Plasma spraying has a number of advantages in comparison with gas-flame spraying and electric arc metallization:

• makes it possible to apply coatings from materials of a wide composition (metals, alloys, oxides, carbides, nitrides, borides, plastics and their various compositions) on a variety of base materials (metals, ceramics, graphite, plastics, etc.);
• Plasma torches make it possible to regulate the energy characteristics of the plasma over a wide range, which facilitates the production of coatings with properties determined by the requirements of the technology;
• the use of inert gases and oxygen-free mixtures in plasma torches helps to reduce the oxidation of the sprayed material and the surface of the part;
• Coatings obtained by plasma spraying are superior in terms of physical and mechanical properties to coatings obtained by gas-flame and arc spraying methods.

Plasma-arc spraying according to the type of filler material used is divided into: powder spraying and wire spraying ( rice. 3.12).

Technological process

Powder atomizers, depending on the properties and particle size, can supply filler material ( rice. 3.13):

• directly into the plasma jet at the outlet of the plasma torch;
• at an angle to the plasma torch nozzle, towards the flow of ionized gas;
• inside the plasma torch nozzle into the anode zone or into the preanode zone of the plasma arc.

The supply of powder into the plasma jet is used in high power plasma torches. Such a supply scheme does not affect the formation of the plasma flow, and plasma torches are characterized by an overestimated power so that the heat of the plasma jet is enough to heat the powder.

The supply of powder to the pre-anode zone is most advantageous in terms of heat transfer, but is associated with overheating of the particles in the nozzle and clogging of the nozzle with molten particles, which leads to the need to put forward increased requirements for the uniformity of powder supply.

The heating efficiency of powder particles can be increased at the same mode parameters by more uniform distribution of powder over the cross section of the hot zone of the plasma jet. This is facilitated by the design of plasma torches, which make it possible to introduce the powder into the plasma jet not through one hole, but, for example, through three, located at an angle of 120°. In this case, the efficiency of powder heating varies from 2 to 30%.

Rice. 3.12
a - powder; b - wire. 1 - supply of plasma gas; 2 - plasma torch cathode; 3 - cathode body; 4 - insulator; 5 - anode body; 6 - powder feeder (Fig. a) or wire feeder (Fig. b); 7 - supply of gas transporting the powder; 8 - plasma jet; 9 - power supply.

Rice. 3.13
1 - into the plasma jet; 2 - at an angle to the plasma jet; 3 - into the nozzle.

Application

For spraying wear-resistant coatings, powders with a granulation not exceeding 200 microns are used. In this case, the dispersion of powder particles should be within narrow limits with a size difference of no more than 50 μm. With a significant difference in particle size, it is impossible to ensure their uniform heating. This is explained by the fact that, despite the high temperature of the plasma jet, coarse powder does not have time to melt during the short time it is in the plasma jet (10 -4 -10 -2 s), fine powder partially evaporates, and its main mass due to the low kinetic energy is pushed aside by the plasma jet, without reaching its central zone. When restoring parts by spraying with powder wear-resistant nickel- and iron-based alloys, the most rational is powder granulation with a particle size of 40-100 microns.

When spraying, as a rule, spherical powder particles are used, since they have the highest flowability. The optimal mode of operation of the plasma torch should be considered the one in which the largest number of particles reaches the substrate (base) of the part in the molten state. Therefore, for highly efficient heating and transportation of powder particles, it is necessary that the design of the plasma torch ensures that a plasma jet of sufficient power is obtained. Currently, installations with a capacity of up to 160-200 kW have been developed, operating on air, ammonia, propane, hydrogen, in dynamic vacuum, in water. The use of special nozzles made it possible to obtain a supersonic outflow of a two-phase flow jet, which, in turn, provided a dense coating. The plasma jet flows out of the plasma torch at a speed of 1000-2000 m/s and imparts a speed of 50-200 m/s to the powder particles.

An increase in the resource of the nozzle apparatus (cathode-anode) of a high-power plasma sputter (50-80 kW) was hampered due to the low erosion resistance of the copper nozzle in the anode spot zone. In order to increase the durability of the nozzle, tungsten inserts were developed, pressed into the copper nozzle in such a way that the heat was effectively removed by the copper sheath and removed by the cooling water. Plasma spraying installations currently produced by the industry are equipped with plasma torches with a power consumption of 25-30 kW at a current strength of 350-400 A.

On the other hand, for coating small parts (surfaces), for example, crowns in dentistry, shroud shelves of GTE blades in the aircraft industry, microplasma burners were developed that operate at currents of 15-20 A at a power of up to 2 kW.

The efficiency of particle heating and their flight speed depend on the type of gas used: diatomic gases (nitrogen, hydrogen), as well as air and their mixtures with argon, increase these parameters.

The technological process of restoring parts by plasma spraying includes the following operations: powder preparation, part surfaces, spraying and machining of sprayed coatings. The preparation of the surface of the part for spraying is of paramount importance, since the adhesion strength of the powder particles to the surface of the part largely depends on its quality. The surface to be restored must be degreased before treatment. Areas adjacent to the surface to be sprayed are protected with a special screen. Coatings should be sprayed immediately after shot-blasting, since already after 2 hours its activity decreases due to an increase in the oxide film on the treated surface.

To increase the adhesion strength of the coating to the base, the process of plasma spraying is carried out with subsequent reflow. The reflow operation completes the coating process. Melting is carried out by the same plasma torch as spraying, at the same power of the compressed arc, with the plasma torch nozzle approaching the part at a distance of 50-70 mm. Fatigue resistance after reflow increases by 20-25%. The adhesion strength after reflow reaches 400 MPa. The zone of mixing of melted and base metals is 0.01-0.05 mm.

Rice. 3.14
a - bar; b - wire ("wire-anode").

disadvantages

A significant disadvantage of plasma heating during reflow is that the plasma jet, having a high temperature and a significant energy concentration, very quickly heats the surface of the coating with insufficient heating of the surface of the part and thereby often leads to the collapse of the melted coating. In addition, as a result of the high velocity of the plasma jet and the significant pressure on the sprayed surface, the coating layer may also be damaged. Plasma spraying with subsequent reflow is recommended for small parts with a diameter not exceeding 50 mm.

When using wire as a filler material, it is possible to use two schemes for connecting a plasma torch: with a current-carrying nozzle ( rice. 3.14, a) or with a current-carrying wire ( rice. 3.14b).

The scheme of wire sputtering with a current-carrying wire - the anode was developed by V.V. Kudinov at the end of the 50s of the last century. Then it was possible to obtain unprecedented productivity - 15 kg / h of tungsten at a power of 12 kW. In plasma spraying, rods are used along with wire. In such a way that the heat is effectively removed by the copper sheath and removed by the cooling water. Plasma spraying installations currently produced by the industry are equipped with plasma torches with a power consumption of 25-30 kW at a current strength of 350-400 A. On the other hand, for coating small parts (surfaces), for example, crowns in dentistry, shroud shelves of GTE blades in the aircraft industry, microplasma burners were developed that operate at currents of 15-20 A at a power of up to 2 kW.

APPLICATION OF POLYMER COATINGS.

CLASSIFICATION OF METHODS.

1. Polymer powder coating

2. Characteristics of polymer powder coating

3. Application of polymer coatings

4. Classification of coating methods

5. The first group of polymer coatings

5.1 Vortex spraying (vibration, vibrovortex method of applying polymer coatings)

2 Pneumatic spraying

3 Flameless spraying

4 Centrifugal powder spraying method

6. The second group of polymer coatings

6.1 Flame spraying

2 Plasma spraying

3 Heat beam method

4 Extrusion method

5 Vacuum coating

7. The third group of polymer coatings

7.1 Electrostatic Powder Coating Technology - Corona Charging Technology

7.2 Tribo spraying - friction charging

3 Coating in an ionized fluidized bed

Conclusion

LIST OF INFORMATION SOURCES USED

APPLICATION OF POLYMER COATINGS. CLASSIFICATION OF METHODS.

1. Polymer powder coating

Polymer coating is the result of surface treatment with powder paint. The latter is a special solid composition, which, when the temperature rises, turns into a continuous film, designed to protect the metal product from corrosion and give it an aesthetic appearance.

Powder polymer coating is widely used today in repair and construction work. It is ideal for façade elements (roofs, window profiles, doors, railings), sports, gardening equipment and office furniture.

Polymer powder coating was developed in the 1950s. in USA. At that time, the automotive industry was just beginning to take shape, which was one of the few that had the honor of testing the latest type of painting. More than 60 years have passed since then, and every person can use the powder-polymer coating of metal every day, including in his kitchen. Today, in terms of production of thermoactive powder coatings, none other than Europe is in the lead. In Russia, the situation is somewhat different, because the mass production of such products began only in 1975. Now polymer powder coating is becoming extremely popular, penetrating many of the layers previously occupied by traditional paint coatings.

The powder coating method is a popular alternative to the application of liquid paints for parts that can be heat treated. Most often, the layer of powder-polymer composition on the product is 0.3 mm.

Powder paints are solid dispersed compositions, which include film-forming resins, hardeners, fillers, pigments and targeted additives. Powder paints are obtained mainly by mixing the components in the melt, followed by grinding the alloy to the maximum particle size.

Powder paints owe their popularity to the absence of solvents and the content of substances that guarantee a thin layer impervious to salts, acids and moisture. At the same time, it meets high quality standards, is abrasion-resistant and high-strength.

Increased resistance to mechanical damage guarantees the preservation of the appearance throughout the entire service life of the metal painted with a polymer-powder coating.

The main advantage of the polymer-powder coating method is the anti-corrosion protection of the metal. And the resulting coating has increased heat resistance, electrical insulating properties, durability, strength, environmental friendliness, retains the original color and meets European standards.

2. Characteristics of polymer powder coating

Coating thickness 60...80 microns;

High resistance to ultraviolet radiation;

Minimum bending radius - 1T;

Possibility of painting in any color.

Increased resistance to mechanical damage, which guarantees the preservation of the appearance throughout the entire service life of the painted metal;

Increased impact strength, bending, abrasion;

High adhesion with the painted surface;

High anti-corrosion resistance to moisture, alkali and acid solutions, organic solvents;

Wide operating range from -60 0С to +150 0С;

Unsurpassed aesthetic performance: the increased thickness of the polymer coating allows you to mask minor surface defects.

In addition, polymer paint has many surface effects that allow you to achieve an impeccable appearance of finished products without tedious and lengthy preparation.

Powder-polymer coating is resistant to atmospheric corrosion and can be confidently used in conditions:

Industrial atmosphere of medium aggressiveness for up to 30 years;

Slightly aggressive atmosphere for up to 45 years;

Seaside urban atmosphere of medium aggressiveness for up to 15 years.

3. Application of polymer coatings

The technology of applying polymer powder paints is an environmentally friendly, waste-free technology for obtaining high-quality protective and protective-decorative polymer coatings. The coating is formed from polymer powders that are sprayed onto the surface of the product, and then the heat treatment (polymerization) process takes place in an oven at a certain temperature.

The coating process by almost all known methods involves the sequential implementation of the following main steps:

1. Cleaning the coated surface from pollution, oxide and hydroxide layers and carrying out activation treatment;

Application of polymeric material to the surface;

Fixing the polymer material on the surface;

Final processing of the coating in order to achieve the necessary service properties;

Quality control of the coating, assessment of the conformity of its properties, geometric parameters with the required ones.

Polymer coatings applied to the surface of a solid body are used to improve the service properties of products.

The quality of coatings depends on the strict observance of the technological regimes of all stages of the process.

Surface preparation.

To clean the surface from rust, scale, old coatings, mechanical and chemical methods are mainly used. Of the mechanical methods, the most common is abrasive blasting with the use of shot blasting, shot blasting and sandblasting machines.

Organic solvents, aqueous washing (alkaline and acidic) solutions are used as degreasing agents. Organic solvents (White spirit, 646), due to their harmfulness and flammability, are used for degreasing by manual wiping with cotton rags that do not leave lint on the surface of products, to a limited extent, mainly when painting small batches. The main industrial method of degreasing is associated with the use of aqueous detergent compositions - concentrates. Basically they are powders. Degreasing is carried out at 40-600C; duration of treatment by dipping 5-15 minutes, by spraying 1-5 minutes. Most formulations are suitable for degreasing both ferrous and non-ferrous metals (aluminum, copper, zinc and magnesium alloys). Degreasing requires not only treatment with a detergent composition, but also their subsequent washing and drying.

Chemical removal of oxides is based on their dissolution or exfoliation using acids (for ferrous metals) or alkalis (for aluminum and its alloys). This operation aims to improve the protection of products, make it more reliable and durable. Phosphating of ferrous metals and oxidation of non-ferrous metals, primarily aluminum and its alloys, are most common. Non-ferrous metals (aluminum, magnesium, their alloys, zinc) are oxidized to improve adhesion and protective properties of coatings. The final stage of obtaining conversion coatings, as well as any operations of wet surface preparation, is the drying of products from water.

Preparation of powder material and compressed air.

Powdered polymeric materials of industrial production, which have not expired, are generally suitable for coating without any preparation. Exceptions may be in cases where the conditions of storage or transportation of the material were violated.

The most typical paint defects associated with their improper storage are clumping, chemical aging; moisture in excess of the allowable norm. The recommended storage temperature for powder coatings is not higher than 30°C. Dried paints with large or even small aggregates are not suitable for use and require processing - grinding to the required particle size and screening. With a small aggregation of particles, sometimes they are limited to sieving. The recommended sieve cell for screening should be in the range of 150-200 microns.

Chemical aging is most susceptible to thermoset paints with high reactivity in case of non-compliance with the conditions of their storage. Paints that show signs of chemical aging should be discarded, their correction is almost impossible. Paints with a high degree of moisture (which can be seen from their reduced flowability, tendency to aggregation, poor chargeability) are subject to - drying at a temperature not exceeding 35 0C on a baking sheet with a layer of 2-3 cm. within 1-2 hours with periodic mixing of the paint.

Polymer powder paints are hygroscopic and absorb water vapor from the surrounding air, as a result of which the paints are poorly transported through the spray pipeline, sprayed, charged (especially for tribostatic spraying). The preparation of compressed air consists in its purification from dripping moisture and oil, followed by drying from their vapors. The air used to spray powder paints must meet the following requirements: oil content - no more than 0.01 mg/m3; moisture content - no more than 1.3 g/m3; dew point - not higher than 7°С; dust content is not more than 1mg/m3. The preparation is carried out by passing compressed air through oil traps and the OSV-30 compressed air dryer, in which the release of compressed air from moisture is achieved by passing the latter through a layer of sorbent, which takes water and oil vapor from the compressed air. Sorbent regeneration is carried out by calcining the sorbent at a temperature of 120-150 0C for 2-3 hours, followed by cooling the latter. The period of use of the sorbent is about 5 years.

4. Classification of coating methods

All methods of applying polymer coatings can be divided into three groups.

I - group - application methods carried out by spraying powder onto products heated above the melting point of the applied polymer:

a) vortex spraying (application in a fluidized bed), vibration, vibrovortex;

b) pneumatic spraying;

c) plasma-free spraying;

d) centrifugal spraying.

II - group - application methods carried out by spraying molten particles of powder polymer onto the surface of a heated product:

a) gas-plasma spraying;

b) heat-beam spraying;

c) extrusion spraying;

III - group - application methods carried out by spraying electrically charged powder particles onto the surface of an oppositely charged surface:

a) electrostatic spraying - charging with a corona charge in an electric field;

b) tribostatic spraying;

c) coating in an ionized fluidized bed.

Let us consider in more detail the methods of applying polymer coatings

5. The first group of polymer coatings

1 Vortex spraying (vibration, vibrovortex method of applying polymer coatings)

It is the most commonly used powder coating method.

The process of whirl spraying is as follows: between the base of the tank and the sintering chamber there is an air- or gas-permeable plate made of cermet or a filter made of synthetic material (pore diameter< 25 мкм). В агломерационную камеру загружается полимерный порошок. Размер частиц, образующихся в результате спекания порошков, составляет от 50 до 300 мкм. Для спекания в нижний отсек резервуара (основание резервуара) вдувается воздух, который, равномерно распределяясь при прохождении через пористую пластину, проникает в агломерационную камеру и создает «кипящий» слой порошка. Необходимое давление воздуха зависит от высоты «кипящего» слоя и плотности порошка и составляет от 2,6 до 2,0 бар. Необходимое количество воздуха равно от 80 до 100 м3 в час и на 1 м2 поверхности днища. Завихренный порошок ведет себя подобно жидкости (он «псевдоожижен»), поэтому предметы, на которые требуется нанести покрытие, могут быть легко в него погружены. Для расплавления порошка необходим предварительный нагрев металлических предметов, на которые предполагается нанести покрытие. Предварительный нагрев целесообразно осуществлять в сушильных печах с циркуляцией воздуха при температурах выше плавления соответствующего полимера (100-200 °С). До предварительного нагрева поверхность обезжиривается. Подготовленные и нагретые металлические изделия опускаются в кипящий слой порошка (рисунок 1). После нанесения покрытия охлаждение полиэфинов должно по возможности осуществляться медленно. Полимерное покрытие может быть доведено до зеркального блеска.

Figure 1. Diagram of a fluidized bed coater:

Tube for air supply, 2 - suspension, 3 - housing, 4 - part to be repaired, 5 - porous partition, 6 - powder

Advantages:

1. in one cycle of application and subsequent curing, a thick-layer coating with high anti-corrosion resistance can be obtained;

2. subject to the technological cycle of application, it is possible to adjust the uniformity of the film thickness;

Low initial equipment cost.

Disadvantages:

1. a large amount of powder is needed to load the bath;

2. the workpiece must be preheated;

This application method is used only when a thick coating is required;

Products to be painted should be simple in shape.

With the vibration method, to create a suspended layer of polymer powder in the working area, the installations are equipped with vibrators - mechanical, electromagnetic or air, forcing the installation body or only the bath bottom connected to the body with a diaphragm to vibrate. The chamber does not have a porous partition. This method has not been widely used, since it does not provide a uniform coating due to the fact that larger particles of powder rise to the surface of the suspended layer during vibration.

The combination of the vortex method with the vibration method is called the vibrovortex spraying method, which provides a uniform structure and density of the suspended layer, and is used to apply polymer powders that have poor flowability or caked.

An electromagnetic vibrator and a membrane with a frequency of 10-100 vibrations per second are mounted in the lower part of the installation under the bath. The powder particles are simultaneously affected by vibration and air currents, which ensures a uniform coating layer. The method is intended for applying protective and decorative coatings.

5.2 Pneumatic spraying

This coating method consists in spraying powder material onto the surface of a preheated product with a pneumatic spray gun. The method makes it possible to apply coatings on products of various overall dimensions and configurations using a small amount of powder. .

The main advantages of the method are high productivity, simplicity of design and versatility. The disadvantages of the method are the need for pre-heating of products, very significant (up to 50%) losses of the sprayed material, the impossibility of obtaining uniform coatings over the film thickness, especially in the presence of sharp edges and non-vertical planes.

All installations for pneumatic spraying of powder polymers consist of a feeder and spray heads, which are equipped with instruments and equipment for regulating and controlling the coating process. The feeder is designed to feed the air-powder suspension into the spray head. The spray head directs the powder onto the surface to be coated.

On fig. 106, a-d shows interchangeable nozzles of a spray gun for applying powder materials. The gun works on the principle of powder ejection suction. The flow rate of the supplied air is regulated by a needle, the air-powder mixture is fed to the gun from the feeder.

3 Flameless spraying

Powdered polymer mixed with air through the spray head is applied to the pre-cleaned heated surface of the product. Compared with the flame spraying method, it uses a simple design of the spray head and the possibility of spraying products of various designs and sizes with a small amount of powder. Flameless spraying is used to cover the outer and inner surfaces of pipes of various diameters up to 12m long.

5.4 Centrifugal powder spraying method

To apply coatings on the inner surfaces of pipes, containers, cylindrical vessels, the centrifugal method of obtaining coatings has become widespread, which consists in applying powder to heated products while simultaneously rotating them.

The powder from the dosing device enters the discs rotating in a horizontal plane in opposite directions. The powder on the disks is sprayed under the action of centrifugal forces, forming a flat jet.

6. The second group of polymer coatings

1 Flame spraying

polymer coating powder coating

The essence of the process of flame deposition of a polymer coating is that a jet of compressed air with powder particles suspended in it is passed through a torch of an acetylene-air flame. In the flame, the powder particles are heated, softened and, hitting the previously prepared and heated surface, stick to it, forming a continuous coating. In repair practice, the application of polymer coatings by the gas-flame method is used to level welds and irregularities on the surfaces of cabs and plumage parts of cars, tractors, combines.

Spraying material - PFN-12 plastic (MRTU6-05-1129-68); TPF-37 (STU12-10212-62). Before use, the powder of these materials must be sifted through a sieve with a mesh No. 016 ... 025 (GOST 3584-53) and, if necessary, dried at a temperature of not more than 60 ° C for 5 ... 6 hours, and then sieved.

Figure 2. Scheme of flame spraying through a spray burner.

Before applying the flame coating, damaged surfaces with dents and irregularities must be straightened, and cracks and holes welded. The surface of the welded seams should be cleaned with a grinder until sharp corners and edges are removed. Surfaces around welds and irregularities are cleaned to a metallic sheen. The prepared surface must be free of scale, rust and contamination. The coating is applied using the UPN-6-63 installation. First, the burner flame heats the damaged surface to a temperature of 220...230 °C. In this case, the speed of movement of the burner is 1.2 ... 1.6 m / min; acetylene pressure - not lower than 0.1004 MPa; compressed air pressure - 0.3 ... 0.6 MPa; the distance from the mouthpiece to the heated surface is 100...120 mm. Then, without turning off the flame of the burner, open the powder supply valve. The powder is applied to the heated surface in two or three passes of the burner. After 5 ... 8 s after spraying, the applied layer of plastic is rolled with a roller moistened with cold water. The rolled surface of the plastic is heated with a burner flame for 5–8 s, a second layer of powder is applied to the heated coating in two or three passes and again rolled with a roller. The sprayed surface is cleaned with a grinding machine so that the transition from the metal surface to the sprayed layer is uniform.

For flame (thermal) powder coating, it is not required to charge the product and powder particles to create an electrostatic field. This means that almost any surface can be painted: not only metals, but also plastics, glass, ceramics, wood and many other materials that would deform or burn in the polymerization chamber.

Flame coating eliminates the need for bulky ovens and curing chambers, and takes powder coating to new frontiers in this technology because spray equipment is portable and versatile. It is also used not only for surface heating, powder spraying, but also for reheating in order to level the surface.

Among the disadvantages of this technology is that the coatings do not always have a flat surface, and their value is more functional than decorative. But for objects such as bridges, ship hulls or water towers, protection against corrosion and rust is more important than a slight unevenness in the coating.

6.2 Plasma spraying

The essence of the method consists in the transfer of powder material to the surface of the product by a high-temperature plasma flow, which is formed as a result of partial ionization of an inert gas (argon, helium or a mixture of helium with nitrogen) when it is passed through an electric arc at a temperature of 3000 to 80000C.

When a powder material is introduced into the plasma flow, the powder melts and, together with the plasma gas, is applied to the surface of the product. The application of powder materials in this way is carried out manually using a plasma sprayer. The installation includes a sprayer, a transformer-rectifier, a device for controlling gas flows, and a container for material. Due to the fact that only powder materials with a narrow range of dispersed distribution of powder particles and withstanding heating of the order of 3500C (such polymers include fluoroplasts, polyamides) can be applied by plasma spraying, this method, despite its advantages (high productivity, safety, etc. ), has not found wide application in industry.

6.3 Heat beam method

More productive and versatile than the flame method. Powdered thermoplastic material is fed into a zone of powerful heat flow, where the material is melted and applied to the surface of the product. The air-powder mixture is formed in the viro-vortex apparatus and directed to the product. This method is more efficient than the flame method, reduces powder consumption and has lower energy consumption. The coating has higher physical and mechanical characteristics and better adhesion to the surface of the product. The disadvantages of the method are significant loss of powder and air pollution.

6.4 Extrusion method

To apply coatings from thermoplastic polymeric materials to electrical wires, cables, steel pipes, wooden planks and other semi-finished products, extrusion lines based on single-screw plasticizing extruders are used, and extrusion units are widely used in the cable industry. For example, for communication technology, copper wires with a diameter of 0.4-1.4 mm are covered with a polyethylene or polyvinyl chloride film 0.15-0.25 mm thick; PVC coatings are used for low-frequency equipment; for cables with a diameter of 20-120 mm, HDPE coatings with a thickness of 4-25 mm are used. .

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Figure 5. Coating with a spray gun

Its popularity is due to the following factors: high charging efficiency of almost all powder coatings, high productivity when powder coating large surfaces, relatively low sensitivity to ambient humidity, suitable for applying various powder coatings with special effects (metallic, shagreen, mauara, etc.). ).

Figure 6. Movement of corona discharge ions in an electric field and their deposition on the surface of particles (“impact charging”).

Along with the advantages of electrostatic spraying, there are several disadvantages, which are caused by a strong electric field between the spray gun and the part, which can make it difficult to apply powder coating in corners and in deep recesses. In addition, the wrong choice of electrostatic parameters of the spray gun and the distance from the spray gun to the part can cause back ionization and degrade the quality of the polymer powder coating.

Powder coating equipment - an electrostatic spray gun is a typical Entente powder coating complex.

Figure 7. Faraday cage effect

The Faraday cage effect is the result of electrostatic and aerodynamic forces.

The figure shows that when applying a powder coating to areas in which the Faraday cage effect acts, the electric field created by the sprayer has a maximum strength at the edges of the recess. The lines of force always go to the nearest ground point and tend to concentrate at the edges of the notch and raised areas rather than penetrating further inward.

This strong field accelerates the settling of the particles, forming a too thick powder coating in these places.

The Faraday cage effect is observed in cases where powder paint is applied to metal products of complex configuration, where an external electric field does not penetrate, so applying an even coating on parts is difficult and in some cases even impossible.

Back ionization

Figure 8. Back ionization

Back ionization is caused by excessive free ion current from the sprayer charging electrodes. When free ions hit the powder-coated surface of a part, they add their charge to the charge accumulated in the powder layer. But the surface of the part accumulates too much charge. At some points, the charge is exceeded so much that micro sparks jump in the thickness of the powder, forming craters on the surface, which leads to a deterioration in the quality of the coating and a violation of its functional properties. Also, back ionization contributes to the formation of orange peel, reducing the efficiency of sprayers and limiting the thickness of the resulting coatings.

To reduce the effect of the Faraday cage and back ionization, special equipment has been developed that reduces the number of ions in ionized air when charged powder particles are attracted to the surface. Free negative ions are diverted to the side by grounding the atomizer itself, which greatly reduces the above-mentioned negative effects. By increasing the distance between the spray gun and the surface of the part, you can reduce the spray gun current and slow down the back ionization process.

7.2 Tribo spraying - friction charging

Static electrification is carried out by exchanging charges due to the difference in the work function of electrons between the particle material and the wall material in the charger or during the exchange of charges between particles due to differences in the chemical composition of impurities, temperature, phase state, surface structure, etc.

Figure 9. Tribotechnical spraying

Unlike electrostatic spraying, this system does not have a high voltage generator for the spray gun. The powder is charged during the friction process.

The main task is to increase the number and strength of collisions between the powder particles and the loading surfaces of the spray gun.

One of the best acceptors in the triboelectric series is polytetrafluoroethylene (Teflon), it provides good charging for most powder coatings, has a relatively high wear resistance and is resistant to particle sticking under impact.

Figure 10. No Faraday cage effect

Tribo-charged nebulizers generate neither a strong electric field nor an ion current, so there is no Faraday cage effect or back ionization. Charged particles can penetrate into deep hidden openings and evenly stain products of complex configuration.

It is also possible to apply multiple coats of paint to obtain thick powder coatings.

Chargers for triboelectric spray guns must meet the following three requirements for efficient charging of the sprayed material:

provide multiple and effective collisions of powder particles with a triboelectric element;

remove the surface charge from the triboelectric element;

ensure the stability of the tribocharging process.

Tribo-charged sprayers are structurally more reliable than corona-charged spray guns because they do not have high voltage converting elements. With the exception of the ground wire, these atomizers are fully mechanical, only sensitive to natural wear and tear.

7.3 Coating in an ionized fluidized bed

The coating device is a chamber with an electric fluidized bed, in which the product - 1 is placed (Figure 5). The chamber is divided by a porous partition - 2 into two parts. Powder material - 3 is poured into the upper part on the porous partition, and compressed air is supplied into the lower part.

Figure 11. Coating in a fluidized bed chamber

At a certain speed of air passing through a porous partition, the powder is transferred to a suspended state, in which the particles seem to hover in an ascending air flow. Due to the randomness of the movement of particles, they collide with each other, which leads to static electrification of the particles and charging them with both negative and positive charges.

The electric field created between the high-voltage electrode placed in the powder layer and the grounded product causes separation of the particles in the fluidized bed according to the signs of the charge. When a negative voltage is applied to the high-voltage electrodes, positively charged particles accumulate around the high-voltage electrode, and negatively charged particles accumulate in the upper part of the fluidized powder bed. Particles with a sufficiently large negative charge are carried out by the electric field from the fluidized bed and are directed to the product. Due to the high concentration of particles in the fluidized bed, the corona discharge at the surface of high-voltage electrodes is in a completely blocked state. As positively charged particles accumulate around high-voltage electrodes, a discharge occurs and pulsed local unlocking of the corona discharge occurs, during which the particles are recharged. Thus, the charging of particles in an electric fluidized bed is complex, combining static electrization of particles and charging in a gas discharge.

The process of transporting powder particles to the product being sprayed is carried out in an air stream. In this case, the ratio of aerodynamic and electrical forces acting on a particle is very different for different devices used for coating. If for atomizers with internal charging, particles are transported exclusively by air flow, then in chambers with an electric fluidized bed, the direction of movement of particles towards the product is mainly created by an electric field. For sprayers with external charging, the movement of particles to the product is equally determined by aerodynamic and electrical forces.

The method of applying coatings from powder materials in an electrostatic field has significant advantages over all the above methods:

No preheating;

Reduced loss of powder material;

Possibility of obtaining coatings of uniform thickness on products of complex configuration;

The possibility of automating the spraying process;

Versatility and high performance;

Ecological cleanliness;

Minimize fire and explosion hazards.

These factors determined the widespread use of the technology of applying polymer coatings in an electrostatic field.

Conclusion

The application of polymer coatings is a rather complex technological process that can be used to protect various types of materials from adverse effects. environment, and to give an attractive appearance to various products. .

As a rule, the application of polymer coatings is carried out using specialized equipment in rooms where certain indicators of the internal environment are maintained. Currently, there are many technological methods for applying polymer coatings to various types of materials.

The most popular technologies that are used in applying various types of polymer coatings are flame and vortex methods, vibration and vibrovortex methods, coating in an electrostatic field, as well as the use of various types of suspensions, emulsions and gumming compositions for surface treatment.

As a rule, polymer coatings are applied during the production of materials or finished products, but in some cases this type of coating can be applied, for example, to a car that has been operated by the owner for several years.

Each technology for applying polymer coatings has its own characteristics, which can be associated with both the process of adhesion of the polymer material and the method of applying the polymer. In any case, before coating any product with a polymer, it is necessary to carefully prepare its surface by removing dirt, old paint or other roughness. .

In addition, when carrying out work on applying a polymer to the surface of any material, it is necessary to strictly observe the technology of this process; in some cases, the temperature at which the coating is applied can reach several hundred degrees. It should also be noted that the room where such work is carried out must be perfectly clean, as dust and other particles can lead to cracking of the polymer coating over time.

Care must be taken when working on polymer coating equipment as there is a possibility of serious injury.

LIST OF INFORMATION SOURCES USED

Panimatchenko A.D. Plastics Recycling, ed. Profession, St. Petersburg 2005.

Karyakina M.I., Poptsov V.E. Technology of polymer coatings: Textbook for technical schools. - M.: Chemistry, 1983 - 336s., ill.

Yakovlev A.D., Zdor V.F., Kaplan V.I. Powder polymeric materials and coatings based on them. L., Chemistry, 1979. 254 p.

4. Meissela L. and Glanga R. Technology of thin films: Handbook / Ed. Per. from English; Ed. Elinson M.I., Smolko. G. G. - M .: Soviet radio, 1977. -T. 1. - 406 p.; T. 2. - 353 p.

Lipin Yu.V., Rogachev A.V., Sidorsky S.S., Kharitonov V.V. Technology of vacuum metallization of polymeric materials - Gomel, 1994. -206 p.

Roikh IL, Kaltunova LN Protective vacuum coatings on steel. M.: Mashinostroenie, 1971. - 280 p.

7. Brook M.A., Pavlov S.A. Polymerization on the surface of solids. - M.: Chemistry, 1990. - 130 p.

Yasuda H. Plasma polymerization. - M.: Mir, 1988. - 376 p.

Krasovsky A.M., Tolstopyatov E.M. Obtaining Thin Films by Sputtering of Polymers in Vacuum / Ed. Bely V.A. - Minsk: Science and Technology, 1989. - 181 p.

The essence of the process. In plasma spraying, the heat of a compressed electric arc (plasma arc) is used to melt the powder fed into the spray torch (plasma torch). The molten powder particles are carried out by the hot gas flow from the nozzle and sprayed onto the surface of the part, to which the burner flame is directed.
The advantages of plasma spraying over flame spraying are as follows: it is possible to spray materials whose melting point exceeds the temperature of an acetylene-oxygen flame; the productivity of deposition of ceramic materials is increased by 6-10 times; does not require the use of oxygen and acetylene. Compared with the electric arc spraying method, the advantage of the plasma method is the possibility of spraying powder materials, including ceramics, while the electric arc method requires the use of a wire made of the metal to be sprayed.
In terms of composition, structure, and properties (strength, degree of oxidation, thermal and electrical conductivity, etc.), plasma coatings have no advantages over those deposited by gas flame and electric arc methods.
Areas of use. Plasma coatings are used, as a rule, for applying heat-resistant coatings required in jet technology. In this way, it is also possible to drip diesel pistons, working blades of smoke exhausters, throttle valves and tuyeres of blast furnaces and other products that require increased heat resistance. When applying coatings to the internal surfaces of parts, the hole diameter must be at least 100 mm. With an increase in the thickness of the coating layer, their strength decreases. For example, when coated with aluminum oxide, the strength of the layer drops sharply at a layer thickness of more than 0.8 mm. Typically, coatings with a layer thickness of 0.2-0.3 mm are used.
To increase the adhesion strength of ceramic coatings with the base metal, they are sprayed onto the sublayer. When spraying aluminum oxide, nichrome or corrosion-resistant steel is the best for the undercoat. The thickness of the sublayer is 0.05 mm. Less suitable for the sublayer, in terms of heat resistance, are molybdenum and tungsten, which form oxides with insufficient strength.
Plasma coatings are also used as electrical insulators, for example, in the manufacture of parts for MHD generators, heat exchangers, strain gauges, electric saw blades, inductors for high-frequency soldering, and other parts in electrical engineering, radio electronics, and instrument making. The porosity of coatings, including ceramic ones, does not prevent their use as electrical insulating materials if they are protected from moisture.
Plasma coatings for protecting parts from corrosion and wear are less effective, since they have a high porosity. To reduce porosity, they need additional impregnation (organic polymeric materials - resins and varnishes) or fusion. The properties of the impregnating materials determine the operating temperature of the part. Impregnation is especially effective when the part is subject to both corrosion and abrasive or erosive wear. Usually, phenol-formaldehyde resin is used for impregnation. For high operating temperatures, impregnation of coatings from sprayed tungsten with copper and silver is used.

Applied materials. For plasma spraying, powders with a particle size of 20-150 microns are used. For aluminum oxide and zirconium dioxide, the particle size should be 40-70 microns, for tungsten 20-100 microns. For high-density coatings, the particle size should be smaller and not exceed 10-40 microns; to obtain the optimal particle size distribution of the powders, they should be sifted before use.
To obtain heat-resistant coatings, the following powders are used: aluminum oxide (alumina) grades GA85 or GA8; zirconia (90% ZrO2); tungsten with particles of 40-100 microns in the form of powder grade B or B-1. As a plasma-forming gas, nitrogen with a concentration of 99.5% or hydrogen with a purity of 99.7% (grade A) or argon is used.
Equipment. For plasma spraying, special installations manufactured by the industry are used, for example, installations of the UMP-4-64 type (Fig. 77). This installation is designed for spraying refractory materials: tungsten, zirconium dioxide, aluminum oxide. In the presence of a chamber with a protective atmosphere, it is also possible to spray carbides, borides, nitrides, silicides and other compounds of refractory materials. The installation consists of a plasma torch, a powder feeder and a control panel.
To power the installation with current, welding converters PSO-500 (2 pcs.) Or semiconductor rectifiers IPN-160/600 are used. on nitrogen 85-90V, with a mixture of nitrogen and hydrogen 100-120V, operating current on nitrogen 320-340 A, on a mixture of nitrogen and hydrogen 270-300 A The device of a torch for plasma spraying is shown in fig. 78.

Rice. 77. Installation UCHP-4-64 for plasma spraying:
1 - burner (plasmatron); 2 - powder feeder; 3 - control panel

Rice. 78. Torch for plasma spraying:
1 - nozzle for cooling the sprayed surface with compressed air; 2 - nozzle-anode; 3 - textolite sleeve; 4 - nipple for gas inlet; 5 - copper body of the cathode; 6 - tungsten cathode with a diameter of 3 mm; 7 - water-cooled cables; 8 - handle; 9 - spark plug; 10 - asbestos cement ring

Plasma spraying technology. Before spraying, the surface of the part is sandblasted, preferably immediately before the coating process. In addition to creating a rough surface, sandblasting removes a film of adsorbed air and moisture that prevents contact between the sprayed particles and the part. Instead of quartz sand, which is harmful because it causes silicosis, corundum powder is used, silicon carbide and white cast iron chips For corrosion-resistant materials, white cast iron chips should not be used, as its particles remaining on the surface of the product can cause local corrosion.
Before spraying the main coating, a sublayer is sprayed from the corresponding materials indicated above. The sublayer can be applied in any way - gas-flame, electric arc.
During plasma spraying, the surface should not be overheated above a temperature of 300 ° C, since this causes internal stresses that can lead to the destruction of the coating. burner mouthpiece.
The use of a cooling nozzle makes it possible to reduce the distance from the torch to the surface from 120 mm to 70 mm. This increases the productivity of the equipment, increases the powder utilization rate, increases the strength and reduces the porosity of the coating. Excessive cooling is unacceptable, as it worsens the properties of the coating. Cooling is not required if the thickness of the coating layer is less than 0.1 mm or the speed of movement of the burner relative to the surface is high enough, and the applied layer has time to cool before the next pass of the burner. This is ensured with massive parts in which intensive heat removal occurs.
Spray angle, i.e. the angle between the axis of the burner nozzle and the surface must be 90-60°. At an angle less than 60°, the energy of impact of particles on the surface is reduced, which worsens the strength of the coating.
To obtain a uniform in thickness and uniform in quality coating, various means of process mechanization are used. The simplest and most accessible of them is a lathe, in the cartridge of which the sprayed part is installed, and in the caliper - the burner.
Nitrogen is recommended as the plasma gas. Adding 5-10% hydrogen to nitrogen increases the productivity of the process, but requires a current source with an operating voltage of 110-120 V instead of 85-95 V with nitrogen alone. Argon can only be used in a mixture with hydrogen or nitrogen, since with one argon the operating voltage does not exceed 35 V, which drastically reduces the thermal power of the burner and its productivity.

Plasma spraying

The method of coating using a plasma flow is superior in its capabilities to metal deposition methods using an oxy-acetylene flame and arc welding. The advantage of this method over others lies in the possibility of melting and deposition of multilayer coatings on materials made of refractory metals, regardless of the melting temperature of the latter, which makes it possible to restore parts that have out of all repair sizes.

Like other methods of high-temperature spraying of coatings, plasma spraying does not cause warping of the part and changes in the structure. The wear resistance of plasma coatings is 1.5...3 times higher, and the coefficient of friction is 1.5...2 times lower than that of hardened steel 45.

The plasma jet is used for surfacing and coating products made of steels, aluminum and its alloys and other materials by melting filler wire or metal powders. Plasma is used for cutting and surface treatment of various materials, heating for soldering and heat treatment. The use of neutral gases for plasma formation and protection - argon, nitrogen and their mixtures - ensures minimal burnout of alloying elements and oxidation of particles. Plasma spraying improves the properties of metal coatings, however, its widespread use is limited by the low adhesion strength of the coating to the surface of the restored part and the reliability of plasma torches, high noise and brightness of the arc. The plasma arc is a high-intensity source of heat, consisting of molecules of atoms, ions, electrons and light quanta in a highly ionized state, the temperature of which can reach 20,000 °C or more.

The plasma jet has a brightly glowing core, the length of which can vary from 2...3 to 40...50 mm depending on the size of the nozzle and channel, gas composition and flow rate, current value and arc length.

The power supply circuit of the installation consists of two sources: one of them is designed to power the plasma arc, and the second - to maintain the main arc. Plasma-forming gas is supplied from the cylinder through the gas equipment located in the control panel. A carrier gas is used to feed the filler powder. Gas equipment consists of cylinders, reducers, flow meters, a mixer, fuses and electromagnetic valves.

For surfacing, it is advisable to use plasma torches in which two arcs burn simultaneously: one is plasma-forming, and the second serves to melt the base metal and melt the filler. When spraying, burners are recommended in which the filler and base metals are heated by a part of the plasma flow that has passed through the hole in the nozzle.

Niresist and bronze powders are used for spraying antifriction coatings. Powders of self-fluxing alloys PG-SRZ, SNGN-50, stainless steel are used in mixtures for spraying wear-resistant coatings, as well as for restoring shafts and bearing seats.

Intermetallic powders (chemical compound of metal with metal) PN55T, PN85Yu15 are used as a sublayer (0.05...0.1 mm) to increase the adhesion strength of coatings and as a component of a powder mixture to increase the cohesive strength of the coating. Plasma coatings have sufficiently high adhesive strength values ​​with a layer thickness of up to 0.6 ... 0.8 mm.

For spraying the main and connecting rod journals of the crankshaft of the ZIL-130 engine, you can use a mixture of powders - 15 ... 25% (by weight) PN85Yu15 + 35 ... 40% PG-SRZ + 35 ... 50% P2X13. For economic reasons, it is advisable to spray with mixtures, the main components of which are cheap powders (ni-resist, stainless steel, bronze). 10…15% powder PN85Yu15 is introduced into their composition.

Powders PR-N70Yu30 and PR-N85Yu15, produced by NPO Tulachermet, can serve as a sublayer and main coating layer in combination with high-carbon powders.

The quality of the coating during plasma spraying largely depends on the power of the burner, gas flow, electric mode, powder supply, spraying conditions (the distance of the burner from the product, the spraying angle is set experimentally for each specific case.

Rice. 1. Scheme of installation for plasma surfacing:
1 - main current source; 2 - current source for excitation; 3 - plasma torch; 4 - gas cylinder transporting welding powder; 5 - gas reducer; 6 - dispenser; 7 - cylinder with plasma gas; 8 - rotameter; 9 - mixer.

Rice. 2. Schemes of plasma torches for surfacing (a) and for spraying (b):
1 - tungsten electrode (cathode); 2 - insulating gasket; 3 - nozzle (anode); 4 - plasma; 5 - deposited layer; 6 - base metal; 7 - channel for supplying welding powder; 8 - channels for cooling water; 9 - sprayed layer.

To restore parts of the “shaft” type (gear shafts, hollow and solid shafts and axles, cardan crosses and differentials) with a wear of not more than 3 mm, the OKS-11231-GOSNITI installation is used by plasma surfacing with hard-alloy materials.

The diameter and length of the welded parts are 20…100 and 100…800 mm, respectively. Applied powders: sor-mite, charged with aluminum powder ASDT; US-25 with aluminum; T-590 with aluminum; PG-L101 with aluminum; gas - argon, compressed air. The hardness of the applied metal is up to 66 HRC3. Overall dimensions of the machine 2225X1236X1815 mm.

According to GOSNITI, the annual economic effect of the installation will be more than 9 thousand rubles.

At the OKS-11192-GOSNITI installation, the chamfers of the valve discs of diesel engines of all brands are successfully restored with PG-SR2 powder material. Its productivity is 80…100 valves per shift.

High reliability in operation was shown by the small-sized plasma torch VSKHIZO-Z, which, in combination with the converted UMP-5-68 installation, is recommended for the restoration of crankshafts of YaMZ-238NB, SMD-14 and A-41 engines using the following compositions: wire Sv-08G2S-80 …85% + PG-SR4-15…20% powder (SMD-14 and A-41) and 15GSTYUTSA-75…80% wire + PG-SR4-20…25% powder. The hardness of the shaft journals in the first case is 46.5 ... 51.5 HRC3, in the second - 56.5 ... 61 HRC3. The wear resistance of the journals and liners is at the level of the crankshaft.

The problem of ensuring the necessary strength of adhesion of the metal coating to the product, the search for new cheap materials and effective methods for preparing worn surfaces of parts before plasma spraying needs to be resolved.

The first can be solved by introducing an additional operation - melting of the sprayed coating, which is performed by a plasma or oxy-acetylene torch immediately after coating, as well as by heating with high-frequency currents. After the coating is melted, its physical and mechanical properties are improved, and the adhesion strength increases by 10 times or more.

The technological process of restoring parts in this way includes cleaning the surface of the product from dirt and oxides (if necessary, preliminary grinding to give the correct geometric shape parts), its degreasing and abrasive blasting (creates work hardening, destroys the oxide film, increases roughness), spraying the part with coating melting and then machining the product.

Compressed air pressure during abrasive blasting - 0.4 ... 0.6 MPa, blowing distance 50 ... 90 mm, abrasive jet attack angle 75 ... 90 °. The duration of treatment depends on the abrasive (powder of white electrocorundum 23A, 24A or black silicon carbide 53C, 54C with a grain size of 80 ... 125 microns GOST 1347-80, steel or cast iron shot DSK and DCHK No. 08K; No. 1.5K GOST 11964-69), the material of the part and its hardness and the area of ​​the machined surface. The time between preparation and spraying should be as short as possible and not exceed 1.5 hours.

The distance from the nozzle exit to the surface of the part during plasma melting is reduced within 50 ... 60 mm.

For cylindrical parts, melting is carried out during their rotation with a frequency of 10 ... 20 min-1.

As a rotator for plasma spraying, installations 011-1-01, 011-109 or a screw-cutting lathe can be used.

When choosing the final layer thickness, one should take into account shrinkage during flashing (10...20%) and machining allowance (0.2...0.3 mm per side).

Plasma coatings sprayed with metal powders are processed on screw-cutting lathes or grinders using standard cutting tools. Grinding with synthetic diamond wheels is especially effective.

The conducted studies have shown that it is possible to restore critical autotractor parts of any shape (poppets and pusher rods, chamfers of poppets and valve stems, crankshafts, water pump rollers) by plasma spraying with coating reflow, which should be taken into account by specialists when developing technological processes for the restoration of these parts.

The use of plasma spraying is advisable in the restoration of wearable working parts of agricultural machines (in this case, the application of carbide powders is desirable). It can be used to apply heat-resistant anti-corrosion coatings for parts operating at high temperatures.

At the same time, the problem of sprayed coatings has not been completely solved yet. For example, control in the process of spraying the thickness of coatings, mechanical processing of sprayed coatings. It is necessary to further improve the existing technology of high-temperature spraying and equipment for its implementation, in-depth and versatile studies of the possibilities and advantages of this technology, and the development of scientifically based recommendations for the use of flux-cored materials on specific parts.

TO Category: - Advanced Repair Methods