Thermal Spray - Plasma Spray

       Our company is mainly engaged in plasma spraying in Chengdu area, which belongs to thermal spraying technology (hard surface technology), which is to send powder materials into plasma (radio frequency discharge) or plasma jet (DC arc) to make powder particles in Among them, after accelerated, melted or partially melted, under the action of impact force, it spreads and solidifies on the substrate to form a layer, and then the layer is laminated to form a coating. 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 application have developed rapidly, and it has become the most important part of thermal spraying technology.



1. 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. The working gas is passed between the nozzle and the electrode, and the arc is ignited by means of high-frequency sparks. The arc heats and ionizes the gas to generate 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, which is heated to a molten state, accelerated by the plasma jet, and sprayed onto the surface of the pretreated substrate at a certain speed to form a coating.



2. Characteristics of coating and process technology



1. Coating structure characteristics



The plasma spray coating has a fine structure, low oxide content and porosity. The bonding between the coating and the substrate and the bonding between the coating particles are mainly based on mechanical bonding, and can also produce micro-domain bonding and physical bonding. Layer bonding strength is high.



2. Process and technical characteristics



Spraying materials are widely used, ranging from low-melting aluminum alloys to high-melting zirconias. ; The coating has high bonding strength, low porosity and less oxide inclusions; the equipment has high control precision and can prepare fine coatings.



3. Main process parameters



1. Selection of plasma gas.



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



2. Delivery quantity



The size of the feeding amount is an important parameter that affects the structure of the coating and the 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 coating quality. If the delivery amount is too small, in addition to increasing the cost of spraying, it may also cause adverse 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 steel industry, automobile manufacturing, petrochemical, textile machinery, Ships, etc. In recent years, the application research of plasma spraying technology in high-tech fields such as nano-coating materials, gradient functional materials, superconducting coatings, and biological functional coatings has gradually attracted people's attention.



1. Nano-coating materials



Nano-WC/Co coatings were prepared 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 the load of 40-60 N, the wear rate of the nano-WC/Co coating is only 1/6 of that of the conventional coating. Connecticut University and others conducted a systematic study on plasma sprayed nanostructured Al2O3-TiO2 coatings, including nano-powder spray drying agglomeration and reconstruction, plasma spraying process parameter optimization, process diagnosis, simulation, and coating structure and performance analysis. The layers have a two-state microstructure and exhibit unique and excellent properties. Compared with the corresponding conventional coatings, the bond strength is increased by 100 %, the abrasive wear resistance is increased by 300 %, and the indentation crack resistance, bending and cupping tests show much higher spall resistance. The structural changes and particle implantation properties of nano-TiO2 during plasma spraying were investigated by Zhu Yingchun et al., Shanghai Institute of Ceramics, China. It was found that the TiO2 nanoparticles were transformed from amorphous to anatase and rutile structures. The coating exhibits good Li+



Injection current and electrochemical stability. Chen Huang et al. prepared zirconia nanocoatings on stainless steel substrates using atmospheric plasma spraying technology. The obtained coating has a dense 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 that of the traditional zirconia coating and the substrate.



2. Functional gradient materials (FGMs)



The preparation of gradient functional materials by plasma spraying is one of the research fields that have received much attention in materials science, and its research scope mainly covers the design, preparation and performance evaluation of gradient functional materials. Due to the high temperature of the plasma flame, it is especially suitable for spraying refractory metal, ceramic and composite material coatings, which provides a broader space for the development of functionally graded materials. At present, the application of ZrO2 coating on metal with NiCrAlY as the intermediate layer has become the focus of most plasma sprayed FGMs structure research, 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, Ni-20 with YSZ coating %Cr, Ni/Al2O3, WC/Co, etc. The study of the YSZ/NiCoCrAlY system by K. A. Khor et al. showed that the functionally graded coating has superior performance compared with the conventional bilayer material. The bond strength of the obtained FGM coating was 18 MPa, and the double-layer coating was only 9 MPa, and the thermal cycle life of the coating was 6 times that of the FGM coating. Sudarshan Rangaraj et al. designed YSZ gradient coatings with five different compositions and studied the effect of coating design on the properties of YSZ coatings. The results showed that the addition of mullite would reduce the driving force of crack growth on the coating surface.



3. Superconducting coating



The arc temperature of plasma spraying is very high, especially suitable for spraying composite oxide ceramics. It does not need protective atmosphere, can spray superconducting parts with complex shapes, 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 are mainly 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 inhomogeneities such as oxygen loss of the material during the spraying process, pores, cracks in the coating structure, and uneven contact between particles. Superconductivity can only be obtained by proper heat treatment of the coating in an oxygen or air atmosphere so that the coating forms a dense, uniform and relatively stable crystal structure. The heat treatment conditions of the YBCO coating were 920 °C × 1 h, then decreased to 400 °C for 1 h. When the Bi2Sr2Cu2CaOy ceramic is quenched or quenched from a high temperature, it develops a superconducting state. This feature is of special significance for plasma spraying, because plasma spraying can enable the coating material to obtain a rapid cooling rate of up to 106 °C/s. 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 biocoating materials. The powder materials of specific components are melted at high temperature and deposited on the surface of metal artificial bone implants to form artificial bones and artificial joints with ductile metal as skeleton and ceramic coating on the surface. This method gives full play to the two types of metal and ceramics. The advantages of the material. There are many research reports on plasma sprayed hydroxyapatite (HA) coatings and titanium coatings at home and abroad, and they have been successfully used in clinical trials. 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 movement