The main direction of thermal spray technology development

       After nearly a century of development, thermal spraying technology has developed from a simple process technology to a complete industrial system, and has become an important part of advanced manufacturing technology. In the process of growth and development, due to the continuous infiltration, crossover and integration between majors and disciplines, technology has become increasingly systematic and integrated, that is, it has developed into a set of majors such as mechanics, materials science, thermodynamic science, high-tech and bioengineering. An integrated emerging interdisciplinary subject has become a complete industrial system in the field of manufacturing. The core of thermal spraying technology is high-quality, high-efficiency and low-consumption surface modification to achieve the purpose of imparting special functions to the surface of the base material. The development of technology mainly includes the discovery of new technology, the innovation of materials, the software system of coating quality control, the basic theoretical research of coating preparation and detection technology.



      1) In recent years, with the development of thermal spraying technology, new process technologies and new application fields have emerged. Coating quality largely depends on the speed of sprayed droplets. Increasing the speed of thermal spray jets and spray particles has become a new trend in the development of current international thermal spray technology. Explosive spraying, high-speed active gas flame spraying (HVAF) , High-speed arc spraying, active arc spraying, high-speed plasma spraying, three-cathode internal powder-feeding plasma spraying, solution plasma spraying (SPS), air-conditioning power spraying (CGDS) and other new technologies. The common feature of these technologies is that the flying speed of the sprayed particles is greatly increased, the porosity of the coating is reduced, and the bonding strength of the coating is improved.



      High-speed plasma spraying technology can spray ceramic materials and superalloy materials with high melting point due to high heat source temperature and high heating and melting efficiency. Compared with ordinary plasma spraying, the jet speed of high-speed plasma spraying exceeds Mach 5, and the sprayed particle speed can reach 500m/s, which makes the coating more dense, with higher hardness, toughness and bonding strength. In particular, the preparation of high-quality ceramic coatings and metal-ceramic coatings has irreplaceable advantages over other spraying techniques.



      Three-cathode internal powder feeding plasma spraying divides the arc into three parts by three parallel cathodes, and feeds the spray powder into the arc through the axis of the spray gun and the arc coaxially, reducing the possibility of local overheating of the nozzle and increasing the arc stability and the service life of the nozzle. ; Extend the powder heating time, improve the deposition rate and powder material utilization, the deposition efficiency can be as high as 90%, the coating quality is uniform, dense, and high bonding strength.



      Solution plasma spraying is a method of preparing coatings by using nano-pioneer solutions or solutions with suspended nanoparticles as spray materials. It effectively solves the technical problem of nano-powder material transportation, expands the application field of coating materials, and provides the possibility for nano-coating preparation and promoting the practical application of nano-coating.



      HVAF high-speed active gas spraying not only has the characteristics of high speed of high-speed flame spraying, but also has the ability to control and change environmental conditions. The active combustion gas uses propane, ethylene or MAPP gas, which can generate a reducing atmosphere; has the best matching of kinetic energy and thermal energy, the sprayed particles are heated to below the melting point, and the particle flying speed is as high as 800m/s, high production efficiency, spraying WC Co material can reach 30kg/h; the prepared metal alloy and carbide coating have very low oxygen content, the coating is very dense, and can obtain compressive stress or stress-free coating, which is one of the most effective methods for preparing thick coatings.



      High-speed active arc spraying is based on ordinary arc spraying. The coating material melted by the arc is atomized into fine particles by the high-speed airflow generated by propane-air combustion, and then accelerated to obtain high-speed flying particle beams, which can effectively By controlling the air flow to form an atmosphere to obtain a reducing environment, the obtained coating has low oxygen content, dense coating, low porosity and high bonding strength.



      Cold air dynamic spraying was first discovered by Russian scientists in wind tunnel tests, and the cold air dynamic spraying technology was invented based on this phenomenon. It is a method of accelerating the deposition of spraying materials on the surface of the substrate by using high pressure and hot air flow at a temperature of 260 to 760 ° C to form a coating. The supersonic gas/particle dual-phase flow and high-pressure gas flow accelerate the particles with a particle size of 1μm≤d≤50μm to 450-1000m/s, and form a coating with low porosity, low oxygen content and low residual stress in the solid state. Completely maintain the original composition of the spray material. Some literatures show that the higher the velocity of the two-phase gas flow, the higher the deposition efficiency of the particles. Now the United States and Germany have successfully developed a complete set of air-conditioning power spraying equipment.



      A variety of composite process technologies have been used in combination. The bonding bottom layer is prepared by high-speed flame spraying, which can effectively control the oxygen content of the bonding layer, and the working surface layer is prepared by plasma spraying; the preset coating layer is prepared by plasma spraying or flame spraying, and the fully dense ceramic coating is prepared by laser irradiation and remelting, so as to achieve the change of the coating layer. the purpose of organizational performance.



      2) The discovery and application of new process technologies promote the development of new materials. New thermal barrier coating materials, using new components and zirconia composites on zirconia coatings, as double-layer composite coatings; nano-coating (nano-pioneer solution, nano-aggregate powder) materials, functional composite coating materials, biological The preparation of functional coating materials, intermetallic compound coating materials, microcrystalline or amorphous coating materials, etc., has become the focus of increasing attention.



      3) Online control quality assurance system. Coating quality is closely related to spraying process methods and process parameters. The reproducibility and stability of standard process parameters are the most basic links to ensure coating quality. There are many factors affecting the change of process parameters. The size of thermal power, the distribution of thermal temperature, the distribution of sprayed particles, and the level of particle velocity have all become important factors for coating quality. Online quality control, detecting the distribution of the temperature field of the spraying heat source, the flying speed and state of the sprayed particles, provides a basic guarantee for optimizing and stabilizing the coating preparation process parameters to obtain high-quality coatings. The SprayWatch thermal spray particle monitoring system developed by Oseir of Finland is designed on the basis of CCD digital camera and is suitable for the detection of most thermal spray processes. It can easily measure and monitor key process parameters in the spraying process, such as: the temperature of the sprayed particles, the state, quantity and distribution of the flying speed; the size, position and direction of the spraying beam. It is expected to research and develop the monitoring system and spraying equipment online to form a new type of intelligent thermal spraying equipment with online control, feedback, diagnosis and correction.



      4) Basic theoretical research on coating preparation and coating performance testing methods. Neural networks, design of experiments, and other optimization methods were used to determine the relationship between thermal spray coating preparation process parameters and coating performance. Research and establish the mathematical model between the heating state, the motion form of the sprayed particles and the conditions at the nozzle outlet; the residual stress formation model in the coating; the coating model and the matrix heat flux model formed by the particle flow and plasma jet in the thermal spraying process. Models are established between various parameters in the spraying process, and methods to control coating quality are theoretically studied. By using optical microscope, electron microscope, X-ray diffraction, high-speed imaging and other detection technologies, the microstructure characteristics of the coating are studied, the influence of spraying parameters on the coating structure and coating performance is analyzed, and the performance of the coating is predicted.