What Is Thermal Spraying Technology?

Thermal spraying technology is defined in the national standard GB/T18719-2002 "Thermal Spraying Terminology and Classification": Thermal spraying technology is a method of using a heat source to heat the spraying material to a molten or semi-molten state, and spraying and depositing it at a certain speed onto the pretreated substrate surface to form a coating. Thermal spraying technology creates a special working surface on the surface of ordinary materials to achieve a series of functions such as corrosion resistance, wear resistance, friction reduction, high temperature resistance, oxidation resistance, heat insulation, insulation, conductivity, and microwave radiation protection, so that it can save materials and energy. We call the special working surface a coating, and the working method of making the coating is called thermal spraying. Thermal spraying technology is one of the important components of surface process technology, accounting for about one-third of surface engineering technology.

 I.Advantages

1. The equipment is light and can be constructed on site.
2. The process is flexible and the operating procedures are few. It can be repaired quickly and the processing time is reduced.
3. It has strong adaptability and is generally not limited by the size of the workpiece and the site.
4. The coating thickness can be controlled.
5. In addition to spray welding, the heating temperature of the substrate is low, the deformation of the workpiece is small, and the crystal structure and performance changes are also small.
6. It is suitable for parts of various base materials and can prepare various protective coatings and functional coatings on the surface of almost all solid materials.

 II.Characteristics

From the analysis of the principle and process of thermal spraying technology, thermal spraying technology has the following characteristics:

1. Since the temperature range of the heat source is very wide, the coating materials that can be sprayed include almost all solid engineering materials, such as metals, alloys, ceramics, metal ceramics, plastics and composites composed of them. Therefore, the substrate can be given a surface with various functions (such as wear resistance, corrosion resistance, high temperature resistance, oxidation resistance, insulation, heat insulation, biocompatibility, infrared absorption, etc.).
2. During the spraying process, the degree of heating on the substrate surface is small and controllable, so it can be sprayed on various materials (such as metal, ceramics, glass, cloth, paper, plastic, etc.), and it has almost no effect on the organization and performance of the substrate, and the deformation of the workpiece is also small.
3. The equipment is simple and flexible to operate. It can spray large areas of large components or spray in designated areas; it can be sprayed indoors in the factory or outdoors.
4. Spraying operation has fewer procedures, shorter construction time, high efficiency and economical. With the improvement of thermal spraying application requirements and the expansion of fields, especially the progress of spraying technology itself, such as the increasingly high-energy and sophisticated spraying equipment, the increasing variety of coating materials, and the gradual improvement of performance, thermal spraying technology has achieved rapid development in the past decade. Not only has the application field been greatly expanded, but the technology has also developed from the early preparation of general protective coatings to the preparation of various functional coatings; from the maintenance of a single workpiece to the manufacture of a large number of products; from a single coating preparation to a spraying system engineering including product failure analysis, surface pretreatment, research and development of coating materials and equipment, selection, coating system design and coating post-processing; it has become a very active discipline in the field of material surface science. And in modern industry, independent material processing technologies such as casting, forging, welding and heat treatment have gradually formed. It has become an important process for the industrial sector to save precious materials, save energy, improve product quality, extend product life, reduce costs, and improve work efficiency. It has been more and more widely used in various fields of the national economy.

III. Technical advantages

1. It can organically combine the toughness and easy processing of metal materials with the high temperature resistance, wear resistance and corrosion resistance of ceramic materials.
2. By rationally selecting coating materials and appropriate spraying processes, surface strengthening coatings with various functions can be obtained.
3. Not limited by the substrate: The substrate material used for thermal spraying can be inorganic materials such as metal, ceramic, cement, refractory materials, stone, gypsum, or organic materials such as plastic, rubber, wood, paper, etc.
4. Not limited by the size of the workpiece and the construction site.
5. The coating deposition rate is fast, the thickness is controllable, and the process is simple.
6. The ceramic coating has good processability, and the coating can be sprayed again after damage.

 IV.Development trend

Thermal spraying technology has been widely promoted and applied in China. The development trends and characteristics in recent years are:

⑴Large-area long-term protection technology has been widely used

For steel structures exposed to the outdoor atmosphere for a long time, spraying aluminum, zinc and its alloy coatings instead of the traditional painting method, and implementing cathodic protection for long-term atmospheric corrosion protection has been rapidly developed in recent years. Large projects such as TV towers, bridges, highway facilities, sluice gates, microwave towers, high-voltage transmission towers, underground cable supports, navigation buoys, shafts, etc., all use spraying aluminum, zinc and their alloys for corrosion protection. There are dozens of professional spraying plants in China engaged in this work, and the spraying area reaches more than several million square meters each year. This technology has not only been widely promoted and applied in China, but also has been well promoted and applied in foreign aid projects.

⑵ Repair and strengthen large-scale equipment and localization of imported parts

Repair and strengthen large-scale equipment and localization of imported parts In recent years, there have been many successful application examples in this regard, such as: 1.7-meter rolling mill, high-speed fan rotor, large extruder plunger, large gear, electrode extrusion molding nozzle, high-power automobile crankshaft, etc. The implementation of these works, first, solves the urgent needs of production; second, saves a lot of foreign exchange.

⑶ Application of supersonic flame spraying technology With the development and improvement of China's thermal spraying technology, the quality requirements for spray coatings are getting higher and higher. In recent years, the high-speed gas (HVAF) method developed in the United States and other countries is a new process for preparing high-quality coatings. Since the supersonic flame spraying method has many advantages, China has successively introduced nearly a dozen equipment from abroad, which plays an important role in various industrial sectors.

⑷Gas deflagration spraying technology is further applied

This spraying technology is superior to other spraying methods in some fields because the particle flight speed can reach more than 800m/s, the bonding strength between the coating and the substrate can reach more than 100MPa, and the porosity is <1%. More than 10 units have been installed in China.

⑸Oxyacetylene flame plastic powder spraying technology has developed rapidly

As mentioned above, in recent years, there have been many domestic manufacturers of oxyacetylene flame plastic powder spraying equipment. Using this process technology, it has been well applied in chemical storage tanks, pipelines, ceramic industry mud machine frame, printing and dyeing industry cloth guide roller, coal industry belt conveyor cast iron support wheel, petroleum industry injection equipment, and surface decoration, which makes up for the shortcomings of electrospraying. It has opened up a new way for the application of plastic coatings.

⑹ Thermal spraying technology is applied in chemical anti-corrosion engineering

Corrosion is one of the main reasons for the failure of mechanical parts due to the chemical or electrochemical action of the surrounding medium. It not only causes a large amount of metal materials to be lost, but also the loss of production suspension caused by it is more difficult to estimate, so people pay special attention to chemical anti-corrosion work. Thermal spray coatings are used in corrosive media, especially strong medium corrosion. The main reason why they have not been able to break through before is that the sealing agent has not been solved. As we all know, there are pores in the spray coating. If the pores are not sealed, various acids, alkalis, and organic media will penetrate into the pores, causing the coating to fall off and affecting the anti-corrosion effect. According to the requirements of anti-corrosion engineering, China has recently successfully developed dozens of types of sealing agents such as polyester type, organic polymer type, resin type, plastic type, adhesive type, etc., which are suitable for the corrosive environment of acid, alkali, salt and organic matter, and the use temperature is 80~350℃. Ceramic coating, oxide coating or metal or alloy coating, according to different media, the appropriate sealing agent is selected, which has been used in many chemical corrosive media with good results. This series of sealing agents has been patented and won the National Invention Award. The successful research of these sealing agents has made new progress in the application of thermal spraying technology in chemical corrosion protection engineering.

⑺Laser remelting technology begins to be applied

High-frequency induction remelting and vacuum induction remelting are only applied within a certain range. Laser remelting technology has been tested on a small area in the past few years and has not been widely used. Recently, Tsinghua University has used laser remelting technology in valve production, and Shanghai Second Textile Machinery Factory has applied laser remelting technology in textile machinery.

⑻It has also been applied in building decoration and medical health

Thermal spraying technology has also been applied in building decoration and medical health. In recent years, Sichuan, Shanghai, Shenyang, Yunnan and other places have adopted thermal spraying technology to spray various statues, ornaments, large wall surfaces, etc., and have received good results. For example, the large mural of the peacock opening its tail in the International Shopping Mall in Shenyang uses thermal spraying technology. With the development and improvement of thermal spraying technology, this technology has penetrated into other fields. For example, in the biological field, thermal spraying methods are used to manufacture artificial bones. More than 200 clinical cases have been used in China, and the effect is very good. In addition, artificial teeth made by thermal spraying methods have also been initially applied.

 VI.Classification method of thermal spraying

As a new type of practical engineering technology, there is no standard classification method. It is generally classified according to the type of heat source, the form of the spraying material and the function of the coating. For example, according to the function of the coating, it can be divided into corrosion-resistant, wear-resistant, heat-insulating and other coatings. According to the heating and bonding method, it can be divided into spraying and spray melting: the former is that the body does not melt, and the coating and the substrate form a mechanical bond; the latter is that the coating is reheated and remelted, and the coating and the substrate are mutually dissolved and diffused to form a metallurgical bond.

A classification method that is more commonly used is to classify according to the type of heat source for heating the spraying material, which can be divided into:

① Flame type, including flame spraying, explosion spraying, and supersonic spraying;

② Arc type, including arc spraying and plasma spraying;

③ Electric heating method, including electric explosion spraying, induction heating spraying and capacitor discharge spraying;

④ Laser type: laser spraying