports about the production of gear wheels for tractors from kapron. These gears can run almost wear-free in contact with steel, in addition, such a system does not need lubrication and is almost silent. Another trend is the complete replacement of metal parts in gearboxes with carbon fiber parts. For them, too, there is a sharp decrease in mechanical losses and an increase in service life.
Another area of application of polymeric materials in mechanical engineering, worthy of special mention, is the manufacture of metal-cutting tools. As the areas of use of strong steels and alloys expand, more and more stringent requirements are placed on the machining tool. And here, too, plastics come to the rescue. But not quite ordinary ultra-hard plastics, but those that can compete even with diamond.
Another example of polymer additives. The dynamic development of sports, peak loads at the goal or at the net lead to the fact that the grass does not have time to grow from one competition to another. And no tricks of gardeners can cope with this. Synthetic materials, such as polyamide film, can come to the rescue. A film 25 µm thick is cut into strips 1.27 mm wide, stretched, crimped, and then interlaced so as to obtain a light bulk mass imitating grass. Flame retardants are added to the polymer to prevent fire. Synthetic grass mats are glued onto the prepared base - and the grass court, football field or other sports ground is ready for use, and as it wears out, certain sections of the playing field can be replaced with new surfaces.