Thermal spray-plasma spray

       Our company is mainly engaged in plasma spraying in Chengdu area. It belongs to thermal spraying technology (hard surface technology). It sends powder materials into plasma (radio frequency discharge) or plasma jet (DC arc) to make the powder particles in Among them, after being accelerated, melted or partially melted, under the action of impact force, it spreads and solidifies on the substrate to form a layer, and then forms a coating layer by layering. It has the advantages of high production efficiency, good coating quality, wide range of sprayed materials, and low cost. Therefore, in recent decades, its technological progress and production applications have developed rapidly, and it has become the most important part of thermal spraying technology.



One, principle



Plasma spraying is realized by a plasma spray gun. The nozzle and electrode of the spray gun are respectively connected to the positive and negative poles of the power supply. A working gas is passed between the nozzle and the electrode, and the arc is ignited with the help of a high-frequency spark. The arc heats and ionizes the gas to produce a plasma arc, and the thermal expansion of the gas is ejected from the nozzle to tell the plasma flow. The powder feeding air pipe sends the powder into the plasma jet, is heated to a molten state, accelerated by the plasma jet, and sprayed on the surface of the pretreated substrate at a certain speed to form a coating.



2. Coating and process technology characteristics



1, coating structure characteristics



Plasma spray coating has fine structure, low oxide content and porosity. In addition to mechanical bonding, the bonding between the coating and the substrate and the bonding between the coating particles can also produce micro-domain bonding and physical bonding. The layer bonding strength is higher.



2, Process technology characteristics



Spraying materials are widely used, from low melting point aluminum alloy to high melting point zirconia can be sprayed. ; The coating has high bonding strength, low porosity, and less oxide inclusions; the equipment has high control precision and can prepare fine coatings.



Three, the main process parameters



1. The choice of plasma gas.



Generally, argon or argon is used as the main gas for plasma spraying, and hydrogen is used as the auxiliary gas. For spraying high melting point materials such as 2ZrO, 23AlO, W, etc., the main gas should be nitrogen and a small amount of hydrogen should be mixed.



2, free portion



The size of the feeding component is an important parameter that affects the coating structure and deposition efficiency. If the feeding amount is too large, it will not only reduce the powder deposition efficiency, but also increase the number of holes and unmelted particles in the coating, resulting in a decrease in the quality of the coating. If the delivery amount is too small, in addition to increasing the cost of spraying, it may also cause undesirable consequences such as overheating of the parts and cracking of the coating.



Fourth, the application of plasma spraying technology



Plasma spraying technology has been widely used in traditional fields such as wear-resistant coatings and corrosion-resistant coatings. Since the 1950s, its application fields have expanded from aviation and aerospace to the steel industry, automobile manufacturing, petrochemicals, textile machinery, Ships and other fields. In recent years, the application research of plasma spraying technology in high-tech fields such as nano-coating materials, functionally gradient materials, superconducting coatings, biological functional coatings, etc. has gradually attracted people's attention.



1, nano-coating materials



Zhu et al. prepared nano-WC/Co coatings by vacuum plasma spraying. It is found that the hardness, toughness and wear resistance of the coating are greatly improved compared with the conventional coating. Under a load of 40-60 N, the wear rate of the nano-WC/Co coating is only 1/6 of that of the conventional coating. The University of Connecticut and others have conducted systematic research on plasma sprayed nanostructured Al2O3-TiO2 coatings, including nanopowder spray drying and agglomeration reconstruction, plasma spraying process parameter optimization, process diagnosis, simulation, and analysis of coating structure and performance. The layer has a two-state microstructure and exhibits unique and excellent performance. Compared with the corresponding conventional coating, the bonding strength is increased by 100%, the abrasive wear resistance is increased by 300%, and the resistance to indentation cracking, bending and cupping tests are much higher. Zhu Yingchun, Shanghai Institute of Ceramics, China, and others studied the structural changes and particle injection characteristics of nano-TiO2 during plasma spraying. The study found that the TiO2 nanoparticles were transformed from amorphous to anatase structure and rutile structure. The coating exhibits good Li+



Injection current and electrochemical stability. Chen Huang et al. used atmospheric plasma spraying technology to prepare zirconia nano-coating on the stainless steel substrate. The obtained coating has a compact structure with a porosity of about 7%, and the bonding strength between the coating and the substrate is 45 MPa, which is significantly better than the bonding strength of the traditional zirconia coating and the substrate.



2, functionally gradient materials (FGMs)



Plasma spraying preparation of functionally gradient materials is one of the research fields that have attracted much attention in materials science. Its research scope mainly covers three aspects: the design, preparation and performance evaluation of functionally gradient materials. Due to the high temperature of the plasma flame, it is especially suitable for spraying coatings of refractory metals, ceramics and composite materials, which provides a broader space for the development of functionally graded materials. At present, coating ZrO2 coating on metal with NiCrAlY as the intermediate layer has become a hotspot in the structure research of most plasma sprayed FGMs, and a good preparation process has been established. In addition, other systems that have been studied include: Cu/W and Cu/B4C, Ni-based alloys combined with Al2O3-Cr2O3, ZrO2 with CoCrAlY or NiCoCrAlY, TiC with Mo, and Ni-20 with YSZ coating %Cr, Ni/Al2O3, WC/Co, etc. The research by K. A. Khor et al. on the YSZ/NiCoCrAlY system shows that, compared with the traditional double-layer materials, the functionally graded coating has better performance. The bonding strength of the obtained FGM coating is 18 MPa, the double-layer coating is only 9 MPa, and the thermal cycle life of the FGM coating is 6 times that of the double-layer coating. Sudarshan Rangaraj et al. designed five YSZ gradient coatings with different compositions and studied the effect of coating design on the performance of YSZ coatings. The results showed that the addition of mullite components would reduce the driving force of crack growth on the coating surface.



3, superconducting coating



Plasma spraying has a high arc temperature, which is especially suitable for spraying composite oxide ceramics. It does not require a protective atmosphere. It can spray superconducting parts with complex shapes. It has high deposition efficiency and is easy to prepare thick film coatings and large area coatings. The superconducting ceramic coating materials suitable for plasma spraying mainly include YBa2Cu3O7-x (YBCO) and Bi2Sr2Cu2CaO. YBCO is a typical superconducting material with a critical temperature of 94 K. The plasma sprayed YBCO coating does not have superconducting properties due to the inhomogeneity of the material's oxygen loss during the spraying process, the pores, cracks in the coating structure, and the uneven contact between particles. Only when the coating is properly heat-treated in oxygen or air atmosphere to form a dense, uniform and stable crystal structure, superconductivity can be obtained. The heat treatment condition of YBCO coating is 920℃×1 h, then it is lowered to 400℃ and kept for 1 h. When Bi2Sr2Cu2CaOy ceramic is quenched or quenched from high temperature, it will produce a superconducting state. This feature is of special significance to plasma spraying, because plasma spraying can achieve a rapid cooling rate of up to 106°C/s for the coating material. As long as the plasma spraying conditions and process parameters are adjusted, it is easy to make the sprayed coating of Bi2Sr2Cu2CaOy Superconducting properties.



4, biological functional coating



Plasma spraying technology is an effective method for preparing medical biological coating materials. The powder material of specific composition is melted at high temperature and deposited on the surface of metal artificial bone implant to form artificial bone and artificial joint with ductile metal as the framework and ceramic coating on the surface. This method makes full use of the two types of metal and ceramics. The advantages of materials. There are many research reports on plasma sprayed hydroxyapatite (HA) coating and titanium coating at home and abroad, and they have been successfully applied in clinical trials. The hydroxyapatite coating is non-toxic to organisms, resistant to body fluid corrosion, and has good adaptability and affinity to biological tissues, and is resistant to wear during long-term exercise