Composite materials consist of a metal matrix(usually A1, Mg, Ni and their alloys), hardened with high-strength fibers (fibrous materials) or finely dispersed refractory particles, insoluble in the base metal (dispersion-strengthened materials). The metal matrix binds the fibers (dispersed particles) into a single whole. Fiber (dispersed particles) plus a bunch (matrix) that make up that
Rice. one
1 - granular (dispersion-strengthened) material (l/d-I): 2 - discrete fibrous composite material; 3 - continuously fibrous composite material; 4 - continuous laying of fibers; 5 - two-dimensional fiber stacking; 6,7 - volumetric laying of fibers
or another composition, received the name composite materials(Fig. 196).
Fibrous composite materials.
On fig. 196 shows the scheme of reinforcement of fibrous composite materials. Composite materials with a fibrous filler (reinforcer) are divided into discrete ones, in which the ratio of fiber length to diameter is l/d ≈ 10-tL03, and with continuous fiber, in which l/d = co. Discrete fibers are randomly arranged in the matrix. The diameter of the fibers is from fractions to hundreds of micrometers. The greater the ratio of length to diameter of the fiber, the higher the degree of strengthening.
Often a composite material is a layered structure in which each layer is reinforced a large number parallel continuous fibers. Each layer can also be reinforced with continuous fibers woven into a fabric, which is the original shape, corresponding in width and length to the final material. It is not uncommon for fibers to be woven into three-dimensional structures.
Composite materials differ from conventional alloys in higher values of tensile strength and endurance limit (by 50-100%), elasticity modulus, stiffness coefficient (Ely) and reduced susceptibility to cracking. The use of composite materials increases the rigidity of the structure while reducing its metal consumption.
Table 44
Mechanical properties of composite materials on metal base
The strength of composite (fibrous) materials is determined by the properties of the fibers; the matrix should mainly redistribute the stresses between the reinforcing elements. Therefore, the strength and modulus of elasticity of the fibers must be significantly greater than the strength and modulus of elasticity of the matrix. Rigid reinforcing fibers perceive the stresses arising in the composition under loading, give it strength and rigidity in the direction of fiber orientation.
To strengthen aluminum, magnesium and their alloys, boron is used (o in \u003d 2500 - * -3500 MPa, E = 38h-420 GPa) and carbon (st in = 1400-g-3500 MPa, E 160-450 GPa) fibers, as well as fibers from refractory compounds (carbides, nitrides, borides and oxides) with high strength and elastic modulus. So, silicon carbide fibers with a diameter of 100 μm have st in = 2500-*m3500 MPa, E= 450 GPa. Often, high-strength steel wire is used as fibers.
To reinforce titanium and its alloys, molybdenum wire, sapphire fibers, silicon carbide and titanium boride are used.
An increase in the heat resistance of nickel alloys is achieved by reinforcing them with tungsten or molybdenum wire. Metal fibers are also used in cases where high thermal and electrical conductivity are required. Promising hardeners for high-strength and high-modulus fibrous composite materials are whiskers made of aluminum oxide and nitride, silicon carbide and nitride, boron carbide, etc., having a b = 15000-g-28000 MPa and E= 400-*-600 GPa.
In table. 44 shows the properties of some fibrous composite materials.
Composite materials based on a metal have high strength (st in, a_ x) and heat resistance, at the same time they have low plasticity. However, fibers in composite materials reduce the rate of propagation of cracks initiating in the matrix and almost completely eliminate sudden
Rice. 197. Dependence of the modulus of elasticity E (a) and temporary resistance o in (b) boron-aluminum composite material along (/) and across (2) reinforcement axis on the volume content of boron fiber
brittle fracture. Anisotropy is a distinctive feature of uniaxial fibrous composite materials. mechanical properties along and across the fibers and low sensitivity to stress concentrators.
On fig. 197 shows the dependence and in and E boron-aluminum composite material from the content of boron fiber along (/) and across ( 2 ) reinforcement axis. The higher the volume content of fibers, the higher a b, a_ t and E along the reinforcement axis. However, it must be taken into account that the matrix can transfer stresses to the fibers only when there is a strong bond at the interface between the reinforcing fiber and the matrix. To prevent contact between the fibers, the matrix must completely surround all the fibers, which is achieved when its content is not less than 15-20%.
The matrix and the fiber should not interact with each other (there should be no mutual diffusion) during manufacture or operation, as this can lead to a decrease in the strength of the composite material.
The anisotropy of the properties of fibrous composite materials is taken into account when designing parts to optimize properties by matching the resistance field with stress fields.
Reinforcement of aluminum, magnesium and titanium alloys with continuous refractory fibers of boron, silicon carbide, titanium diboride and aluminum oxide significantly increases the heat resistance. A feature of composite materials is the low rate of softening over time (Fig. 198, a) with an increase in temperature.
Rice. 198. Long-term strength of a boron-aluminum composite material containing 50% boron fiber, in comparison with the strength of titanium alloys (a) and long-term strength of a nickel composite material in comparison with the strength of precipitation hardening alloys (b):
/ - boron-aluminum composite; 2 - titanium alloy; 3 - dispersion-strengthened composite material; 4 - precipitation hardening alloys
The main disadvantage of composite materials with one- and two-dimensional reinforcement is the low resistance to interlaminar shear and transverse shear. This shortcoming is deprived of materials in bulk reinforcement.
- Polymer, ceramic and other matrices are widely used.
GENERAL CHARACTERISTICS AND CLASSIFICATION
Traditionally used metallic and non-metallic materials have largely reached their structural strength limit. At the same time, the development of modern technology requires the creation of materials that work reliably in a complex combination of force and temperature fields, under the influence of aggressive media, radiation, deep vacuum and high pressures. Often, the requirements for materials can be contradictory. This problem can be solved by using composite materials.
composite material(CM) or composite is called a bulk heterogeneous system consisting of mutually insoluble components that differ greatly in properties, the structure of which allows you to use the advantages of each of them.
Man borrowed the principle of construction of CM from nature. Typical composite materials are tree trunks, plant stems, human and animal bones.
CMs make it possible to have a given combination of heterogeneous properties: high specific strength and rigidity, heat resistance, wear resistance, heat-shielding properties, etc. The spectrum of CM properties cannot be obtained using conventional materials. Their use makes it possible to create previously inaccessible, fundamentally new designs.
Thanks to CM, a new qualitative leap has become possible in increasing engine power, reducing the mass of machines and structures, and increasing the weight efficiency of vehicles and aerospace vehicles.
Important characteristics of materials operating under these conditions are specific strength σ in /ρ and specific stiffness E/ρ, where σ in - temporary resistance, E is the modulus of normal elasticity, ρ is the density of the material.
High-strength alloys, as a rule, have low ductility, high sensitivity to stress concentrators, and relatively low resistance to fatigue crack development. Although composite materials may also have low ductility, they are much less sensitive to stress concentrators and better resist fatigue failure. This is due to the different mechanism of crack formation in high-strength steels and alloys. In high-strength steels, a crack, having reached a critical size, then develops at a progressive rate.
In composite materials, another mechanism operates. The crack, moving in the matrix, encounters an obstacle at the matrix-fiber interface. Fibers inhibit the development of cracks, and their presence in the plastic matrix leads to an increase in fracture toughness.
Thus, the composite system combines two opposite properties required for structural materials - high strength due to high-strength fibers and sufficient fracture toughness due to the plastic matrix and the fracture energy dissipation mechanism.
CMs consist of a relatively plastic matrix material-base and harder and stronger components that are fillers. The properties of CM depend on the properties of the base, fillers and the strength of the bond between them.
The matrix binds the composition into a monolith, gives it a shape and serves to transfer external loads to reinforcement from fillers. Depending on the base material, CMs are distinguished with a metal matrix, or metal composite materials (MCM), with a polymer - polymer composite materials (PCM) and with a ceramic - ceramic composite materials (CMC).
The leading role in the strengthening of CMs is played by fillers, often referred to as hardeners. They have high strength, hardness and modulus of elasticity. According to the type of reinforcing fillers, CMs are divided into dispersion-strengthened,fibrous and layered(Fig. 28.2).
Rice. 28.2. Schemes of the structure of composite materials: a) dispersion-strengthened; b) fibrous; in) layered
Fine, uniformly distributed refractory particles of carbides, oxides, nitrides, etc., which do not interact with the matrix and do not dissolve in it up to the melting point of the phases, are artificially introduced into dispersion-hardened CMs. The smaller the filler particles and the smaller the distance between them, the stronger the CM. Unlike fibrous, in dispersion-strengthened CMs, the main bearing element is the matrix. The ensemble of dispersed filler particles strengthens the material due to the resistance to the movement of dislocations under loading, which hinders plastic deformation. Effective resistance to dislocation motion is created up to the melting temperature of the matrix, due to which dispersion-strengthened CMs are characterized by high heat resistance and creep resistance.
Reinforcement in fibrous CM can be fibers of various shapes: threads, tapes, meshes of various weaves. Reinforcement of fibrous CM can be carried out according to a uniaxial, biaxial and triaxial scheme (Fig. 28.3, a).
The strength and stiffness of such materials is determined by the properties of the reinforcing fibers that take the main load. Reinforcement gives a greater increase in strength, but dispersion hardening is technologically easier to implement.
Layered composite materials (Fig. 28.3, b) are made up of alternating layers of filler and matrix material (sandwich type). The filler layers in such CMs can have different orientations. It is possible to alternately use layers of filler from different materials with different mechanical properties. For layered compositions, non-metallic materials are usually used.
Rice. 28.3. Fibrous reinforcement schemes ( a) and layered ( b) composite materials
DISPERSION-HARDENED COMPOSITE MATERIALS
During dispersion strengthening, the particles block the sliding processes in the matrix. The effectiveness of hardening, under the condition of minimal interaction with the matrix, depends on the type of particles, their volume concentration, as well as the uniformity of distribution in the matrix. Apply dispersed particles of refractory phases such as Al 2 O 3 , SiO 2 , BN, SiC, having a low density and a high modulus of elasticity. CM is usually produced by powder metallurgy, an important advantage of which is the isotropy of properties in different directions.
In industry, dispersion-strengthened CMs on aluminum and, more rarely, nickel bases are usually used. Characteristic representatives of this type of composite materials are materials of the SAP type (sintered aluminum powder), which consist of an aluminum matrix reinforced with dispersed particles of aluminum oxide. Aluminum powder is obtained by spraying molten metal, followed by grinding in ball mills to a size of about 1 micron in the presence of oxygen. With an increase in the duration of grinding, the powder becomes finer and the content of aluminum oxide in it increases. Further technology for the production of products and semi-finished products from SAP includes cold pressing, pre-sintering, hot pressing, rolling or extrusion of sintered aluminum billet into the form of finished products that can be subjected to additional heat treatment.
Alloys of the SAP type are satisfactorily deformed in the hot state, and alloys with 6–9% Al 2 O 3 are also deformed at room temperature. From them, cold drawing can be used to obtain foil with a thickness of up to 0.03 mm. These materials are well machined and have high corrosion resistance.
SAP grades used in Russia contain 6–23% Al 2 O 3 . SAP-1 is distinguished with a content of 6-9, SAP-2 - with 9-13, SAP-3 - with 13-18% Al 2 O 3. With an increase in the volume concentration of aluminum oxide, the strength of composite materials increases. At room temperature, the strength characteristics of SAP-1 are as follows: σ in = 280 MPa, σ 0.2 = 220 MPa; SAP-3 are as follows: σ in \u003d 420 MPa, σ 0.2 \u003d 340 MPa.
SAP type materials have high heat resistance and outperform all wrought aluminum alloys. Even at a temperature of 500 °C, their σ is not less than 60–110 MPa. Heat resistance is explained by the retarding effect of dispersed particles on the recrystallization process. The strength characteristics of SAP-type alloys are very stable. Long-term strength tests of SAP-3 type alloys for 2 years had practically no effect on the level of properties both at room temperature and when heated to 500 °C. At 400 °C, SAP strength is 5 times higher than that of aging aluminum alloys.
SAP type alloys are used in aviation technology for the manufacture of parts with high specific strength and corrosion resistance, operating at temperatures up to 300–500 °C. Piston rods, compressor blades, shells of fuel elements and heat exchanger tubes are made from them.
CM is obtained by powder metallurgy using dispersed particles of silicon carbide SiC. The chemical compound SiC has a number of positive properties: high melting point (more than 2650 ° C), high strength (about 2000 MPa) and elastic modulus (> 450 GPa), low density (3200 kg / m 3) and good corrosion resistance. The production of abrasive silicon powders has been mastered by the industry.
Powders of aluminum alloy and SiC are mixed, subjected to preliminary compaction under low pressure, then hot pressing in steel containers in vacuum at the melting temperature of the matrix alloy, i.e., in a solid-liquid state. The resulting workpiece is subjected to secondary deformation in order to obtain semi-finished products of the required shape and size: sheets, rods, profiles, etc.
Composite materials consist of a metal matrix (more often Al, Mg, Ni and their alloys) reinforced with high-strength fibers (fibrous materials) or finely dispersed refractory particles that do not dissolve in the base metal (dispersion-strengthened materials). The metal matrix binds the fibers (dispersed particles) into a single whole. Fiber (dispersed particles) plus a binder (matrix) that make up a particular composition are called composite materials.
Composite materials with non-metallic matrix
Composite materials with a non-metallic matrix have found wide application. As non-metallic matrices, polymer, carbon and ceramic materials. Of the polymer matrices, the most widely used are epoxy, phenol-formaldehyde and polyamide.
Carbon matrices coked or pyrocarbon obtained from synthetic polymers subjected to pyrolysis. The matrix binds the composition, giving it form. Strengtheners are fibers: glass, carbon, boron, organic, based on whiskers (oxides, carbides, borides, nitrides, and others), as well as metal (wires), which have high strength and rigidity.
The properties of composite materials depend on the composition of the components, their combination, quantitative ratio and bond strength between them.
Reinforcing materials can be in the form of fibers, tows, threads, tapes, multilayer fabrics.
The content of the hardener in oriented materials is 60-80 vol.%, in non-oriented (with discrete fibers and whiskers) - 20-30 vol.%. The higher the strength and modulus of elasticity of the fibers, the higher the strength and stiffness of the composite material. The properties of the matrix determine the strength of the composition in shear and compression and resistance to fatigue failure.
According to the type of hardener, composite materials are classified into glass fibers, carbon fibers with carbon fibers, boron fibers and organ fibers.
In laminated materials, fibers, threads, tapes impregnated with a binder are laid parallel to each other in the laying plane. Flat layers are assembled into plates. The properties are anisotropic. For the work of the material in the product, it is important to take into account the direction of the acting loads. You can create materials with both isotropic and anisotropic properties. You can lay the fibers at different angles, varying the properties of composite materials. The bending and torsional stiffness of the material depends on the order of laying the layers along the thickness of the package.
The laying of reinforcing elements of three, four or more threads is used.
The structure of three mutually perpendicular threads has the greatest application. Hardeners can be located in axial, radial and circumferential directions.
Three-dimensional materials can be of any thickness in the form of blocks, cylinders. Bulky fabrics increase peel strength and shear resistance compared to layered fabrics. A system of four strands is built by expanding the reinforcing agent along the diagonals of the cube. The structure of four threads is balanced, has increased shear rigidity in the main planes.
However, creating four directional materials is more difficult than creating three directional ones.
This type of composite materials includes materials such as SAP (sintered aluminum powder), which are aluminum reinforced with dispersed particles of aluminum oxide. Aluminum powder is obtained by spraying molten metal, followed by grinding in ball mills to a size of about 1 micron in the presence of oxygen. With an increase in the duration of grinding, the powder becomes finer and the content of aluminum oxide in it increases. Further technology for the production of products and semi-finished products from SAP includes cold pressing, pre-sintering, hot pressing, rolling or extrusion of sintered aluminum billet into the form of finished products that can be subjected to additional heat treatment.
SAP-type alloys are used in aviation technology for the manufacture of parts with high specific strength and corrosion resistance, operating at temperatures up to 300–500 °C. Piston rods, compressor blades, shells of fuel elements and heat exchanger tubes are made from them.
Reinforcement of aluminum and its alloys with steel wire increases their strength, increases the modulus of elasticity, fatigue resistance and extends the temperature range of the material.
Reinforcement with short fibers is carried out by powder metallurgy methods, consisting of pressing followed by hydroextrusion or rolling of blanks. When reinforced with continuous fibers of sandwich-type compositions consisting of alternating layers aluminum foil and fibers, rolling, hot pressing, explosion welding, diffusion welding are used.
A very promising material is the aluminum-beryllium wire composition, which realizes the high physical and mechanical properties of beryllium reinforcement and, first of all, its low density and high specific rigidity. Compositions with beryllium wire are obtained by diffusion welding of packages from alternating layers of beryllium wire and matrix sheets. Aluminum alloys reinforced with steel and beryllium wires are used to make rocket body parts and fuel tanks.
In the "aluminum - carbon fiber" composition, the combination of low density reinforcement and matrix makes it possible to create composite materials with high specific strength and rigidity. The disadvantage of carbon fibers is their fragility and high reactivity. The aluminum-carbon composition is obtained by impregnating carbon fibers with liquid metal or by powder metallurgy methods. Technologically, it is most simply feasible to pull bundles of carbon fibers through a melt of aluminum.
The aluminum-carbon composite is used in the design of the fuel tanks of modern fighters. Due to the high specific strength and rigidity of the material, the mass of fuel tanks is reduced by 30%. This material is also used for the manufacture of turbine blades for aircraft gas turbine engines.
Composite materials with non-metallic matrix
Composite materials with a non-metallic matrix are widely used in industry. Polymer, carbon and ceramic materials are used as non-metallic matrices. Of the polymer matrices, the most widely used are epoxy, phenol-formaldehyde, and polyamide. Carbon matrices are coked or obtained from synthetic polymers subjected to pyrolysis (decomposition, disintegration). The matrix binds the composition, giving it form. Strengtheners are fibers: glass, carbon, boron, organic, based on whiskers (oxides, carbides, borides, nitrides, etc.), as well as metal (wires), which have high strength and rigidity.
The properties of composite materials depend on the composition of the components, their combination, quantitative ratio and bond strength between them.
The content of the hardener in oriented materials is 60–80 vol. %, in non-oriented (with discrete fibers and whiskers) - 20 - 30 vol. %. The higher the strength and modulus of elasticity of the fibers, the higher the strength and stiffness of the composite material. The properties of the matrix determine the strength of the composition in shear and compression and resistance to fatigue failure.
According to the type of hardener, composite materials are classified into glass fibers, carbon fibers with carbon fibers, boron fibers and organ fibers.
In laminated materials, fibers, threads, tapes impregnated with a binder are laid parallel to each other in the laying plane. Planar layers are assembled into plates. The properties are anisotropic. For the work of the material in the product, it is important to take into account the direction of the acting loads. You can create materials with both isotropic and anisotropic properties. You can lay the fibers at different angles, varying the properties of composite materials. The bending and torsional stiffness of the material depends on the order of laying the layers along the thickness of the package.
The stacking of reinforcing elements of three, four or more threads is used (Fig. 7). The structure of three mutually perpendicular threads has the greatest application. Hardeners can be located in axial, radial and circumferential directions.
Three-dimensional materials can be of any thickness in the form of blocks, cylinders. Bulky fabrics increase peel strength and shear resistance compared to layered fabrics. A system of four strands is built by placing the reinforcement along the diagonals of the cube. The structure of four threads is balanced, has increased shear rigidity in the main planes. However, creating four directional materials is more difficult than creating three directional ones.
Rice. 7. Scheme of reinforcement of composite materials: 1 - rectangular, 2 - hexagonal, 3 - oblique, 4 - with curved fibers, 5 - a system of n threads
The most effective in terms of use in the most severe conditions of dry friction are anti-friction materials based on polytetrafluoroethylene (PTFE).
PTFE is characterized by a rather high static friction coefficient, however, during sliding friction, a very thin layer of highly oriented polymer is formed on the surface of PTFE, which helps to equalize the static and dynamic friction coefficients and smooth movement when sliding. When the direction of sliding is changed, the presence of an oriented surface film causes a temporary increase in the coefficient of friction, the value of which decreases again as the surface layer is reoriented. This behavior of PTFE under friction has led to its widespread use in industry, where unfilled PTFE is mainly used for the production of bearings. In many cases, non-lubricated bearings must operate at higher frictional speeds. At the same time, unfilled PTFE is characterized by high values of friction coefficient and wear rate. As materials for non-lubricated bearings operating in such conditions, composite materials, most often based on PTFE, have found wide application.
The simplest way to reduce the relatively high wear rate of PTFE during dry friction is the introduction of powdered fillers. In this case, the creep resistance under compression increases and a significant increase in wear resistance under dry friction is observed. The introduction of the optimal amount of filler makes it possible to increase the wear resistance up to 10 4 times.
Polymers and composite materials based on them have a unique set of physical and mechanical properties, due to which they successfully compete with traditional structural steels and alloys, and in some cases it is impossible to provide the required functional characteristics and performance of special products and machines without the use of polymeric materials. The high manufacturability and low energy consumption of technologies for processing plastics into products, combined with the above-mentioned advantages of PCM, make them very promising materials for machine parts for various purposes.
This type of composite materials includes materials such as SAP (sintered aluminum powder), which are aluminum reinforced with dispersed particles of aluminum oxide. Aluminum powder is obtained by spraying molten metal, followed by grinding in ball mills to a size of about 1 micron in the presence of oxygen. With an increase in the duration of grinding, the powder becomes finer and the content of aluminum oxide in it increases. Further technology for the production of products and semi-finished products from SAP includes cold pressing, pre-sintering, hot pressing, rolling or extrusion of sintered aluminum billet into the form of finished products that can be subjected to additional heat treatment.
SAP-type alloys are used in aviation technology for the manufacture of parts with high specific strength and corrosion resistance, operating at temperatures up to 300–500 °C. Piston rods, compressor blades, shells of fuel elements and heat exchanger tubes are made from them.
Reinforcement of aluminum and its alloys with steel wire increases their strength, increases the modulus of elasticity, fatigue resistance and extends the temperature range of the material.
Reinforcement with short fibers is carried out by powder metallurgy methods, consisting of pressing followed by hydroextrusion or rolling of blanks. When reinforcing with continuous fibers of sandwich-type compositions consisting of alternating layers of aluminum foil and fibers, rolling, hot pressing, explosion welding, and diffusion welding are used.
A very promising material is the composition "aluminum - beryllium wire", which implements the high physical and mechanical properties of beryllium reinforcement, and first of all, its low density and high specific rigidity. Compositions with beryllium wire are obtained by diffusion welding of packages from alternating layers of beryllium wire and matrix sheets. Aluminum alloys reinforced with steel and beryllium wires are used to make rocket body parts and fuel tanks.
In the composition "aluminum - carbon fibers" the combination of low density reinforcement and matrix allows you to create composite materials with high specific strength and rigidity. The disadvantage of carbon fibers is their fragility and high reactivity. The composition "aluminum - carbon" is obtained by impregnating carbon fibers with liquid metal or by powder metallurgy methods. Technologically, it is most simply feasible to pull bundles of carbon fibers through a melt of aluminum.
Composite "aluminum - carbon" is used in the design of the fuel tanks of modern fighters. Due to the high specific strength and rigidity of the material, the mass of fuel tanks is reduced by
thirty %. This material is also used for the manufacture of turbine blades for aircraft gas turbine engines.
