Material surface treatment of thermal spray technology

       1 Overview



The rapid development of high and new technology has put forward higher and higher requirements to improve the performance of metal materials and prolong the service life of parts and components in instruments and equipment. The requirements of these two aspects are faced with the problem that the cost of high-performance structural materials is rising 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 and accelerates a certain solid material onto the surface of the machine part through special equipment to form a special thin layer to improve the corrosion resistance, wear resistance, high temperature resistance and other properties of the machine part. Because thermal spraying 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, flexible construction, strong adaptability, and wide application. The economic benefits are outstanding, especially in improving product quality, prolonging 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 technology has been continuously improved, some new technologies have also emerged. At present, thermal spraying technology has been widely used in many fields including aviation, aerospace, atomic energy equipment, electronics and other cutting-edge technologies, and has achieved good economic benefits.



2 History of thermal spraying process



As early as 1910, the Swiss scholar Schoop, inspired by children's play with lead shot toy guns, invented the first metal spraying device - metal solution spraying, which was named "metal spraying" at that time. It is to inject the solution of low melting point metal into the 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 processes of thermal spraying, creating the field of thermal spraying technology. In 1913, Schoop proposed the design of electric arc spraying, and in 1916 the practical arc spray gun. In the 1930s, after the successful development of the Schort powder flame gun in the United Kingdom, the METCO-P powder flame spray gun appeared. Other thermal spraying professional companies also developed a series of powder flame 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. In the late 1940s, thermal spray technology had made great strides. In the late 1950s, the gas repeated explosive spraying was successfully developed by the United Carbide Company for the preparation of 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 appearance 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 coating quality. 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-stabilized plasma spraying equipment. In the 1980s, thermal spraying technology had new development, which was highlighted by the successful development of supersonic flame spraying and the application of electronic computers to thermal spraying equipment, so that the sprayed coating developed in the direction of higher quality and precision. The United Kingdom and the United States form a huge economic industrial chain of thermal spraying. In the late 1940s and 1950s, we began to manufacture wire flame spray guns and powder air spray guns, developed wire arc spraying, established the first professional spraying factory in China in Shanghai, and developed oxyacetylene flame wire spraying. And arc spraying equipment, and external spraying business. In the early 1960s, the closed nozzle fixed arc spray gun and the ceramic powder air spray gun were successfully developed. In the late 1960s, he began to study plasma spray welding technology, and began to apply powder flame spray welding technology. In the 1970s, spraying equipment and materials with complete varieties and models appeared, but in general, the progress was slow.



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



At the meeting, it was announced that under the leadership of the National Economic Committee, the "National Thermal Spraying Collaboration Group" had been established. In 1991, the Wuhan Institute of Materials Protection of the Ministry of Mechanical and Electrical Engineering applied to the Ministry of Civil Affairs for the "China Surface Engineering Association Thermal Spray Professional Committee".



After 20 years of development, in the field of thermal spraying, new equipment, new materials, new processes and new application technologies continue to emerge, forming a scientific research, design, manufacturing and construction quasi-team and promotion and application network. The development of my country has laid a solid foundation. It can be expected that thermal spraying technology will develop more rapidly in my country and be more widely used in various sectors of the national economy.



3



The principle and characteristics of thermal spraying process



Thermal spraying technology heats certain materials to a molten or semi-molten state through a certain heat source, and then sprays 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 several surface properties that the original matrix material does not have a membranous structure.



The formation of the spray layer includes the heating and melting stage of the spray material, the atomization stage of the droplet, the flight stage of the particles and the spraying of the particles.



coating stage. The combination of coating and 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 matrix, forming a metallurgical bond of micro-regions and improving the bonding strength of the coating and the matrix. The bonding between the particles in the spray coating is mechanically



Mechanical bonding is the main method, and diffusion bonding, physical bonding, metallurgical bonding, etc. also work together. Since 1910 Swiss Chopp (



Dr. Schoop) invented a flame spraying device (that is, 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 spray 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 substrate temperature 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 sizes and shapes, especially suitable for large-area coating, and can be used in the field.



4. Wide range of coating thickness. 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 during spraying is 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 difficulty in non-destructive inspection of the coating. Low thermal efficiency, low material utilization, large waste and low coating and substrate strength.



4



Types of thermal spraying methods



The general process flow for completing the thermal spraying of the workpiece is: surface preparation of the workpiece - spraying of the workpiece - post-spraying treatment of the workpiece. The whole process is not only related to the type of equipment, but also the spray material, heat source and spray speed. Spray materials include metals, ceramics, and plastics. Thermal spraying methods can be roughly divided into flame spraying method, explosive spraying method, supersonic spraying method, arc spraying method, plasma spraying method and laser spraying method.



1 The flame spraying method is a widely used process in spraying technology at present. 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 great technical difficulty and strong technological performance, and it is also a high-energy spraying method. In contrast to normal flame spraying, a sufficiently high gas pressure must be provided to generate up to 5



The flame is 1830m/S twice the speed of sound. The consumption of gas is also very large, in terms of oxygen, it is usually 10 times that 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. Currently, arc spraying has evolved from a crude high spray rate technique to a more sophisticated means of producing high quality coatings at low cost.



4. Supersonic plasma spraying



The principle of supersonic plasma spraying is to input the main gas (argon gas, the flow rate is small) from the rear gun body, while a large amount of secondary gas (nitrogen or a mixture of nitrogen and hydrogen) is passed through the gas swirl ring. The main gas is ejected from the Laval tube-shaped secondary nozzle (tungsten electrode is connected to the negative electrode, and the primary nozzle is connected to the positive electrode during arc ignition), and the arc is struck 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 elongated to the outer edge of the nozzle, forming a high-pressure expanding plasma arc. The high-power extended plasma arc effectively heats the primary and secondary gases, resulting in a stable, concentrated supersonic plasma jet from the nozzle. The sprayed powder is added to the supersonic plasma flow through the powder feeding nozzle to obtain a high temperature and kinetic energy, and impact on the surface of the workpiece to form a coating. This method is characterized by extremely high heat source temperature (plasma arc temperature up to 16 000°C) and power, so that ceramic powder can be heated above its melting point in a short time to obtain high-quality coatings. It has the characteristics of high heating temperature of plasma arc spraying and fast flying speed of spraying materials of gas explosion spraying and HVOF spraying.



5. Laser spraying



The principle of action is to heat the top of the welding wire (or powder) with a high-energy density beam until it melts, and then use the sprayed high-pressure gas to make the melted part particle, and spray it to the surface of the substrate to form a coating. The atmosphere of the spraying environment can be selected in the atmospheric atmosphere, and the spraying can also be carried out 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; the porosity of the coating, even when prepared with welding wire, is higher than that of plasma spraying



Low porosity of coatings prepared as powders