Overview of Thermal Spraying Technology
It is well known that, except for a few precious metals, metal materials will be corroded by chemical and electro
chemical reactions with the surrounding medium. In addition, metal surfaces are subjected to a variety of mechanical action caused by wear and tear is also extremely
serious. A large number of metal components fail due to corrosion and wear, resulting in great waste and loss. According to the statistics of some
industrialized countries, the annual loss of steel due to corrosion and abrasion accounts for about
10 % of the total steel production, and the amount of loss accounts for about 2 - 4 % of the total value of national economic output. If the metal corrosion and wear and tear
cause of work stoppage, production stoppage and the corresponding cause of industrial injuries, fires, explosions and other losses in the statistics
words, its value is even more alarming. Therefore, the development of metal surface protection and strengthening technology, is a common concern in all countries
major issues.
With the development of cutting-edge science and modern industry, various industrial sectors increasingly require machinery and equipment
to be able to operate stably for a long period of time under high parameters (high temperature, high pressure, high speed, and high degree of automation) and severe working conditions (such as severe
wear and corrosion). As a result, higher demands are placed on material properties. The use of
high-performance advanced materials to manufacture the whole equipment and parts to obtain the effect of surface protection and strengthening, obviously
is not economical, and sometimes even impossible. Therefore, the research and development of material surface treatment technology is
has great technical and economic significance. And surface treatment technology is also driven by this need to get
rapid development and improvement.
Thermal spraying technology is one of the surface protection and strengthening technology, is an important discipline in surface engineering
. The so-called thermal spraying, is the use of a heat source, such as electric arc, plasma arc, combustion flame
powder or filamentary metal and non-metallic coating materials heated to a molten or semi-molten state, and then
with the help of the flame itself or the external high-speed airflow atomization and a certain speed of injection to the surface of the substrate after the
treatment of the substrate material, and the substrate material Combined with the substrate material to form a surface coating with various functions
a technology.
I. Classification of thermal spray technology
According to the type of heat source thermal spray technology is mainly categorized as:
Heat source temperature ℃ Spraying method
Powder flame spraying (welding)
Fire wire flame spraying
Approx. 3000 ceramic rods flame spraying
Flame High Velocity Flame (HVOF)
Explosive spraying (D - GUN) (D - GUN)
Electric Arc Approx. 5000 Arc Spraying
Atmospheric Plasma Spraying (APS)
Plasma Arc 10,000 Above Low Pressure Plasma Spraying (LPPS)
Water Stabilized Plasma Spraying
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Heat Source Temperature and Flow Rate of Various Thermal Spraying Methods
II. Thermal Spraying Equipment
Although the equipment varies according to the method of thermal spraying, based on the principle of thermal spraying technology,
the equipment mainly consists of spray gun, heat source, coating material supplying device, as well as control system and cooling system.
The following figure shows the temperature of heat source and flow rate of various thermal spraying methods. The following diagram shows the equipment configuration of plasma spraying.
Three. Thermal spraying process
The thermal spraying process is as follows:
Workpiece surface pretreatment → workpiece preheating → spraying → coating post-treatment
1. Surface pretreatment
In order to make the coating and the substrate material to combine well, the surface of the substrate material must be clean and rough, purification and
Roughening the surface of the method, the selection of the method according to the design requirements and the substrate material, material and the surface of the substrate material. The choice of the method depends on the design requirements and the material, shape, thickness, original condition of the surface and the construction conditions of the substrate.
The purpose of purging is to remove all dirt from the surface of the workpiece, such as oxidized skin, oil, paint and
Low Speed Flame Spraying
250 500 750 750 1000 m/s
2500 5000 7500 10000 oC
0
0
Plasma Spraying
Plasma Spraying
Plasma Spraying
Plasma Spraying
Plasma Spraying
Plasma Spraying
Plasma Spraying
Plasma sprayingHigh-speed flame spraying
Temperature
Speed
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Other contaminants, the key is to remove the surface of the workpiece and the oils and greases that have penetrated it. Decontamination methods include solvent cleaning, steam cleaning, alkaline cleaning and heat degreasing.
The purpose of roughening is to increase the contact surface between the coating and the substrate, to increase the mechanical
bite force between the coating and the substrate, and to activate the decontaminated surface to improve the bonding strength between the coating and the substrate. At the same time
substrate surface roughening also changes the residual stress distribution in the coating, which is also beneficial to improve the bonding strength of the coating. Roughening treatment methods include sandblasting, machining method (such as threading, knurling), electric drawing, etc.
Which sandblasting is the most commonly used roughing treatment method, commonly used sandblasting media are aluminum oxide, silicon carbide
and cold hard cast iron. Sand blasting, sand blasting media type and particle size, sand blasting wind pressure size and other conditions
Must be based on the hardness of the workpiece material, the shape and size of the workpiece and so on to make a reasonable choice. For a variety of
metal substrate, the recommended sand grain size of about 16-60 sand, coarse sand for solid and heavy parts
blasting, sand blasting pressure of 0.5-0.7Mpa, thin workpiece is easy to deformation, sand blasting pressure of 0.3-0.4
Mpa.
Mpa. It is worth noting that the compressed air used for sandblasting must be waterless and oil-free,
otherwise it will seriously affect the quality of the coating. The degree of roughness of the surface of the workpiece before spraying is sufficient for most metallic materials
2.5-13 μmRa. As surface roughness increases, the bond between the coating and the substrate material increases, but as surface roughness exceeds 10 μmRa, the increase in coating bond strength decreases
.
For some of the coating material and the substrate bonding is not good, should also choose a good bonding with the substrate material
Spraying a layer of transition layer, called bonding layer, commonly used as a bonding layer of materials such as Mo, NiAl,
NiCr and aluminum bronze. The thickness of the bonding layer is generally 0.08-0.18μm.
2. Preheating
The purpose of preheating is to eliminate the water and moisture on the surface of the workpiece, to increase the interface temperature of the sprayed particles in contact with the workpiece, to improve the bonding strength of the coating and the substrate, and to reduce stresses caused by differences in thermal expansion of the substrate and the coating material.
This is a good way to improve the bonding strength of the coating and the substrate. The temperature of the interface between the coating and the substrate is increased to improve the bonding strength between the coating and the substrate, and to reduce the cracking of the coating due to the
stress caused by the
difference between the substrate and the coating material. The preheating temperature depends on the size, shape and material of the workpiece, as well as the coefficients of thermal expansion of the substrate and coating materials, etc. Generally, the preheating temperature is controlled
between 60 - 120 ℃.
3. Spraying
The method of spraying mainly depends on the choice of spraying material, the working conditions of the workpiece
and the quality of the coating requirements. For example, if the ceramic coating, it is best to use plasma spraying; such as
fruit is a carbide metal ceramic coating is best to use high-speed flame spraying; if the spraying of plastics can only be used
with the flame spraying; and if you want to outdoor spraying of large-area anticorrosive projects, it is not a flexible and high-
effective arc spraying or wire flame spraying is none other than. In short, the choice of spraying methods is generally diverse, but there is always a method that is best for a particular application.
Pre-treated workpieces should be sprayed in the shortest possible time, spraying parameters should be based on the coating
materials, gun performance and workpiece specifics, the optimized spraying conditions can improve the spraying efficiency,
and obtain a high density, high bonding strength of high-quality coatings.
4. Post-treatment of coatings
The coatings obtained from spraying are sometimes not ready for use, and must undergo a series of post-treatment.
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In order to prevent the corrosive medium from reaching the substrate through the pores of the coating and causing corrosion of the substrate
, it is necessary to seal the pores of the coating. There are many materials used as sealers, including paraffin, epoxy
resin, silicone resin and other organic materials and inorganic materials such as oxides, so how to choose the right sealer should be considered according to the workpiece's working medium, environment, temperature and cost and many other factors.
For workpieces subjected to high stress loads or impact abrasion, in order to improve the bonding strength of the coating, remelting treatment (e.g., flame remelting, induction remelting, laser remelting, and hot isostatic pressing) is performed on the sprayed layer, so that the porous coating that is mechanically bonded to the substrate only becomes metallurgically bonded to the substrate and becomes a dense coating.
Where dimensional accuracy is required, the coating is mechanically processed. Because the spray coating has different characteristics from general
metal and ceramic materials, such as the coating has microporous, not conducive to heat dissipation; the coating itself is
low strength, can not withstand a lot of cutting force; the coating has a lot of hard texture, the wear of the tool is very fast, etc.,
Therefore, the formation of the coating is difficult to form a spray coating different from the general material machining characteristics. Therefore, must choose reasonable
processing methods and the corresponding process parameters to ensure the smooth progress of the spray coating layer machining and ensure that
to the required dimensional accuracy.
Four. Characteristics of thermal spray technology
From the principle of thermal spray technology and process analysis, thermal spray technology has the following characteristics.
1. Because of the wide temperature range of the heat source, the coating materials that can be sprayed include almost all solid
engineering materials, such as metals, alloys, ceramics, cermets, plastics, and complexes composed of them.
Thus can give the substrate with a variety of functions (such as wear-resistant, corrosion-resistant, heat-resistant, oxidation-resistant, insulating, thermal insulation,
biocompatible, infrared absorption, etc.) of the surface.
2. Spraying process of the substrate surface heat to a lesser extent and can be controlled, so you can spray in a variety of
materials (such as metal, ceramics, glass, cloth, a bolt, paper, plastics, etc.) and the substrate's
organization and performance has almost no impact on the deformation of the workpiece is also small.
3. Simple equipment, flexible operation, both large components for large-area spraying, but also in the designated
local spraying; both in the factory indoor spraying can also be in the outdoor construction site.
4. Spraying operation program is less, shorter construction time, high efficiency, more economical.
With the improvement of thermal spraying application requirements and the expansion of the field, especially the progress of the spraying technology itself,
such as the spraying equipment is increasingly high-powered and sophisticated, the gradual increase in the number of varieties of coating materials, gradually improve the performance of the thermal
spraying technology in the past ten years to obtain the rapid development of the field of application has been greatly expanded, and the technology
has been developed from the early preparation of general protective coatings. It has not only expanded its application fields, but also developed from the preparation of general protective coatings to the preparation of various functional coatings; from the maintenance of individual workpieces to the manufacture of large quantities of products; from the preparation of a single coating to the coating system engineering, including the failure analysis, surface
pre-treatment, the development and selection of coating materials and equipments, the design of the coating system, and the processing of the coating; and become a very active field in the field of material and surface sciences. Surface science has become a very active discipline in the field of material science. And in the modern industry
gradually formed like casting, forging, welding and heat treatment of independent material processing technology. Become
Industrial sector to save valuable materials, energy saving, improve product quality, extend product life, reduce
low cost, improve the efficiency of the important means of technology, in all areas of the national economy to get more and more
widely used.
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V. Overview of various thermal spraying methods
1. Oxyacetylene flame spraying (welding)
is one of the earliest spraying methods. It is the use of oxygen and acetylene combustion flame will be powder or wire
shaped, rod-shaped coating material heated to a molten or semi-molten state and sprayed to the surface of the substrate to form a coating
a method. It is characterized by simple equipment, mature technology, flexible operation, low investment and quick results.
It can prepare a variety of metals, alloys, ceramics and plastic coatings, is currently the most commonly used spraying method
one of the domestic. However, the porosity of the coating prepared by this method is large, and the bonding strength with the base material is
low. However, for the self-melting alloy, if the combustion flame will be a spray melt or spray coating layer into
secondary remelting (flame remelting, induction remelting and furnace melting, etc.) method is called spray welding, spray welding coating
because of the metallurgical bonding with the substrate material is a state of bonding with the base material, and thus the bonding strength of the substrate material is greatly improved, and can be
applied to the impact of a large, heavy loads, heavy working conditions, the coating is also a good combination with the base material. Heavy working conditions, such as continuous casting straightening rolls, hot rolling straightening roll surface using
Nickel-based fusion alloy spray welding coating for strengthening, are very good corrosion resistance, wear resistance and resistance to thermal fatigue
labor strengthening effect.
2. Explosive spraying (D - GUN)
This method is the use of a mixture of oxygen and flammable gases, after ignition in the spray gun explosion, the use of
pulsed gas explosion energy, will be sprayed powder material heated, accelerated bombardment of the surface of the workpiece and
the formation of coatings. The result of gas combustion and explosion can produce supersonic high-energy gas flow, explosion wave propagation speed
degree as high as 3000 m / s, its center temperature up to 3450 ℃, the powder particles flying speed of up to 1200
m / s. Therefore, the explosion coating layer coating dense, high bonding strength with the substrate, up to 24 kg /
mm2. The disadvantage of the method is that the noise, and the explosion of the powder material to be sprayed to the surface of the workpiece to form a coating. The disadvantage of this method is that the noise, and the explosion is not continuous, and therefore less efficient. Explosive spraying
was invented in the 1950s by the United States Union Carbide Corporation, but after the introduction of many years by the company's
monopoly, do not sell the technology and equipment, but only in its service companies for the user to carry out spraying processing, mainly
To spraying of ceramics and cermets, for the repair of aircraft engines.
3. High velocity flame spraying (HVOF)
High velocity flame spraying (or supersonic flame spraying) is a high
energy spraying method that appeared in the 1980s, and its development is the most creative progress in the thermal spraying industry after plasma spraying.
Although the high-speed flame spraying method can spray a lot of materials, but because of its flame contains less oxygen temperature moderate,
flame speed is very high, can effectively prevent the oxidation and decomposition of the powder coating materials, so it is particularly suitable for carbonized
Other types of coatings spraying. The development of this equipment to the third generation, the performance has been greatly improved, such as
JP-5000, DJ - 2700 and other equipment, the room pressure of 8 -12 bar, power up to 100 -120 kw, spray
coating efficiency of up to 10 kg / h (WC -Co), coating thickness of up to a few millimeters, the performance of the coatings can be achieved
explosive spraying level. The coating performance has reached the level of explosion spraying. It is widely used in many industrial sectors. For example, wear-resistant coatings in aerospace engines
layers, mirror coatings for paper machines, etc. In recent years, as a result of electrochromic plating, the coating thickness can reach several mm. In recent years, due to the environmental pollution of the chromium plating process, chromium plating
chromium industry in some industrially developed countries are strictly limited and gradually eliminated, the use of high-speed flame
spray coating instead of chromium plating layer of the application of more and more attention and importance of the industrial community.
4. Arc spraying
Arc spraying is in the two wire-like metal material between the arc, the heat generated by the arc so that the metal
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wire melting, the melted part of the compressed air stream by the atomization and sprayed to the surface of the substrate and the formation of coatings. The process
also has a small investment in equipment, easy to use, high efficiency, but the spray material must be electrically conductive
metal and alloy wires, and thus its application has been somewhat limited, but its high efficiency makes it in the spray
coating Al, Zn and stainless steel and other large areas of the anticorrosive application of the process of choice.
5. Plasma spraying (APS)
When a gas such as nitrogen, argon, hydrogen and helium, etc. through a compressed arc when the ionization and the formation of electrically
nature of the plasma (is the material in addition to the gas, liquid, solid state of the fourth state). The energy concentration of the plasma arc is
high, with a flame stream of more than 10,000 degrees, which can melt all solid engineering materials. This high
temperature plasma as a heat source to melt the coating material to prepare the coating process is plasma spraying. At home and abroad
hundreds of materials have been used for plasma spraying, is the more common application of the coating method.
Plasma spray coating density and bonding strength with the substrate material are higher than flame spray coating
and arc spray coating, but also the best process for the preparation of ceramic coatings.
Plasma technology in the striking point is the equipment of large capacity and high output power, the current gas
Plasma spraying equipment has 200 kw equipment for sale, not only greatly improve the efficiency of the spraying, but also
To make the coating quality is more improved, and thus can realize the large area of high-quality coatings of continuous production, such as flexible
Printing with the
The preparation of mirror ceramic layers for anilox rollers and ceramic insulating coatings for corona treatment of polymer films
, etc.
6.
6. Low Pressure Plasma Spraying (LPPS)
Plasma spraying can be realized in different atmospheres and pressures, and when the spraying operation is carried out in a negative-pressure sealed container with a controlled atmosphere
it is called low pressure plasma spraying. The advantages of low-pressure plasma spraying are:
High flame speed, particle kinetic energy, the formation of dense coatings, high bonding strength; low-pressure environment can be
substrate preheating and reverse transfer arc cleaning, further improve the bonding of the coating and the substrate
degree; due to the absence of atmospheric pollution, the coating material does not oxidize the composition of the changes in the small, and therefore can be carried out in the active Because of the absence of atmospheric pollution, the coating material does not change its oxidized composition, so it is possible to spray reactive metals such as Ti, Ta, Nb, etc.; it is also possible to make the plasma gas react with the coating material during the spraying process to form special compound coatings. Because of the above characteristics, low-pressure plasma spraying is mainly
used to prepare coatings for the aerospace industry and other high-tech fields, such as aircraft turbine engine blades to resist high-temperature oxygen
and thermal corrosion of the MCrAlY (M = Co, Ni, Fe) coatings, as well as the preparation of artificial implants with biological
objects functional coatings.
7. Water-stabilized plasma spraying
Water-stabilized plasma spraying is a high-power and high-speed plasma spraying method that uses a plasma working gas in which water vapor decomposes to form O2 and H2 in an arc created by a tunnel formed by high-speed spinning
water
. Compared with the gas plasma spraying method, the flame stream is higher in temperature, larger in volume, and longer in length, and
especially higher in energy, making it particularly suitable for the mass coating of high melting point oxide ceramics. Its main advantages
are: high power output (150 -200 kw), coating bonding strength is 2 -
3 times higher than that of gas plasma spraying coatings, and the coating is dense, and its hardness, abrasion resistance and thermal shock resistance have been greatly improved; spraying
higher efficiency, with a spraying capacity of up to 50 kg / h, and a coating thickness of up to 20 mm, and it can be used to spray a large quantity of high melting point ceramics. Spraying
dispersible powder, and therefore particularly suitable for ceramic parts of the spraying molding; only water and air, low operating
costs, than other spraying methods economic. The disadvantage of this method is that the flame flow is oxidizing flame, which is not suitable for spraying
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coating materials that are easy to oxidize. In addition, the spray gun is large and bulky.
Six. Thermal Spraying Principle and Coating Properties
Thermal Spraying Principle1. Formation of Thermal Spraying Coating
When thermal spraying, the particles of the coating material are heated by the heat source to a molten state or a highly plastic state, and then under the thrust of the external
added gases or the flame flow itself, they are atomized and sprayed onto the surface of the substrate at high speeds, and the particles of the coating material
collide violently with the substrate and deform, flatten and deposit onto the substrate. Deformation, flattening deposited on the surface of the substrate, at the same time cold and rapid solidification,
Particles so that Su layer deposition and accumulation of coatings.
2. Thermal spray coating structure
Thermal spray coating formation process determines the structural characteristics of the coating, the spray layer is composed of countless deformed particles
Sub interlaced with each other as a wave stacked together in the layered organizational structure, the coating particles and particles between the unavoidable
There are a number of pore spaces and voids, and accompanied by oxides inclusions. The typical structure of the coating profile is shown in the figure below, which is characterized as follows:
* Layered
* Containing oxide inclusions
* Containing pores or pores
Typical Coating Profile
3. Combination Mechanism of Thermal Spraying Coatings
The combination of coatings consists of combination of coatings with the substrate and the combination of the coatings within the coating. The bonding mechanism of the coating includes the bonding between the coating and the substrate and the bonding within the coating. The bond between the coating and the surface of the substrate
is called bonding force, and the bond within the coating is called cohesive force. The bonding between particles in the coating and the substrate
and the mechanism of bonding between particles is not yet conclusive, and is usually considered to be in the following
ways.
(1) mechanical bonding
Collision into a flat and undulating with the surface of the substrate particles and uneven surfaces embedded
combined with each other, Bei to the mechanical interlocking of the particles and the formation of the bond (anchoring effect), in general, the coating and the substrate
body of the combination of the mechanical bonding is dominated.
(2) metallurgical-chemical bonding
This is when the coating and the substrate surface to produce the main metallurgical reaction, such as the emergence of diffusion and alloying
a type of bonding. When the coating is remelted after spraying, i.e., welded, the bond between the sprayed layer and the substrate is primarily a metallurgical
bond.
Substrate Roughness
Oxide Addition Pore or Pore Interparticle Bonding Particle Substrate Roughness
Substrate
Coating
Adhesion to the Substrate
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(3) Physical Bonding
Bonds between the particles and substrate surfaces that are formed by van der Waals forces or secondary valence bonds.
4. Residual stresses in the coating
When the molten particles collide with the surface of the substrate, they are subjected to radical cooling and solidification while deforming, from
which contraction stresses are generated. The outer layer of the coating is subjected to tensile stresses, and the substrate, and sometimes the inner layer of the coating, is subjected to
compressive stresses. This residual stress in the coating is caused by the thermal spraying conditions and the differences in the physical
properties of the sprayed material and the substrate material. It affects the quality of the coating, limiting the thickness of the coating. The process should take
measures to eliminate and reduce the residual stress of the coating.
Thermal spray coating performance1. Chemical composition
As the coating material in the melting and spraying process, at high temperatures will be with the surrounding medium to generate oxides, nitrides, and decomposition occurs at high temperatures, and thus the composition of the coating and the composition of the coating
layer material is a certain degree of difference and to some extent, the coating material will be used to generate oxides, nitrides and decomposition occurs at high temperatures. affect the performance of the coating. For example,
MCrAlY oxidation will affect its corrosion resistance, and WC-Co oxidation and high temperature decomposition of its wear resistance
will be reduced. This phenomenon can be avoided and mitigated by the choice of spraying method. For example, the use of low-pressure
plasma spraying can greatly reduce the oxidation of the coating material, while high-speed flame spraying can prevent the high-temperature decomposition of carbide
.
2. Porosity
Thermal spray coating inevitably exists in the pores, the size of the porosity and particle temperature
and speed, as well as spraying distance and spray angle and other spraying parameters. Generally speaking, the temperature and speed are
low flame spraying and arc spraying coating porosity are higher, generally up to a few percent, and even
up to more than ten percent. The high-temperature plasma spray coating and high-speed supersonic flame spray coating is
lower porosity. The lowest can be up to 0.5% or less.
3. Hardness
Because of the thermal spray coating in the formation of cold and high-speed impact, coating grain refinement and crystal
lattice distortion so that the coating is strengthened, and thus the hardness of the thermal spray coating than the general hardness of the material to be
some of the size of the spraying method will be different and there are differences.
4. Bonding strength
The bonding of thermal spray coatings to the substrate relies mainly on mechanical occlusion with the rough surface of the substrate (throw
stroke effect). The cleanliness of the surface of the substrate, the temperature of the particles of the coating material and the speed at which the particles hit the substrate
degree as well as the magnitude of the residual stresses in the coating affect the bonding strength between the coating and the substrate, and thus the
bonding strength of the coating also relates to the spraying method used.
5. Cold and heat fatigue performance
For some of the workpieces used in the hot and cold cycle state, the coating's cold and heat fatigue (or
heat shock) performance is critical, such as the coating's anti-heat shock performance is not good, then the workpiece in the process of
will be very soon cracked or even peeled. Coating thermal shock resistance is mainly dependent on the coating material and the base
material thermal expansion coefficient of the difference in size and the coating and the base material combination of the strength of the weak.
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VII. Thermal spraying materials and coatings function and application
Thermal spraying materialsThe current practical application has realized industrial production of spraying materials are metals, alloys and ceramics, etc., mainly
The use of powder, wire, rod state, of which the spraying powder accounted for 70% of the total amount of spraying materials
On. Used as coating materials are:
1. Thermal spray powder
Pure metal powder: W, Mo, Al, Cu, Ni, Ti, Ta, Nb, etc.
Alloy powder: Al-Ni, Ni-Cr, Ti-Ni, Ni-Cr-Al, Co-Cr-W,
MCrAlY (M = Co, Ni, Fe), Co-Cr-Al, Co-Cr-Al, Co-Cr-W, Co-Cr-W, Co-Cr-W, Co-Cr-W, Co-Cr-W, Co-Cr-Al, Co-Cr-Al (M = Co, Ni, Fe), Fe), Co-based, Ni-based, Fe-based self-fusing alloys, etc.
Oxide ceramic powders: Al2O3, ZrO2, Cr2O3, TiO2, etc.
Carbide powders: WC, TiC, Cr3C2, etc.
Metal ceramic powders: WC-Co, Cr3C2-NiCr, etc.
Plastics Powder: Nylon, polyethylene, polyphenylene sulfide, etc.
2. Thermal Spraying Wires
Al, Cu, Zn, Al-Zn alloys, pasteurized alloys, stainless steel, Ni-Al wires, etc.
3. Thermal Spraying Rods
Al2O3, Cr2O3, ZrO2, etc.
Coatings Functions and Applications1. Anti-wear coatings
Wear is one of the main causes of damage to equipment in the industrial sector, and the working conditions in which wear can occur
include vibration, sliding, impact, abrasion, erosion, and so on. Antiwear coatings should be hard and
have heat and chemical resistance. Fe, Ni, Co-based self-fusing alloys, as well as cermets such as WC-Co and Cr3C2-NiCr
, and ceramics such as Al2O3 and Cr2O3, have these properties. The use of coating technology
to improve the wear resistance of the surface of the workpiece is very wide range of applications, such as piston rings, gear synchronizer ring spraying Mo coating,
Textile machinery in the rollers, guide hooks and other parts of the spraying of fiber-resistant ceramic coatings of Al2O3, Al2O3 - TiO2
, pumps and valves, sealing surfaces of the spraying of wear-resistant coatings such as Cr2O3, WC-Co. Coating, high-horsepower trucks
crankshafts and large coal mills, exhaust shafts and other Fe-based alloys for wear repair and wear-resistant reinforcement
and so on.
2. Anti-corrosion coatings
Long-term exposure in the outdoor atmosphere (marine, industrial and urban and rural atmosphere) and different media (seawater, river water,
solvents and oils, etc.) in the environment of large-scale iron and steel components, such as power transmission pylons, steel bridges, offshore drilling
platforms, coal derricks, and a variety of chemical containers such as tanks, etc., subject to different degrees of environmental oxidation and erosion. The environmental oxidation and erosion.
The use of Al, Zn, Al - Zn alloys and stainless steel coatings for protection can be up to 20 years
on the long-term protective effect. Some parts subject to gas corrosion and chemical corrosion, according to the specific conditions
(such as medium, concentration, temperature, pressure, etc.) to choose the appropriate metal, alloy, ceramic and plastic coatings
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Materials for protection.
3. Anti-temperature oxidation and heat corrosion resistant coatings
For some parts exposed to high temperature corrosive gases, by high temperature, gas corrosion and airflow
brush effect, seriously affecting the life of the equipment and the operation of the safety. High-temperature oxidation and high-temperature corrosion-resistant materials
Materials must be resistant to high-temperature oxidation and corrosion, but also must have a similar thermal expansion coefficient
number of base materials, so as not to be due to cyclical changes in temperature and localized overheating caused by the coating of the thermal fatigue resistance to performance degradation. The coating materials used to resist high
temperature oxidation and high-temperature corrosion include NiCr, NiAl, MCrAl, MCrAlY (M = Co, Ni,
Fe) and Hastiloy and Stellite alloys. Typical applications of such coatings, such as power plant boiler four tubes
(water-cooled wall tube, reheater tube, superheater tube and economizer tube) and water-cooled walls and other high-temperature oxidative corrosion
has always been the electric power, papermaking, chemical and other industrial boilers need to solve the problem of the user, after many years of research, practice
Meaning that the use of arc spraying of Ni-Cr, Fe - Cr - Al, Ni - Cr - Al, 45C -MCrAlY coating is used for aviation turbine engine blade coating
layer as well as as turbine engine combustion chamber, flame tube and other thermal barrier coating of the bonding layer.
4. Thermal barrier coating
ZrO2, Al2O3 and other ceramic coatings, high melting point, low thermal conductivity, at high temperatures on the substrate metal
has a good thermal insulation protection is called thermal barrier coating. This kind of coating generally consists of two systems,
One is made of metal as the bottom layer, the other is made of ceramic as the surface layer. Sometimes, in order to reduce the difference in thermal expansion between the metal and the ceramic and to improve the stress distribution in the coating, a transition layer is added between the bonded base layer and the ceramic top layer, which is either a multilayer coating consisting of a mixture of the base metal and the ceramic top layer in different proportions
or a coating consisting of a succession of variations in the composition of the metal and the ceramic to form a so-called compositional (or functional) gradient
or a coating consisting of successive variations in the composition of the metal and the ceramic to form a so-called compositional (or functional) gradient. functional) gradient
coatings. The metal-bonded substrates are alloys of Co or Ni with Cr, Al, and Y. The ceramic materials are preferably
stabilized by Y2O3 and ZrO2. Thermal barrier coatings are generally used for diesel engine pistons, turbine engine combustion chambers, valves, and flame stabilizers, among others.
5. Insulation coating
Ceramic materials not only have high hardness and excellent wear resistance, but also has a very good insulation
performance, the use of high-energy plasma spraying of Al2O3 coatings coating dense, high dielectric strength, it is the ideal insulation
edge coating. If the sprayed layer is sealed with organic or inorganic substances, even better insulation results can be obtained.
These highly insulating coatings have been used in the past for many years. At present, this highly insulating coating has been used for the polymer material film to activate
corona discharge roller surface treatment, the effect is good.
6. Gap control coatings
The use of composite powder, sprayed on the substrate of the soft abrasive sealing coating is aviation, aerospace industry
Rapidly developing high-temperature sealing, gap-control technology, is one of the important application of modern thermal spray coatings.
The use of abradable coatings in the contact motion of the mating parts can make the mating parts automatically form the necessary gap,
providing the best sealing condition. Currently, high-tech abradable coating materials consist of a mixture of two powders or agglomerates, prepared by flame or plasma spraying. Generally, the abradable coatings consist of a metallic body and a non-metallic filler, which is usually graphite, polyester, boron nitride, etc. The role of the filler in the abradable coatings is to provide the material with a high degree of flexibility. The role of the filler
is to weaken the monolithic nature of the coating itself, thereby enhancing the wearability of the coating. A range of abrasive coating materials have been developed for spraying, and these coatings have been used to obtain good seals in air-sealing areas on surfaces, between compressor or turbine
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blades and metal surface structures or magazines. Wearable coatings can also be used for labyrinth seals, which are used to divert cooling air, minimize engine compressed air losses, and
maintain pressure balance on the rotor shaft.
7. Dimensional Restoration Coatings
Thermal spraying is a cost-effective method of dimensional restoration of components. Whether it is due to operational wear
or poor machining that results in an undersized part, it can be restored using thermal spray technology. This method does not have the distortion problems associated with welding and is not as expensive as special plating processes. At the same time, the new surface can be made of wear- or corrosion-resistant materials, or it can be made of the same materials as the workpiece. Typical applications include the repair of various shafts
and plungers, including major rotor shafts, automobile axles, reciprocating plungers, journals, rolls, paper drying cylinders, and impeller blades and housings for pumps in the petrochemical industry. Generator cylinder center parting surface present
Field thermal spray repair is a successful application of thermal spray to restore the dimensions of flat workpieces. Generator cylinder
In the long-term use of its parting surface due to micro-vibration, hot vapor flow corrosion and erosion, etc. and the occurrence of a number of different shapes
Different shapes, different areas and depth of damage, causing leakage and affect the efficiency of the generator. Thermal
spraying method is used to spray and fill the damages respectively, and then grinding makes the cylinder plane to restore flatness
and achieve the required dimensional accuracy. Thermal spraying technology is a safe (no deformation), simple and efficient method for on-site repair of the center parting surface of heavy, complex and expensive
cylinders.
8. Biofunctional coating
In stainless steel or titanium substrate sprayed with biofunctional ceramic coatings, such as hydroxyapatite, etc., can effectively
overcome the incompatibility of metal-type artificial bone and biological tissues and corrosion of bodily fluids, and improve the human body
tissue and the combination of the artificial implant.
9. Far-infrared radiation coating
Some oxides have a high thermal emissivity, in the heat can emit far-infrared waves, this wave
energy is very easy to be absorbed by the molecules of polymers of organic matter (such as paint), water, air and other substances to produce *** vibration
and generate endothermic, which accelerates the process. Spraying this coating on the heating element, the power saving
rate is generally about 25-40% on average.