<****> Thermal spraying technology is one of the important technologies in the field of surface engineering. Its principle is to use a variety of different heat sources to pre-spray various materials such as metals, alloys, ceramics, plastics and various composites. The material is heated to a molten or molten state, and a high-speed atomization of the gas stream forms a coating of particulate mist deposited on the surface of the pretreated workpiece to form a coating which is tightly bonded to the substrate. The coating having metallurgical bonding characteristics formed by remelting the Ni-Cr-B-Si series sprayed layer is called a sprayed layer or a remelted layer. Thermal spraying technology can be used to spray almost all solid engineering materials, such as cemented carbide, ceramics, metals, graphite and nylon, to form wear, corrosion, heat, oxidation, insulation, electrical, radiation and so on. Specially functional coating. The technology also has the advantages of flexible process, convenient construction, strong adaptability and good economic benefits. It is widely used in aerospace, machinery, chemical, metallurgy, geology, transportation, construction and other industrial sectors, and has achieved rapid development. Thermal Spray Schematic: From the spraying of the material into the heat source to the formation of the coating, the spraying process generally takes four stages of heating, atomizing, flying and depositing. The first is that the spray material is heated to melt or soften. When the end material enters the high temperature region of the heat source, that is, it is heated to form a droplet, and the formed droplet is atomized into fine molten particles under the action of the external compressed air stream or the heat source self jet. The second stage of the fused flow is accelerated during flight. When these particles with a certain temperature and velocity impact the surface of the substrate with a certain kinetic energy, a strong collision occurs. At the moment of collision, the kinetic energy of the particles is converted into heat energy and transmitted to the substrate, and the deformation occurs along the uneven surface. It condenses rapidly and shrinks, and is continuously deposited on the surface of the substrate in a flat shape to form a coating. It is well known that, with the exception of a few precious metals, metallic materials undergo chemical and electrochemical reactions with surrounding media and are subject to corrosion. In addition, the wear of the metal surface caused by various mechanical actions is also extremely serious, and a large number of metal members fail due to corrosion and wear, resulting in great waste and loss. According to statistics from some industrialized countries, the annual loss of steel due to corrosion and wear accounts for about 10% of the total steel output, and the amount of losses accounts for about 2-4% of the gross national product. If the damage caused by metal corrosion and wear and the corresponding work-related injuries, fires, explosions, etc. are counted, the value is even more alarming. Therefore, the development of metal surface protection and strengthening technology is a major issue of common concern to all countries. With the development of cutting-edge science and modern industry, various industrial sectors are increasingly demanding high equipment (high temperature, high pressure, high speed and high automation) and harsh working conditions (such as severe wear and corrosion). Long-term stable operation. Therefore, higher requirements are placed on the performance of the material. It is obviously uneconomical and sometimes impossible to manufacture high-performance materials and parts to obtain surface protection and reinforcement. Therefore, the research and development of surface treatment technology for materials has significant technical and economic significance. Surface treatment technology has also been rapidly developed and improved by this demand. Thermal spraying technology is one of the techniques of surface protection and strengthening, and is an important subject in surface engineering. The so-called thermal spraying is to use a heat source such as an electric arc, a plasma arc, a combustion flame, etc. to heat the powdered or filamentous metal and non-metal coating materials to a molten or semi-molten state, and then by means of the power of the flame flow itself or The high velocity gas stream is atomized and sprayed onto the surface of the pretreated substrate material at a rate which combines with the matrix material to form a surface covering coating having various functions. Although the thermal spraying equipment has different equipments due to different thermal spraying methods, according to the principle of thermal spraying technology, the equipment is mainly composed of a spray gun, a heat source, a coating material supply device, a control system and a cooling system. Thermal spraying process The thermal spraying process is as follows: pretreatment of the workpiece surface → preheating of the workpiece → spraying → post-coating treatment 1. surface pretreatment In order to make the coating and the substrate material well combined, the surface of the substrate must be clean and rough, purified There are many ways to roughen the surface. The choice of method depends on the design requirements of the coating and the material, shape, thickness, surface condition and construction conditions of the substrate. The purpose of the purification process is to remove all dirt on the surface of the workpiece, such as scales, oil stains, paints and other contaminants. The key is to remove the surface of the workpiece and the grease that penetrates into it. The purification treatment methods include a solvent cleaning method, a steam cleaning method, an alkali washing method, and a heating degreasing method. The purpose of the roughening treatment is to increase the contact surface between the coating and the substrate, increase the mechanical occlusion force of the coating and the substrate, and further activate the purified surface to improve the bonding strength between the coating and the substrate. At the same time, the roughening of the surface of the substrate also changes the residual stress distribution in the coating, which is also advantageous for improving the bonding strength of the coating. The methods of roughening treatment include sand blasting, mechanical processing (such as threading, knurling), and electric drawing. Among them, sandblasting is the most commonly used roughing method. Commonly used sandblasting media are alumina, silicon carbide and chilled cast iron. When sandblasting, the type and size of the blasting medium, and the magnitude of the wind pressure during blasting must be reasonably selected according to the hardness of the workpiece material, the shape and size of the workpiece. For various metal substrates, the recommended sand particle size is about 16-60 sand. The coarse sand is used for sandblasting of solid parts and heavy parts. The blasting pressure is 0.5-0.7Mpa, the thin workpiece is easy to be deformed, and the blasting pressure is 0.3-0.4 Mpa. It is particularly noteworthy that the compressed air used for sandblasting must be water-free and oil-free, otherwise the quality of the coating will be seriously affected. The degree of roughening of the surface of the workpiece before spraying is sufficient for most metallic materials of 2.5-13 μm Ra. As the surface roughness increases, the adhesion of the coating to the matrix material is enhanced, but when the surface roughness exceeds 10 μm Ra, the degree of improvement in the bonding strength of the coating is reduced. For some coating materials that do not bond well with the substrate, a material that is bonded to the substrate material should be sprayed with a transition layer called a bonded bottom layer. The material commonly used as a bonding underlayer is Mo, NiAl. , NiCr and aluminum bronze. The thickness of the bonding underlayer is generally from 0.08 to 0.18 μm. 2. The purpose of preheating preheating is to eliminate the moisture and moisture on the surface of the workpiece, to improve the interface temperature when the sprayed particles are in contact with the workpiece, to improve the bonding strength between the coating and the substrate; and to reduce the thermal expansion of the substrate and the coating material. Cracking of the coating caused by the stress caused by the difference. The preheating temperature depends on the size, shape and material of the workpiece, as well as the thermal expansion coefficient of the substrate and the coating material. Generally, the preheating temperature is controlled between 60 and 120 °C. 3. The spraying method used for spraying depends mainly on the selected spraying materials, the working conditions of the workpiece and the requirements on the coating quality. For example, if it is a ceramic coating, it is best to use plasma spraying; if it is a carbide cermet coating, it is best to use high-speed flame spraying; if it is sprayed plastic, it can only be sprayed with flame; if it is to be used for large-area corrosion protection outdoors If the project is sprayed, then it is a non-flexible and efficient arc spray or wire flame spray. In short, the choice of spray method is generally diverse, but there is always one way to best for an application. The pre-processed workpiece should be sprayed in the shortest possible time. The spray parameters should be determined according to the coating material, the performance of the spray gun and the specific conditions of the workpiece. The optimized spray conditions can improve the spray efficiency and achieve high density and combination. High quality coating with high strength. 4. Post-Coating Treatment The resulting coating is sometimes not used directly and must be subjected to a series of post-treatments. The coating used for corrosion protection must be sealed to prevent the corrosive medium from penetrating the pores of the coating to the substrate to cause corrosion of the substrate. There are many materials used as sealing sealants, such as organic materials such as paraffin, epoxy resin and silicone resin, and inorganic materials such as oxides. How to choose a suitable sealing agent depends on the working medium, environment, temperature and cost of the workpiece. A variety of factors are considered. For workpieces subjected to high stress loads or impact wear, in order to improve the bonding strength of the coating, the sprayed layer should be remelted (such as flame remelting, induction remelting, laser remelting, and hot isostatic pressing) to make the porous The coating that is only mechanically bonded to the substrate becomes a dense coating that is metallurgically bonded to the substrate. If dimensional accuracy is required, the coating should be machined. Because the spray coating has different characteristics from ordinary metal and ceramic materials, such as micropores in the coating, it is not conducive to heat dissipation; the coating itself has low strength and cannot withstand large cutting forces; there are many hard coatings in the coating. The particle point, the wear of the tool is very fast, etc., thus forming a difficult coating characteristic of the spray coating different from the general material. Therefore, reasonable processing methods and corresponding process parameters must be selected to ensure the smooth operation of the spray coating and to ensure the required dimensional accuracy. The characteristics of thermal spray technology are analyzed from the principle of thermal spray technology and process analysis. Thermal spray technology has the following characteristics: 1. Due to the wide temperature range of the heat source, the sprayable coating material includes almost all solid engineering materials, such as metal. , alloys, ceramics, cermets, plastics, and composites composed of them. Therefore, the surface of the substrate can be imparted with various functions such as abrasion resistance, corrosion resistance, high temperature resistance, oxidation resistance, insulation, heat insulation, biocompatibility, infrared absorption, and the like. 2. The surface of the substrate is less heated and can be controlled during spraying, so it can be sprayed on various materials (such as metal, ceramic, glass, cloth, paper, plastic, etc.), and the structure and properties of the substrate. Almost no effect, the workpiece deformation is also small. 3. The equipment is simple and flexible in operation. It can spray large-scale components in large areas or in specific parts. It can be sprayed in the factory or on the outdoor site. 4. There are fewer procedures for spraying operations, shorter construction time, high efficiency and economical. With the improvement of thermal spraying application requirements and the expansion of the field, especially the advancement of the spraying technology itself, such as the increasing energy and precision of the spraying equipment, the increasing variety of coating materials and the gradual improvement of the performance, the thermal spraying technology has been obtained in the past ten years. The rapid development has not only greatly expanded the application field, but also developed from the early preparation of general protective coatings to the preparation of various functional coatings; from the maintenance of individual workpieces to the manufacture of a large number of products; from a single coating The preparation progressed to the coating system engineering including product failure analysis, surface pretreatment, coating material and equipment development, selection, coating system design and post-coating processing; it has become a very active subject in the field of material surface science. And in the modern industry, independent material processing techniques such as casting, forging, welding and heat treatment are gradually formed. It has become an important process for the industrial sector to save valuable materials, save energy, improve product quality, extend product life, reduce costs, and improve work efficiency. It has been widely used in various fields of the national economy.
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