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

Rice. 14.2 Dependence of temperature change on load during running-in of steel-alloy pairs.
1-bronze BrOZO, 2-aluminum alloy AO20-1



Rice. 14.3 Change in wear rate of self-lubricating material over time.
I-friction of adsorbed films, II and III-transfer films in the solid and molten states, IV-friction with an unstable transfer film
During period I, the friction surfaces are covered with adsorbed films and oxides. Then, a transfer film is formed in the friction pair: first in the solid state (II), and later in the molten state (III). With intensive wear, it is removed from the friction zone (IV). After that, intensive wear of the pair occurs. The patterns of frictional transfer constitute the scientific basis for the development of self-lubricating materials and the design of friction pairs operating without lubrication.
Lecture number 15.
Materials used in rubbing pairs.
15.2 Friction materials
In order to give the friction material a positive gradient of mechanical properties, it is necessary to create a plastic “third body” in the friction zone, the shear resistance of which would be lower than that of the materials of rubbing parts. This idea can be implemented in two ways:
1) introduce into the friction material components that form plastic layers on the friction surface - metal oxides, solid lubricants, non-ferrous metals, etc.;
2) use as friction materials such materials that soften or thermally decompose under the action of shear stresses and frictional heating.
We will consider examples of such solutions by analyzing the range of friction materials. It includes metal, asbestos-rubber materials, asboplastics, organic and cermet materials. The counterbody for them is usually steels with a high carbon content and alloyed heat-treated cast irons.
Metallic friction materials are predominantly gray cast irons in pairs with high-carbon steels U7, U8. Cast iron is an alloy of iron with carbon, the content of which is more than 2.14%. Wear products of gray cast iron create a “third body” in the friction zone with low shear resistance. The disadvantage of such pairs - a low coefficient of friction - is covered by such advantages as low cost, reliability, and long service life. A pair of gray cast iron - high-carbon steel is used in the brakes of rolling stock in railway transport.
Asbestos -rubber materials are composite materials based on asbestos with a rubber binder and various fillers. Asbestos is a mineral, hydrous magnesium silicate (3MgO 2SiO ₂ 2H ₂ O). It has a fibrous structure, high heat resistance (up to 900K) and a coefficient of friction that does not depend much on temperature. Rubber binds the components into a single material and, like asbestos, has a significant coefficient of friction. Red lead (Pb ₃ O ₄ ) and lead oxide (PbO) powders are used as fillers , which give the material extreme pressure properties.
Asbestos-rubber materials do not conduct heat well, therefore, in pairs with their participation, the “third body” is formed during friction rather quickly. The disadvantages of such materials are due to the low heat resistance of the binder: as the temperature rises, the friction coefficient decreases significantly due to the softening of the rubber, and at 640-670K the material degrades and crumbles. Therefore, asbestos-rubber materials are used for relatively light braking conditions, for example, in passenger cars.
Asboplastics contain a binder based on synthetic polymers, mainly those that form spatial networks during curing due to cross chemical bonds between adjacent macromolecules. During frictional heating, they do not melt, but decomposing at 660-680K, they form a coke film, the pores of which are filled with liquid decomposition products of the binder. The coefficient of friction of such materials passes through a minimum (670K) and increases with increasing temperature.
One of the best asboplastics is retinax , a composite based on asbestos and phenol-formaldehyde resin. It operates up to temperatures T 1300K, at which the counterbody material usually softens and is transferred to the harder coke. To overcome this shortcoming, retinax brake pads are reinforced with brass wire. At T 900K, brass is smeared on the counterbody, creating an intermediate layer in the friction zone. Retinaks are used in heavily loaded brakes of drawworks and aircraft wheels. The use of expensive retinaxes at T 700K is ineffective.
Organic friction materials (wood, leather, cork, etc.) are most often used for light loads, mainly in devices. This class of materials also includes rubber, which is formed during the vulcanization of rubbers. Products from it are obtained by processing the so-called rubber mixture, which contains, in addition to rubber and vulcanizing agents, fillers, stabilizers, plasticizers and other components. Belt drives and friction safety devices based on rubber have a wide range of applications from agricultural machines to hydraulic presses. The main disadvantages of this class of materials are high cost and low heat resistance.
Metal-ceramic materials (cermets) are composite materials consisting of ceramic and metal phases. They are processed into products by powder metallurgy methods, mainly by pressing in molds followed by sintering. The content of ceramic phases (borides, carbides, nitrides, oxides) in friction cermets exceeds 40%, the rest is metals (iron, cobalt, molybdenum, nickel, etc.). Sometimes additives are introduced into the powder composition that form a protective film on the friction surface - graphite, molybdenum disulfide, lead. Combining ductile metals and strong refractory compounds in cermets, it is possible to obtain friction materials that are distinguished by high heat resistance, wear resistance, strength and hardness. Their heat capacity and thermal conductivity are higher than those of asbestos materials. The disadvantages of cermets as friction materials are the tendency to seize and the decrease in the coefficient of friction with increasing temperature. Nevertheless, they are operable up to 900K, and the volumes of their use in tribological engineering are increasing every year.