Application and Development of Thermal Spraying Technology in the Metallurgical Industry

Thermal spraying technology involves using a heat source to heat certain materials to a molten or semi-molten state. , Then it is sprayed onto the coated substrate surface. , Form a coating with performance superior to that of the original matrix. , Thus, the original workpiece exhibits even superior surface performance. , Or it can impart to the workpiece a surface property film structure that the original matrix material did not possess. . These surface properties include wear resistance, corrosion resistance, heat resistance, antioxidant properties, thermal insulation, electrical insulation, electrical conductivity, sealing, disinfection, microwave radiation shielding, and a variety of other specialized physicochemical characteristics. Currently, thermal spraying technology has been widely adopted in many fields—including cutting-edge technologies such as aviation, aerospace, nuclear equipment, and electronics—and has yielded significant economic benefits.

The spraying materials include metals, ceramics, and plastics. The formation of a spray coating involves several stages: the heating and melting stage of the spraying material, the atomization stage of molten droplets, the flight stage of particles, and the deposition stage of particles onto the substrate. The bonding between the coating and the substrate can be categorized into mechanical bonding, diffusion bonding, physical bonding, and metallurgical bonding. When using exothermic spraying materials or employing high-temperature heat sources for spraying, the molten particles of the spraying material will undergo welding with the molten substrate, forming localized metallurgical bonds that significantly enhance the adhesion strength between the coating and the substrate. Within the spray coating, the primary type of bonding among particles is mechanical bonding; however, diffusion bonding, physical bonding, and metallurgical bonding also play complementary roles.

1. Characteristics of Thermal Spraying Technology

Thermal spraying methods can generally be categorized into flame spraying, detonation spraying, supersonic spraying, arc spraying, and plasma spraying, among others.

1 . 1 Flame Spraying

Using a flame as the heat source, metallic and non-metallic materials are heated to a molten state. Under the propulsion of a high-speed gas flow, they form an aerosol stream that is sprayed onto a substrate. When the tiny molten particles in the spray strike the substrate, they undergo plastic deformation and accumulate in layered deposits, forming a coating. This process is known as flame spraying.

Flame spraying is currently one of the most widely used coating techniques in the field of surface engineering. Using flame-spraying technology, it is possible to produce coatings made of various pure metals, alloys, ceramics, and other materials.

1 . 2 Explosive spraying

Explosive spraying involves first feeding a specific ratio of oxygen and acetylene into the combustion chamber of a cold-spray gun through a gas supply port. The mixture is ignited by a spark plug, causing an explosive combustion of the oxygen-acetylene gas. Then, a powder feeder delivers the spray powder into the combustion chamber, where the thermal energy heats the powder to a certain state. Finally, under the influence of the explosive shock wave, the powder particles are sprayed onto the surface of the workpiece, forming a coating.

The explosive spraying method is a new technology that is technically challenging yet boasts strong process performance; it is also a high-energy spraying technique. Compared with conventional flame spraying, this method requires significantly higher gas pressure and involves much greater gas consumption.

1 . 3 Arc Spraying

Arc spraying is a technique that uses an electric arc ignited between two continuously fed metal wires to melt the metal. A high-speed gas flow atomizes the molten metal and accelerates the atomized metal particles, propelling them onto the workpiece to form a coating. Arc spraying can be used to coat a wide variety of metallic materials and is extensively applied in engineering fields such as corrosion protection and wear resistance. Today, arc spraying has evolved from a rough, high-spray-rate technique into a more sophisticated method capable of producing high-quality coatings at low cost.

1 . 4 Off the spray.

Plasma spraying uses a plasma generator (plasma torch) to produce plasma. Simultaneously, the powder fed through the powder feed tube is heated to a molten state in the plasma jet and then sprayed onto the workpiece surface at high velocity, forming a coating. Plasma spraying is a new, versatile, and precision-based spraying technique that has developed rapidly following flame spraying, and it has seen relatively fast growth in China over the past few years.

2. Application of Thermal Spraying Technology in the Metallurgical Industry

The metallurgical industry is characterized by the following: From ore to metallic materials, every stage of metallurgical production involves key equipment operating under extremely harsh conditions—high wear, heavy load, high temperature, and corrosive environments. These factors often lead to equipment failure, making the metallurgical industry a major consumer of both materials and energy in the national economy. Thermal spraying technology finds extensive applications in areas such as the short-side copper plates of continuous casting molds and cold-rolling processes.

Bi Gang from Baosteel conducted a study on the thermal spraying of copper plates used in continuous casting crystallizers. The thermal-sprayed coating samples were taken from actual crystallizer copper plate products and consisted of a supersonic flame-sprayed nickel-based metal-ceramic coating with a specified thickness. 1mm The experimental results show that thermal-sprayed coatings significantly outperform nickel plating coatings—in terms of both high-temperature hardness degradation and high-temperature friction and wear performance. Thermal-sprayed coatings exhibit superior overall performance compared to nickel, with a high-temperature microhardness approximately ... times that of nickel plating coatings. 4 Double, high-temperature wear-resistant performance of paper cigarette nickel plating layer. 5 Therefore, with the breakthrough in thermal spraying technology for the long-side copper plates of crystallizers, green and environmentally friendly thermal spraying technology will gradually replace traditional electroplating technology.

During the hot-dip galvanizing process of strip steel, the immersion rolls and stabilizing rolls are not submerged in the zinc bath. Typically, these rolls are made of stainless steel. However, stainless steel has relatively poor resistance to corrosion by molten zinc in its liquid state. Therefore, the current common practice is to use... WC-Co With the cobalt-based tungsten carbide coating, the service life of the rollers has been significantly extended, and the surface quality of the steel strip has also been improved.

For hot-dip galvanizing and hot-dip aluminizing, since the temperature of the plating bath is higher ( 60 (Above 0ºC), its erosive capability is stronger, significantly shortening the service life of the submerged rolls and increasing costs. The existing WC-Co coatings can no longer meet the requirements, making it urgently necessary to develop new coating materials.

Due to the prolonged exposure to high temperatures, the surface of furnace rolls in continuous annealing furnaces for cold-rolled steel tends to develop scale buildup. This scale buildup is the root cause of scratches, dents, and pitting defects on the strip steel as it moves through the furnace. Therefore, preventing scale buildup on furnace rolls is crucial for ensuring the quality of the strip steel within the furnace. Maintaining the surface roughness of furnace rolls is an important measure to ensure smooth operation of the strip steel inside the furnace and to address issues such as slippage and deviation. Additionally, furnace rolls must exhibit excellent resistance to thermal shock. Selecting effective thermal spray coatings can help address these challenges; commonly used coating materials include: M c rAl, r3C-NiCrey MCrAl+Al ₂ O ₃ Wait.

As The development of new high-strength steels such as DP and TRIP is continuously raising the demands on roller systems for continuous annealing furnaces. As the strength of strip steel increases, so does the tension within the furnace, which in turn requires rollers with higher load-bearing capacities. Moreover, these new high-strength steels typically demand higher annealing temperatures, thus placing even greater demands on the heat resistance of furnace rollers. In addition, these high-strength steels generally contain relatively high levels of alloying elements—particularly Si and Mn. The oxides of Si tend to accumulate on the surface of the strip steel and, through friction and adhesion, transfer to the surface of the furnace rollers. Meanwhile, the Mn element interacts with certain chemically less stable Cr components in the coating. ₂ O ₃、 Al ₂ O ₃ The uniform synthesis of certain complex oxides can lead to the formation of nodules. Therefore, there is a need to develop new thermal spray coating materials with improved performance that better meet the requirements of continuous annealing furnace processes.

3. Types of Coatings and Their Application Status

3.1 Thermal Barrier Coatings

A thermal barrier coating is a ceramic material with excellent thermal insulation properties. It is applied to the surfaces of critical hot-end components in aircraft engines, and its typical thickness does not exceed— 0.5 mm, It can effectively prevent the hot-end components of an aero-turbo engine from coming into direct contact with high-temperature gases. , This thereby provides effective protection for the hot-end components of the engine. In recent years, as aeroengines have evolved toward higher bypass ratios, higher turbine inlet temperatures, and higher thrust-to-weight ratios, the gas temperatures in engines have continued to rise, making thermal barrier coating technology even more critical. As a result, research on coating preparation methods has become increasingly active both domestically and internationally. Common thermal spraying techniques used for preparing thermal barrier coatings include flame spraying, detonation spraying, and plasma spraying; among these, the detonation spraying process has become the primary focus of research into thermal barrier coating preparation technologies.

3 . 2 Nano-coating

During thermal spraying, the melting behavior of nanoparticles differs from that of conventional particles. Conventional particles undergo only surface melting during spraying, whereas nanoparticles, due to their large specific surface area, high reactivity, and relatively lower melting point, are easily heated and melted. Because of their superior degree of melting, nanoparticles exhibit significant deformation upon impact with the substrate, resulting in markedly better spreading compared to conventional particles. This leads to a dense coating with low porosity. Moreover, thanks to the unique properties inherent in nanomaterials themselves, thermally sprayed nanostructured coatings demonstrate enhanced performance characteristics, including high bonding strength, simultaneous improvements in hardness and ductility, excellent corrosion resistance, superior fracture toughness, and increased scratch resistance.

The development and research of nano-coatings are still not fully mature. However, the surface nanostructures formed by nanomaterial powders hold immense potential for applications across a wide range of critical sectors—including high-tech industries, civil industries, and national defense. From thermal barrier coatings for turbine blades to wear- and corrosion-resistant coatings for rotating components, as well as stealth coatings for high-performance fighter jets, the potential economic scale is truly enormous.

Currently, there are two main approaches for preparing nano-coatings using thermal spraying technology:

1. Prepare macroscopic coatings with partial nanoscale characteristics by incorporating a relatively small amount of nanoparticles into conventional coatings during the preparation process, thereby transforming the conventional coatings into composite coatings that are nanoparticle-dispersion strengthened and exhibit certain nanoscale features.
2. Prepare macroscopic coatings entirely composed of nanoparticles—specifically, macroscopic coatings with thicknesses ranging from micrometers to even millimeters—that are completely made up of nanoparticles, thereby endowing the coatings with exceptionally outstanding engineering performance.

There are two main issues with thermally sprayed nanostructured coatings:

1. The issue of nanoparticle delivery. Nanopowders cannot be directly used in thermal spraying; otherwise, problems such as burn-off and dust generation may occur. Moreover, due to the surface and interface effects inherent in nanomaterials themselves, nanoparticles tend to agglomerate easily, resulting in poor flowability and potentially causing blockages in the delivery pipelines. To fully harness the superior properties of nanoparticles and meet the requirements of existing processing technologies, it is necessary to prepare nanomaterials into micron-sized nanostructured feedstocks that can be directly used in thermal spraying.
2. The sintering and growth of nanoparticles. Nanoparticles have a large specific surface area and high surface activity, which leads to a lower melting point. During the spraying process, they tend to sinter and grow, altering their properties and ultimately affecting the preservation of the nanocrystalline structure in the coating.

3 . 3 Amorphous coating

Amorphous alloys exhibit high strength, hardness, and excellent wear resistance, corrosion resistance, and magnetic properties. Extensive research has been conducted both domestically and internationally on thermal-sprayed amorphous alloy coating materials. In nickel-based systems, there is... Ni2Zr2Si2Sn.Ni2Cr2MoB Amorphous coatings; iron-based amorphous coating systems Fe2CrMo2(C.B).Fe2Cr2P2C.Fe2Cr2Si2B2Mn All amorphous coatings exhibit excellent corrosion resistance. Recently, additional components have also been discovered. Fe2Cr2Mn2Mo2W2B2C2Si Amorphous metals with a series structure exhibit excellent corrosion resistance.

Preparing bulk, three-dimensional amorphous alloys is technically very challenging. However, by employing techniques such as plasma spraying and supersonic flame spraying, amorphous powders can be deposited onto inexpensive, low-performance metal substrates to form dense amorphous coatings with high bonding strength—making this an excellent approach for surface amorphization of materials. During plasma spraying, the cooling rate of molten particles can reach... 105-106K/S This rapid cooling process can induce an amorphous phase microstructure in the coating. Abroad, research on amorphous alloy coatings for thermal spraying has focused primarily on their corrosion- and wear-resistant properties, and these coatings have already begun to be commercially promoted. In China, however, the research has mainly centered on the preparation processes of amorphous alloy coating materials, with relatively little attention paid to their application performance in terms of corrosion resistance and wear resistance; thus, there have been no reported practical applications yet.

3 . 4 Bioactive coating

Bioactive hydroxyapatite exhibits excellent biocompatibility with biological tissues and can be used to fabricate various joint and dental implants. Plasma spraying. HAP The powder is on a metal matrix (typically... You The metal-based material forms a biological coating, which highlights... HAP It exhibits excellent biological activity and leverages the outstanding mechanical properties of metallic materials, thereby avoiding... HAP The issue of brittleness and fatigue sensitivity. Due to... HAP The significant differences in physical properties between the powder and the metal matrix impose certain limitations on the adhesion strength of the coating. In Ti26Al24V Add between the alloy matrix and the coating Zro2 As an enhancing phase is formed ( HA+ ZrO2 ) Composite coatings can significantly enhance the bonding strength between the coating and the substrate.

3 . 5 Anti-corrosion coating

For the long-term protection of steel structural components—especially large and critical steel structures that are designed to require little or no maintenance during their service life—thermal spraying technology represents a highly effective approach for durable corrosion protection. Currently, the primary thermal-sprayed coatings used in China for corrosion prevention are zinc coatings and aluminum coatings. However, practical experience has shown that aluminum coatings are relatively sensitive to pitting corrosion and mechanical damage, whereas zinc coatings perform better in weakly acidic environments and environments containing... SO2 These coatings exhibit poor corrosion resistance in industrial atmospheres and marine environments, and their zinc-sprayed coatings have relatively high corrosion rates. Moreover, during the application process, they result in significant material loss and pose considerable health risks to humans. In recent years, several new types of coatings have been developed—such as zinc-aluminum-magnesium alloy coatings, aluminum-zinc-silicon alloy coatings, and zinc-aluminum alloy coatings—which have found widespread use. Additionally, in recent times, a number of thermal spraying techniques have been explored. Ni.Cr.Si Alloy, spray coating, or laser cladding WC/Co Alloy, nano-doping (Al2O3 + TiO2) Plasma spraying.

4. Post-processing of thermal spray coatings

Post-processing by remelting thermal spray coatings can eliminate the layered structure of the coating, reduce porosity, and significantly enhance its microstructure as well as its wear resistance, corrosion resistance, and heat resistance. This process has been successfully applied in actual production, thereby extending the service life of parts and yielding substantial economic benefits.

Post-processing of thermal spray coatings primarily refers to remelting treatment. In remelting, a heat source is used to melt the alloy's most easily fusible constituents, and the resulting liquid phase facilitates the enhancement of diffusion processes and promotes the penetration of alloying elements. As a result of melting, the bonding zone between the thermal spray coating and the substrate transforms from its original layered structure into a denser and more uniform microstructure, with porosity reduced or even eliminated altogether. By employing appropriate remelting treatments, the bond strength between the coating and the substrate as well as the intrinsic quality of the coating can be improved, thereby enhancing the overall performance of the coating. Currently, the main technological shortcomings in remelting treatments include limitations associated with laser and electron-beam techniques. TIG Remelting, flame remelting, full-body heating, and induction remelting, among others.

4 . 1 Laser remelting

Laser remelting is a process in which a laser beam is used to scan and melt thermal spray coatings under a protective atmosphere. During laser remelting, the specimen—when exposed to a high-energy laser beam—causes the thin surface layer of the substrate material, along with any ceramic or alloy coating added as needed, to rapidly melt and mix together, forming a layer of specified thickness. 10-1000 mu m the surface melting layer. The cooling rate attained during solidification of the melting layer can reach 100 Approximately °C/S. Moreover, due to physical phenomena such as diffusion within the molten layer and surface tension effects, a surface alloying layer with the desired concentration and chemical composition is formed on the material surface in a very short time. This alloying layer exhibits certain properties that are superior to those of the base material; thus, this technology can achieve the purpose of surface modification.

Problems with laser remelting: Due to the poor thermal shock resistance and low fracture toughness of ceramic materials, cracks tend to form easily under the rapid heating and cooling conditions encountered during laser remelting. In laser surface remelting processes, the melting point of the ceramic coating material used is significantly higher than that of the metal substrate, and there are substantial differences among these materials in terms of thermal expansion coefficient, elastic modulus, and thermal conductivity. The resulting thermal stresses can readily lead to cracking and coating spalling. During plasma spraying, the compatibility between the metal substrate and ceramic powder is poor; the molten metal fails to wet the solid ceramic powder effectively, which also makes the coating prone to cracking and porosity. Adjusting dimensions, reducing the thermal expansion coefficient, and improving ductility and toughness have little effect on addressing coating cracking, and from a process perspective, it remains difficult to achieve a fundamental solution. For large-area laser remelting, since the laser spot area is small, it is necessary to employ either multi-pass overlapping techniques or large-area spot techniques (such as defocusing, broadband, and rotating-mirror methods). When using multi-pass overlapping, each adjacent scan track exhibits... There is one overlapping region; consequently, the microhardness values in each region fluctuate. From the perspective of metallographic structure, the overlapped coating exhibits a macroscopic, periodic variation in properties throughout its entirety. For large-area spot techniques, when the output power remains constant, a larger spot area results in lower power density. Increasing the beam diameter may diminish the laser’s advantages of high energy density and ultra-fast heating. Therefore, the application of large-area spot techniques has its limitations.

4.2 Electron Beam Remelting Technology

Electron-beam remelting is a technique that utilizes a high-speed, directed electron beam. After impacting the coating surface, the beam converts part of its kinetic energy into thermal energy, thereby enhancing the surface properties. Electron-beam remelting can increase the solid-solution solubility of alloys, refine grain size, and reduce segregation. It also achieves vacuum degassing, dissolving inclusions such as oxides and sulfides, thus producing solid-solution strengthening effects. As a result, this process effectively improves the material's resistance to wear, impact, corrosion, and high-temperature oxidation.

When using electron-beam heating for remelting, a vacuum chamber is essential. Remelting large components and parts with deep holes is limited and lacks flexibility; however, this method boasts a high thermal conversion rate, and the energy of the electron beam... 75% of the energy is absorbed by the coating (while the laser beam absorbs only 2% to 8%), and the cost of electron-beam remelting is significantly lower than that of laser remelting. Thanks to the combined effects of electromagnetic waves (with shorter wavelengths) and electron beams (controlled by electric field strength), as well as the rapid advancements in electron optics, the electron-beam spot can be confined to an extremely small area—often even smaller than the laser spot. Therefore, when the sample is small or precise control over the remelting zone is required, electron-beam remelting proves to be a highly effective post-processing technique.

4.3 Tungsten Inert Gas (TIG) Remelting

Tungsten inert gas (TIG) remelting technology uses the energy of an arc beam to surface-enhance workpieces. Due to its low cost, excellent strengthening effect, simple operation, and ease of implementation, this technique has been increasingly adopted both domestically and internationally for surface enhancement of gray cast iron and ductile iron components, such as automobile engine cylinder blocks, cylinder liners, camshafts, and numerous pump and valve bodies.

4.4 Flame Remelting

The flame remelting heat source is not limited to oxygen. —Acetylene. Due to its abundant gas resources, simple equipment, and ease of operation, when performing flame remelting, the travel speed of the remelting torch and the distance from the workpiece must be strictly controlled; otherwise, the heat-affected zone in the base material will become excessively large, leading to deformation and cracking. This method is often used for self-fluxing alloys and is both simple and practical, allowing operations to be carried out directly at the construction site. If the parts are small and the quantity is not large, heating with a handheld oxygen-fuel flame torch is the most cost-effective option. However, heating with a handheld flame torch is labor-intensive and makes it difficult to achieve uniform heating.

4.5 Induction Remelting

Coating induction remelting technology involves pre-applying a coating onto the workpiece substrate and then using the alternating magnetic field generated by an induction coil to induce eddy currents within the workpiece. The heat produced by these eddy currents is sufficient to melt the coating. The skin effect of eddy currents constitutes the primary advantage of induction heating, enabling heat to be concentrated precisely in the desired heating zone. Since the coating can be melted, the bonding strength between the coating and the substrate is significantly improved. Moreover, because the heat is concentrated on the surface layer, the thermal impact on the substrate itself is minimal, thus satisfying the requirements of minimal thermal influence and strong bonding strength.

Generally speaking, alloy powders can be used in flame spraying only if they meet two basic requirements: their melting point must be lower than that of the substrate, and they must possess self-melting properties. The effectiveness of induction remelting of coatings is closely related to the relative permeability and resistivity of the coating. During the remelting of various alloy coatings, high-temperature zones invariably appear at the interface between the coating and the substrate as well as on the coating surface. This non-uniform temperature distribution is a fundamental characteristic of the temperature field during induction remelting of coatings; it not only affects the stability of the remelting process but also determines the microstructure and properties of the coating.

4.6 Overall Heating and Remelting

The spray-coated layer is reheated to a temperature between the solid and liquid states for remelting. At this point, the material becomes semifluid, densifies, and melts completely. As a result, metallurgical reactions induce the precipitation of numerous hard phases (carbides, borides, and their compounds) in the form of a mixed coating. For overall heating and remelting, the furnace temperature must be carefully controlled—typically above the alloy’s solidus temperature. 10–30ºC. At this temperature, the viscosity is sufficiently high to prevent the coating from flowing. After remelting and subsequent cooling of the surface, these retained hardness levels remain in the coating, providing excellent resistance to friction and wear. Based on their widespread applications and cost considerations, nickel-based alloys are the most valuable among fusion alloys and have been the subject of the most research on remelting in recent years.

5. Development of Thermal Spraying Technology

Thermal spray materials are the core of all thermal spray technologies. The diversity of thermal spray coating functions is determined by the variety of materials with differing properties. The diversity of thermal spray materials paves the way for both the advanced nature and cost-effectiveness of thermal spray technology. With the development and application of ultra-high-strength steels, the requirements for metallurgical equipment in terms of wear resistance, corrosion resistance, and heat resistance are continually increasing. Therefore, the development of thermal spray coatings must keep pace with these growing demands. Currently, both domestic and international thermal spray equipment is evolving toward higher energy, higher speed, and greater efficiency. Axial powder feeding technology, multi-functional integrated technologies, and real-time control technologies will also become key directions for the advancement of thermal spray equipment. The continuous advancement of thermal spray equipment will broaden the range of applicable coating materials. The preparation of functional coatings—such as nanostructured, amorphous, thermally conductive, catalytic, electrically conductive, insulating, superconducting, and stealth coatings—will become a fluid-dynamic feature of the thermal spray process, enabling the establishment of temperature and velocity fields for sprayed particles and facilitating a deeper understanding of the coating formation process. This, in turn, will allow us to determine the specific effects of various spray parameters on the structure and performance of the coatings. Furthermore, the adoption of various advanced non-destructive testing techniques—such as acoustic, optical, and electrical methods—will enable online diagnostics of coating performance, assessment of coating weight, and prediction of coating service life. These approaches represent important research directions for future quality monitoring of thermal spray coatings.

6. Conclusion

With the continuous advancement of thermal spraying technology, thermal-sprayed coatings will undoubtedly find increasingly widespread applications across all sectors of the national economy. As one of the areas where thermal spraying has achieved particularly successful applications, the metallurgical industry should place even greater emphasis on the development and implementation of thermal spraying technologies.

Application and Development of Thermal Spraying Technology in the Metallurgical Industry

Thermal spraying technology involves using a heat source to heat certain materials to a molten or semi-molten state. , Then it is sprayed onto the coated substrate surface. , Form a coating with performance superior to that of the original matrix. , Thus, the original workpiece exhibits even superior surface performance. , Or it can impart to the workpiece a surface property film structure that the original matrix material did not possess. . These surface properties include wear resistance, corrosion resistance, heat resistance, antioxidant properties, thermal insulation, electrical insulation, electrical conductivity, sealing, disinfection, microwave radiation shielding, and a variety of other specialized physicochemical characteristics. Currently, thermal spraying technology has been widely adopted in many fields—including cutting-edge technologies such as aviation, aerospace, nuclear equipment, and electronics—and has yielded significant economic benefits.

The spraying materials include metals, ceramics, and plastics. The formation of a spray coating involves several stages: the heating and melting stage of the spraying material, the atomization stage of molten droplets, the flight stage of particles, and the deposition stage of particles onto the substrate. The bonding between the coating and the substrate can be categorized into mechanical bonding, diffusion bonding, physical bonding, and metallurgical bonding. When using exothermic spraying materials or employing high-temperature heat sources for spraying, the molten particles of the spraying material will undergo welding with the molten substrate, forming localized metallurgical bonds that significantly enhance the adhesion strength between the coating and the substrate. Within the spray coating, the primary type of bonding among particles is mechanical bonding; however, diffusion bonding, physical bonding, and metallurgical bonding also play complementary roles.

1. Characteristics of Thermal Spraying Technology

Thermal spraying methods can generally be categorized into flame spraying, detonation spraying, supersonic spraying, arc spraying, and plasma spraying, among others.

1 . 1 Flame Spraying

Using a flame as the heat source, metallic and non-metallic materials are heated to a molten state. Under the propulsion of a high-speed gas flow, they form an aerosol stream that is sprayed onto a substrate. When the tiny molten particles in the spray strike the substrate, they undergo plastic deformation and accumulate in layered deposits, forming a coating. This process is known as flame spraying.

Flame spraying is currently one of the most widely used coating techniques in the field of surface engineering. Using flame-spraying technology, it is possible to produce coatings made of various pure metals, alloys, ceramics, and other materials.

1 . 2 Explosive spraying

Explosive spraying involves first feeding a specific ratio of oxygen and acetylene into the combustion chamber of a cold-spray gun through a gas supply port. The mixture is ignited by a spark plug, causing an explosive combustion of the oxygen-acetylene gas. Then, a powder feeder delivers the spray powder into the combustion chamber, where the thermal energy heats the powder to a certain state. Finally, under the influence of the explosive shock wave, the powder particles are sprayed onto the surface of the workpiece, forming a coating.

The explosive spraying method is a new technology that is technically challenging yet boasts strong process performance; it is also a high-energy spraying technique. Compared with conventional flame spraying, this method requires significantly higher gas pressure and involves much greater gas consumption.

1 . 3 Arc Spraying

Arc spraying is a technique that uses an electric arc ignited between two continuously fed metal wires to melt the metal. A high-speed gas flow atomizes the molten metal and accelerates the atomized metal particles, propelling them onto the workpiece to form a coating. Arc spraying can be used to coat a wide variety of metallic materials and is extensively applied in engineering fields such as corrosion protection and wear resistance. Today, arc spraying has evolved from a rough, high-spray-rate technique into a more sophisticated method capable of producing high-quality coatings at low cost.

1 . 4 Off the spray.

Plasma spraying uses a plasma generator (plasma torch) to produce plasma. Simultaneously, the powder fed through the powder feed tube is heated to a molten state in the plasma jet and then sprayed onto the workpiece surface at high velocity, forming a coating. Plasma spraying is a new, versatile, and precision-based spraying technique that has developed rapidly following flame spraying, and it has seen relatively fast growth in China over the past few years.

2. Application of Thermal Spraying Technology in the Metallurgical Industry

The metallurgical industry is characterized by the following: From ore to metallic materials, every stage of metallurgical production involves key equipment operating under extremely harsh conditions—high wear, heavy load, high temperature, and corrosive environments. These factors often lead to equipment failure, making the metallurgical industry a major consumer of both materials and energy in the national economy. Thermal spraying technology finds extensive applications in areas such as the short-side copper plates of continuous casting molds and cold-rolling processes.

Bi Gang from Baosteel conducted a study on the thermal spraying of copper plates used in continuous casting crystallizers. The thermal-sprayed coating samples were taken from actual crystallizer copper plate products and consisted of a supersonic flame-sprayed nickel-based metal-ceramic coating with a specified thickness. 1mm The experimental results show that thermal-sprayed coatings significantly outperform nickel plating coatings—in terms of both high-temperature hardness degradation and high-temperature friction and wear performance. Thermal-sprayed coatings exhibit superior overall performance compared to nickel, with a high-temperature microhardness approximately ... times that of nickel plating coatings. 4 Double, high-temperature wear-resistant performance of paper cigarette nickel plating layer. 5 Therefore, with the breakthrough in thermal spraying technology for the long-side copper plates of crystallizers, green and environmentally friendly thermal spraying technology will gradually replace traditional electroplating technology.

During the hot-dip galvanizing process of strip steel, the immersion rolls and stabilizing rolls are not submerged in the zinc bath. Typically, these rolls are made of stainless steel. However, stainless steel has relatively poor resistance to corrosion by molten zinc in its liquid state. Therefore, the current common practice is to use... WC-Co With the cobalt-based tungsten carbide coating, the service life of the rollers has been significantly extended, and the surface quality of the steel strip has also been improved.

For hot-dip galvanizing and hot-dip aluminizing, since the temperature of the plating bath is higher ( 60 (Above 0ºC), its erosive capability is stronger, significantly shortening the service life of the submerged rolls and increasing costs. The existing WC-Co coatings can no longer meet the requirements, making it urgently necessary to develop new coating materials.

Due to the prolonged exposure to high temperatures, the surface of furnace rolls in continuous annealing furnaces for cold-rolled steel tends to develop scale buildup. This scale buildup is the root cause of scratches, dents, and pitting defects on the strip steel as it moves through the furnace. Therefore, preventing scale buildup on furnace rolls is crucial for ensuring the quality of the strip steel within the furnace. Maintaining the surface roughness of furnace rolls is an important measure to ensure smooth operation of the strip steel inside the furnace and to address issues such as slippage and deviation. Additionally, furnace rolls must exhibit excellent resistance to thermal shock. Selecting effective thermal spray coatings can help address these challenges; commonly used coating materials include: M c rAl, r3C-NiCrey MCrAl+Al ₂ O ₃ Wait.

As The development of new high-strength steels such as DP and TRIP is continuously raising the demands on roller systems for continuous annealing furnaces. As the strength of strip steel increases, so does the tension within the furnace, which in turn requires rollers with higher load-bearing capacities. Moreover, these new high-strength steels typically demand higher annealing temperatures, thus placing even greater demands on the heat resistance of furnace rollers. In addition, these high-strength steels generally contain relatively high levels of alloying elements—particularly Si and Mn. The oxides of Si tend to accumulate on the surface of the strip steel and, through friction and adhesion, transfer to the surface of the furnace rollers. Meanwhile, the Mn element interacts with certain chemically less stable Cr components in the coating. ₂ O ₃、 Al ₂ O ₃ The uniform synthesis of certain complex oxides can lead to the formation of nodules. Therefore, there is a need to develop new thermal spray coating materials with improved performance that better meet the requirements of continuous annealing furnace processes.

3. Types of Coatings and Their Application Status

3.1 Thermal Barrier Coatings

A thermal barrier coating is a ceramic material with excellent thermal insulation properties. It is applied to the surfaces of critical hot-end components in aircraft engines, and its typical thickness does not exceed— 0.5 mm, It can effectively prevent the hot-end components of an aero-turbo engine from coming into direct contact with high-temperature gases. , This thereby provides effective protection for the hot-end components of the engine. In recent years, as aeroengines have evolved toward higher bypass ratios, higher turbine inlet temperatures, and higher thrust-to-weight ratios, the gas temperatures in engines have continued to rise, making thermal barrier coating technology even more critical. As a result, research on coating preparation methods has become increasingly active both domestically and internationally. Common thermal spraying techniques used for preparing thermal barrier coatings include flame spraying, detonation spraying, and plasma spraying; among these, the detonation spraying process has become the primary focus of research into thermal barrier coating preparation technologies.

3 . 2 Nano-coating

During thermal spraying, the melting behavior of nanoparticles differs from that of conventional particles. Conventional particles undergo only surface melting during spraying, whereas nanoparticles, due to their large specific surface area, high reactivity, and relatively lower melting point, are easily heated and melted. Because of their superior degree of melting, nanoparticles exhibit significant deformation upon impact with the substrate, resulting in markedly better spreading compared to conventional particles. This leads to a dense coating with low porosity. Moreover, thanks to the unique properties inherent in nanomaterials themselves, thermally sprayed nanostructured coatings demonstrate enhanced performance characteristics, including high bonding strength, simultaneous improvements in hardness and ductility, excellent corrosion resistance, superior fracture toughness, and increased scratch resistance.

The development and research of nano-coatings are still not fully mature. However, the surface nanostructures formed by nanomaterial powders hold immense potential for applications across a wide range of critical sectors—including high-tech industries, civil industries, and national defense. From thermal barrier coatings for turbine blades to wear- and corrosion-resistant coatings for rotating components, as well as stealth coatings for high-performance fighter jets, the potential economic scale is truly enormous.

Currently, there are two main approaches for preparing nano-coatings using thermal spraying technology:

1. Prepare macroscopic coatings with partial nanoscale characteristics by incorporating a relatively small amount of nanoparticles into conventional coatings during the preparation process, thereby transforming the conventional coatings into composite coatings that are nanoparticle-dispersion strengthened and exhibit certain nanoscale features.
2. Prepare macroscopic coatings entirely composed of nanoparticles—specifically, macroscopic coatings with thicknesses ranging from micrometers to even millimeters—that are completely made up of nanoparticles, thereby endowing the coatings with exceptionally outstanding engineering performance.

There are two main issues with thermally sprayed nanostructured coatings:

1. The issue of nanoparticle delivery. Nanopowders cannot be directly used in thermal spraying; otherwise, problems such as burn-off and dust generation may occur. Moreover, due to the surface and interface effects inherent in nanomaterials themselves, nanoparticles tend to agglomerate easily, resulting in poor flowability and potentially causing blockages in the delivery pipelines. To fully harness the superior properties of nanoparticles and meet the requirements of existing processing technologies, it is necessary to prepare nanomaterials into micron-sized nanostructured feedstocks that can be directly used in thermal spraying.
2. The sintering and growth of nanoparticles. Nanoparticles have a large specific surface area and high surface activity, which leads to a lower melting point. During the spraying process, they tend to sinter and grow, altering their properties and ultimately affecting the preservation of the nanocrystalline structure in the coating.

3 . 3 Amorphous coating

Amorphous alloys exhibit high strength, hardness, and excellent wear resistance, corrosion resistance, and magnetic properties. Extensive research has been conducted both domestically and internationally on thermal-sprayed amorphous alloy coating materials. In nickel-based systems, there is... Ni2Zr2Si2Sn.Ni2Cr2MoB Amorphous coatings; iron-based amorphous coating systems Fe2CrMo2(C.B).Fe2Cr2P2C.Fe2Cr2Si2B2Mn All amorphous coatings exhibit excellent corrosion resistance. Recently, additional components have also been discovered. Fe2Cr2Mn2Mo2W2B2C2Si Amorphous metals with a series structure exhibit excellent corrosion resistance.

Preparing bulk, three-dimensional amorphous alloys is technically very challenging. However, by employing techniques such as plasma spraying and supersonic flame spraying, amorphous powders can be deposited onto inexpensive, low-performance metal substrates to form dense amorphous coatings with high bonding strength—making this an excellent approach for surface amorphization of materials. During plasma spraying, the cooling rate of molten particles can reach... 105-106K/S This rapid cooling process can induce an amorphous phase microstructure in the coating. Abroad, research on amorphous alloy coatings for thermal spraying has focused primarily on their corrosion- and wear-resistant properties, and these coatings have already begun to be commercially promoted. In China, however, the research has mainly centered on the preparation processes of amorphous alloy coating materials, with relatively little attention paid to their application performance in terms of corrosion resistance and wear resistance; thus, there have been no reported practical applications yet.

3 . 4 Bioactive coating

Bioactive hydroxyapatite exhibits excellent biocompatibility with biological tissues and can be used to fabricate various joint and dental implants. Plasma spraying. HAP The powder is on a metal matrix (typically... You The metal-based material forms a biological coating, which highlights... HAP It exhibits excellent biological activity and leverages the outstanding mechanical properties of metallic materials, thereby avoiding... HAP The issue of brittleness and fatigue sensitivity. Due to... HAP The significant differences in physical properties between the powder and the metal matrix impose certain limitations on the adhesion strength of the coating. In Ti26Al24V Add between the alloy matrix and the coating Zro2 As an enhancing phase is formed ( HA+ ZrO2 ) Composite coatings can significantly enhance the bonding strength between the coating and the substrate.

3 . 5 Anti-corrosion coating

For the long-term protection of steel structural components—especially large and critical steel structures that are designed to require little or no maintenance during their service life—thermal spraying technology represents a highly effective approach for durable corrosion protection. Currently, the primary thermal-sprayed coatings used in China for corrosion prevention are zinc coatings and aluminum coatings. However, practical experience has shown that aluminum coatings are relatively sensitive to pitting corrosion and mechanical damage, whereas zinc coatings perform better in weakly acidic environments and environments containing... SO2 These coatings exhibit poor corrosion resistance in industrial atmospheres and marine environments, and their zinc-sprayed coatings have relatively high corrosion rates. Moreover, during the application process, they result in significant material loss and pose considerable health risks to humans. In recent years, several new types of coatings have been developed—such as zinc-aluminum-magnesium alloy coatings, aluminum-zinc-silicon alloy coatings, and zinc-aluminum alloy coatings—which have found widespread use. Additionally, in recent times, a number of thermal spraying techniques have been explored. Ni.Cr.Si Alloy, spray coating, or laser cladding WC/Co Alloy, nano-doping (Al2O3 + TiO2) Plasma spraying.

4. Post-processing of thermal spray coatings

Post-processing by remelting thermal spray coatings can eliminate the layered structure of the coating, reduce porosity, and significantly enhance its microstructure as well as its wear resistance, corrosion resistance, and heat resistance. This process has been successfully applied in actual production, thereby extending the service life of parts and yielding substantial economic benefits.

Post-processing of thermal spray coatings primarily refers to remelting treatment. In remelting, a heat source is used to melt the alloy's most easily fusible constituents, and the resulting liquid phase facilitates the enhancement of diffusion processes and promotes the penetration of alloying elements. As a result of melting, the bonding zone between the thermal spray coating and the substrate transforms from its original layered structure into a denser and more uniform microstructure, with porosity reduced or even eliminated altogether. By employing appropriate remelting treatments, the bond strength between the coating and the substrate as well as the intrinsic quality of the coating can be improved, thereby enhancing the overall performance of the coating. Currently, the main technological shortcomings in remelting treatments include limitations associated with laser and electron-beam techniques. TIG Remelting, flame remelting, full-body heating, and induction remelting, among others.

4 . 1 Laser remelting

Laser remelting is a process in which a laser beam is used to scan and melt thermal spray coatings under a protective atmosphere. During laser remelting, the specimen—when exposed to a high-energy laser beam—causes the thin surface layer of the substrate material, along with any ceramic or alloy coating added as needed, to rapidly melt and mix together, forming a layer of specified thickness. 10-1000 mu m the surface melting layer. The cooling rate attained during solidification of the melting layer can reach 100 Approximately °C/S. Moreover, due to physical phenomena such as diffusion within the molten layer and surface tension effects, a surface alloying layer with the desired concentration and chemical composition is formed on the material surface in a very short time. This alloying layer exhibits certain properties that are superior to those of the base material; thus, this technology can achieve the purpose of surface modification.

Problems with laser remelting: Due to the poor thermal shock resistance and low fracture toughness of ceramic materials, cracks tend to form easily under the rapid heating and cooling conditions encountered during laser remelting. In laser surface remelting processes, the melting point of the ceramic coating material used is significantly higher than that of the metal substrate, and there are substantial differences among these materials in terms of thermal expansion coefficient, elastic modulus, and thermal conductivity. The resulting thermal stresses can readily lead to cracking and coating spalling. During plasma spraying, the compatibility between the metal substrate and ceramic powder is poor; the molten metal fails to wet the solid ceramic powder effectively, which also makes the coating prone to cracking and porosity. Adjusting dimensions, reducing the thermal expansion coefficient, and improving ductility and toughness have little effect on addressing coating cracking, and from a process perspective, it remains difficult to achieve a fundamental solution. For large-area laser remelting, since the laser spot area is small, it is necessary to employ either multi-pass overlapping techniques or large-area spot techniques (such as defocusing, broadband, and rotating-mirror methods). When using multi-pass overlapping, each adjacent scan track exhibits... There is one overlapping region; consequently, the microhardness values in each region fluctuate. From the perspective of metallographic structure, the overlapped coating exhibits a macroscopic, periodic variation in properties throughout its entirety. For large-area spot techniques, when the output power remains constant, a larger spot area results in lower power density. Increasing the beam diameter may diminish the laser’s advantages of high energy density and ultra-fast heating. Therefore, the application of large-area spot techniques has its limitations.

4.2 Electron Beam Remelting Technology

Electron-beam remelting is a technique that utilizes a high-speed, directed electron beam. After impacting the coating surface, the beam converts part of its kinetic energy into thermal energy, thereby enhancing the surface properties. Electron-beam remelting can increase the solid-solution solubility of alloys, refine grain size, and reduce segregation. It also achieves vacuum degassing, dissolving inclusions such as oxides and sulfides, thus producing solid-solution strengthening effects. As a result, this process effectively improves the material's resistance to wear, impact, corrosion, and high-temperature oxidation.

When using electron-beam heating for remelting, a vacuum chamber is essential. Remelting large components and parts with deep holes is limited and lacks flexibility; however, this method boasts a high thermal conversion rate, and the energy of the electron beam... 75% of the energy is absorbed by the coating (while the laser beam absorbs only 2% to 8%), and the cost of electron-beam remelting is significantly lower than that of laser remelting. Thanks to the combined effects of electromagnetic waves (with shorter wavelengths) and electron beams (controlled by electric field strength), as well as the rapid advancements in electron optics, the electron-beam spot can be confined to an extremely small area—often even smaller than the laser spot. Therefore, when the sample is small or precise control over the remelting zone is required, electron-beam remelting proves to be a highly effective post-processing technique.

4.3 Tungsten Inert Gas (TIG) Remelting

Tungsten inert gas (TIG) remelting technology uses the energy of an arc beam to surface-enhance workpieces. Due to its low cost, excellent strengthening effect, simple operation, and ease of implementation, this technique has been increasingly adopted both domestically and internationally for surface enhancement of gray cast iron and ductile iron components, such as automobile engine cylinder blocks, cylinder liners, camshafts, and numerous pump and valve bodies.

4.4 Flame Remelting

The flame remelting heat source is not limited to oxygen. —Acetylene. Due to its abundant gas resources, simple equipment, and ease of operation, when performing flame remelting, the travel speed of the remelting torch and the distance from the workpiece must be strictly controlled; otherwise, the heat-affected zone in the base material will become excessively large, leading to deformation and cracking. This method is often used for self-fluxing alloys and is both simple and practical, allowing operations to be carried out directly at the construction site. If the parts are small and the quantity is not large, heating with a handheld oxygen-fuel flame torch is the most cost-effective option. However, heating with a handheld flame torch is labor-intensive and makes it difficult to achieve uniform heating.

4.5 Induction Remelting

Coating induction remelting technology involves pre-applying a coating onto the workpiece substrate and then using the alternating magnetic field generated by an induction coil to induce eddy currents within the workpiece. The heat produced by these eddy currents is sufficient to melt the coating. The skin effect of eddy currents constitutes the primary advantage of induction heating, enabling heat to be concentrated precisely in the desired heating zone. Since the coating can be melted, the bonding strength between the coating and the substrate is significantly improved. Moreover, because the heat is concentrated on the surface layer, the thermal impact on the substrate itself is minimal, thus satisfying the requirements of minimal thermal influence and strong bonding strength.

Generally speaking, alloy powders can be used in flame spraying only if they meet two basic requirements: their melting point must be lower than that of the substrate, and they must possess self-melting properties. The effectiveness of induction remelting of coatings is closely related to the relative permeability and resistivity of the coating. During the remelting of various alloy coatings, high-temperature zones invariably appear at the interface between the coating and the substrate as well as on the coating surface. This non-uniform temperature distribution is a fundamental characteristic of the temperature field during induction remelting of coatings; it not only affects the stability of the remelting process but also determines the microstructure and properties of the coating.

4.6 Overall Heating and Remelting

The spray-coated layer is reheated to a temperature between the solid and liquid states for remelting. At this point, the material becomes semifluid, densifies, and melts completely. As a result, metallurgical reactions induce the precipitation of numerous hard phases (carbides, borides, and their compounds) in the form of a mixed coating. For overall heating and remelting, the furnace temperature must be carefully controlled—typically above the alloy’s solidus temperature. 10–30ºC. At this temperature, the viscosity is sufficiently high to prevent the coating from flowing. After remelting and subsequent cooling of the surface, these retained hardness levels remain in the coating, providing excellent resistance to friction and wear. Based on their widespread applications and cost considerations, nickel-based alloys are the most valuable among fusion alloys and have been the subject of the most research on remelting in recent years.

5. Development of Thermal Spraying Technology

Thermal spray materials are the core of all thermal spray technologies. The diversity of thermal spray coating functions is determined by the variety of materials with differing properties. The diversity of thermal spray materials paves the way for both the advanced nature and cost-effectiveness of thermal spray technology. With the development and application of ultra-high-strength steels, the requirements for metallurgical equipment in terms of wear resistance, corrosion resistance, and heat resistance are continually increasing. Therefore, the development of thermal spray coatings must keep pace with these growing demands. Currently, both domestic and international thermal spray equipment is evolving toward higher energy, higher speed, and greater efficiency. Axial powder feeding technology, multi-functional integrated technologies, and real-time control technologies will also become key directions for the advancement of thermal spray equipment. The continuous advancement of thermal spray equipment will broaden the range of applicable coating materials. The preparation of functional coatings—such as nanostructured, amorphous, thermally conductive, catalytic, electrically conductive, insulating, superconducting, and stealth coatings—will become a fluid-dynamic feature of the thermal spray process, enabling the establishment of temperature and velocity fields for sprayed particles and facilitating a deeper understanding of the coating formation process. This, in turn, will allow us to determine the specific effects of various spray parameters on the structure and performance of the coatings. Furthermore, the adoption of various advanced non-destructive testing techniques—such as acoustic, optical, and electrical methods—will enable online diagnostics of coating performance, assessment of coating weight, and prediction of coating service life. These approaches represent important research directions for future quality monitoring of thermal spray coatings.

6. Conclusion

With the continuous advancement of thermal spraying technology, thermal-sprayed coatings will undoubtedly find increasingly widespread applications across all sectors of the national economy. As one of the areas where thermal spraying has achieved particularly successful applications, the metallurgical industry should place even greater emphasis on the development and implementation of thermal spraying technologies.