Material surface treatment of thermal spraying technology

       1 Overview



The rapid development of high and new technology has put forward higher and higher requirements for improving the performance of metal materials and extending the service life of parts and components in instruments and equipment. The requirements of these two aspects are faced with the problem of increasing the cost of high-performance structural materials year by year. In recent years, surface engineering has developed rapidly, especially thermal spraying technology has made great progress, which provides a new method to solve the above problems. Thermal spraying is a surface engineering technology that melts a certain solid material and sprays it on the surface of a machine part through special equipment to form a special thin layer to improve the performance of the machine part in terms of corrosion resistance, wear resistance, and high temperature resistance. Because the thermal spray technology can spray most solid engineering materials such as various metals and alloys, ceramics, plastics and non-metals, it can be made into functional coatings with various properties, and it is flexible in construction, adaptable and has a wide range of applications. The economic benefits are outstanding, especially for improving product quality, extending product life, improving product structure, saving energy, saving precious metal materials, improving work efficiency, and reducing costs. With the development of industry and technology, people have put forward higher and higher requirements for thermal spraying technology. While the existing thermal spraying process has been continuously improved, some new processes have also emerged. At present, thermal spraying technology has been widely used in many fields, including cutting-edge technologies such as aviation, aerospace, atomic energy equipment, and electronics, and has achieved good economic benefits.



2 History of thermal spraying process



As early as 1910, Swiss scholar Schoop was inspired by children playing with lead shot toy guns and invented the first metal spraying device-metal solution spraying, which was named "metal spraying" at that time. It is to inject a low melting point metal solution into a heated compressed air stream to atomize the metal solution and spray it onto the surface of the substrate to form a coating. Although this device is huge and inefficient, it already contains the basic principles and process of thermal spraying, creating the field of thermal spraying technology. In 1913, Schoop proposed the design of arc spraying, a practical arc spray gun in 1916. The METCO-P powder flame spray gun appeared after the successful development of the Schort powder flame gun in the United Kingdom in the 1930s. Other thermal spray professional companies also developed a series of powder guns, which also made this thermal spray method widely used. In 1943, the American METCO company published the "Metal Spraying" manual for the first time. At the end of the 1940s, thermal spraying technology made great progress. At the end of the 1950s, the United Carbide Corporation successfully developed gas repetitive explosive spraying, which was used to prepare high-quality carbide and oxide ceramic coatings, and was first applied to the aviation industry. In the early 1960s, plasma spraying technology has been applied in industry. It has changed the face of thermal spraying technology, greatly expanded the scope of spraying materials and coating applications, solved the spraying problems of insoluble metal materials and ceramic materials, and greatly improved the quality of coatings. In the 1970s, thermal spraying technology developed in the direction of high energy, high speed and high efficiency, 80KW



high-energy plasma spraying equipment, low-pressure plasma spraying equipment and 200KW liquid stable plasma spraying equipment. In the 1980s, thermal spraying technology had a new development, which was prominently manifested in the successful development of supersonic flame spraying and the application of electronic computers to thermal spraying equipment, so that spray coatings developed in the direction of higher quality and precision. Britain and the United States have formed a huge thermal spraying economic industry chain. Application of Thermal Spray Technology in my country Appetite At the end of the 1940s, in the 1950s, it began to manufacture wire flame spray guns and powder air spray guns, developed wire arc spraying, established the first domestic professional spraying plant in Shanghai, and developed oxyacetylene flame wire spraying And arc spraying equipment, and develop spraying business externally. In the early 1960s, the closed nozzle fixed arc spray gun and ceramic powder air spray gun were successfully developed. In the late 1960s, plasma spray welding technology began to be researched, and powder flame spray welding technology began to be applied at the same time. In the 1970s, more complete varieties and models of spraying equipment and materials appeared, but overall progress was slow.



The first National Thermal Spraying Conference was held in Beijing in 1981 under the auspices of the National Economic Commission and the National Science and Technology Commission.



It was announced at the meeting that the "National Thermal Spray Cooperative Group" was established under the leadership of the National Economic Committee. In 1991, the Wuhan Institute of Material Protection of the Ministry of Mechanical and Electrical Engineering applied to the Ministry of Civil Affairs for the “Thermal Spraying Professional Committee of China Surface Engineering Association”.



After two decades of development, in the field of thermal spraying, new equipment, new materials, new processes and new application technologies have continuously emerged, forming a scientific research, design, manufacturing, and construction team and a popularization and application network, which is my country's thermal spraying technology. The development has laid a solid foundation. It can be expected that thermal spraying technology will be developed more rapidly in our country and will be more widely promoted and applied in various sectors of the national economy.



3



The principle and characteristics of thermal spraying process



Thermal spraying technology is to heat certain materials to a molten or semi-molten state through a certain heat source, and then spray them on the surface of the coated substrate to form a coating with better performance than the original substrate, so that the original workpiece has an excellent surface Performance, or to make the workpiece obtain one or more surface properties that the original matrix material does not possess.



The formation of the spray coating includes the heating and melting phase of the sprayed material, the atomization phase of the droplets, the flight phase of the particles, and the spraying of the particles.



Tu stage. The combination of the coating and the substrate is generally considered to have mechanical bonding, diffusion bonding, physical bonding and metallurgical bonding. When using exothermic spraying materials or spraying with high-temperature heat sources, the molten sprayed material particles will weld with the molten substrate, forming a micro-area metallurgical bond, and improving the bonding strength of the coating and the substrate. The combination of particles in the spray coating is



Mechanical bonding is dominant, while diffusion bonding, physical bonding, metallurgical bonding, etc. also work together. Since 1910 Swiss Chopp (



Dr. 　　Schoop) invented a flame spraying device (ie thermal spraying, thermal spraying technology has been greatly developed, especially



Since the 1980s, the application of thermal spraying technology has made great achievements. Compared with various other surface technologies, thermal spraying technology has its own characteristics:



1 Coatings of various materials can be prepared on various substrates, including almost all solid materials such as metals, ceramics, cermets, engineering plastics, glass, wood, cloth, and paper.



2 The substrate temperature is low. The temperature of the substrate is generally between 30 and 200°C, the deformation is small, and the heat-affected zone is shallow.



3 Flexible operation. It can spray objects of various specifications and shapes, especially suitable for large-area coatings, and can be used in the field.



4 The coating thickness range is wide. Coatings ranging from tens of microns to several millimeters can be prepared, and are easy to control, with high spraying efficiency and low cost. The production efficiency of spraying is from several kilograms to tens of kilograms per hour.



As a surface treatment technology, thermal spraying also has many shortcomings. For example, the coating has problems such as low bonding force, high porosity, poor uniformity, and it is not easy to perform non-destructive inspection of the coating, which are mainly reflected in Low thermal efficiency, low material utilization, large waste and low strength of coating and substrate.



4



Types of thermal spraying methods



The general process for completing the thermal spraying of the workpiece is: surface preparation of the workpiece-spraying of the workpiece-post-treatment of the workpiece after spraying. In addition to the type of equipment, the whole process also involves spraying materials, heat sources and spray speed. Spraying materials include metals, ceramics and plastics. Thermal spraying methods can be roughly divided into flame spraying, explosive spraying, supersonic spraying, arc spraying, plasma spraying and laser spraying.



1 The flame spraying method is a widely used process in the current spraying technology. Using flame spraying technology, various coatings of pure metals, alloys, ceramics and plastics can be prepared.



2 The explosive spraying method is a new technology with greater technical difficulty and strong process performance. It is also a high-energy spraying method. Compared with general flame spraying, it is necessary to provide high enough gas pressure to produce up to 5



The flame current is 1830m/S times the speed of sound. The consumption of gas is also very large. As far as oxygen is concerned, it is usually 10% of general flame spraying.



times.



3 The arc spraying method can spray all kinds of metal materials, and is widely used in anti-corrosion, wear-resistant and other projects. At present, the arc spraying method has evolved from a rough technology with high spraying rate to a more precise method that can produce high-quality coatings at low cost.



4 supersonic plasma spraying



The principle of supersonic plasma spraying is to input the main gas (argon gas, with a small flow rate) from the rear gun body, and a large amount of secondary gas (nitrogen or a mixture of nitrogen and hydrogen) through the gas swirl ring, and The main gas is ejected from the Laval tube-shaped secondary nozzle (the tungsten electrode is connected to the negative electrode, and the primary nozzle is connected to the positive electrode when the arc is piloted), and the arc is triggered by high-frequency in the primary gas, and then the positive electrode is transferred to the secondary nozzle, that is An arc is generated between the tungsten electrode and the inner wall of the secondary nozzle. Under the strong action of the rotating secondary gas, the arc is pressed in the center of the nozzle and stretched to the outer edge of the nozzle, forming a high-pressure extended plasma arc. The high-power extended plasma arc effectively heats the main gas and the secondary gas, so as to obtain a stable and concentrated supersonic plasma jet from the nozzle. The sprayed powder is added to the supersonic plasma flow through the powder feeding nozzle to obtain high temperature and kinetic energy, and impact on the surface of the workpiece to form a coating. The method is characterized by extremely high heat source temperature (plasma arc temperature up to 16 000 DEG C) and power, so the ceramic powder can be heated above its melting point in a short time to obtain a high-quality coating. It has the characteristics of high heating temperature of plasma arc spraying and fast flying speed of sprayed materials of gas explosion spraying and HVOF spraying.



5 laser spraying



The principle of action is to heat the top of the wire (or powder) with a high-energy density beam to melt, and then use the sprayed high-pressure gas to granulate the molten part and spray it onto the surface of the substrate to form a coating. The atmosphere of the spraying environment can be selected in an atmospheric atmosphere, or sprayed in an inert atmosphere or a vacuum state. Advantages of laser spraying: the coating structure obtained by spraying is the same as the original powder;



can spray most materials, ranging from low melting point coating materials to ultra-high melting point coating materials; even when the porosity of the coating is prepared by welding wire, it is better than that of plasma spraying



The coating prepared in powder form has a low porosity.