Non-ferrous metals are often used as a matrix - 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. Schemes of structures of composite materials with a metal matrix:
A- granular; b- fibrous; V- layered with continuous laying of fibers;
G- tissue; d- with volumetric fiber laying
IN fiber composites filler is a hardener. If the ratio of the fiber length to its diameter L/ d= 10…10 3 , then fibrous composites are called discrete. Discrete fibers are arranged randomly in the matrix, and the greater the ratio L/ d, the higher the degree of hardening. If L/ d→ ∞, then the composites will be with a continuous fiber.
For aluminum and magnesium fibrous composites, boron, carbon, silicon carbide fibers, as well as carbides, nitrides and oxides of refractory metals and high-strength steel are used. To reinforce titanium and its alloys, molybdenum wire, sapphire fibers, silicon carbide, and titanium boride are used. For heat-resistant nickel fiber composites, tungsten or molybdenum wire fibers are used.
Fibrous composites differ from conventional alloys in their high strength properties, 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 stresses between them. At the same time, the mechanical properties of fibrous composites along the fibers are much better than in the transverse direction.
Fibrous composites have low plasticity, however, the rate of propagation of cracks 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 in time. The disadvantage of such composites is also a relatively low resistance to interlaminar shear, however, this resistance is much higher in fiber composites with bulk fiber stacking.
In contrast to fibrous materials, in dispersion-strengthened composites, the matrix is the basis that perceives the load, while the dispersed particles, which are the filler, inhibit the movement of dislocations in the matrix. The most optimal is the particle size of 10...15 nm and the distance between them 100...150 nm with a uniform distribution of particles. Similar composites can be obtained on the basis of almost all metals and alloys used in engineering, 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. Composites with a non-metallic matrix
The following materials are often used as a matrix for such composites:
polymeric (epoxy, phenol-formaldehyde, polyamide and other resins),
carbonaceous,
ceramic.
Fibers serve as reinforcers:
organic,
based on whiskers (oxides, borides, carbides, nitrides),
metal wire.
glass,
carbon,
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 strength of the composite in shear and compression, resistance to fatigue wear. The properties of the hardener determine mainly the strength and stiffness of the material.
In the laboratory work, we will consider profiled products made of reinforced fiberglass.
Experts predict that 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 atmospheric influences, ultraviolet radiation and the aquatic environment,
low thermal coefficient of linear expansion,
wide operating temperature range 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 based on a metal matrix
According to the structure and geometry of reinforcement, composites based on a metal matrix are presented in the form of fibrous (MVKM), dispersion-hardened (DKM), pseudo- and eutectic alloys (EKM), and such metals as Al, Mg, Ti, Ni, Co are most widely used as the base material.
Properties and methods for obtaining MVKM based on aluminum. MVKM Al-steel fibers. When obtaining CMs consisting of alternating layers of aluminum foil and fibers, rolling, dynamic hot pressing, explosion welding, and diffusion welding are most often used. The strength of this type of composite is mainly determined by the strength of the fibers. The introduction of high-strength steel wires into the matrix increases the endurance limit of the composite.
MVKM Al-silica fibers are obtained by passing the fibers through the matrix melt, followed by hot pressing. The creep rate of these MVCMs at temperatures of 473-573 K is two orders of magnitude lower than the creep of an unreinforced matrix. Composites Al - SiO 2 have good damping ability.
MVKM Al-boron fibers are among the most promising structural materials, since they have high strength and rigidity at temperatures up to 673-773 K. Diffusion welding is widely used in the manufacture. Liquid-phase methods (impregnation, various types of casting, etc.), due to the possibility of chemical interaction of boron with aluminum, are used only in cases where protective coatings are previously applied to the boron fibers - silicon carbide (boron fibers) or boron nitride.
MVKM Al-carbon fibers have high strength and stiffness at low density. At the same time, a big disadvantage of carbon fibers is their lack of technology associated with the fragility of the fibers and their high reactivity. Usually MVKM Al - carbon fibers are obtained by impregnation with liquid metal or by powder metallurgy. Impregnation is used for reinforcement with continuous fibers, and powder metallurgy methods for reinforcement with discrete fibers.
Properties and methods for obtaining MVKM based on magnesium. The use of magnesium and magnesium alloys as a matrix reinforced with high-strength and high-modulus fibers makes it possible to obtain lightweight structural materials with increased specific strength, heat resistance, and modulus of elasticity.
MVKM Mg-boron fibers are characterized by high strength properties. For the manufacture of MKM, methods of impregnation and casting can be used. Mg – B sheet compositions are produced by diffusion welding. The disadvantage of MKM Mg - B is a reduced corrosion resistance.
MVKM Mg-carbon fibers are obtained by impregnation or hot pressing in the presence of a liquid phase; there is no solubility of carbon in magnesium. To improve the wetting of carbon fibers with liquid magnesium, they are pre-coated with titanium (by plasma or vacuum deposition), nickel (electrolytically), or a combined Ni-B coating (chemical deposition).
Properties and methods of obtaining MVKM based on titanium. Reinforcement of titanium and its alloys increases rigidity and extends the operating temperature range up to 973-1073 K. Metal wires, as well as silicon and boron carbide fibers, are used to reinforce the titanium matrix. Composites based on titanium with metal fibers are obtained by rolling, dynamic hot pressing and explosion welding.
MVKM Ti – Mo (fibers) is obtained by dynamic hot pressing of ʼʼsandwichʼʼ blanks in evacuated containers. Such reinforcement allows to increase the long-term strength compared to the matrix and maintain strength at high temperatures. One of the disadvantages of Ti-Mo MVKM is its high density, which reduces the specific strength of these materials.
MVCM Ti – B, SiC (fibers) have increased not only absolute, but also specific characteristics of MVCM based on titanium. Since these fibers are brittle, vacuum diffusion welding is most often used to obtain compact compositions. Long-term holding of Ti – B MVKM at temperatures above 1073 K under pressure leads to the formation of brittle titanium borides, which weaken the composite. Silicon carbide fibers are more stable in the matrix. Ti-B composites have high short-term and long-term strength. To increase the thermal stability of boron fibers, they are coated with silicon carbide (borsik). Ti-SiC composites have high values of off-axis creep strength.
In the Ti-Be MVKM system (fibers), there is no interaction at temperatures below 973 K. Above this temperature, the formation of a brittle intermetallic compound is possible, while the strength of the fibers remains practically unchanged.
Properties and methods for obtaining MVKM based on nickel and cobalt. The existing types of hardening of industrial nickel alloys (dispersed hardening, carbide hardening, complex alloying and thermomechanical treatment) make it possible to maintain their performance only up to the temperature range of 1223-1323 K. For this reason, it was important to create nickel MVKM reinforced with fibers and capable of operating for a long time at higher temperatures. The following hardeners are used:
In the Ni-Al 2 O 3 MVKM system (fibers), when heated in air, nickel oxide is formed, which interacts with the reinforcement, due to which NiAl 2 O 4 spinel is formed at the interface. In this case, the connection between the components is broken. To increase the bond strength, thin coatings of metals (W, Ni, nichrome) and ceramics (yttrium and thorium oxides) are applied to the reinforcement. Since liquid nickel does not wet Al 2 O 3 , Ti, Zr, Cr are introduced into the matrix, which improve impregnation conditions.
At room temperature, the strength of the composite Nickel - Al 2 O 3 whiskers, obtained by electrodeposition of Nickel on the fibers, significantly exceeds the strength of the matrix.
MVKM Ni - C (fibers). Nickel is practically insoluble in carbon. In the Ni – C system, a metastable Ni 3 C carbide is formed, which is stable at temperatures above 1673 K and below 723 K. Having a high diffusion mobility, carbon saturates the nickel matrix in a short time; therefore, the main weakening factors in the Ni – C MVCM are the dissolution of carbon fibers and their recrystallization due to the penetration of nickel into the fiber. The introduction of carbide formers (Cr, Al, Ti, Mo, W, Nb) into the nickel matrix enhances the interaction of the matrix with fibers. To increase the structural stability, anti-diffusion barrier coatings of zirconium carbide, zirconium nitride, and titanium carbide are applied to the fibers.
MVKM N - W, Mo (fibers) are obtained by dynamic hot pressing, diffusion welding, explosion welding, rolling. Due to the fact that W, Mo are intensively oxidized when heated, the composites are obtained in a vacuum or a protective atmosphere. When MVKM is heated in air, the tungsten or molybdenum fibers located on the surface of the composite are oxidized. If the fibers do not come to the surface, then the heat resistance of MVKM is determined by the heat resistance of the matrix.
Areas of application of MVKM. Composite fibrous materials with a metal matrix are used at low, high and ultra-high temperatures, in aggressive environments, under static, cyclic shock, vibration and other loads. MVKM are most effectively used in structures, special conditions, the operation of which does not allow the use of traditional metallic materials. At the same time, most often, at present, by reinforcing metals with fibers, they seek to improve the properties of the matrix metal in order to increase the operating parameters of those structures in which unreinforced materials were previously used. The use of aluminum-based MVKM in aircraft structures, due to their high specific strength, makes it possible to achieve an important effect - weight reduction. Replacing traditional materials with MVKM in the basic parts and assemblies of aircraft, helicopters and spacecraft reduces the weight of the product by 20-60%.
The most urgent task in gas turbine construction is to increase the thermodynamic cycle of power plants. Even a small temperature increase in front of the turbine significantly increases the efficiency of a gas turbine engine. It is possible to ensure the operation of a gas turbine without cooling, or at least with cooling that does not require large structural complications of a gas turbine engine, by using high-temperature nickel and chromium-based MVCM reinforced with Al 2 O 3 fibers.
An aluminum alloy reinforced with glass fiber containing uranium oxide has increased strength at a temperature of 823 K and should be used as fuel plates for nuclear reactors in the power industry.
Fibrous metal composites are used as sealing materials. For example, static seals made of Mo or steel fibers impregnated with copper or silver withstand a pressure of 3200 MPa at a temperature of 923 K.
As a wear-resistant material in gearboxes, disc clutches, starting devices, MVKM reinforced with mustaches and fibers can be used. In hard magnetic materials reinforced with W-wire, it is possible to combine magnetic properties with high resistance to shock loads and vibrations. The introduction of W, Mo armature into a copper and silver matrix makes it possible to obtain wear-resistant electrical contacts designed for heavy-duty high-voltage circuit breakers, which combine high thermal and electrical conductivity with increased resistance to wear and erosion.
The principle of reinforcement can be used as the basis for the creation of superconductors, when a frame is created from fibers of alloys with superconductivity, for example, Nb - Sn, Nb - Zr, in matrices of Al, Cu, Ti, Ni. Such a superconducting composite can transmit current with a density of 10 5 -10 7 A/cm 2 .
Composite materials based on a metal matrix - concept and types. Classification and features of the category "Composite materials based on a metal matrix" 2017, 2018.
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; V) 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 phase melting temperature, 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, the strength of SAP is 5 times higher than the strength 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.
38.1. Classification
Composite materials are materials reinforced with fillers located in a certain way in the matrix. Fillers are most often substances with high energy of interatomic bonds, high strength and high modulus, however, in combination with brittle matrices, highly plastic fillers can also be used.
Binder components, or matrices, in composite materials can be different - polymeric, ceramic, metal or mixed. In the latter case, one speaks of polymaterial composite materials.
According to the morphology of the reinforcing phases, composite materials are divided into:
zero-dimensional (designation: 0,), or hardened by particles of different fineness, randomly distributed in the matrix;
one-dimensional fibrous (symbol: 1), or reinforced with unidirectional continuous or discrete fibers;
two-dimensional layered (symbol: 2), or containing equally oriented reinforcing lamellas or layers (Fig. 38.1).
The anisotropy of composite materials, "designed" in advance for the purpose of using it in appropriate structures, is called structural.
According to the size of the reinforcing phases or the size of the reinforcement cell, composite materials are divided as follows:
submicrocomposites (reinforcement cell size, fiber or particle diameter<С 1 мкм), например, дисперсноупрочненные сплавы или волокнистые композиционные материалы с очень тонкими волокнами:
microcomposites (reinforcement cell size, fiber diameter, particles or layer thickness ^1 µm), for example, materials reinforced with particles, fibers of carbon, silicon carbide, boron, etc., unidirectional eutectic alloys;
macrocomposites (diameter or thickness of reinforcing components -100 microns), for example parts made of copper or aluminum alloys, reinforced with tungsten or steel wire or foil. Macrocomposites are most often used to improve the wear resistance of friction parts in production tooling.
38.2. Interfacial interaction in composite materials
38.2.1. Physicochemical and thermomechanical compatibility of components
The combination in one material of substances that differ significantly in chemical composition and physical properties brings to the fore the problem of thermodynamic and kinetic compatibility of components in the development, manufacture and connection of composite materials. Under the germ
dynamic compatibility is understood as the ability of the matrix and reinforcing fillers to be in a state of thermodynamic equilibrium for an unlimited time at temperatures of production and operation. Almost all artificially created composite materials are thermodynamically incompatible. The only exceptions are a few metallic systems (Cu-W, Cu-Mo, Ag-W), where there is no chemical and diffusion interaction between the phases for an unlimited time of their contact.
Kinetic compatibility - the ability of the components of composite materials to maintain a metastable equilibrium in certain temperature-time intervals. The problem of kinetic compatibility has two aspects: 1) physical and chemical - ensuring a strong bond between the components and limiting the processes of dissolution, hetero- and reactive diffusion on the interfaces, which lead to the formation of brittle interaction products and degradation of the strength of the reinforcing phases and the composite material as a whole; 2) thermomechanical - achieving a favorable distribution of internal stresses of thermal and mechanical origin and reducing their level; ensuring a rational relationship between the strain hardening of the matrix and its ability to relax stresses, preventing overload and premature failure of the hardening phases.
There are the following possibilities for improving the physicochemical compatibility of metal matrices with reinforcing fillers:
I. Development of new types of reinforcing fillers that are resistant in contact with metal matrices at high temperatures, for example, ceramic fibers, whiskers and dispersed particles from silicon carbides, titanium, zirconium, boron, aluminum oxides, zirconium, silicon nitrides, boron, etc.
II Deposition of barrier coatings on reinforcing fillers, for example, coatings of refractory metals, titanium carbides, hafnium, boron, titanium nitrides, boron, yttrium oxides on carbon fibers, boron, silicon carbide. Some barrier coatings on fibers, mainly metallic ones, serve as a means of improving the wetting of fibers by matrix melts, which is especially important when obtaining composite materials by liquid-phase methods. Such coatings are often referred to as technological
No less important is the plasticization effect found during the application of technological coatings, which manifests itself in the stabilization and even increase in the strength of the fibers (for example, when boron fibers are aluminized by pulling through a melt bath or when carbon fibers are nickel-plated with subsequent heat treatment).
III. The use in composite materials of metal matrices alloyed with elements with a greater affinity for the reinforcing filler than the matrix metal, or with surface-active additives. The resulting change in the chemical composition of the interfaces should prevent the development of interfacial interaction. Alloying of matrix alloys with surface-active or carbide-forming additives, as well as the deposition of technological coatings on fibers, can improve the wettability of the reinforcing filler with metal melts.
IV. Alloying of the matrix with elements that increase the chemical potential of the reinforcing filler in the matrix alloy, or with additives of the reinforcing filler material to saturation concentrations at temperatures of obtaining or operating the composite material. Such doping prevents the dissolution of the reinforcing phase, i.e., increases the thermal stability of the composition.
V. Creation of "artificial" composite materials according to the type of "natural" eutectic compositions by choosing the appropriate composition of the components.
VI. The choice of optimal durations of contacting components in a particular process of obtaining composite materials or in their service conditions, i.e., taking into account temperature and force factors. The duration of contact, on the one hand, should be sufficient for the emergence of strong adhesive bonds between the components; on the other hand, it does not lead to intense chemical interaction, the formation of brittle intermediate phases, and a decrease in the strength of the composite material.
Thermomechanical compatibility of components in composite materials is ensured by:
selection of matrix alloys and fillers with a minimum difference in elastic moduli, Poisson's ratios, thermal expansion coefficients;
the use of intermediate layers and coatings and reinforcing phases, which reduce differences in the physical properties of the matrix and phases;
the transition from reinforcement with a component of one type to polyreinforced - iiu, i.e., a combination in one composite material of reinforcing fibers, particles or layers that differ in composition and physical properties;
changing the geometry of parts, scheme and scale of reinforcement; morphology, size and volume fraction of reinforcing phases; replacement of a continuous filler with a discrete one;
the choice of methods and modes of production of a composite material that provides a given level of bond strength of its components.
38.2.2. Reinforcing fillers
For the reinforcement of metal matrices, high-strength, high-modulus fillers are used - continuous and discrete metal, non-metal and ceramic fibers, short fibers and particles, whiskers (Table 38.1).
Carbon fibers are one of the most developed and promising reinforcing materials in production. An important advantage of carbon fibers is their low specific gravity, thermal conductivity close to that of metals (R=83.7 W/(m-K)), and relatively low cost.
The fibers are supplied in the form of smooth or twisted myogofilament bundles, fabrics or ribbons from them. Depending on the type of feedstock, the diameter of the filaments varies from 2 to 10 microns, the number of filameites in the bundle varies from hundreds to tens of thousands of pieces.
Carbon fibers have high chemical resistance to atmospheric conditions and mineral acids. The heat resistance of the fibers is low: the temperature of long-term operation in air does not exceed 300-400 °C. To increase chemical resistance in contact with metals, barrier coatings of titanium and zirconium borides, titanium carbides, zirconium, silicon, and refractory metals are applied to the surface of the fibers.
Boron fibers are obtained by deposition of boron from a gas mixture of hydrogen and boron trichloride on a tungsten wire or carbon monofilaments heated to a temperature of 1100-1200 ° C. When heated in air, boron fibers begin to oxidize at temperatures of 300-350 ° C, at 600-800 ° C they completely lose strength. Active interaction with most metals (Al, Mg, Ti, Fe, Ni) begins at temperatures of 400-600 °C. To increase the heat resistance of boron fibers, thin layers (2-6 μm) of silicon carbide (SiC / B / W), boron carbide (B4C / B / W), boron nitride (BN / B / W) are deposited in the gas-phase method
Silicon carbide fibers with a diameter of 100-200 microns are produced by deposition at 1300 ° C from a vapor-gas mixture of silicon tetrachloride and methane, diluted with hydrogen in a ratio of 1: 2: 10, and a tungsten wire
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Carbon fibers
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TABLE 38.2 ALLOYS USED AS MATRIX IN COMPOSITE MATERIALS
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or pitch carbon fibers. The best fiber samples have a strength of 3000-4000 MPa at 1100 °C
Coreless silicon carbide fibers in the form of multifilameite bundles, obtained from liquid organosilanes by drawing and pyrolysis, consist of ultrafine f)-SiC crystals.
Metal fibers are produced in the form of a wire with a diameter of 0.13; 0.25 and 0.5 mm. Fibers from high-strength steels and beryllium alloys are intended mainly for reinforcing matrices made of light alloys and titanium. Fibers from refractory metals alloyed with rhenium, titanium, oxide and carbide phases are used to harden heat-resistant and nickel-chromium, titanium and other alloys.
The whiskers used for reinforcement can be metal or ceramic. The structure of such crystals is single-crystal, the diameter is usually up to 10 microns with a length-to-diameter ratio of 20-100. Whiskers are obtained by various methods: growth from coatings, electrolytic deposition, deposition from a vapor-gas medium, crystallization from the gas phase through the liquid phase. by the mechanism of vapor - liquid - crystal, pyrolysis, crystallization from saturated solutions, viscerization
38.2.3. Matrix alloys
In metal composite materials, matrices are mainly used from light wrought and cast alloys of aluminum and magnesium, as well as from alloys of copper, nickel, cobalt, zinc, tin, lead, silver; heat-resistant nickel-chromium, titanium, zirconium, vanadium alloys; alloys of refractory metals of chromium and niobium (table 38 2).
38.2.4. Bond Types and Interface Structures in Composite Materials
Depending on the material of the filler and matrices, methods and modes of obtaining on the interfaces of composite materials, six types of bonds are realized (Table 38.3). The strongest bond between the components in compositions with metal matrices is provided by chemical interaction. A common type of bond is mixed, represented by solid solutions and intermetallic phases (for example, the “aluminum-boron fibers” composition obtained by continuous casting) or solid solutions, intermetallic and oxide phases (the same composition obtained by pressing plasma semi-finished products), etc. .
38.3. Methods for the production of composite materials
The technology for the production of metal composite materials is determined by the design of products, especially if they have a complex shape and require the preparation of joints by welding, soldering, gluing or riveting, and, as a rule, is multi-junction.
The elemental basis for the production of parts or semi-finished products (sheets, pipes, profiles) from composite materials is most often the so-called prepregs, or tapes with one layer of reinforcing filler impregnated or coated with matrix alloys; metal-impregnated fiber tows or individual fibers coated with matrix alloys.
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TYPES OF COMMUNICATION ON INTERFACE SURFACES IN COMPOSITE MATERIALS
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Parts and semi-finished products are obtained by joining (compacting) the original prepregs by impregnation, hot pressing, rolling or drawing packages from prepregs. Sometimes both prepregs and products from composite materials are made by the same methods, for example, by powder or casting technology, and under different modes and at different technological stages.
Methods for obtaining prepregs, semi-finished products and products from composite materials with metal matrices can be divided into five main groups: 1) vapor-gas-phase; 2) chemical and electrochemical; 3) liquid phase; 4) solid phase; 5) solid-liquid phase.
38.4. Properties of metal matrix composite materials
Composite materials with metal matrices have a number of undeniable advantages over other structural materials intended for operation in extreme conditions. These advantages include: high strength and. stiffness combined with high fracture toughness; high specific strength and rigidity (ratio of ultimate strength and modulus of elasticity to specific gravity a/y and E/y); high fatigue limit; high heat resistance; low sensitivity to thermal shocks, to surface defects, high damping properties, electrical and thermal conductivity, manufacturability in design, processing and connection (Table 38 4).
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COMPOSITE MATERIALS WITH METAL MATRIXS IN COMPARISON WITH THE BEST METAL STRUCTURAL MATERIALS |
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TABLE 385 |
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MECHANICAL PROPERTIES OF COMPOSITE MATERIALS WITH METAL MATRIX
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In the absence of special requirements for materials in terms of thermal conductivity, electrical conductivity, cold resistance and other properties, the temperature intervals for the operation of composite materials are determined as follows:<250 °С - для материалов с полимерными матрицами; >1000 °С - for materials with ceramic matrices; composite materials with metal matrices transcend these limits
The strength characteristics of some composite materials are given in Table 38-5.
The main types of joints of composite materials today are bolted, riveted, glued, soldered and welded joints, and combined. Soldered and welded joints are especially promising, since they open up the possibility to most fully realize the unique properties of a composite material in a structure, however, their implementation is a complex scientific and technical task and in many cases has not yet left the experimental stage.
38.5. Problems of weldability of composite materials
If weldability is understood as the ability of a material to form welded joints that are not inferior to it in their properties, then composite materials with metal matrices, especially fibrous ones, should be classified as difficult-to-weld materials. There are several reasons for this.
I. Welding and soldering methods involve joining composite materials along a metal matrix. Reinforcing filler in a welded or soldering seam or completely absent (for example, in butt seams located across the reinforcement in the fibrous or layered composite materials), or is present in a reduced voluminous share (when welding dispersed materials containing discrete reinforcing phase), or there is a violation Zionic welding of fibrous compositions across the direction of reinforcement). Therefore, a welded or brazed seam is a weakened section of a composite material structure, which must be taken into account when designing and preparing the joint for welding. There are proposals in the literature for offline welding of composition components to maintain continuity of reinforcement (for example, pressure welding of tungsten fibers in a tungsten-copper composition), however, offline butt welding of fibrous composite materials requires special edge preparation, strict adherence to the reinforcement step and is suitable only for materials reinforced with metal fibers. Another suggestion is to prepare butt joints with overlapping fibers at a length greater than the critical length, however, there are difficulties in filling the joint with matrix material and ensuring a strong bond along the fiber-matrix interface.
II. The influence of welding heating on the development of physicochemical interaction in a composite material is conveniently considered using the example of a joint formed during arc penetration of a fibrous material across the direction of reinforcement (Fig. 38.2). If the matrix metal does not have polymorphism (for example, Al, Mg, Cu, Ni, etc.), then 4 main zones can be distinguished in the joint: 1 - zone heated to the return temperature of the matrix (by analogy with welding of homogeneous materials, we will call this area the main material); 2 - zone limited by the return and recrystallization temperatures of the matrix metal (return zone); 3-zone,
limited by the temperatures of recrystallization and melting of the matrix (recrystallization zone); 4 - heating zone above the melting temperature of the matrix (let's call this zone a weld). If the matrix in the composite material is alloys of Ti, Zr, Fe, and other metals that have polymorphic transformations, then subzones with complete or partial phase recrystallization of the matrix will appear in zone 3, and for this consideration this point is not significant.
Changes in the properties of the composite material begin in zone 2. Here, the recovery processes remove the strain hardening of the matrix achieved during solid-phase compaction of the composite material (in compositions obtained by liquid-phase methods, softening in this zone is not observed).
In zone 3, recrystallization and grain growth of the matrix metal occur. Due to the diffusion mobility of matrix atoms, further development of interfacial interaction, which was initiated in the production of a composite material, becomes possible, the thickness of brittle interlayers increases and the properties of the composite material as a whole deteriorate. Material fusion welding
porosity along the fusion boundary and adjacent interfacial boundaries is possible, which worsens not only the strength properties, but also the tightness of the welded joint.
In zone 4 (weld), 3 sections can be distinguished:
Plot 4", adjacent to the axis of the weld, where due to strong overheating under the arc of the metal matrix melt and the longest stay of the metal in the molten state, the reinforcing phase is completely dissolved;
Segment 4", characterized by a lower heating temperature of the melt and a shorter duration of contact of the reinforcing phase with the melt. Here, this phase is only partially dissolved in the melt (for example, the diameter of the fibers decreases, shells appear on their surface; the unidirectionality of the reinforcement is violated);
Segment 4"", where there is no noticeable change in the size of the reinforcing phase, but intense interaction with the melt develops, interlayers or islands of brittle interaction products are formed, and the strength of the reinforcing phase decreases. As a result, zone 4 becomes the zone of maximum damage to the composite material during welding.
III. Due to differences in the thermal expansion of the matrix material and the reinforcing phase, additional thermoelastic stresses arise in welded joints of composite materials, causing the formation of various defects: cracking, destruction of brittle reinforcing phases in the most heated zone 4 of the joint, delamination along interfacial boundaries in zone 3.
To ensure high properties of welded joints of composite materials, the following is recommended.
Firstly, among the known joining methods, preference should be given to solid phase welding methods, in which, due to the lower energy input, a minimum degradation of the properties of the components in the joining zone can be achieved.
Secondly, pressure welding modes must be chosen so as to exclude displacement or crushing of the reinforcing component.
Thirdly, in fusion welding of composite materials, methods and modes should be chosen that ensure minimal heat input into the joint zone.
Fourth, fusion welding should be recommended for joining composite materials with thermodynamically compatible components, such as copper-tungsten, copper-molybdenum, silver-tungsten, or reinforced with heat-resistant fillers, such as silicon carbide fibers, or fillers with barrier coatings, such as boron fibers coated with boron carbide or silicon carbide.
Fifth, the electrode or filler material or the material of intermediate gaskets for fusion welding or soldering must contain alloying additives that limit the dissolution of the reinforcing component and the formation of brittle interfacial interaction products during the welding process and during the subsequent operation of welded assemblies.
38.5.1. Composite welding
Fibrous and layered composite materials are most often overlapped. The ratio of the length of the floor to the thickness of the material usually exceeds 20. Such connections can be additionally reinforced with riveted or bolted connections. Along with lap joints, it is possible to make butt and fillet welds in the direction of reinforcement and, more rarely, across the direction of reinforcement. In the first case, with the right choice of methods and modes of welding or soldering, it is possible to achieve equal strength of the joint; in the second case, the bond strength usually does not exceed the strength of the matrix material.
Composite materials reinforced with particles, short fibers, whiskers are welded using the same techniques as precipitation hardening alloys or powder materials. The equal strength of welded joints to the base material in this case can be achieved provided that the composite material is made by liquid-phase technology, reinforced with heat-resistant fillers and when choosing the appropriate welding modes and welding materials. In some cases, the electrode or filler material may be similar or close in composition to the base material.
38.5.2. Arc welding in shielding gases
The method is used for fusion welding of composite materials with a matrix of reactive metals and alloys (aluminum, magnesium, titanium, nickel, chromium). Welding is carried out with a non-consumable electrode in an atmosphere of argon or a mixture with helium. To control the thermal impact of welding on materials, it is advisable to use a pulsed arc, a compressed arc, or a three-phase arc.
To increase the strength of the joints, it is recommended to perform seams with composite electrodes or filler wires with a volume content of the reinforcing phase of 15-20%. As reinforcing phases, short fibers of boron, sapphire, nitride or silicon carbide are used.
38.5.3. electron beam welding
The advantages of the method are in the absence of oxidation of the molten metal and reinforcing filler, vacuum degassing of the metal in the welding zone, high energy concentration in the beam, which makes it possible to obtain joints with a minimum width of the melting zone and the near-weld zone. The latter advantage is especially important when making connections of fibrous composite materials in the reinforcement direction. With special preparation of joints, welding using filler spacers is possible.
38.5.4. Contact spot welding
The presence of a reinforcing phase in a composite material reduces its thermal and electrical conductivity compared to the matrix material and prevents the formation of a cast core. Satisfactory results were obtained in spot welding of thin sheet composite materials with cladding layers. When welding sheets of different thicknesses or composite sheets with homogeneous metal sheets, in order to bring the core of the weld point into the plane of contact between the sheets and balance the difference in the electrical conductivity of the material, electrodes with different conductivity are selected, with compression of the peripheral zone, the diameter and radius of the electrode curvature, the thickness of the cladding layer are changed, additional spacers are used.
The average strength of the weld point when welding uniaxially reinforced boron aluminum plates with a thickness of 0.5 mm (with a volume fraction of fibers of 50%) is 90% of the strength of boron - aluminum of the equivalent section. The bonding strength of boro-aluminum sheets with cross reinforcement is higher than that of sheets with uniaxial reinforcement.
38.5.5. Diffusion welding
The process is carried out at high pressure without the use of solder. Thus, boron-aluminum parts to be joined are heated in a sealed retort to a temperature of 480 °C at a pressure of up to 20 MPa and kept under these conditions for 30–90 minutes. The technological process of diffusion resistance spot welding of boron-aluminum with titanium is almost the same as fusion spot welding. The difference is that the welding mode and the shape of the electrodes are chosen so that the heating temperature of the aluminum matrix is close to the melting temperature, but below it. As a result, a diffusion zone with a thickness of 0.13 to 0.25 µm is formed at the contact point.
Specimens overlapped by diffusion spot welding, when tested for tension in the temperature range of 20-120 ° C, are destroyed along the base material with a tear along the fibers. At a temperature of 315 °C, the samples are destroyed by shear at the junction.
38.5.6. wedge-press welding
To connect end pieces made of conventional structural alloys with pipes or bodies made of composite materials, a method has been developed for welding dissimilar metals that differ sharply in hardness, which can be called micro-clinopress welding. Press-in pressure is obtained due to thermal stresses arising from heating of the mandrel and holder of a device for thermocompression welding, made of materials with different coefficients of thermal expansion (K. TP). The ending elements, on the contact surface of which a wedge thread is applied, are assembled with a pipe made of a composite material, as well as with a mandrel and a ferrule. The assembled fixture is heated in a protective environment to a temperature of 0.7-0.9 of the melting point of the most fusible metal. The fixture mandrel has a higher CTE than the clip. During the heating process, the distance between the working surfaces of the mandrel and the holder is reduced, and the protrusions ("wedges") of the thread on the tip are pressed into the cladding layers of the pipe. The strength of a solid-phase joint is not lower than the strength of the matrix or cladding metal.
38.5.7. Explosion welding
Explosive welding is used to join sheets, profiles and pipes made of metal composite materials reinforced with metal fibers or layers having sufficiently high plastic properties to avoid crushing of the reinforcing phase, as well as to join composite materials with flashings of various metals and alloys. The strength of the joints is usually equal to or even higher (due to work hardening) than the strength of the weakest matrix material used in the parts to be joined. To increase the strength of the joints, intermediate gaskets made of other materials are used.
Joints are usually free of pores or cracks. Melted areas in the transition zone, especially during the explosion of dissimilar metals, are mixtures of phases of the eutectic type.
38.6. Soldering of composite materials
Brazing processes are very promising for joining composite materials, since they can be carried out at temperatures that do not affect the reinforcing filler and do not cause the development of interfacial interaction.
Soldering is carried out by conventional techniques, i.e. solder dipping or in an oven. The question of the quality of surface preparation for soldering is very important. Brazing joints with fluxes are susceptible to corrosion, so the flux must be completely removed from the joint area.
Soldering with hard and soft solders
Several options for soldering boron aluminum have been developed. Solders for low temperature soldering were tested. Solder composition 55% Cd -45% Ag, 95% Cd -5% Ag, 82.5% Cd-17.5% Zn are recommended for parts operating at temperatures not exceeding 90 ° C; solder composition 95% Zn - 5% Al - for operating temperatures up to 315 °C. To improve the wetting and spreading of the solder, a nickel layer 50 µm thick is applied to the surfaces to be joined. High-temperature soldering is performed using eutectic solders of the aluminum-silicon system at temperatures of the order of 575-615 ° C. Soldering time must be kept to a minimum due to the danger of degradation of the strength of the boron fibers.
The main difficulties in brazing carbon-aluminum compositions both among themselves and with aluminum alloys are associated with poor wettability of carbon-aluminum with solders. The best solders are alloy 718 (A1-12% Si) or alternating layers of foil from alloy 6061. Soldering is carried out in an oven in an argon atmosphere at a temperature of 590 ° C for 5-10 minutes. Solders of the aluminum-silicon-magnesium system can be used to join boron-aluminum and carbon-aluminum with titanium. To increase the strength of the connection, it is recommended to apply a nickel layer on the titanium surface.
Eutectic diffusion soldering. The method consists in applying a thin layer of a second metal to the surface of the welded parts, which forms a eutectic with the matrix metal. For matrices of aluminum alloys, layers of Ag, Cu, Mg, Ge, Zn are used, the eutectic temperature of which with aluminum is 566, 547, 438, 424 and 382 °C, respectively. As a result of the diffusion process, the concentration of the second element in the contact zone gradually decreases, and the melting point of the compound rises, approaching the melting point of the matrix. Thus, solder joints can operate at temperatures higher than the temperature of the punch.
During diffusion soldering of boron aluminum, the surfaces of the parts to be joined are coated with silver and copper, then compressed and held under pressure up to 7 MPa at a temperature of 510-565 ° C in a steel retort in a vacuum or an inert atmosphere.
The powder filler is introduced into the matrix of the composite material in order to realize the properties inherent in the filler substance in the functional properties of the composite. In powder composites, the matrix is mainly metals and polymers. The name stuck behind polymer matrix powder composites "plastics".
Composites with a metal matrix
Composites with a metal matrix. 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 matrix metal. Sometimes pressing and sintering processes are combined. The technology for producing powder composites is called "powder metallurgy". Powder metallurgy methods 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 a matrix. Cermets combine the hardness and heat resistance and heat resistance of ceramics with the high toughness and thermal conductivity of metals. Therefore, cermets, unlike ceramics, are less brittle and are able to withstand large temperature differences without breaking.
Cermets are widely used in the production of metalworking tools. Powdered carbides are called tool cermets.
Powder filler of hard alloys are carbides or carbonitrides in the 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 are divided into four groups:
- 1) WC-Co - single carbide type B K;
- 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 titanium carbide and carbonitride - tungsten-free type TN and CNT.
Alloys VK. Alloys are marked with the letters VK and a number indicating the content of cobalt. For example, the composition of the alloy VK6: 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.
Alloys TK. 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 alloy T15K6: TiC - 15%, Co - 6%, the rest, 79%, - WC. The hardness of TK alloys due to the introduction of a harder titanium carbide into its filler is greater than the hardness of VK alloys. They also have an advantage in heat resistance - 1000 ° C, however, their strength is lower with an equal cobalt content.
TTK alloys (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 are superior to 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 mainly used for severe cutting conditions with high force and temperature loads.
Alloys TN, KNT. These are tungsten-free hard alloys (BVTS) based on titanium carbide and carbonitride with a nickel-molybdenum bond rather than a cobalt bond.
In terms of heat resistance, BVTS are 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 are lower than those of traditional alloys.
Despite the relatively low cost, the widespread use of BVTS for the manufacture of cutting tools is problematic. It is most expedient to use tungsten-free alloys for the manufacture of measuring (end blocks, gauges) and drawing tools.
The metal matrix is also used to bind the powder filler of diamond and cubic boron nitride, which are collectively referred to as "superhard materials" (SHM). Composite materials filled with STM are used as a processing tool.
The choice of matrix for diamond powder filler is limited by the low heat resistance of diamond. The matrix must provide a thermochemical regime of reliable binding of grains of diamond filler, excluding combustion or graphitization of diamond. Tin bronzes are most widely used for bonding diamond filler. The higher heat resistance and chemical inertness of boron nitride allow the use of binders based on iron, cobalt, and hard alloy.
The 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 being machined 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 application. This is determined by the differences in their physico-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 determining factors are the high diffusivity of diamond with respect 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
The applicability of 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 a diamond tool compared to a tool based on cubic boron nitride. Significant oxidation of boron nitride in air is observed only after one hour exposure at a temperature of 1200°C.
The temperature limit of diamond performance 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 extensive area of application of cermets is their use as a high-temperature structural material for new technology objects.
Service properties of powder composites with a metal matrix are determined mainly by the properties of the filler. Therefore, for powder composite materials with a special property, the classification by application is most common.
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