Km based on a metal matrix. Chapter xxiv. composite materials with a metal matrix. Reinforced laminated plastics

The powder filler is introduced into the matrix of the composite material in order to realize the properties inherent in the filler material in the functional properties of the composite. In powder composites, the matrix is mainly metals and polymers. Powder composites with a polymer matrix have the name "Plastics".

Metal matrix composites

Metal matrix composites. Powder composites with a metal matrix are obtained by cold or hot pressing of a mixture of matrix and filler powders, followed by sintering the resulting semi-finished product in an inert or reducing environment at temperatures of about 0.75 T pl metal matrix. Sometimes the pressing and sintering processes are combined. The technology for producing powder composites is called "Powder metallurgy". Powder metallurgy is used to produce cermets and alloys with special properties.

Cermets called composite materials with a metal matrix, the filler of which are dispersed particles of ceramics, such as carbides, oxides, borides, silicides, nitrides, etc. Cobalt, nickel and chromium are mainly used as the matrix. Cermets combine the hardness and heat resistance and heat resistance of ceramics with high viscosity and thermal conductivity of metals. Therefore, cermets, in contrast to ceramics, are less brittle and are able to withstand large temperature drops without destruction.

Cermets are most widely used in the production of metalworking tools. Powdered hard alloys are called tool cermets.

Powder filler of hard alloys are carbides or carbonitrides in an amount of 80% or more. Depending on the type of filler and the metal that serves as the matrix of the composite, powder hard alloys divided into four groups:

1) WC-Co - single-carbide type VK;
2) WC-TiC-Co - two-carbide type TK,
3) WC-TiC-TaC-Co - three-carbide type TTK;
4) TiC and TiCN- (Ni + Mo) - alloys based on carbide and titanium carbonitride - tungsten-free type TH and KNT.

VK alloys. Alloys are marked with the letters VK and a number showing the cobalt content. For example, the composition of the VK6 alloy is 94% WC and 6% Co. The heat resistance of VK alloys is about 900 ° C. Alloys of this group have the highest strength compared to other hard alloys.

TK alloys. Alloys are designated by a combination of letters and numbers. The number after T indicates the content of titanium carbide in the alloy, after K - cobalt. For example, the composition of the T15K6 alloy: TiC - 15%, Co - 6%, the rest, 79% - WC. The hardness of TK alloys due to the introduction of more than solid carbide titanium is higher than the hardness of the VK alloys. They also have an advantage in heat resistance - 1000 ° C, but their strength is lower with an equal cobalt content.

Alloys TTK (TT7K12, TT8K, TT20K9). The designation of TTK alloys is similar to TK. The number after the second letter T indicates the total content of TiC and TaC carbides.

With equal heat resistance (1000 ° C), TTK alloys surpass TK alloys with the same cobalt content both in hardness and strength. The greatest effect of alloying with tantalum carbide is manifested under cyclic loads - impact fatigue life increases up to 25 times. Therefore, tantalum-containing alloys are used mainly for severe cutting conditions with high power and temperature loads.

Alloys TN, KNT. These are tungsten-free hard alloys (BHTS) based on titanium carbide and carbonitride with nickel-molybdenum, not cobalt binder.

In terms of heat resistance, BVTS is inferior to tungsten-containing alloys, the heat resistance of BVTS does not exceed 800 ° C. Their strength and modulus of elasticity are also lower. The heat capacity and thermal conductivity of BVTS is lower than that of traditional alloys.

Despite the relatively low cost, the widespread use of BVTS for the manufacture of cutting tool problematic. It is most expedient to use tungsten-free alloys for the manufacture of measuring (gauge blocks, gauges) and drawing tools.

The metal matrix is also used to bind powder filler made of diamond and cubic boron nitride, which are united under the common name “ superhard materials"(STM). Composite materials filled with STM are used as processing tools.

The choice of matrix for diamond powder filler is limited by the low heat resistance of diamond. The matrix should provide a thermochemical mode of reliable bonding of diamond filler grains, excluding combustion or diamond graphitization. Tin bronzes are most widely used to bind diamond filler. Higher heat resistance and chemical inertness of boron nitride make it possible to use binders based on iron, cobalt, and hard alloy.

Tool with STM is made mainly in the form of circles, the processing of which is carried out by grinding the surface of the material to be processed with a rotating circle. Abrasive wheels based on diamond and boron nitride are widely used for sharpening and finishing cutting tools.

When comparing abrasive tools based on diamond and boron nitride, it should be noted that these two groups do not compete with each other, but have their own areas of rational use. This is determined by the differences in their physical, mechanical and chemical properties.

The advantages of diamond as a tool material over boron nitride include the fact that its thermal conductivity is higher and the coefficient of thermal expansion is lower. However, the decisive factor is the high diffusion capacity of diamond in relation to iron-based alloys - steels and cast irons and, on the contrary, the inertness of boron nitride to these materials.

At high temperatures, an active diffusion interaction of diamond with iron-based alloys is observed. At temperatures below os

The suitability of a diamond in air has temperature limitations. Diamond begins to oxidize at a noticeable rate at a temperature of 400 ° C. At higher temperatures, it burns with the release of carbon dioxide. It also limits the performance of diamond tools compared to cubic boron nitride tools. A noticeable oxidation of boron nitride in air is observed only after an hour's exposure at a temperature of 1200 ° C.

The temperature limit of the performance of diamond in an inert environment is limited by its transformation into a thermodynamically stable form of carbon - graphite, which begins when heated to 1000 ° C.

Another wide area of application of cermets is their use as a high-temperature structural material for new technology objects.

The service properties of powder composites with a metal matrix are mainly determined by the properties of the filler. Therefore, for powder composite materials with a special property, the most common classification is by field of application.

As a matrix, non-ferrous metals are often used - aluminum, magnesium, nickel, or their alloys. The structure of composites depends on the filler used. There are the following types of structures (Fig. 1):

granular (Fig. 1, a);

fibrous (Fig. 1, b);

layered with continuous laying of filler fibers (Fig. 1, v);

tissue (Fig. 1, G);

volumetric (Fig. 1, d).

Fig. 1. Structural diagrams of composite materials with a metal matrix:

a- grainy; b- fibrous; v- layered with continuous laying of fibers;

G- fabric; d- with bulk packing of fibers

V fiber composites the filler is a hardener. If the ratio of the length of the fiber to its diameter L/ d= 10 ... 10 3, then fiber composites are called discrete... Discrete fibers are randomly located in the matrix, and, the greater the value of the ratio L/ d, the higher the degree of hardening. If L/ d→ ∞, then the composites will be continuous fiber.

For aluminum and magnesium fiber composites, boron, carbon fibers, from silicon carbides, as well as from carbides, nitrides and oxides of refractory metals and high-strength steel are used. For the reinforcement of titanium and its alloys, molybdenum wire, sapphire fibers, silicon carbide, and titanium boride are used. For high-temperature nickel fiber composites, tungsten or molybdenum wire fibers are used.

Fiber composites differ from conventional alloys with high strength characteristics, reduced tendency to crack formation and high specific strength. Their strength is determined by the properties of the fibers. The matrix holds the fibers together and distributes the stresses between them. Moreover, the mechanical properties of fiber composites along the fibers are much better than in the transverse direction.

Fibrous composites are low-plastic, however, the rate of crack propagation in them is so low that the possibility of their sudden destruction is practically excluded. Another feature of them is the low rate of softening over time. The disadvantage of such composites is also a relatively low resistance to interlayer shear, but this resistance is much higher in fibrous composites with bulk packing of fibers.

In contrast to fibrous materials, in dispersion-strengthened composites, the matrix is the basis that receives the load, while the dispersed particles, which are fillers, inhibit the movement of dislocations in the matrix. The most optimal is the particle size 10 ... 15 nm and the distance between them 100 ... 150 nm with a uniform distribution of particles. Such composites can be obtained on the basis of almost all metals and alloys used in technology, for example, SAP - sintered aluminum powder. In SAP, the matrix is aluminum, and the filler is small particles of aluminum oxide Al 2 O 3 (6 - 8%). With an increase in the Al 2 O 3 content, the tensile strength of SAP increases and its relative elongation decreases.

1.2. Non-metallic matrix composites

The following materials are often used as a matrix for such composites:

polymer (epoxy, phenol-formaldehyde, polyamide and other resins),

carbonaceous,

ceramic.

Fibers serve as hardeners:

glass,

carbon,

organic,
based on whiskers (oxides, borides, carbides, nitrides),
metal wire.

The properties of composites depend on the composition of the composition, the combination of components, and the strength of the bonds between them. The properties of the matrix determine the shear and compressive strength of the composite and the resistance to fatigue wear. The properties of the hardener are mainly determined by the strength and stiffness of the material.

In the laboratory work, we will consider profiled products made of reinforced fiberglass.

Experts predict: over time, this material will replace both expensive metal and rotting wood. It has no analogues and differs from traditional materials in higher properties and qualities:

improved physical and mechanical characteristics,

low specific gravity (4 times lighter than steel),

high corrosion and biological resistance,

high resistance to weathering, ultraviolet radiation and water environment,

low thermal coefficient of linear expansion,

wide range of operating temperatures from -60 to +80 0 С,

seismic resistance - 100% elastic recovery after deformation, resistance to wind loads at wind speeds up to 300 km / h,

Applications: construction, housing and communal services, consumer goods, energy, medicine, chemical industry etc.

Pultruded fiberglass is a unique composite material of the 21st century with a service life of at least 50 years.

Composite materials consist of a metal matrix (more often Al, Mg, Ni and their alloys), reinforced with high-strength fibers (fibrous materials) or fine-dispersed refractory particles that do not dissolve in the base metal (dispersion-hardened materials). The metal matrix binds the fibers (dispersed particles) into a single whole. Fiber (dispersed particles) plus a binder (matrix) that make up

Rice. 196. Diagram of the structure (a) and continuous fiber reinforcement (b) of composite materials: 1 - granular (dispersion-hardened) material (l / d = 1); 2 - discrete fibrous composite material; 3 - continuously fibrous composite material; 4 - continuous laying of fibers; 5 - two-dimensional laying of fibers; 6.7 - bulk packing of fibers

or another composition, called composite materials (Fig. 196).

Fibrous composite materials. In fig. 196 shows the schemes of reinforcement of fibrous composite materials. Composite materials with a fibrous filler (hardener), according to the mechanism of reinforcing action, are divided into discrete ones, in which the ratio of fiber length to diameter and with continuous fiber, in which discrete fibers are randomly located in the matrix. The diameter of the fibers is from fractions to hundreds of micrometers. How more attitude the length to the diameter of the fiber, the higher the degree of hardening.

Often the composite material is a layered structure in which each layer is reinforced with a large number of parallel continuous fibers. Each layer can also be reinforced with continuous fibers woven into a fabric, which is the original shape, in width and length corresponding to the final material. Often, the fibers are woven into three-dimensional structures.

Composite materials differ from conventional alloys in higher values of ultimate tensile strength and endurance limit (by 50-100%), elastic modulus, stiffness coefficient () and a reduced tendency to crack formation. The use of composite materials increases the rigidity of the structure while reducing its metal consumption.

Table 44 (see scan) Mechanical properties of metal-based composites

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 elastic modulus of the fibers must be significantly greater than the strength and elastic modulus of the matrix. Rigid reinforcing fibers absorb the stresses arising in the composition during loading, impart strength and rigidity to it in the direction of fiber orientation.

To strengthen aluminum, magnesium and their alloys, boron and carbon fibers are used, as well as fibers from refractory compounds (carbides, nitrides, borides and oxides) with high strength and modulus of elasticity. So, silicon carbide fibers with a diameter of 100 microns are often used as fibers of high-strength steel wire.

For the reinforcement of 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 is required. Promising hardeners for high-strength and high-modulus fibrous composite materials are whiskers of aluminum oxide and nitride, silicon carbide and nitride, boron carbide, etc.

Table 44 shows the properties of some fibrous composites.

Composite materials on a metal basis have high strength and heat resistance, at the same time they are low plastic. However, fibers in composites reduce the rate of propagation of cracks originating in the matrix, and almost completely eliminate sudden

Rice. 197. Dependence of the modulus of elasticity E (a) and ultimate resistance (b) of boron-aluminum composite material along (1) and across (2) the axis of reinforcement on the volumetric content of boron fiber

fragile destruction. A distinctive feature of uniaxial fibrous composite materials is the anisotropy of mechanical properties along and across the fibers and low sensitivity to stress concentrators.

In fig. 197 shows the dependence and E of boron-aluminum composite material on the content of boron fiber along (1) and across the reinforcement axis. The higher the volumetric content of fibers, the higher and E along the axis of the reinforcement. However, it should be borne in mind that the matrix can transfer stresses to the fibers only when there is a strong bond at the reinforcing fiber-matrix interface. To prevent contact between the fibers, the matrix must completely surround all the fibers, which is achieved with a content of at least 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 6 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 in time (Fig. 198, a) with increasing temperature.

Rice. 198. Long-term strength of boron-aluminum composite material containing 50% boron fiber in comparison with the strength of titanium alloys (a) and long-term strength of nickel composite material in comparison with the strength of precipitation-hardening alloys (b): 1 - boron-aluminum composite; 2 - titanium alloy; 3 - dispersion-strengthened composite material; 4 - dispersion-hardening alloys

The main disadvantage of composite materials with one- and two-dimensional reinforcement is low resistance to interlayer shear and transverse breakage. Materials in bulk reinforcement are devoid of this drawback.

Dispersion-hardened composite materials. In contrast to fibrous composite materials, in dispersion-hardened composite materials, the matrix is the main element that carries the load, and dispersed particles inhibit the movement of dislocations in it. High strength is achieved with a particle size of 10-500 nm with an average distance between them of 100-500 nm and their uniform distribution in the matrix. The strength and heat resistance, depending on the volumetric content of the strengthening phases, do not obey the law of additivity. The optimal content of the second phase for different metals is not the same, but usually does not exceed

The use as strengthening phases of stable refractory compounds (oxides of thorium, hafnium, yttrium, complex compounds of oxides and rare earth metals), which do not dissolve in the matrix metal, allows maintaining the high strength of the material up to. In this regard, such materials are often used as heat-resistant materials. Dispersion-hardened composite materials can be obtained on the basis of most metals and alloys used in technology.

The most widely used alloys based on aluminum - SAP (sintered aluminum powder). SAP consists of aluminum and dispersed flakes Particles effectively inhibit the movement of dislocations and thereby increase strength

alloy. The content in SAP varies from and to. As the content increases, it rises from 300 to for and the elongation, correspondingly, decreases from 8 to 3%. The density of these materials is equal to the density of aluminum, they are not inferior to it in corrosion resistance and can even replace titanium and corrosion-resistant steels when operating in the temperature range. In long-term strength, they are superior to wrought aluminum alloys. Long-term strength for alloys at is

Nickel dispersion-hardened materials have great prospects. Nickel-based alloys with 2-3 vol. thorium dioxide or hafnium dioxide. The matrix of these alloys is usually a solid solution. Alloys (nickel hardened with thorium dioxide), (nickel hardened with hafnium dioxide) and (matrix hardened with thorium oxide) are widely used. These alloys have high heat resistance. At temperature, the alloy has an alloy Dispersion-hardened composite materials, as well as fibrous ones, are resistant to softening with increasing temperature and duration of exposure at a given temperature (see Fig. 198).

The fields of application of composite materials are not limited. They are used in aviation for highly loaded aircraft parts (skins, spars, ribs, panels, etc.) and engines (compressor and turbine blades, etc.), in space technology for units of power structures of vehicles exposed to heating, for elements stiffness, panels, in the automotive industry to lighten bodies, springs, frames, body panels, bumpers, etc., in the mining industry (drilling tools, parts for combines, etc.), in civil engineering (bridge spans, prefabricated elements high-rise buildings, etc.) and in other areas of the national economy.

The use of composite materials provides a new qualitative leap in increasing the power of engines, power and transport installations, and reducing the weight of machines and devices.

The technology for producing semi-finished products and products from composite materials is well developed.

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 μm 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 a sintered aluminum billet in the form of finished products that can be subjected to additional heat treatment.

SAP 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. They are used to make piston rods, compressor blades, shells of fuel elements and heat exchanger tubes.

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 material service.

Reinforcement with short fibers is carried out by powder metallurgy methods, consisting of pressing followed by hydroextrusion or rolling of billets. When reinforcing sandwich-type compositions with continuous fibers, 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 "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 stiffness. Compositions are obtained with beryllium wire by diffusion welding of packages of alternating layers of beryllium wire and matrix sheets. From aluminum alloys, reinforced with steel and beryllium wires, rocket body parts and fuel tanks are made.

In the composition "aluminum - carbon fibers" the combination of low density of reinforcement and matrix allows creating composite materials with high specific strength and rigidity. The disadvantage of carbon fibers is their fragility and high reactivity. Composition "aluminum - carbon" is obtained by impregnation of carbon fibers with liquid metal or by powder metallurgy methods. Technologically, it is most simply feasible to pull bundles of carbon fibers through an aluminum melt.

The aluminum-carbon composite is used in the designs of the fuel tanks of modern fighters. Due to the high specific strength and rigidity of the material, the mass of the fuel tanks is reduced by
thirty %. This material is also used for the manufacture of turbine blades for aircraft gas turbine engines.

SAP 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. They are used to make piston rods, compressor blades, shells of fuel elements and heat exchanger tubes.

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 material service.

A very promising material is the aluminum-beryllium wire composition, which realizes the high physical and mechanical properties of the beryllium reinforcement and, first of all, its low density and high specific stiffness. Compositions are obtained with beryllium wire by diffusion welding of packages of alternating layers of beryllium wire and matrix sheets. From aluminum alloys, reinforced with steel and beryllium wires, rocket body parts and fuel tanks are made.

In the composition "aluminum - carbon fibers" the combination of low density of reinforcement and matrix allows creating 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 an aluminum melt.

The aluminum-carbon composite is used in the construction of the fuel tanks of modern fighters. Due to the high specific strength and rigidity of the material, the mass of the fuel tanks is reduced by 30%. This material is also used for the manufacture of turbine blades for aircraft gas turbine engines.

Non-metallic matrix composites

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 widespread are epoxy, phenol-formaldehyde, and polyamide. Carbon matrices are coked or obtained from synthetic polymers subjected to pyrolysis (decomposition, decomposition). The matrix binds the composition, giving it shape. Strengtheners are fibers: glass, carbon, boric, organic, based on filamentary crystals (oxides, carbides, borides, nitrides, etc.), as well as metal (wires) with 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 elastic modulus of the fibers, the higher the strength and stiffness of the composite material. The properties of the matrix determine the shear and compressive strength of the composition and the resistance to fatigue failure.

By the type of hardener, composite materials are classified into fiberglass, carbon fiber with carbon fiber, boron fiber and organic fiber.

In layered materials, fibers, threads, tapes impregnated with a binder are laid parallel to each other in the laying plane. Plane layers are collected into plates. The properties are obtained anisotropic. For the material to work 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. It is possible to lay the fibers at different angles by varying the properties of the composites. The bending and torsional stiffnesses of the material depend on the order of stacking the layers along the thickness of the package.

The laying of hardeners of three, four or more strands is used (Fig. 7). The structure of three mutually perpendicular threads has the greatest application. Strengtheners 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 strength compared to layered fabrics. The four-strand system is constructed by placing the hardener along the diagonals of the cube. The structure of four strands is in equilibrium, has increased shear rigidity in the main planes. However, creating four directional materials is more difficult than creating three directional materials.

Rice. 7. Scheme of reinforcement of composite materials: 1- rectangular, 2-hexagonal, 3- oblique, 4- with bent fibers, 5 - a system of n strands

Antifriction materials based on polytetrafluoroethylene (PTFE) are the most effective from the point of view of use in the harshest conditions of dry friction.

PTFE is characterized by a fairly high static coefficient of friction; however, in the process of sliding friction, a very thin layer of highly oriented polymer forms on the surface of PTFE, contributing to the equalization of the static and dynamic coefficients of friction and smooth motion during sliding. When the sliding direction changes, the presence of an oriented surface film causes a temporary increase in the friction coefficient, the value of which decreases again as the surface layer is reoriented. This friction behavior of PTFE 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 friction rates. In this case, unfilled PTFE is characterized by high values of the coefficient of friction and wear rate. Composite materials, most often based on PTFE, have found wide application as materials for non-lubricated bearings operating under such conditions.

The simplest way to reduce the relatively high wear rate of PTFE during dry friction is to introduce powdered fillers. This increases the compressive creep resistance and a significant increase in dry friction wear resistance 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, without the use of polymeric materials, it is impossible to provide the required functional characteristics and performance of special products and machines. High manufacturability and low energy consumption of technologies for processing plastics into products in combination with the above-mentioned advantages of PCM make them very promising materials for machine parts for various purposes.