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

15.3 Anti-friction materials
Elements of a friction pair made of antifriction materials have a positive gradient of mechanical properties, which is created during friction without lubrication due to the material's own components.
The main groups of antifriction materials are: metal, natural, rubber, plastics, carbon, metal-ceramic.
Metal materials are divided into anti-friction non-ferrous alloys and cast irons. Metal alloys are macroscopically homogeneous systems consisting of two or more metals (more rarely, metals and non-metals). The group of alloys includes the following materials.
Babbits are alloys based on tin or lead (with the addition of antimony, copper and other elements). They are named after the inventor I. Babbit. A thin layer of babbitt is applied to the working surface of the plain bearing shell. These bearings perform well when lubricated with oils. During start-stop periods (i.e. with poor lubrication), they have satisfactory tribotechnical characteristics due to the positive gradient of mechanical properties that the babbitt components soften during friction create. The use of babbits declined sharply after the industrial production of rolling bearings began in the 1930s. Currently, the main area of application of babbits is the rolling stock of railway transport.
Copper-based anti-friction alloys are bronzes and brasses.
Bronze is an alloy of copper with tin, aluminum, beryllium, lead, cadmium or chromium. Accordingly, bronze is called tin, beryl, lead, etc. Until now, bronze has been the main material for low-speed plain bearings. It ensures the performance of bearings even with poor lubrication.
Brass is an alloy of copper and zinc. Brass may contain additives (up to 10%) of aluminum, tin, manganese, etc., which increase its strength, corrosion resistance and casting properties. Brass is well processed by pressure. In friction pairs copper alloy - steel, a film of copper oxides is transferred to the surface of the steel part, which acts as a lubricant.
Aluminum alloys contain refractory (chromium, iron, manganese, silicon) and fusible (tin, antimony, lead, cadmium, magnesium) additives. The technology of processing such alloys into products that is optimal from the point of view of tribological engineering is as follows. The mixture of components is melted and granules are obtained by melt spraying. The latter have a fine-grained structure, because. the high cooling rate of the melt prevents the growth of large crystals of lead or tin. Then parts are made from granules by pressing in molds. Such parts work well with liquid lubrication, for example, in the bearings of tractor engines. Refractory additives give bearings strength, low-melting additives provide a positive gradient of mechanical properties when the oil layer breaks through. Advantages of aluminum alloys - low cost, strength, high thermal conductivity, disadvantages - significant thermal expansion and the ability to seize during friction without lubrication.
Anti-friction cast irons contain graphite, which has a certain structure: gray cast iron - lamellar particles, high-strength - spherical, malleable cast iron - flaky particles of graphite. Cast irons are cheaper than other antifriction materials, but inferior to them in terms of wear resistance and friction coefficient. The values of the latter correspond to the area between friction and antifriction materials (region 3 in Fig. 15.1). Cast iron is used in low-responsibility friction units operating at low loads and sliding speeds.
Natural anti-friction materials - wood and precious stones.
Wood is the tissue of higher plants. The advantage of wood is the continuous and rapid renewal of its resources in nature. In its pure form, wood as an anti-friction material is rarely used. It is usually impregnated with oils or suspensions containing self-lubricating components and then pressed to improve mechanical properties. It is advisable to place the friction surface at the end of the wood, where the channels of the capillary-porous system go. Lubricants flow through them to the friction surface.
There are three groups of wood-based anti-friction materials. The first includes wood impregnated with oils. Its technical resource is limited by the time of depletion of the oil supply. The materials of the second group are wood filled with oils containing suspended fine particles of graphite, molybdenum disulfide, and fluoroplast. During impregnation, the particles are unevenly distributed in the surface layer of the product, so its tribological characteristics depend on the degree of wear. The materials of the third group are obtained by impregnating wood with a mixture of oils and stearates of polyvalent metals. Stearates are salts of stearic acid and metals with variable valence (zinc, iron, copper, etc.).
Gemstones are minerals with special properties used for jewelry purposes. Diamond, ruby, sapphire, emerald are expensive anti-friction materials used for sliding bearings in watches, gyroscopes and other devices. In recent decades, the synthesis of crystals of these stones has been established, which makes it possible to reduce their cost. Synthetic diamonds have the trade names ballas, impregnite, megadaimon, etc. In terms of hardness, materials based on dense modifications of boron nitride - amborite, belbor, elbor, etc. approach them. The advantages of antifriction materials of this class: weak molecular interaction with the counterbody, small area of actual contact , high wear resistance with boundary lubrication.
Rubber based on a number of synthetic rubbers (siloxane, fluorosiloxane, fluoroelastomers, etc.) is an antifriction material that is used in liquid-lubricated moving seals. The state and departmental standards for seals regulate rubber groups (heat-resistant, oil- and petrol-resistant, vacuum, etc.), their performance and acceptable friction conditions.
A new type of anti-friction rubber is the so-called “slippery” rubber. The friction surface of the rubber part is covered with a layer of fluoroplastic and treated in a vacuum with high-energy particles. As a result, a thin fluoroplastic coating is formed on the friction surface, which is chemically bonded to the substrate. The part, while retaining the inherent elasticity of rubber, acquires a low coefficient of friction characteristic of PTFE.
Plastics are filled, reinforced or otherwise modified polymers. Pure polymers are rarely used in engineering, preferring plastics.
According to the characteristics of the processes accompanying the molding of products, plastics are divided into thermoplastics and thermoplastics. The processing of thermoplastics is accompanied by chemical reactions of the formation of a three-dimensional structure - curing, as a result of which the plastic loses the ability to pass into a viscous state. Thermoplastics do not lose this property during processing, they can be subjected to repeated remelting.
From thermoplastic antifriction plastics, we will consider materials based on polyamides, polyolefins, and fluoroplastics.
Polyamides are heterochain high-molecular compounds containing nitrogen in the main chain. The trade names for plastics based on polyamides are capron, nylon, caprolon, anid, etc. Their advantage is a combination of good technological properties, high strength and wear resistance. Plain bearing bushings are made from polyamides.
A typical representative of polyolefins is polyethylene, the main chain of which consists of carbon atoms, and side links - of hydrogen atoms. Depending on the polymerization conditions, polyethylene of various molecular weights is obtained. Parts of joint endoprostheses are made from super high molecular weight polyethylene. Polyethylene has high chemical resistance and low cost, but is less heat resistant than polyamides.
Fluoroplastics - consist of macromolecules in the form of a carbon chain, to which fluorine atoms are attached. Such macromolecules weakly interact with each other. Therefore, fluoroplastics are chemically inert, have a low coefficient of friction, and practically do not absorb moisture. The disadvantage of fluoroplastics is the tendency to creep - continuous plastic deformation under the action of constant stress. Because of this, they are rarely used in their pure form, preferring to fill them with powders of coke, graphite, nitrides, etc., and reinforce them with fibers, wire, nets, etc. Fluoroplastics are low-volatile and do not lose their antifriction properties in medium vacuum. Plastics based on fluoroplastics are indispensable for friction units operating in aggressive environments of the chemical industry. Bearing materials are widely used (Fig. 7.5) in the form of a steel tape (1) with a porous metal coating (bronze, titanium, stainless steel, etc.) (2) impregnated with fluoroplastic (3).