Classification of coatings according to functional properties and method of application. Method of surface modification Purpose and areas of application of coatings
synthetic fibers (RSV)
Modification- this is a directed change in the properties of melt synthetic fibers (RSV), which can be implemented in various ways:
- physical modification is achieved by a directional change in the conditions of molding, orientational stretching and heat treatment. The goal is to obtain fibers with new, predetermined, reproducible properties. At the same time, the primary structure of the fiber remains unchanged. Thus, physical modification can be achieved by changing the rheological properties of spinning polymer melts, the conditions of their extrusion, spinneret drawing, varying the drawing ratios and the conditions of orientational drawing and heat treatments (thermofixation or thermorelaxation).
The main shape of the cross section of the filaments (f) is round. But this circumstance does not allow in a number of cases to achieve the necessary textile-technological characteristics, such as covering, given air, gas, water tightness, etc.
It is known that such an important property as comfort - the ability to remove moisture, heat or store them, if required, in the space between clothing and the body - depends on the number of voids in the textile material. This circumstance predetermined great interest in the possibility of obtaining fibers, mainly based on RSV, with a non-circular (profiled) cross section. Professor Jambrich (Slovak Technical University) was constantly working on this problem.
The production of profiled fibers is complicated by two circumstances:
Technical difficulties in the manufacture of holes for dies of a figured profile;
Physico-chemical circumstances, which are due to the desire of the liquid to minimize its surface.
If the shape of the die opening is an open ring, then the fiber is hollow.
Even greater technical complications arise when obtaining profiled fibers with a low linear density of a single filament (less than 0.1 tex).
The shape of the cross section of the fiber does not change during the drawing or heat-setting treatments. Threads, yarn from profiled fibers make it possible to obtain light, soft, comfortable textile materials.
In recent years, technologies for producing thin and very thin threads and fibers have been intensively developed. We are talking about fibers with a linear density of a single filament (T T f) in the range of 0.1-0.3 decitex (dtex). Complex threads and yarn from such fibers are capable of creating qualitatively new types of textile materials, while it is possible to obtain thin textile fabrics even based on hydrophobic polypropylene (PP, PP). These fibers with T T f = 0.01-0.02 tex make it possible to obtain yarn, products from which are very comfortable and light.
The transition to microfibers (MF) means not only a decrease in equipment productivity, but an increase in energy and labor costs, an increase in polymer consumption rates. However, these fibers have a very big future;
- methods of physico-chemical modification are based on the introduction of various additives (additives) into the polymeric fiber substrate.
For this purpose, the method of introducing additives through the spinning melt (technology "master batch", "nanotechnology") is used.
The introduction of additives according to this method is carried out by various technological methods. Additives can be introduced at the beginning of the preparation of the spinning melt, i.e. at the stage of polymer synthesis, or by directly mixing the main spinning melt with a concentrated polymer melt containing this additive, i.e. with additive polymer concentrate (PKD) immediately before molding (master-batch technology).
The additives introduced can impart various properties to the fibers. These may be pigments, i.e. dyes (dyeing "in mass"), flame retardant additives that reduce the combustibility of fibers, bactericidal and other bioactive additives, various linear polymers introduced into the base polymer to control properties;
- dyeing "in mass".
The dye additives introduced may be soluble in the spinning melt or may be heterogeneous fillers. In the second case, these are dispersed pigment additives.
The main types of pigments used for dyeing "in mass" are: titanium dioxide TiO 2 (white standard), highly dispersed carbon black C (black standard), various other dye pigments.
The most important technological requirement is the high dispersion of the introduced pigments (particle sizes cannot exceed 10-15% of the filament radius, therefore they are conditionally called "nanoparticles"). Particles of large sizes will disrupt the stability of the thread formation process and the uniformity of the fiber structure, worsening its physical and mechanical properties. The largest pigment particles are filtered out in the spunbond kit before entering the extrusion through the holes of the spunbond, but this leads to a change in the pigment content in the fiber, and consequently, to a change in color intensity.
The introduction of matting agents (TiO 2 and others) is achieved by obtaining a fiber with a muted sheen. To slightly reduce the gloss, micromatting is used (the introduction of a matting agent is hundredths of a percent). The most widespread TiO 2, which has the following three crystallographic structures: rutile, anatase, brookite. These crystallographic modifications of titanium dioxide differ in the sizes of elementary crystallographic lattices. The anatase form is characterized by the most developed specific surface area. It is she who is the most important component in matting.
For coloring in gray and black colors, the addition of soot is used. The size requirements for carbon black particles are the same as for all pigments.
The introduction of TiO 2 , carbon black and other pigments is aimed not only at achieving a color effect, but is also an essential factor in structure formation.
Previously, it was found that a layer of sorbed polymer molecules is formed on the surface of a dispersed particle. As is known, the packing density of macromolecular segments is different and depends on the flexibility of the polymer, the regularity of the structure of its primary structure, and other factors. As a result of sorption by the surface of macromolecules of polyethylene terephthalate (PET, PET) of TiO 2 particles, a layer of sorbed polymer appears on the surface of the particles. Under the influence of the surface forces of TiO 2 particles, polymer chain segments are packed into layers whose density is higher than the density in the surrounding polymer liquid (PET melt). At the phase boundary, a sorption layer of the polymer appears, the segments of which can not only be more densely packed, but also mutually ordered.
The kinetics of polymer crystallization is described by the Avrami equation, and the mechanism is characterized by different values of the constants in this equation, mutual ordering (crystallization) can occur according to the "nucleation" mechanism. In this case, the crystallographic characteristics of the "nucleus" should correspond to the crystallographic characteristics of the polymer. In this regard, pigment particles can only be "seeds" of crystallization when their crystallographic cell is identical to the crystallographic cell of the crystalline phase of the polymer.
However, the parameters of the crystallographic cells of pigments, TiO 2 , carbon black are very far from the parameters of the crystallographic cells of PET. Therefore, they are not "seeds" of crystallization, but they are the factors that change the dynamics of the crystallization process as a result of the formation of an ordered layer of sorbed polymer on their surface. Therefore, with the introduction of pigments, the crystallization process is accelerated and the structure of the formed thread changes. The introduction of approximately 0.05-0.5% (wt.) titanium dioxide with a particle size of not more than 0.5-0.7 microns (μ, μm) is a factor that changes the mechanical properties of polyester (PEF, PES) threads, increasing uniformity of their physical and mechanical characteristics. Not being "germs" of crystallization, pigment particles are the centers of structure formation. In this case, fibers with higher fatigue properties are obtained, with a smaller spread (dispersion, coefficient of variation) in terms of physical and mechanical parameters.
Thus, pigments are not only dyes, but also substances that improve the physical structure of the fibers.
The introduction of dyes soluble in polymeric liquids (melts) is also an important method of physicochemical modification. In this case, not only a coloristic effect is achieved, but the structure of the fibers changes.
The most important requirement for soluble dyes is their stability in the spinning stock at high melt temperatures.
The introduced dyes also affect the properties of the "polymer-dye" system. Dyes can be plasticizers or anti-plasticizers (ie reduce or increase the glass transition temperature (T g)). This must be taken into account when developing new technological schemes.
The most important method of physicochemical modification is obtaining fibers from mixtures of polymers (obtaining composite fibers).
When small amounts of the second polymer, which is incompatible with the main one, are introduced into the polymer substrate, the effects of strengthening and strengthening the structure are achieved (the effect of "small polymer additives").
These polymer additives (up to 5% wt.) are the centers of structure formation, increasing the uniformity of the structure of the formed thread and improving its properties.
By mixing melts of polyamide (PA, RA) and PET in various proportions (while the content of the second polymer is small), a fairly homogeneous mixture of polymers is obtained. As a result of a sharp change in the velocity gradients, when such a mixed melt enters the die hole, a microheterogeneous (if it is an incompatible pair of polymers), but rather homogeneous fiber structure arises.
But technically, another mixing option has been implemented, when the mixture of polymers is macroheterogeneous (approximately equal ratio of two different polymers). Accordingly, the resulting filaments are built from two polymers of different chemical nature.
These are the so-called bicomponent fibers (BKV) or bicomponent yarns (BKN), which can be obtained by all known molding methods. In this case, two polymers in the form of melts are extruded through special dies, the holes in which are arranged in such a way that melt flows of each component are fed into them through individual channels. As a result, the fiber consists, as it were, of two parts. In a cross section, the distribution of these components can be presented in the form of two lobules or in the form of various concentric arrangements. All technological operations remain normal. But bicomponent fibers have an interesting feature. During thermal relaxation, the lower T c polymer component is capable of shrinking more than the second component. The fiber thus acquires a stable crimp. Therefore, this is one of the techniques for texturing fibers and threads.
The cost of such fibers is higher. But bicomponent fibers based on polyamides, polyesters and other polymeric substrates have sufficient consumer demand in the world market;
- chemical modification processes can be carried out by carrying out reactions:
polymer-analogous transformations;
Copolymerization (SPM);
Copolycondensation (SPK);
- "grafting" to the outer surface of the fiber side chains of polymers of a different chemical nature.
During surface treatments of the fiber, the chemical nature of the fiber changes along the cross section (the outer layers acquire a different chemical nature).
Changes in the primary structure through polymer-analogous transformations, SPM, SPC lead to the emergence of new types of fiber-forming polymers.
Surface modification is carried out on finished fibers (under heterogeneous conditions).
For example, carbon chain polymers, polycaproamide (PCA, PCA, PA6, PA6), polyesters can be grafted onto the surface of cellulose fibers. To reduce the hydrophobicity of polyamide fibers, hydrophilic monomers are “grafted” (for example, itaconic acid (ITA), etc.). The grafting of nitrofuran and other compounds onto the surface of nylon socks makes it possible to impart antifungal properties to them.
Surface grafting can be carried out as a result of a recombination addition reaction.
By chemically modifying fibers, materials with completely different properties can be obtained.
Ministry of Education and Science of the Russian Federation
Federal State Autonomous Educational Institution of Higher Professional Education "Ural Federal University named after the first President of Russia B.N. Yeltsin"
Department "Heat treatment and physics of metals"
"Classification of coatings by functional properties and method of application"
Teacher:
Assoc., Ph.D. Rossina N.G.
Student: Trapeznikov A.I.
Group: Mt 320701
Yekaterinburg 2015
Introduction
Classification of coatings and methods for their preparation
1 Changes in the physical and chemical properties of surfaces during coating
2 Internal coatings
3 External coatings
4 Surface preparation for coating
Chemical and electrochemical coating methods
1 Classification of chemical and electrochemical coatings
2 The essence of the chemical coating method
3 Coating the product
Vacuum condensation coating
Application of hardfacing coatings with concentrated heat sources
1 Classification of deposited coatings
2 Applications for hardfacing
Coating by cladding
Thermal gas coating methods
1 Classification methods
Plasma spraying of coatings
1 Advantages and disadvantages of the plasma spraying method
Flame spraying of coatings
Conclusion
Introduction
The coatings available to modern technology are very diverse both in properties and in methods of production. The use of protective, protective-decorative and special coatings allows us to solve many problems. By choosing the coating material, the conditions for their application, combining metallic and non-metallic coatings, it is possible to give the surface of products a different color and texture, the necessary physical, mechanical and chemical properties: increased hardness and wear resistance, high reflectivity, improved anti-friction properties, surface electrical conductivity, etc. . But the optimal choice of coatings or methods of their finishing is impossible without a comprehensive consideration of their properties and features of production.
Coating technology, along with other science-intensive and energy-saving industries, is one of the main directions in the development of modern production in the leading countries of the world community.
Currently, the improvement and search for new coating methods continues. Study of coating methods, their varieties; thermodynamics of processes in the creation of coatings of various types on metal and non-metal surfaces; structure, structure and operational properties of coatings; the main equipment for gas-thermal and electro-thermal coating of metal products.
Study of methods for improving the quality of products by the formation of multilayer and reinforced coatings; metrological control technological parameters formations and their properties.
The role and place of coatings in modern production
Coatings are a single or multilayer structure applied to a surface to protect it from external influences (temperature, pressure, corrosion, erosion, and so on).
Distinguish between external and internal coatings.
External coatings have a border between the coating and the surface of the product. Accordingly, the size of the product increases by the thickness of the coating, while the weight of the product increases.
In internal coatings, there is no interface and the dimensions and weight of the product remain unchanged, while the properties of the product change. Internal coatings are also called modifying coatings.
There are two main tasks that can be solved when applying a coating
Changing the initial physical and chemical properties of the surface of products that provide the specified operating conditions;
Restoration of properties, dimensions, mass of the surface of the product, violated by the operating conditions.
Purpose and scope of coatings
The main reason for the emergence and development of the technology of applying protective coatings was the desire to increase the durability of parts and assemblies of various mechanisms and machines. Optimization of the coating system involves the appropriate selection of coating composition, its structure, porosity and adhesion, taking into account both the temperature of application and operating temperature, the compatibility of substrate and coating materials, the availability and cost of the coating material, as well as the possibility of its renewal, repair and proper maintenance. during operation.
The use of an insufficiently strong coating, the thickness of which noticeably decreases during operation, can lead to a decrease in the strength of the entire part due to a decrease in the effective area of its full cross section. Mutual diffusion of components from the substrate into the coating and vice versa can lead to depletion or enrichment of alloys in one of the elements. Thermal exposure can change the microstructure of the substrate and cause residual stresses to appear in the coating. Taking into account all of the above, the optimal choice of the system should ensure its stability, i.e., the preservation of such properties as strength (in its various aspects), ductility, impact strength, fatigue and creep resistance after any impact. The strongest influence on the mechanical properties is exerted by operation under conditions of rapid thermal cycling, and the most important parameter is the temperature and time of its effect on the material; interaction with the surrounding working environment determines the nature and intensity of chemical exposure.
Mechanical methods of bonding the coating to the substrate often do not provide the desired quality of adhesion. Diffusion bonding methods usually give much better results. A good example of a successful diffusion coating is the aluminizing of ferrous and non-ferrous metals.
1. Classification of coatings and methods for their preparation
Currently, there are many different coatings and methods for obtaining them.
Many publications offer various classification schemes for inorganic coatings according to various criteria. Coatings can be classified according to the following basic principles:
By purpose (anti-corrosion or protective, heat-resistant, wear-resistant, anti-friction, reflective, decorative, etc.);
By physical or chemical properties (metallic, non-metallic, refractory, chemically resistant, reflective, etc.);
By the nature of the elements (chromium, chromium-aluminum, chromium-silicon and others);
By the nature of the phases formed in the surface layer (aluminide, silicide, boride, carbide and others)
Consider the most important coatings, classified by purpose.
Protective coatings - the main purpose is associated with their various protective functions. Corrosion-resistant, heat-resistant and wear-resistant coatings are widely used. Heat-shielding, electrically insulating and reflective coatings are also widely used.
Structural coatings and films - play the role of structural elements in products. They are also especially widely used in the manufacture of products in instrumentation, electronic equipment, integrated circuits, in turbojet engines - in the form of actuated seals in a turbine and compressor, etc.
Technological coatings - designed to facilitate technological processes in the manufacture of products. For example, applying solders when soldering complex structures; production of semi-finished products in the process of high-temperature deformation; welding of dissimilar materials, etc.
Decorative coatings - extremely widely used in the manufacture of household products, decorations, enhancing aesthetics industrial installations and devices, prosthetics in medical technology, etc.
Restorative coatings - give a huge economic effect when restoring worn surfaces of products, such as propeller shafts in shipbuilding; necks of crankshafts of internal combustion engines; blades in turbine engines; various cutting and pressing tools.
Optical coatings - reduce reflectivity compared to bulk materials, mainly due to surface geometry. Profiling shows that the surface of some coatings is a collection of roughness, the height of which varies from 8 to 15 microns. On separate macroroughnesses, microroughnesses are formed, the height of which varies from 0.1 to 2 μm. Thus, the height of the irregularities is commensurate with the wavelength of the incident radiation. Reflection of light from such a surface occurs in accordance with the Frenkel law.
In the literature, there are various principles for classifying coating methods. Although it should be noted that there is no unified classification system for coating methods. Hawking and a number of other researchers have proposed three classifications of coating methods:
According to the phase state of the medium from which the coating material is deposited;
According to the condition of the applied material;
By the state of the processes that define one group of methods
coatings.
More detailed classifications of coating methods are presented in Table 1.
Table 1 Advantages and disadvantages various methods coating
MethodAdvantagesDisadvantagesPVDVersatility; all solid elements and materials can be deposited. It is possible to obtain thin films and sufficiently thick coatings. There are various modifications of the method. Н = 5-260 microns. It is possible to apply coatings only on the visible part of the surface. Poor scattering power. Expensive equipment. CVD Competes with the physical deposition method. Elements and compounds can be applied chemically active and in a vapor state. Good scattering ability. H = 5-260 µm. An important role is played by the heating source. Deposition is usually carried out at higher temperatures than in the physical deposition method. Substrate may overheat. Undesirable direct deposition possible. Solid phase deposition Good uniformity and close dimensional tolerances of the coating. High economic efficiency of the process. The most common coating materials are Al and Cr. High coating hardness. H = 5 - 80 µm. Limited dimensions of the substrate. Not applicable for heat sensitive substrates. Thinner than with other diffusion methods, coatings. Brittleness of coatings is possible. Spraying Possibility to control the spraying conditions and the quality of the applied material during the process. Possibility of obtaining thick homogeneous coatings. H = 75 - 400 microns. The quality depends on the skill of the operator. The substrate must be resistant to heat and impact. The coatings are porous with a rough surface and possible inclusions. Cladding Thick coatings are possible. Large substrates can be processed. H = 5 - 10% of the thickness of the substrate Warping of the substrate is possible. Suitable for rigid substrates. Electrodeposition (including chemical and electrophoresis) Cost-effective process using aqueous electrolytes. It is possible to apply precious metals and refractory coatings from salt melts. It is used for industrial production of cermets. Chemical deposition and electrophoresis are applicable only for some elements and types of substrates. H = 0.25 - 250 µm. Careful design of equipment is required to ensure good scattering power. The use of molten salts as electrolytes requires strict control to prevent moisture ingress and oxidation. Harmful fumes above the melt. Coatings can be porous and stressed. Limited to specific areas of high temperatures. Hot dip Relatively thick coatings. Rapid coating method. H = 25 - 130 microns. Limited only by the application of A1 to obtain high-temperature coatings. Coatings can be porous and discontinuous.
Table 2. Classification of coating methods according to the phase state of the medium
Solid stateMechanical bonding Cladding SinteringLiquid stateHot dipping Spraying HardfacingSemi-liquid or pasty stateSol-gel process Slip solderingGas medium (atomic, ionic or electron interaction)Physical vapor deposition Chemical vapor depositionSolutionChemical Galvanic ElectrogalvanicPlasmaSurface treatment
Table 3. Classification of coating methods according to the state of the processes defining one group of methods
Mechanical Cladding Connection Physical Physical Vapor Deposition Vacuum Coatings Thermal Evaporation Sputtering Ion Deposition Chemical Chemical Vapor Deposition Electrolyte Deposition without Electric Field Electrochemical In Aqueous Solutions In Molten Salt Spray Detonation Gun Electric Arc Metallization Plasma Gas Flame Using Wire Surfacing Laser Manual Electric Welding Oxygen Inert Gas Welding Plasma welding Sputter fusion Submerged arc Other between tungsten electrodes in an inert atmosphere
Table 4. Classification of methods according to the state of the applied material and manufacturing methods
Group 1 Atomic or ionic state Vacuum methods: Vacuum evaporation Ion beam deposition Epitaxial molecular beam deposition Plasma methods: Sputtering (ion, magnetron) Ion deposition Plasma polymerization Activated reactive evaporation Cathode arc deposition Chemical interaction in reagent vapors: Vapor phase deposition Recovery Decomposition Plasma Deposition Spray Pyrolysis Electrolyte Deposition: Electroplating Chemical Deposition Molten Salt Deposition Chemical SubstitutionGroup 2 Explosive rolling Laser melting Wetting: Brushing Hot dipping Electrostatic methods: Spin coating Spray patterning Groups a 4 Surface structure modification Laser surface modification Heat treatment Ion implantation Surface alloying: Diffusion from the bulk Sputtering Leaching Chemical converse liquid vapor diffusion (heating, plasma) Electrolytic anodizing Heat treatment in molten salts Mechanical methods: Shot blasting
1.1 Changes in the physicochemical properties of surfaces during coating application
The surface layer (coating) plays a decisive role in the formation of operational and other properties of products; its creation on the surface of a solid body almost always changes the physicochemical properties in the right direction. Coating allows you to restore previously lost properties during the operation of products. However, the properties of the initial surfaces of products obtained during their production are most often changed. In this case, the properties of the material of the surface layer differ significantly from the properties of the original surface. In the overwhelming majority, the chemical and phase composition of the newly created surface changes, resulting in products with the required performance characteristics, such as high corrosion resistance, heat resistance, wear resistance, and many other indicators.
Changing the physical and chemical properties of the original surfaces of products can be carried out by creating both internal and external coatings. Combined options are also possible (Fig. 1).
coating chemical vacuum cladding
When applying internal coatings, the dimensions of the products remain unchanged (L And = const). Some methods also ensure the constancy of the mass of the product, in other methods - the mass increment is negligible and can be neglected. As a rule, there is no clear boundary of the modified surface layer ( ?m ? const). When applying external coatings, the size of the product increases (L And ?const) on the coating thickness ( ?PC ). The mass of the product also increases. In practice, there are also combined coatings. For example, when applying heat-shielding coatings, which are characterized by an increased number of discontinuities in the outer layer, heat resistance is provided by an internal non-porous coating.
1.2 Internal coatings
Internal coatings are created by various methods of influencing the surface of the original material (modification of the original surfaces). In practice, the following methods of influence are widely used: mechanical, thermal, thermal diffusion and high-energy with penetrating flows of particles and radiation.
There are also combined methods of influence, for example, thermomechanical, etc. In the surface layer, processes occur that lead to a structural change in the source material to a depth from the nanometer range to tenths of a millimeter or more.
Depending on the method of influence, the following processes take place:
change in the grain structure of the material;
Distortion of the crystal lattice, changing its parameters and type;
destruction of the crystal lattice (amorphization);
change in the chemical composition and synthesis of new phases.
1.3 External coatings
The practical value of external coatings is very high. The application of external coatings allows not only solving problems of changing the physicochemical properties of the original surfaces, but also restoring them after operation.
The formation mechanism and kinetics are shown in Figs. 3. External coatings often play the role of a structural element, for example, film coatings in the production of integrated circuits. To date, a large number of methods have been developed for applying coatings for various purposes from many inorganic materials.
To analyze the physicochemical processes associated with the deposition of coatings, it is advisable to systematize them according to the conditions of formation, it seems possible to distinguish the following groups of coatings formed on a solid surface: solid-phase, liquid-phase, powder and atomic.
1.4 Surface preparation for coating
Surface preparation determines the main quality indicator - the adhesion strength of the coating to the base material of the product, or adhesive strength. Some exceptions are coatings formed on a molten surface, for example, when coatings are deposited with concentrated heat sources. However, even in this case, contaminated surfaces adversely affect the properties of the coating material. Its embrittlement is observed, the tendency to form defects increases: cracks, porosity, etc. In this regard, surface preparation is a key operation in the technological process of applying any coatings.
When preparing the surface, two important tasks must be solved:
) removal from the surface of adsorbed substances - pollution;
) surface activation.
Removal of contaminants and activation of the surface can be carried out both in a single technological process and separately. In principle, any removal of physically or chemically adsorbed substances from a surface already activates that surface.
Broken bonds of surface atoms are restored, their asymmetry is restored and, accordingly, the level of surface energy increases. The greatest effect in surface preparation is achieved when, along with the removal of contaminants, its highest activation occurs. In real technological processes, such surface preparation is not always possible. Usually use two or three-stage separate preparation. The final stage is mainly aimed at activating the surface to its maximum values.
In the practice of applying coatings, the following main methods of preparing the surface of products have been used: washing with cold or hot water; degreasing; etching; mechanical impact; thermal and chemical-thermal effects; electrophysical impact; exposure to light fluxes; dehydration.
2. Chemical and electrochemical coating methods
The production of coatings from solutions by chemical and electrochemical methods is a classic example of processes that make it possible to trace the formation of deposited layers in a relatively pure way by successively attaching atoms to the surface of the coated product during its interaction with the ion-reaction medium.
There are standard definitions of methods for obtaining coatings from aqueous solutions - electrolytes (GOST 9.008-82).
The chemical method of obtaining coatings is the production of a metallic or non-metallic inorganic coating in a salt solution without electric current from an external source. Examples of obtaining coatings by a chemical method are: for metal coatings obtained by reduction - nickel plating, copper plating, silvering, etc.; for non-metallic coatings obtained by oxidation - oxidation, phosphating, chromating, etc. The latter are also used for additional processing of the coating.
The electrochemical method of obtaining a coating is the production of a metallic or non-metallic inorganic coating in an electrolyte under the action of an electric current from an external source.
Metal cathodic reduction is an electrochemical method for producing a metal coating on the metal that is the cathode.
Anodic oxidation is an electrochemical method for producing a non-metallic inorganic coating on a metal that is an anode.
The contact method of obtaining a coating is the preparation of a coating from a solution of salts of the deposited metal by immersing the coated metal in contact with a more electronegative metal.
2.1 Classification of chemical and electrochemical coatings
Chemical and electrochemical coatings can be classified based on the following basic principles:
According to the method of production (chemical, electrochemical, galvanic, cathodic, anodic oxide and contact);
By type of applied material (metallic, non-metallic and composite);
According to the requirements for the coating (protective, protective-decorative, decorative, special);
In relation to the external chemically active medium (cathode, anode, neutral);
According to the coating design (single-layer, multi-layer).
2.2 The essence of the chemical coating method
The coatings obtained by the chemical method are characterized by lower porosity than those deposited by the galvanic method at the same thickness and high uniformity.
Chemical deposition of metals is a reduction process proceeding according to the equation:
Mez+ + Ze?M
where Me z+ - metal ions in solution; z is the valency of the metal; Ze is the number of electrons; Me - coating metal.
Metal ions in solution (Me z+ ) combine (depending on valence) with the appropriate number of electrons (Ze) and turn into a metal (Me).
In the case of chemical deposition, the necessary electrons are generated as a result of a chemical process occurring in the solution used to obtain the coating. During galvanic deposition, the electrons necessary for the reduction of metal ions are supplied by an external current source .Depending on the chemical process that occurs during the deposition of the coating, the following methods are distinguished.
The contact method (immersion), in which the coated metal is immersed in a solution containing a salt of a more electropositive metal, and the coating in this case is deposited due to the potential difference that occurs between the coated metal and the ions in the solution. The contact-chemical method (internal electrolysis), in which the deposition is carried out due to the potential difference that occurs when the coated metal is in contact with a more electronegative metal in the process of immersion in a metal salt solution, which is used for coating. A chemical reduction method in which the metal to be coated is immersed in a solution containing a salt of the metal to be deposited, buffering and complexing additives and a reducing agent, while the ions of the deposited metal are reduced as a result of interaction with the reducing agent and deposited on the metal to be coated, and this reaction occurs only on the metal surface, which is catalytic for this process.
2.3 Coating the product
Applicable in domestic or foreign enterprises technological equipment for the deposition of coatings by chemical reduction, they are designed based on specific production tasks: large parts are hung into baths using special devices, small ones are covered with bulk in drums, pipes (straight lines or coils) - at installations that provide the possibility of pumping the solution through internal cavities, etc. e. Often, chemical coating plants are located in galvanizing shops, which makes it possible to use the equipment available there for degreasing, insulating, pickling, washing, drying and heat treating parts.
A simplified diagram of the apparatus for applying chemical coatings is shown in fig. 4.
Chemical coating is carried out in non-flowing or flowing solutions. In some cases, the solution after processing 1-2 batches of parts in it is poured out and replaced with fresh; in others, the solution is filtered, adjusted, and reused. An installation for a one-time coating of parts in a non-flowing solution usually has a welded iron or porcelain bath, which is inserted into a larger container - a thermostat. The space between the walls of both baths is filled with water or oil, which is heated by electric heaters or live steam. Outside, the thermostat has a heat-insulating layer (for example, from sheet asbestos, on which a casing is put on). A contact thermometer with a thermostat is placed in the bath to maintain the required temperature of the working solution.
3. Vacuum condensation coating
The methods and technological features of vacuum condensation coating (VKNP) have much in common, and in this regard, it is advisable to consider a generalized scheme of the process. A generalized scheme of the vacuum condensation coating process is shown in Fig. 1. five.
It is known that coatings during vacuum condensation deposition are formed from a stream of particles in the atomic, molecular, or ionized state. Neutral and excited particles (atoms, molecules, clusters) with normal and high energies and ions with a wide energy range pass into the coatings. The flow of particles is obtained by evaporation or spraying of the material by exposing it to various energy sources. Particle flows of the applied material are obtained by thermal evaporation, explosive evaporation - sputtering and ion sputtering of a solid material. The application process is carried out in rigid sealed chambers at a pressure of 13.3 - 13.3 10-3Pa due to which they provide the necessary length of free path of particles and protection of the process from interaction with atmospheric gases. The transfer of particles towards the condensation surface is carried out as a result of the difference in the partial pressures of the vapor phase. The highest vapor pressure (13.3 Pa or more) near the spraying (evaporation) surface causes particles to move towards the product surface, where the vapor pressure is minimal. Other transfer forces act in a stream of particles in an ionized state; ionized particles have more energy, which facilitates the formation of coatings.
Vacuum condensation coating methods are classified according to various criteria:
According to the methods of obtaining a vapor flow from the coating material and forming particles: thermal evaporation of a material from a solid or molten state, explosive (intensified) evaporation - spraying; ion sputtering of a solid material;
According to the energy state of particles: application by neutral particles (atoms, molecules) with different energy states; ionized particles, ionized accelerated particles (in real conditions, various particles are present in the flow);
According to the interaction of particles with the residual gases of the chamber: application in an inert rarefied medium or high vacuum (13.3 MPa); and in an active rarefied medium (133 - 13.3 Pa).
The introduction of active gases into the chamber makes it possible to proceed to the method of vacuum reaction deposition of coatings. Particles in the flow or on the condensation surface enter into chemical interaction with active gases (oxygen, nitrogen, carbon monoxide, etc.) and form the corresponding compounds: oxides, nitrides, carbides, etc.
The classification of vacuum condensation coating is shown in fig. 6. The choice of the method and its varieties (methods) is determined by the requirements for coatings, taking into account economic efficiency, productivity, ease of control, automation, etc. The most promising methods are vacuum condensation deposition with ionization of the flow of sprayed particles (plasma stimulation); often these methods are called ion-plasma.
The following basic requirements are imposed on products obtained by vacuum condensation methods:
Compliance with the requirements of the modern industry;
Low pressure of saturated vapors of the product material at the process temperature;
Possibility of heating the surface to increase the adhesive strength of coatings.
Vacuum condensation coatings are widely used in various fields of technology. The vacuum reaction process creates wear-resistant coatings for products for various purposes: friction pairs, pressing and cutting tools, etc.
Vacuum condensation deposition makes it possible to obtain coatings with high physical and mechanical properties; from synthesized compounds (carbides, nitrides, oxides, etc.); thin and uniform; using a wide class of inorganic materials.
Technological processes associated with vacuum condensation application do not pollute the environment and do not violate the environment. In this respect, they compare favorably with chemical and electrochemical methods for applying thin coatings.
The disadvantages of the method of vacuum condensation deposition include the low productivity of the process (the condensation rate is about 1 μm/min), the increased complexity of the technology and equipment, and the low energy coefficients of spraying, evaporation and condensation.
It is advisable to consider the process of vacuum condensation deposition of coatings as consisting of three stages:
The transition of the condensed phase (solid or liquid) into the gaseous (steam);
Formation of the flow and transfer of particles to the condensation surface;
Vapor condensation on the surface of the product - the formation of a coating.
To obtain high-quality coatings, flexible process control is necessary by creating optimal conditions for their flow.
4. Application of hardfacing coatings with concentrated heat sources
The deposition of coatings by welding with concentrated heat sources is carried out in the form of separate passes, each of which forms a bead of molten material with a width b. Roller overlap ?b usually is (1/4 - 1/3)3. The coating material consists of a molten base material and a filler material, which is fed into the bath. If the base material is not melted, then the weld bead is formed only from the filler material, in which case the share of the base material in the formation of the deposited coating is zero. The most widespread methods of surfacing by concentrated heat sources with a slight melting of the base material with a height h n . Bead height h n usually is 2 - 5 mm. When the rollers overlap, longitudinal grooves (irregularities) with a depth of 1–2 mm are formed.
Knowing chemical composition the base and filler material and the proportion of their participation in the formation of the coating material, it is possible to determine the chemical composition of the deposited layer.
Under the action of a concentrated source of heat, the base material is locally heated, especially when it is melted. The heat flow is removed to the base material, forming a heat-affected zone (HAZ) in it. In the high-temperature region of the HAZ, as a rule, grain growth is observed, a hardened structure, hot and cold cracks are formed. In practice, surfacing tends to minimize the length of the HAZ.
Under the influence of a heat source, the molten metal is displaced from the bath in separate portions, which, during crystallization, form a bead of deposited material. The crystallization process takes place on the basis of melted grains of the base material, the main axis of the crystallites is oriented in accordance with the direction of heat removal to the base material. During crystallization, the formation of defects is possible: hot and cold cracks, porosity, slag inclusions, etc. The nature of the formation of the coating from individual deposited beads (passages) with overlap does not allow obtaining thin and uniform overlays. The minimum coating thickness of 1 - 2 mm can only be achieved using precision technology. For surfacing coatings, metal materials are mainly used, sometimes various refractory non-metallic compounds are introduced into the molten metal.
4.1 Classification of deposited coatings
The classification of deposited coatings is carried out according to various criteria. Best classified according to:
concentrated heat sources;
the nature of the protection of the molten metal;
degree of mechanization.
According to the sources of heat, the surfacing of coatings is divided into:
flame;
plasma;
light beam;
electron beam;
induction;
electroslag.
By the nature of the protection of the molten metal, they distinguish: surfacing with slag, gas and gas-slag protection. According to the degree of mechanization, manual and mechanized surfacing with automation elements will be changed.
4.2 Applications for hardfacing
Surfacing with concentrated heat sources is used to restore worn surfaces, while coatings, as a rule, give a high economic effect. However, surfacing can also be used to create the initial surfaces of new products with a wide range of physical and chemical properties, for example, when creating exhaust valves in internal combustion engines, in the production of drilling tools, etc.
It is especially advisable to use hardfacing to create wear-resistant surfaces in friction pairs, and minimal wear can be achieved due to both an increase in hardness in the hardfaced layer and a decrease in the friction coefficient. A large economic effect is known when creating a cutting tool. High-speed steel in the deposited coating was obtained by argon-arc surfacing with the supply of filler wire from tungsten-molybdenum alloys with a high carbon content (0.7 - 0.85 wt.%). For surfacing heavily loaded dies during hot stamping, coated electrodes were used, for example, TsI-1M (type EN - 80V18Kh4F - 60, type F). Surfacing of wear-resistant coatings is widely used in the production of earth-moving equipment. In general, surfacing methods are highly effective, their disadvantages include:
a large thickness of the deposited layer (with some exceptions);
the presence of an extended heat-affected zone in the base material;
high surface roughness, which requires subsequent machining;
a limited range of deposited materials, mainly metal.
5. Coating by cladding
Cladding includes a wide range of coating methods. These include:
Explosive shock;
Magnetic shock;
Hot isostatic pressing, or cladding;
Obtaining a mechanical bond by extrusion.
With such a classification, cladding methods and methods with the formation of a diffusion bond overlap somewhat. Cladding methods are classified according to the rate of bond formation between the coating and the substrate:
1. Very fast processes (explosion cladding, impact electromagnetic);
Moderately fast processes (rolling, extrusion);
Slow processes (diffusion welding, hot isostatic pressing).
More commonly, cladding is used to coat iron alloys with nickel-based alloys. Cobalt cladding of steel is less common, mainly due to high costs.
Among the cladding methods, rolling and extrusion seem to be the most widely used. Explosive coatings were discovered by accident in 1957. Hot isostatic pressing and electromagnetic impact coatings are relatively new techniques. Diffusion bond coatings were developed in the early 20th century for the purpose of coating iron with nickel alloys and other high temperature alloys for special applications.
6. Gas-thermal coating methods
Assuming the type of heat source as the basis for separation, the following spraying methods have been applied in practice: plasma, gas-flame, detonation-gas, arc and high-frequency metallization.
The first thermal coatings were obtained at the beginning of the 20th century. M. W. Shoop, who sprayed the molten metal with a gas jet and, directing this flow onto the base sample, obtained a coating layer on it. This process was called shoping after the author's name, and it was patented in Germany, Switzerland, France and England. The design of the first gas-flame wire metallizer Schoop dates back to 1912, and the first electric arc wire metallizer - to 1918.
In the domestic industry, gas-flame metallization began to be used from the end of the 20s. In the late 1930s, it was successfully replaced by electric arc plating. The equipment for electric arc metallization was created by N. V. Katz and E. M. Linnik.
Thermal spraying of coatings in world practice began to develop actively in the late 50s. This was facilitated by the creation of a reliable technique for generating low-temperature plasma; detonation gas explosive devices, improvement of arc discharge processes.
Many scientific teams of the USSR Academy of Sciences, technical higher educational institutions, branch institutes and industrial enterprises have been involved in the development of the theory, technology and equipment for thermal spraying. At a similar pace, work was developing in the main leading foreign countries.
6.1 Classifications of methods
There is much in common in the methods and technology of thermal spraying. The scheme of the thermal spraying process is shown in fig. 7.
Sprayed material in the form of powder, wire (cords) or rods is fed into the heating zone. Distinguish between radial and axial material feed. The heated particles are sprayed with gas, the main purpose of which is to accelerate the sprayed particles in the axial direction, but along with this, it can also perform other functions. When wire or rods are fed into the heating zone, the spraying gas disperses the molten material; in a number of spraying methods, it also performs the function of heating.
The heating of particles, their dispersion and acceleration by a gas flow predetermined the name of the process - thermal spraying. Particles entering the coating formation surface must ensure the formation of strong interatomic bonds during the contacting process, which requires their heating and the corresponding speed. It is known that the temperature of the particles determines the thermal activation in the contact area; the velocity of particles upon impact on the surface creates the conditions for the mechanical activation of the surface contact. It should be taken into account that at high particle velocities, at the moment of their contact, part of the kinetic energy is converted into thermal energy, which also contributes to the development of thermal activation.
The developed methods of gas-thermal spraying make it possible to regulate within sufficient limits the temperatures and velocities of particles entering the coating formation surface.
Methods of thermal spraying classify:
by types of energy;
by type of heat source;
by type of sprayed material;
by types of protection;
according to the degree of mechanization and automation;
by the periodicity of the particle flow.
According to the type of energy, there are methods using electrical energy (gas-electric methods) and methods in which thermal energy is generated due to the combustion of combustible gases (gas-flame methods). The following types of heat source are used to heat the sprayed material: arc, plasma, high-frequency discharges and gas flame. Accordingly, the spraying methods are called: electric arc plating, plasma spraying, high-frequency plating, flame spraying, detonation gas spraying. The first three methods are gas-electric, the last - gas-flame.
According to the type of sprayed material, powder, wire (rod) and combined spraying methods are used. In combined methods, flux-cored wire is used. The following spraying methods are known according to the type of protection: without process protection, with local protection and with general protection in sealed chambers. With general protection, a distinction is made between conducting the process at normal (atmospheric) pressure, elevated and at rarefaction (in low vacuum).
The degree of mechanization and automation of the process. With manual methods of spraying, only the supply of the sprayed material is mechanized. In mechanized methods, the sprayer is also moved relative to the product to be sprayed. Often use the movement of sprayed products relative to a fixed sprayer. The level of automation of deposition processes depends on the design of the installation, in the simplest versions there is no automation, and in complex complexes full automation of the process is possible.
Flow frequency. Most sputtering methods are carried out with a continuous flow of particles. For some methods, only cyclic process control is possible. The coating is formed in a pulsed deposition mode, alternating with pauses. Gas-thermal spraying methods are widely used for applying coatings for various purposes. The main advantages of thermal spraying methods include high productivity of the process with satisfactory quality of coatings.
7. Plasma spraying of coatings
The plasma jet is widely used as a source of heating, atomization and acceleration of particles in the deposition of coatings. Due to the high flow rate and temperature, the plasma jet allows you to spray almost any material. A plasma jet is obtained in various ways: by arc heating of the gas; high frequency induction heating, electric explosion, laser heating, etc.
A generalized scheme of the process of plasma spraying of coatings is shown in fig. 8. With plasma spraying, both radial and axial supply of the sprayed material in the form of powder or wire (rods) is possible. Various types of plasma jets are used: turbulent, laminar, subsonic and supersonic, swirling and non-swirling, axisymmetric and plane-symmetrical, continuous and pulsed, etc.
Laminar jets provide much larger outflow lengths (l n ,l from ), due to which the heating time of the sprayed particles increases, and are characterized by higher values of the ratio of the input energy to the flow rate of the plasma-forming gas. Laminar jets should be classified as high-enthalpy. In addition, they are characterized by a high flow rate and a lower noise level (up to 40 - 30 dB). At present, solutions have not yet been found that allow the widespread use of laminar jets for spraying. Difficulties are associated mainly with the supply of powder. The theory and practice of applying coatings with laminar jets was developed by A. V. Petrov.
Supersonic plasma jets are also quite promising for sputtering. High speeds of sprayed particles (800 - 1000 m/s and more) make it possible to form coatings mainly without their melting
The modern level of plasma spraying is mainly based on the use of subsonic and supersonic, turbulent, axisymmetric, plasma jets with a wide range of thermal properties. About half of the power supplied to the atomizer is spent on heating the plasma-forming gas. Typically, the thermal efficiency of the atomizer is 0.4-0.75. It should also be noted the weak use of the plasma jet as a source of heat for heating powder particles. Effective efficiency of heating powder particles by plasma ?P is in the range of 0.01 - 0.15. When spraying the wire, the effective efficiency is much higher and reaches 0.2 -0.3.
The most important thermophysical characteristics of plasma jets, which determine the optimal conditions for heating, spraying, and accelerating sprayed particles, include specific enthalpy, temperature, and velocity in various sections along the flow axis. Flexible control of the thermophysical parameters of the jet determines the manufacturability of the process and its capabilities.
According to the degree of protection of the process, plasma spraying is distinguished: without protection, with local protection and general protection.
7.1 Advantages and disadvantages of the plasma spraying method
The main advantages of the plasma spraying method:
high process productivity from 2 - 8 kg / h for plasma torches with a power of 20 - 60 kW to 50 - 80 kg / h with more powerful atomizers (150 - 200 kW);
versatility in sprayed material (wire, powder with different melting points;
a large number of parameters that provide flexible control of the deposition process;
regulation of the quality of sprayed coatings over a wide range, including obtaining especially high-quality process leads with general protection;
high values of KIM (when spraying wire materials 0.7 - 0.85, powder - 0.2 - 0.8);
the possibility of complex mechanization and automation of the process;
wide availability of the method, sufficient efficiency and low cost of the simplest equipment.
The disadvantages of the method include:
low values of the energy utilization factor (for wire spraying ?to = 0.02 - 0.18; powder - ?And = 0,001 - 0,02);
the presence of porosity and other types of discontinuities (2 - 15%);
relatively low adhesive and cohesive strength of the coating (maximum values are 80 - 100 MPa);
high noise level during open process (60 - 120 dB).
As the plasma spraying method improves, the number of disadvantages decreases. Promising, for example, is the development of spraying with a supersonic outflow of a plasma jet, which makes it possible to form coatings mainly from particles without melting, which are in a viscoplastic state. Compared with the radial, the most efficient is the axial supply of the sprayed material in arc plasma sprayers.
Of considerable interest is plasma spraying using two-arc or three-phase plasma torches. The use of high-frequency plasma torches promises great advantages. In these cases, a plasma is obtained that is not contaminated with electrode materials, and the axial supply of the sprayed material is simplified.
8. Flame spraying of coatings
A gas flame is produced by the combustion of combustible gases in oxygen or air. In special burners-sprayers, a combustible mixture is supplied along the periphery of the nozzle, the central part is designed to supply the sprayed material into the formed gas-flame jet. Near the nozzle exit, the gas flame is a cone; as it moves away from the nozzle exit, the gas flame forms a continuous flow of high-temperature gas. There are laminar (R e < Recp ) and turbulent jets (R e > R ecr ). The transition of the combustion regime and jet flow from laminar to turbulent depends on the nature of the combustible gas and is determined by the Reynolds numbers (Re =2200 - 10000).
Gas-flame jets as a source of heating, spraying and acceleration during coating deposition are similar to plasma jets. However, the temperature, enthalpy, and velocity of the gas-flame jet are much lower. The sprayed particles interact with a complex gas phase consisting of combustible gases, products of their combustion and dissociation, oxygen and nitrogen. The redox potential in the initial section of the jet is easily controlled by changing the ratio between the combustible gas and oxygen. Conventionally, three modes of flame formation can be distinguished: neutral, oxidizing and reducing.
The following combustible gases are used for spraying coatings: acetylene (C 2H 2), methane (CH 4), propane (C 3H 8), butane (C 4H1 0), hydrogen (H 2), etc. Sometimes mixtures are used, for example, propane-butane, etc.
Flame spraying is carried out in an open atmosphere. Air enters the gas flame torch, and therefore the amount of oxygen is greater than required for the complete oxidation of the combustible gas elements according to the above reactions. To balance the compositions, the amount of oxygen in the combustible gas-oxygen mixture is reduced.
The highest flame temperature is achieved when using acetylene-oxygen mixtures. However, the calorific value is higher for propane and butane. Therefore, standard technical acetylene or a propane-butane mixture is most often used for spraying. With the formation of gas-plasma jets, the thermal efficiency of the atomizer is quite high ( ?t.r. \u003d 0.8 - 0.9). In this case, most of the supplied energy is spent on heating the gas. However, the effective efficiency of heating powder particles ( ?And ) composition is only 0.01 - 0.15.
1 Flame spraying methods
The generalized scheme of the flame spraying process is shown in fig. nine.
Combustible gas and oxygen (rarely air) enters the mixing chamber 3, the combustible mixture then enters the nozzle device 7, at the outlet of which the mixture is ignited and forms a flame 2. To compress the gas flame, an additional nozzle 4 is used, into which compressed gas is supplied, usually air or nitrogen. The outer wake of the gas flow 5 lengthens the high-temperature gas jet, increases its temperature, enthalpy and speed, in addition, the gas can be used to cool the heat-stressed elements of the atomizer.
The sprayed material in the form of powder or wire (rods) is fed along the axis of the gas-flame jet inside the torch, which contributes to more intense heating and spraying of the material.
Flame spraying methods are classified according to the following criteria:
Type of sprayed material. Flame spraying is distinguished by powder and wire (rod) materials.
combustible gas type. Known methods of spraying using acetylene or gases, substitutes for acetylene (propane, butane, mixtures thereof, etc.).
degree of mechanization. Apply manual spraying and mechanized (machine). With manual methods, only the supply of the sprayed material is mechanized. In fully mechanized methods, the sprayed product is moved relative to the sprayer or vice versa, and automation elements are introduced.
2 Flame sprayers
In our country, a number of installations for flame spraying with wire and powder materials are produced. Acetylene and propane-butane mixture are used as energy gases. Acetylene (or a substitute), oxygen, and in some cases additional gas (air) for spraying is supplied to the atomizer from the gas supply unit. The gas supply unit is not included in the set of the manufactured apparatus. It is mounted directly on the working area. Flame sprayers are usually equipped with a sprayer (gun), a wire or powder feeder and a control panel. Often, the wire feed mechanism is located in the same housing as the sprayer, on which the powder feeder is attached.
Conclusion
Modern production taking into account modern achievements in science and technology, it requires the creation of a powerful base for the implementation of new methods for applying coatings from various groups of inorganic materials. Coatings with a wide range of physical and chemical properties are required: for protection in various environments; wear-resistant; optical; heat protection and many others. Significant efforts are also required to improve existing and long-established coating methods.
To solve these problems, an integrated approach is required, not only related to the solution of specific scientific and technical aspects of creating new technologies in the field of coatings, but the task of optimizing and coordinated storage and dissemination of information is becoming increasingly important.
List of used literature
1. Grilikhes, S.Ya., Tikhonov, K.I. Electrolytic and chemical coatings. L.: Chemistry, 1990. -288 p.
Kovensky, I.M., Povetkin, V.V. Methods for the study of electrolytic coatings. -M.: Nauka, 1994. -234 p.
Molchanov V.F. Combined electrolytic coatings - Kyiv: Technique, 1976. -176 p.
Dasoyan, M.A., Palmskaya, I.Ya., Sakharova, E.V. Technology of electrochemical coatings. -L.: Mashinostroenie, 1989. -391 p.
Eichis, A.P. Coatings and technical aesthetics. - Kiev: Technique, 1971. - 248 p.
Biront, V.S. Coating: a textbook for university students. - Krasnoyarsk. GATsMiZ, 1994. - 160 p.
Bobrov, G.V. Application of inorganic coatings (theory, technology, equipment): a textbook for university students. / G.V.Bobrov, A.A. Ilyin. - M.: Intermet Engineering, 2004. - 624 p.
8. Liner, V.I. Protective coatings of metals / V.I. Liner, - M.: Metallurgy, 1974. - 560 p.
9.. Nikandrova, L.I. Chemical methods for obtaining metal coatings./ L.I. Nikandrov. - L.: Mashinostroenie, 1971. 101 p.
Corrosion.: Reference edition. / Ed. L.L. Schreier. - M.: Metallurgy. 1981. - 632 p.
Chemical-thermal treatment of metals and alloys.: Handbook / Ed. L.S. Lyakhovich. M.: Metallurgy, 1981.-.424 p.
Kolomytsev, P.T. Heat-resistant diffusion coatings / P.T. Kolomytsev. - M.: Metallurgy, 1979. - 272 p.
Hawking, M. Metallic and ceramic coatings / M. Hawking, V. Vasantasri, P. Sidki. - M.: Mir, 2000. - 516 p.
Physical and chemical modification is understood as a purposeful change in surface properties as a result of technological external influence. This refers to a change in the structure of the material in thin surface layers due to physical impact (ion and electron beams, low-temperature and high-temperature plasma, electric discharge, etc.) or chemical impact, leading to the formation on the surface of layers of chemical compounds based on the base material (chemical , electrochemical and thermal oxidation, phosphating, sulfiding, plasma nitriding, etc.).
Obviously, there is no pronounced classification boundary between the processes of physicochemical modification and surface hardening.
Among the many methods of physicochemical modification, the most promising are ion implantation, anodizing, in particular, pulsed (treatment in an electrolyte plasma), and laser hardening.
Ion implantation is a relatively new method of physical and chemical modification, based on the introduction of accelerated ions of alloying elements into the surface layer.
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Rice. 19.12. Scheme of the installation for ion implantation with a linear accelerator (but ) from D-implants (b):
1 - ion source; 2 - ion extraction system; 3 - separator; 4 - focusing magnets; 5 - linear accelerator; 6 - electrostatic deflecting system; 7 - ion flow; 8 - hardened parts
Implanted ions have a shallow penetration depth, but their influence extends much farther from the surface.
The following features of ion implantation can be distinguished:
The possibility of forming on the surface of alloys that cannot be obtained under normal conditions due to the limited solubility or diffusion of the components. In a number of cases, the equilibrium solubility limits are exceeded by several orders of magnitude;
Doping is not associated with diffusion processes, except for the modification of ion implantation materials at high current density, when radiation-stimulated diffusion of components is observed;
The process proceeds at low temperatures (less than 150 °C), without changing the mechanical properties of the material. The method allows the processing of heat-sensitive materials;
There is no noticeable change in the dimensions of parts after implantation;
Modified surfaces do not require further finishing;
The process is well controlled and reproducible;
Environmental cleanliness of processes;
Only exposed surfaces that are directly exposed to ion irradiation are hardened;
Shallow depth of the modified layer;
Relatively high cost of equipment.
The installation for implantation with an ion beam contains an ion source, an ion "pulling" system 2, an ion separator 3, magnetic focusing lenses 4, a linear accelerator 5, an electrostatic deflection system b. In practice, continuous and pulsed ion sources of various designs are used, generating gas ions (from hydrogen to krypton) and metals (with a hot and cold cathode, magnetron, diaplasmotron, etc.). The ions leaving the source are heterogeneous in composition. To separate foreign ions, a magnetic mass separator is used, which deflects ions having a different mass and charge from the main axis. The "purified" ion beam is focused and accelerated in a linear accelerator. Scanning of the ion beam over the surface of the hardened part is performed by a deflecting system 6 .
To ensure uniform hardening, the part rotates and rotates relative to the beam.
Ion implantation with plasma ions - sometimes called 3D implantation - is performed in vacuum chambers, where an ionized medium is created by a glow or arc discharge, and a pulsed high voltage is applied to the part, which ensures the acceleration of ions in the direction of the bombarded surfaces. A high-energy ion flux can be formed directly in the process of burning a pulsed self-sustained discharge between a grounded vacuum chamber and a product that is a cathode.
Ions accelerated in the cathode fall field of small thickness effectively modify the surface of the product, which can have a complex three-dimensional shape. The incident ions generate an electron beam from the product surface, which, interacting with the plasma, ensures the self-sustaining of the discharge. This method has certain advantages over beam methods due to its simplicity and relatively low cost of implementing technological processes. It can be combined with other ion-plasma processing methods, such as magnetron, vacuum-arc and plasma-thermal spraying, ion nitriding, etc.
High-energy ion implantation uses gas ions with energies up to 100 keV to harden metals and alloys, ceramics, and polymers.
Treatment with high-energy nitrogen ions effectively increases the durability of cutting and stamping tools, and the fatigue strength of parts.
The implantation of interstitial atoms (nitrogen, carbon, and boron) improves the wear resistance and fatigue resistance of steels. These elements have the property of dislocation segregation even at room temperature, which blocks their movement and strengthens the surface layer, which in turn prevents the development of fatigue cracks.
With ion implantation of nickel with boron, the fatigue strength increases by more than 100%.
The increase in fatigue strength is due not to the action of residual compressive stresses arising during ion implantation, as previously thought, but to the inhibition of the development of fatigue cracks due to a decrease in the mobility of dislocations.
To improve the anti-friction properties, molybdenum ions and a double amount of sulfur ions can be implanted. Joint implantation can become a new method for the formation of anti-friction and other special alloyed layers.
By implanting titanium, an amorphous Ti-C-Fe phase is obtained on the surface, which leads to a decrease in friction and wear.
Ion implantation is widely used to improve the corrosion resistance of steel parts. For this purpose, ions are implanted.
Local heat treatment carries out modification of the structure of the surface layer. At the same time, such temperature-time conditions and hardening results are provided that are difficult or impossible to obtain by traditional heat treatment methods, namely:
High heating and cooling rates (heating rates reach 10 4 ... 10 8 K / s, and cooling rates - 10 3 ... 10 4 K / s, depending on the exposure time and radiation energy, as well as on the operating modes of the laser ). Such modes of heating and cooling lead to a nonequilibrium course of phase transformations, to a shift in critical points A with And BUT, the formation of supersaturated solid solutions with finely dispersed structures up to amorphous. As a result, a layer with increased hardness is formed (exceeds by 15 ... 20% hardness after hardening by existing methods), with good resistance to wear and seizure during friction;
The possibility of hardening surfaces in hard-to-reach places (cavities, recesses), where the laser beam can be introduced using optical devices;
The use of a laser makes it possible to sharply reduce the depth of the hardened layer and effectively control its size.
laser hardening used to harden cutting and measuring tools, working edges of dies and punches to a depth of up to 0.15 mm (pulsed radiation) and up to 1.5 mm (continuous radiation). On tool steels, the hardness is 63 ... 67 HRC. The roughness of the machined surface does not change.
It has been established that the use of laser radiation as a heating source in the thermoplastic hardening of nickel alloys makes it possible to obtain residual compressive stresses up to 10 GPa in the surface layer.
With laser heat treatment, it is possible to create conditions for the selective evaporation of the protrusions of irregularities, which lead to a decrease in surface roughness.
Laser hardfacing is one of the most promising ways to restore critical parts of gas turbine engines, in particular turbine and compressor blades. Its main advantages are the ability to eliminate small defects without heating the surface adjacent to the defect and the absence of a lead during surfacing.
Laser surfacing is carried out in chambers with a protective atmosphere or with inert gas blowing. Wire, foil or powder materials are used as filler materials.
Laser cladding with powder metal alloys with minimal thermal impact makes it possible to increase the performance of parts several times under severe temperature, erosion and other operating conditions.
INTRODUCTION
The processes of surface modification of conductive materials are widely used to create special properties of various products in optics, electronics, and also as a finishing treatment for a wide range of household and technical products. Existing mechanical polishing methods are time-consuming, complex and often lead to undesirable structural changes in the surface layer of products, the creation of additional stresses, which can be of decisive importance in the formation of thin films with special properties in microelectronics. The widely used electrochemical methods of polishing metal products are expensive, mainly due to the use of expensive acid electrolytes, which also cause great environmental damage to the environment. In this regard, the greatest importance is attached to the development and implementation of new technological processes that make it possible to maintain the quality and structure of the surface, to have high productivity and good environmental and economic indicators. Such processes include the polishing of various conductive materials by the electrolytic-plasma method. Unlike traditional electrochemical polishing in acids, the electrolytic-plasma technology uses environmentally friendly aqueous solutions of salts of low concentration (3–6%), which are several times cheaper than toxic acid components.
Disposal of spent electrolytes does not require special treatment facilities. The polishing time is 2–5 minutes and the deburring time is 5–20 seconds. This method allows processing products in four main areas:
- surface preparation before applying thin films and coatings;
- polishing of complex-profile surfaces of critical parts;
- deburring and blunting sharp edges;
- decorative polishing of metal products;
Currently, electrolytic-plasma processing of various steels and copper alloys is used at a number of enterprises in Belarus, Russia, Ukraine, as well as in China and other countries. The wide application of this technology is constrained by the limited range of polished materials and products, since electrolytes and modes of polishing products of complex shape and such metals as aluminum and titanium, as well as semiconductor materials, have not been developed. The search for effective electrolytes requires a deeper study of the mechanism for removing roughness and the formation of surface gloss during electrolytic-plasma action on conductive materials.
ISO-CHEMICAL PROCESSES UNDER ELECTROLYTE-PLASMA EXPOSURE
The operation of electrolytic-plasma processing installations is based on the principle of using pulsed electrical discharges that occur along the entire surface of a product immersed in an electrolyte. The combined effect of a chemically active medium and electrical discharges on the surface of a part creates the effect of polishing products. In the technology of electrolytic-plasma polishing, the workpiece is an anode, to which a positive potential is applied, and a negative potential is applied to the working bath. After some critical current and voltage densities are exceeded, a vapor-plasma shell is formed around the metal anode, pushing the electrolyte away from the metal surface. The phenomena occurring in the near-electrode region do not fit into the framework of classical electrochemistry, since a multiphase metal-plasma-gas-electrolyte system arises near the anode, in which ions and electrons serve as charge carriers /3/.
Polishing of metals takes place in the range of voltages of 200–350 V and current densities of 0.2–0.5 A/cm 2 /2.3/. At a voltage of more than 200 V, a stable thin (50–100 μm) vapor-plasma shell (VPO) is formed around the anode, which is characterized by small current fluctuations at U = const. The electric field strength in the shell reaches 10 4 –10 5 V/cm 2 . At a temperature of about 100 0 C, such an intensity can cause vapor ionization, as well as the emission of ions and electrons, which is necessary to maintain a stationary glow electric discharge in the near-electrode sheath. Near the microprotrusions, the electric field strength increases significantly, and pulsed spark discharges occur in these areas with the release of thermal energy.
It has been established by research that the stability and continuity of PPO, being a necessary condition for the implementation of the process of smoothing microroughness, are determined by a combination of various physicochemical parameters: electrical characteristics of the circuit, thermal and structural conditions on the treated surface, chemical and phase composition of the treated material, molecular properties of the electrolyte and hydrodynamic parameters liquids in the near-electrode area /1–4/.
ADVANTAGES OF ELECTROLYTE PLASMA TREATMENT
In the Republic of Belarus, for the first time, a new high-performance and environmentally friendly method of electrolytic-plasma processing of metal products made of stainless steels and copper alloys in aqueous salt solutions has found industrial application. This method is largely devoid of the disadvantages that are inherent in mechanical and electrochemical polishing, and additionally allows saving material and financial resources. The electrolytic-plasma technology has higher technical characteristics of the process, such as the processing speed of the product, the class of cleanliness of its surface, the absence of the introduction of abrasive particles and the degreasing of the surface. The process can be fully automated, large production areas are not required to accommodate the equipment (Fig. 1).
Figure 1. Scheme of installation for polishing conductive products. 1 - working bath; 2 - electric pump; 3 - preparatory bath; 4 - transformer; 5 - electrical cabinet; 6 - control panel.
The use of more high-performance methods of electrolytic-plasma polishing will make it possible to replace labor-intensive mechanical and toxic electrochemical processing. The process of polishing metals is environmentally friendly and meets sanitary standards; special treatment facilities are not required to clean the spent electrolyte.
The main technical solutions for the electrolytic-plasma technology of polishing a number of metals have been developed and patented in Germany and Belarus. Known electrolytes are suitable for processing a limited class of metals and do not polish aluminum, titanium, etc. The Institute of Energy Problems of the National Academy of Sciences of Belarus (now the Joint Institute for Energy and Nuclear Research - Sosny of the National Academy of Sciences of Belarus) has developed a new composition of electrolytes for polishing wrought aluminum alloys, which does not contain concentrated acid, non-aggressive to equipment, durable and low cost, an application for an invention was filed on May 20, 2002.
ECONOMIC INDICATORS OF ELECTROLYTE-PLASMA TREATMENT
When polishing 1 m 2 of a product by the classical electrochemical method, about 2.5 kg of acids are consumed at a cost of 3 conventional units, and when polished by an electrolytic-plasma method, about 0.1 kg of salts are consumed at a cost of 0.02 conventional units. Calculations show that with two-shift operation of electrolytic-plasma equipment for 200 days, the savings financial resources per year is about 30,000 USD, thus, with an installation cost of 26,000 USD. its payback does not exceed one year. In addition, this calculation does not take into account the savings obtained due to the absence of costs for treatment facilities.
In addition to the fact that the electrolytic-plasma technology has a higher productivity and is environmentally friendly, it has better economic performance compared to mechanical and electrochemical processing methods. Although the power consumption during electrolytic-plasma polishing (operating voltage is 220-320 V) is significantly higher than during processing by the traditional electrochemical method at low voltages, nevertheless, the total operating costs when using this technology are on average six times lower and this economic The gain is achieved primarily by replacing the expensive acidic electrolyte with a cheap aqueous salt solution. It should be noted that high purity reagents (salts) are not required to obtain the polishing effect, which significantly affects their cost. The economic indicators of the electrolytic-plasma technology are noticeably improved by a simplified scheme for the disposal of spent electrolyte and the absence of special treatment facilities.
Cost calculations using the technology under consideration show that with an increase in the power of the installation, when the maximum area of the polished surface per one load increases, the total unit costs (per 1 m 2 of surface) decrease, including the reduction of capital and operating costs separately. In this case, there is a shared redistribution of costs for individual items of expenditure. The given data are valid for continuous seven-hour operation of the installation per shift for twenty working days per month. The practice of using the proposed method shows that, depending on the size, shape, volume of a batch of processed products and the mode of operation of the installation, one should choose the appropriate capacity of the installation, which gives lowest cost and the shortest payback period.
PROSPECTS FOR ELECTROLYTE-PLASMA PROCESSING OF CONDUCTING MATERIALS
The Joint Institute for Energy and Nuclear Research - Sosny of the National Academy of Sciences of Belarus (JIPNR-Sosny) conducts research on the development of effective electrolytes for polishing a wide range of conductive materials and products, work is underway to refine the technology, create and implement equipment. Theoretical and experimental studies are aimed at optimizing the process taking into account thermophysical factors, such as the boiling crisis, as well as the physical parameters of the electrolyte (surface tension coefficient, viscosity, wetting angle) in order to develop scientifically based approaches to the search for electrolyte compositions that provide a given quality of processing a wide range of materials at minimum costs of the resources used (material, energy, time, labor, etc.).
For polishing stainless steels and copper alloys by the electrolytic-plasma method, JIPINR-Sosny has developed a power range of equipment EIP-I, EIP-II, EIP-III, EIP-IV costing from 4000 c.u. up to 22000 c.u. various productivity from 400 cm 2 to 11000 cm 2 for one loading. This product is export-oriented. Such installations have been supplied to many Belarusian, Russian and Ukrainian enterprises. In the manufacture of electrolytic-plasma equipment, materials and components manufactured in Belarus are used.
In order to save additional energy, a new economical power source and a two-stage polishing method have been developed using high operating voltages at the first stage of removing surface roughness and carrying out the second final stage of processing in an electrolyte at lower voltages. The energy-saving effect of equipping installations with a new power source and the use of a two-stage mode of polishing conductive products can be from 40 to 60% of the consumed electricity compared to the standard power sources used at a constant fixed voltage.
CONCLUSIONS
The most significant factors influencing the technological regime of electrolytic-plasma processing of conductive materials have been determined. It is shown that the new method of processing in an electrolyte has a number of technical and economic advantages compared to existing technologies for polishing surfaces of a wide range of products.
Widespread development in various industries of environmentally friendly methods of processing conductive materials will not only save material and labor resources and dramatically increase labor productivity in metalworking, but also solve a significant social task of significantly improving working conditions for engineering and technical personnel and creating a more favorable environmental situation at enterprises. and in the regions.
LITERATURE
- Patent No. 238074 (GDR).
- I.S.Kulikov, S.V.Vashchenko, V.I.Vasilevsky Peculiarities of electro-pulse polishing of metals in electrolytic plasma // VESTSI ANB ser. Phys-techn. Sciences. 1995. No. 4. pp. 93–98.
- B.R. Lazarenko, V.N. Duradzhi, Bryantsev I.V. On the structure and resistance of the near-electrode zone during heating of metals in an electrolyte plasma // Elektronnaya obrabotka materialov. 1980. No. 2. pp. 50–55.
- Patent of the Republic of Belarus No. 984 1995.
Kulikov I.S., Vashchenko S.V., Kamenev A.Ya.
The invention relates to the field of chemical-physical treatment of the surface layer of metal products made of titanium and its alloys in order to change their surface properties. The method includes physical and chemical surface treatment of products and aluminizing, while physical and chemical surface treatment of products is carried out by electrochemical polishing in an electrolyte of the following composition: perchloric acid - 1 part; acetic acid - 9 parts, at a temperature of 30-35 ° C, current density 2 A / dm 2, voltage 60 V, for 3 minutes. EFFECT: activation of the interaction of the surface of metal products with contacting media and substances, high scale resistance and corrosion resistance, high antifriction properties. 1 tab.
The invention relates to the field of chemical-physical treatment of the surface layer of metal products made of titanium and its alloys in order to change their surface properties.
Surface phenomena - an expression of the special properties of surface layers, i.e. thin layers of matter at the interface between bodies (mediums, phases). These properties are due to the excess free energy of the surface layer, the features of its structure and composition. The molecular nature and properties of the surface can radically change as a result of the formation of surface monomolecular layers or phase (polymolecular) films. Any "modification" of the surface (interfacial) layer usually leads to an increase or decrease in the molecular interaction between the contacting phases (lyophilicity and lyophobicity). Lyophilicity means good (often complete) wetting, low interfacial tension, resistance of surfaces to mutual sticking. Lyophobicity is the opposite concept.
When two solid bodies or a solid body come into contact with liquid and gaseous media, the properties of the surface determine the conditions for such phenomena as adhesion, wetting, and friction. Physical or chemical transformations in surface layers strongly influence the nature and rate of heterogeneous processes - corrosion, catalytic, membrane, etc. Surface phenomena largely determine the ways of obtaining and durability of the most important building and structural materials, in particular, those produced in metallurgy.
Wetting (lyophilicity) is a necessary condition for the surface saturation of titanium with aluminum and other elements (diffusion saturation with metals). The product, the surface of which is enriched with these elements, acquires valuable properties, which include high scale resistance, corrosion resistance, increased wear resistance, hardness and weldability.
The non-wettability (lyophobicity) of an unprotected metal increases its resistance to aggressive media.
The patent (RF patent 2232648, IPC B 05 D 5/08, publ. 2004.07.20) indicates that the properties of the surfaces manifest themselves in different ways. This is due to the fact that surfaces are made from a variety of materials, and in most cases they have a different structure. In particular, metals selected from the group consisting of beryllium, magnesium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, niobium, molybdenum, technetium, have the most lyophobic properties. ruthenium, rhenium, palladium, silver, cadmium, indium, tin, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, thallium, lead, bismuth, especially titanium, aluminium, magnesium and nickel, or a corresponding alloy of the named metals.
Carbide and oxide films have a great influence on the surface properties. Particularly dense films of carbide and oxide are observed in reactive metals, such as titanium and zirconium.
There is a known method for changing the surface properties of titanium-based alloys (W. Zwinger, "Titanium and its alloys", translated from German, Moscow, "Metallurgy", 1979, p. 326), in which the author claims that "the oxide layer, always existing on the surface of titanium, most often not wetted by metals. At elevated temperatures in melts, wetting occurs in cases of preliminary annealing of titanium in vacuum, when an oxide-free surface is formed. When bending such samples, cracks are formed.
The disadvantage of this method of preparing the surface for metallization is a complex and difficult to implement mechanism for processing multi-ton ingots, slabs, large-sized billets. In addition, the method does not take into account the impact on the same wettability of the surface of another interstitial element - carbon. Established (Kurapov V.N., Trubin A.N., Kurapova L.A., Saveliev V.V. “Study of the features of carbon distribution in titanium alloys by the method of radioactive tracers (RAI), Collection “Metallurgy and processing of titanium and heat-resistant alloys” Moscow, 1991; V.V. Tetyukhin, V.N. Kurapov, A.N. Trubin, L.A. Kurapova, "Investigation of ingots and semi-finished products of titanium alloys using the method of radioactive tracers (RAI)" Scientific and technical magazine "Titan", No. 1(11), 2002), that when the alloys are heated, carbon is transported to the surface layers from the underlying volumes, but does not leave the titanium crystal lattice, unlike steel, where, during high-temperature heating, carbon forms a volatile compound along formula:
C (TV) + O2 (gas) CO2 (gas) .
Therefore, in contrast to steel, where surface decarburization occurs, in titanium only its redistribution occurs in the surface layers. It has also been established that a similar redistribution of carbon in the surface layers of blanks and products occurs during metal cutting, which is a consequence of its local heating and deformation. This redistribution is observed in various types of cutting, including chiseling and filing, even in the “softest” modes, such as turning.
In contrast to the redistribution of carbon in the surface layers during high-temperature heating, which is visible on photographic film with the naked eye, in the case of metal cutting, redistribution is observed with an increase. This redistribution in the most superficial layer is more chaotic. In the depths of the metal, undulating curves of carbon redistribution in the surface layer, equivalent to mechanical and thermal loads arising during material processing, are revealed, which makes the physicochemical properties of the surface after cutting completely unstable. This instability, as shown above, is not eliminated by vacuum annealing.
A known method of cleaning the surface of silicon (RF Patent No. 1814439, publ. 1995.02.27, IPC H 01 L 21/306). The essence of the invention: silicon wafers are processed in a liquid etchant. The resulting oxide layer and the silicon surface are removed at room temperature by etching in xenon difluoride. This achieves a high degree of surface decarburization. Then the silicon wafers are transferred without contact with the atmosphere into a vacuum chamber and the fluorides adsorbed on the surface are removed by heating and holding at 600°C in an ultrahigh vacuum. To recrystallize the damped layer on the silicon surface, annealing can be carried out at a higher temperature.
This method is expensive and can be used when processing parts of small geometric dimensions.
A known method of surface chemical-thermal modification of friction units (RF Patent No. 2044104, publ. 20.09.1995, IPC C 23 C 8/40). The method includes interaction with the reaction liquid followed by heat treatment.
The disadvantages of this method include the fact that it is used to increase the wear resistance of structural materials, and fluorinated carbon, which is highly lyophobic, is used as a surface modifier, the surface is practically not wetted.
A known method of hot aluminizing products made of titanium and its alloys (SU 160068, publ. 01/14/1964) is a prototype in which the products are etched with solutions of sulfuric (35-65%) or hydrochloric (30-37%) acid at a temperature of 50-70 °C for 30-40 minutes or at room temperature for 2-3 hours to obtain a hydride film on them instead of an oxide one, after which the products are immersed in molten aluminum at a temperature of 800-850°C.
The disadvantage of this method is the properties of the hydride film, which has a brittle, porous nature, with a large number of microcracks and cavities that can penetrate to a depth of 0.2-0.3 mm, forming areas with a porous structure between the base metal and the coating. In addition, in the process of contacting molten aluminum with titanium hydrides, they decompose with the release of hydrogen, which predetermines the formation of pores in the aluminum coating. The combination of these factors sharply reduces the durability of the resulting coating.
The objective of the present invention is to increase the lyophilicity of the surface layer of workpieces and products made of titanium-based alloys by removing the surface layer containing oxides and carbides without the use of machining and annealing.
The technical result achieved in the implementation of the invention is the activation of the interaction of the surface of metal products with contacting media and substances, which gives them qualitatively new properties - high scale resistance and corrosion resistance, high anti-friction properties.
The specified technical result is achieved by the fact that in the method of modifying the surface layer of products made of titanium and its alloys, including physical and chemical surface treatment of products and aluminizing, physical and chemical surface treatment of products is carried out by electrochemical polishing in an electrolyte of the following composition: perchloric acid - 1 part; acetic acid - 9 parts, at a temperature of 30-35 ° C, current density 2 A / dm 2, voltage 60 V, for 3 minutes.
During electrochemical treatment, under the action of an electric current in the electrolyte, the anode material (the surface layer of the product) dissolves, and the protruding parts of the surface dissolve most quickly, which leads to its leveling. At the same time, the material, incl. an oxide or carbide film is removed from the entire surface, in contrast to mechanical polishing, where only the most protruding parts are removed. Electrolytic polishing makes it possible to obtain surfaces of very low roughness. An important difference from mechanical polishing is the absence of any changes in the structure of the processed material, which does not cause a redistribution of carbon over the thickness of the product and its focal concentration on the surface.
There is a complete removal of the surface layer containing oxides and carbides, and the surface of products made of reactive metals acquires a high lyophilicity, which allows high-quality chemical-thermal treatment of the surface layer, such as aluminizing.
The proposed method was tested when aluminating samples of titanium alloy VT8 in a melt of aluminum brand A85 for 4 hours at a temperature of 850°C. Four samples were made with different methods of surface preparation, and the following results were obtained (table):
| Tab. | ||
| № | Surface preparation method | Aluminizing quality |
| 1 | fine turning | No sticking of aluminum on the surface. |
| 2 | mechanical polishing | Focal adhesion (thin layer on approximately 42-57% of the surface). |
| 3 | Electrochemical polishing in the electrolyte of the following composition: perchloric acid - 1 part, acetic acid - 9 parts. At an electrolyte temperature of 30-35°C, current density - 2 A / dm 2, voltage - 60 V, within 3 min. | Aluminum sticking over the entire surface.* |
*Local determination of aluminum in a plane perpendicular to the axis of the sample showed:
a) its uniform circumferential penetration deep into the sample,
b) revealed the diffusion zone of aluminum enrichment of the titanium sample,
c) found on the sample surface a zone of titanium soluble in aluminum.
Thus, the elimination of the surface layer, which is enriched in carbon (from the depth of the metal) and oxygen from the atmosphere after any mechanical treatment of workpieces and parts made of titanium and its alloys by electropolishing, is a simple and reliable way to activate the interaction of contacting metals during metallization. EFFECT: invention makes it possible to convert a lyophobic surface into a lyophilic one at insignificant material and labor costs. Surface activation makes it possible, for example, to improve adhesion during diffusion alloying of the surface with metal, to increase the rate of diffusion of atoms of the introduced metal into the crystal lattice of workpieces and products, which gives their surfaces qualitatively new performance qualities, in particular:
High scale resistance and corrosion resistance - aluminum coating reduces the oxidation rate of titanium alloys at a temperature of 800-900°C by 30-100 times. This occurs as a result of the formation on the surface of the coating layer -Al 2 O 3 (E.M. Lazarev and others, Oxidation of titanium alloys, M., Nauka, 1985, p. 119);
High anti-friction properties, because the coefficient of friction of aluminum is much lower than that of titanium alloys.
CLAIM
A method for modifying the surface layer of products made of titanium and its alloys, including physical and chemical surface treatment of products and aluminizing, characterized in that the physical and chemical surface treatment of products is carried out by electrochemical polishing in an electrolyte of the following composition: perchloric acid - 1 part; acetic acid - 9 parts, at a temperature of 30-35 ° C, a current density of 2 A / dm 2, a voltage of 60 V for 3 minutes.
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