Thermal spraying

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Thermal spraying technique is a coating process in which the melted (or heated) material is sprayed onto the surface. The "feedstock" (coating pioneer) is heated by electricity (plasma or arc) or chemical means (fire combustion).

Thermal spraying can provide a thick layer (a thickness range of about 20 microns to several mm, depending on process and raw material), over a large area at high deposition levels compared to other coating processes such as electroplating, physical and chemical coatings of vapors. The coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites. They are fed a powder or wire form, heated to a liquid or semimolten state and accelerated toward a substrate in the form of micrometer-sized particles. Discharging or discharging an electric arc is usually used as an energy source for thermal spraying. The resulting coatings are made by the accumulation of many sprayed particles. The surface may not heat up significantly, allowing a layer of flammable substances.

The quality of the coating is usually assessed by measuring porosity, oxide content, macro and micro-hardness, bond strength and surface roughness. Generally, the quality of the coating increases with increasing particle speed.

Some variations of thermal spraying are distinguished:

• Plasma Spray
• Spraying detonation
• Wire spraying
• Spraying flame
• High-speed oxy-fuel layer spraying (HVOF)
• High speed air fuel (HVAF)
• Spraying warm
• Cold spraying

In the classic (developed between 1910 and 1920) but still widely used processes such as spraying fire and spraying arc wire, the particle velocity is generally low (& lt; 150 m/s), and the raw material must be liquid to be stored. Plasma spraying, developed in the 1970s, used high-temperature plasma jets produced by arc discharge with typical temperature & gt; 15000 K, which allows to spray refractory materials such as oxide, molybdenum, etc.

Video Thermal spraying

Gambaran sistem

The typical thermal spray system consists of the following:

• Torque spray (or spray gun) - a core device that performs smelting and particle acceleration to be stored
• Feeder - to supply powder, wire or liquid to a torch through a tube.
• Media supplies - gas or liquids for fires or plasma plasma, gas for carrying powder, etc.
• Robot - to manipulate the torch or substrate to be coated
• Power supply - often stand alone for torch
• Control console - either integrated or individual for all of the above

Maps Thermal spraying

The thermal spray detonation process

The detonation gun consists of a water-cooled barrel length with an inlet valve for gas and powder. Oxygen and fuel (the most common acetylene) are inserted into the vat along with the powder charge. A spark is used to ignite the gas mixture, and the resulting detonation heats and accelerates the powder to supersonic speed through the barrel. The nitrogen pulses are used to clean the barrel after each detonation. This process repeats many times every second. The high kinetic energy of the heat powder particles in impact with the substrate results in a very solid and strong layer buildup.

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Plasma spray

In the plasma spraying process, the material to be stored (the raw material) - usually as a powder, sometimes as a liquid, a suspension or a wire - is inserted into a plasma jet, originating from a plasma torch. In jets, where temperatures are in the order of 10,000 K, the material is melted and pushed onto the substrate. There, liquid droplets flatten, quickly harden and form a deposit. Generally, the deposits remain attached to the substrate as a coating; Free parts can also be produced by removing the substrate. There are a large number of technology parameters that affect the interaction of particles with plasma jets and substrates and therefore their storage properties. These parameters include raw material type, plasma gas composition and flow rate, energy input, torch ignition distance, substrate cooling, etc.

Deposit Property

The sediment consists of many 'flakes' like pancakes called lamellae, formed by the spread of liquid droplets. Since raw material powders typically have sizes from micrometers up to over 100 micrometers, lamellae have thicknesses in the micrometer range and lateral dimensions from several to hundreds of micrometers. Among these lamellae, there are small holes, such as pores, cracks and areas of incomplete bonding. As a result of this unique structure, the sediment can have significantly different properties than bulk materials. These are generally mechanical properties, such as lower strength and modulus, higher strain tolerances, and lower thermal and electrical conductivity. Also, due to rapid compaction, the metastable phase may be present in the precipitate.

Apps

This technique is widely used to produce coatings on structural materials. Such coatings provide protection against high temperatures (eg thermal barrier layers for waste heat management), corrosion, erosion, wear and tear; they can also change the appearance, electrical properties or surface tribology, replace the material used, etc. When sprayed on a substrate of various shapes and disposed of, free standing parts in the form of plates, tubes, shells, etc. can be produced.. It can also be used for powder processing (spheroidization, homogenization, chemical modification, etc.). In this case, the substrate for precipitation is absent and the particles harden during flight or in a controlled environment (eg, water). Techniques with this variation can also be used to create a porous structure, suitable for bone growth, as a coating for medical implants. A polymer dispersion aerosol can be injected into a plasma discharge to create this polymer grafting onto the substrate surface. This application is mainly used to modify the polymer surface chemistry.

Variations

Plasma spraying systems can be categorized based on several criteria.

Generation of plasma jets:

• direct current (DC plasma), where energy is transferred to plasma jet by direct current, high power arc
• induction plasma or plasma RF, where energy is transferred by induction from a coil around a plasma jet, where the radio frequency alternating current passes through

Plasma-forming medium:

• gas-stable plasma (GSP), in which the plasma is formed from gas; usually argon, hydrogen, helium or mixtures thereof
• water-stable plasma (WSP), in which the plasma is formed from water (by evaporation, dissociation and ionization) or other suitable liquids
• Hybrid plasma - with combined gas and liquid stabilization, usually argon and water

Spraying environment:

• atmospheric plasma spraying (APS), performed in ambient air
• controlled plasma spraying (CAPS), usually done in enclosed spaces, filled with inert gas or evacuated
• CAPS variations: high pressure plasma spraying (HPPS), low pressure plasma spraying (LPPS), extreme cases are vacuum plasma spraying (VPS, see below)
• underwater plasma spraying

Another variation consists of having a liquid feedstock instead of a solid powder for melting, this technique is known as plasma spray precursor solution

Vacuum plasma spraying

Vacuum plasma spraying (VPS) is a technology for etching and surface modification to create porous layers with high reproducibility and for cleaning and engineering plastic surfaces, rubber and natural fibers as well as for replacing CFCs for cleaning metal components. This surface engineering can improve properties such as friction behavior, heat resistance, surface electrical conductivity, lubrication, cohesive film strength, or dielectric constant, or can create hydrophobic or hydrophobic materials.

This process usually operates at 39-120 ° C to avoid thermal damage. It can induce a thermally activated surface reaction, causing surface changes that can not occur with molecular chemistry at atmospheric pressure. Plasma treatment is performed in a controlled environment in a confined space on a moderate vacuum, about 13-65 Pa. The gas or gas mixture is energized by an electric field from DC to microwave frequency, typically 1-500 W at 50 V. The treated component is usually electrically isolated. The volatile plasma side product is evacuated from space by the vacuum pump, and if necessary it can be neutralized in a chimney scrubber.

Unlike molecular chemistry, plasma uses:

• Molecules, atoms, metastables and free radicals for chemical effects.
• Positive ions and electrons for kinetic effects.

Plasma also produces electromagnetic radiation in the form of UV vacuum photons to penetrate the bulk polymer to a depth of about 10 Âμm. This can lead to chain imaging and cross linking.

Plasma affects materials at the atomic level. Techniques such as X-ray photoelectron spectroscopy and scanning electron microscopy are used for surface analysis to identify the processes needed and to assess their effects. As a simple indication of the surface energy, and hence adhesion or wettability, frequent angle contact drip tests are used. The lower the contact angle, the higher the surface energy and the more hydrophilic material.

Change the effect with plasma

At higher energy ionization tends to occur more than chemical dissociation. In typical reactive gases, 1 in 100 molecules form free radicals while only 1 in 10 ⁶ are ionized. The main effect here is the formation of free radicals. Ionic effects can dominate with the selection of process parameters and if necessary the use of noble gases.

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Wire spray

Spray arc wire is a form of thermal spray in which two metal wires are consumed independently inserted into a spray gun. These cables are then filled and arcs are produced between them. The heat from this arc melts the incoming wire, which is then inserted into the air jet from the gun. The liquid raw material is then deposited onto the substrate with the help of compressed air. This process is commonly used for metal coating and heavy.

Arc wire transferable plasma

The plasma transfer wire arc (PTWA) is another form of spray arc wire that stores the coating on the internal surface of the cylinder, or on the external surface of a portion of the geometry. It is especially known for its use in coating the engine cylinder holes, allowing the use of Aluminum engine blocks without the need for heavy cast iron sleeves. Single conductive wire is used as "raw material" for the system. Supersonic plasma jets melt the wire, spray it and push it onto the substrate. Plasma jets are formed by arcs that are transferred between the non-consumption cathode and the wire type. After atomization, forced air transports the flow of liquid droplets to the bore wall. Particles taper as they hit the surface of the substrate, due to high kinetic energy. Particles quickly harden after contact. The stacked particles form a high wear-resistant coating. PTWA thermal spray process uses one wire as raw material. All conductive cables up to and including 0.0625 "(1.6 mm) can be used as raw materials, including PTWA cored cable can be used to apply coating to the wear surface of the machine or transmission components to replace the bushing or bearing For example, using PTWA to coat the bearing surface of the connecting rod offers a number of benefits including weight reduction, cost, friction potential, and pressure on the connecting rod.

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High-speed oxygen fuel spraying (HVOF)

During the 1980s, a thermal spraying process class called high-speed oxy-fuel spraying was developed. Mixtures of gas and liquid fuels and oxygen are fed into the combustion chamber, where they are ignited and burned continuously. The hot gas produced at a pressure approaching 1 MPa radiates through the convergent-convergent nozzle and moves through the straight portion. Fuel may be gaseous (hydrogen, methane, propane, propylene, acetylene, natural gas, etc.) or fluids (kerosene, etc.). The jet velocity at the exit of the barrel (& gt; 1000 m/s) exceeds the speed of sound. A feed feed is injected into the gas stream, which accelerates the powder to 800 m/s. The flow of hot gas and powder is directed to the surface to be coated. The powder partially melts in the flow, and settles on the substrate. The resulting layer has a low porosity and high bond strength.

The HVOF coating can be as thick as 12 mm (1/2 ").This is commonly used for storing wear resistant and rust resistant coatings on materials, such as ceramic and metallic layers.Products include WC-Co, chrome carbide, MCrAlY, and alumina. managed to store mirror materials (WC-Co, etc.) and other corrosion-resistant alloys (stainless steels, nickel-based alloys, aluminum, hydroxyapatite for medical implants, etc.).

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Cold spraying

In the 1990s, cold spraying (often called dynamic cold spray gas) was introduced. This method was originally developed in Russia, with an unintentional observation of rapid layer formation. This occurs when experimenting with particle erosion from targets exposed to high velocity flow filled with fine powder in the wind tunnel. In cold spraying, particles are accelerated to very high speed by the carrier gas being forced through the converging-divergent de nozzle Laval. After collision, the solid particles with sufficiently kinetic energy are plastic and bind mechanically to the substrate to form a layer. The critical speed required to form bonds depends on material properties, powder size and temperature. Metals, polymers, ceramics, composite materials and nanocrystalline powders can be stored by cold spraying. Soft metals such as Cu and Al are best suited for cold spraying, but other coating materials (W, Ta, Ti, MCrAlY, WC-Co, etc.) with cold spraying have been reported.

The deposition efficiency is usually low for alloy powders, and the process parameter window and corresponding powder size are narrow. To speed up the powder to a higher speed, fine powder (& lt; 20 micrometers) is used. It is possible to speed up the powder particles at much higher speeds using processing gases that have high sound speed (helium, not nitrogen). However, expensive helium and its flow rate, and thus consumption, are higher. To improve the acceleration capacity, nitrogen gas is heated to about 900 C. As a result, the deposition efficiency and tensile strength of the deposit increases.

Spraying warm

Warm spraying is a new modification of high-speed oxy-fuel spraying, where the combustion gas temperature is lowered by mixing nitrogen with combustion gases, thus bringing the process closer to cold spraying. The resulting gas contains a lot of water vapor, unreacted hydrocarbons and oxygen, and thus more dirty than cold spraying. However, the coating efficiency is higher. On the other hand, a lower temperature of warm spray reduces melting and chemical reactions than feed powders, compared to HVOF. This advantage is particularly important for coating materials such as Ti, plastics, and metallic glass, which rapidly oxidize or worsen at high temperatures.

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Applications

• Conditioning or conditioning Crankshaft
• Corrosion protection
• Fouling protection
• Changes thermal conductivity or electrical conductivity
• Wear control: either the hardfacing layer or the abradable
• Fixed a damaged surface
• Temperature/oxidation protection (thermal barrier coating)
• Medical implants
• Production of functionally graded materials (for any of the above apps)

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Limitations

Thermal spraying is the process of line vision and mechanical bond mechanisms. The application of thermal spray is not compatible with the substrate if the applied area is complex or blocked by another body.

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Security

Thermal spraying does not need to be a dangerous process, if equipment is properly maintained, and proper spraying practices are followed. As with industrial processes, there are a number of dangers, which the operator must pay attention to, and what specific precautions to take. Ideally, equipment should be operated automatically, in enclosures designed specifically to extract smoke, reduce noise levels, and prevent direct view of the spray head. Such a technique will also produce a more consistent layer. There are times when the type of component being treated, or their low production level, requires manual equipment operation. Under these conditions, a number of hazards, specifically for thermal spraying, are experienced, other than those commonly encountered in the production or processing industry.

Noise

Metal spraying equipment uses compressed gas, which creates noise. The noise levels vary with the type of spraying equipment, sprayed material, and operating parameters. Typical sound pressure levels are measured at 1 meter behind the arc.

UV light

Combustion spraying equipment produces an intense flame, which may have a peak temperature of over 3,100 ° C, and is very bright. Spraying electric arc produces ultra-violet light, which can damage delicate tissues. Spray booths and covers should be equipped with ultra-violet absorbent dark glass. If this is not possible, the operator, and others around him must use protective goggles containing a BS grade 6 green glaze. An opaque screen should be placed around the spraying area. The bow gun nozzle should not be seen directly, unless it is certain that no power is available for the equipment.

Dust and smoke

Atomization of liquids produces large amounts of dust and smoke made of very fine particles (about 80-95% particles of> 100 nm). Proper extraction facilities are essential, not just for personal safety, but to minimize the traps of frozen particles in sprayed coatings. The use of a respirator, equipped with an appropriate filter, is strongly recommended, where equipment can not be isolated. Certain materials offer certain known hazards:

1. Finely divided metal particles are potentially pyrophoric and harmful when accumulated in the body.
2. Certain materials eg aluminum, zinc, and other base metals can react with water to form hydrogen. This is potentially explosive and special precautions are required in smoke extraction equipment.
3. The smoke of certain materials, especially zinc and copper alloys, has an unpleasant odor and can cause fever-type reactions in certain individuals (known as metal smoke fever). This can happen sometime after spraying and usually subside quickly. If not, medical advice should be sought.
4. Smoke from reactive compounds can dissociate and create harmful gases. Respirators should be worn in this area and the gas meter should be used to monitor the air before the respirator is removed.

Hot

Combustion guns use oxygen and fuel gas. The fuel gas is potentially explosive. In particular, acetylene may only be used under approved conditions. Oxygen, while not explosive, will retain combustion, and many materials will ignite spontaneously, if excess oxygen levels are present. Care should be taken to avoid leakage, and to isolate the supply of oxygen and fuel gas, when not in use.

Shocking hazard

The electric arc rifle operates at a low voltage (below 45 V dc), but at a relatively high current. They can be safely held hands. The power supply unit is connected to a 440 V AC source, and should be treated with care.

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See also

• List of coating techniques
• Thin film

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References

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Further reading

• Pawlowski L, "Thermal Spray Layer Science and Engineering" (New York: Wiley, 1995)
• Papyrin A, Kosarev V, Klinkov S, Alkhimov A, and Fomin V "Cold Spray Technology" (Oxford: Elsevier, 2007)
• Alternative Metal Scaling Method

Source of the article : Wikipedia