Plasma arc spraying technologies have significantly evolved, offering a spectrum of applications in surface engineering. In this article, we will come to see different types of Plasma Arc Spraying.
Air Plasma Arc Spraying (APS):
Air plasma arc spraying is conducted in atmospheric conditions using a direct current power source and inert gas as the working gas. The power of the APS gun typically ranges from 40 to 80 kW, and the coating’s porosity is between 1% and 5%. The spraying distance between the gun and the workpiece varies between 80 and 150 mm, depending on the spraying material and parameters. Because spraying is performed in the atmosphere, heated spray particles interact with air, leading to coatings with a significant amount of oxides. Hence, this method is not suitable for spraying materials prone to oxidation. However, APS has lower equipment costs and operating expenses, making it applicable in various coating protection scenarios. It is currently the most commonly used plasma arc spraying method.
Low-power (2-12 kW) internal powder feed plasma arc spraying is a variant of APS, with a spraying distance of around 40 mm. This method reasonably exploits the characteristics of the plasma jet, where the temperature and speed have maximum values near the plasma outlet. With optimized internal powder feeder positions and angles, the spraying quality can reach satisfactory levels. Specially designed guns can accomplish spraying on the inner surfaces of small-diameter pipes. Presently, internal bore plasma spraying methods can coat the inner walls of holes with diameters of approximately 40 mm.
Please note that technical details and figures may vary depending on the source and specific application context.
It is a technology developed to meet the increased demands for coating density, bond strength, and spraying efficiency for ceramic materials.
Here are some key points about this technology:
1. Principle of Operation:
HPPS utilizes a higher working voltage (up to several hundred volts) to increase power under conditions similar to the arc current of conventional atmospheric plasma spraying. Simultaneously, a larger gas flow is used to increase the jet velocity, achieving Mach numbers greater than 5. This combination results in a higher-energy spray jet, enhancing coating density and bond strength.
–High Efficiency: HPPS is characterized by high efficiency, allowing for the rapid completion of coating processes in a relatively short time.
–High Deposition Rate: The technology achieves a high deposition rate, forming thicker and more substantial coatings on the surface.
–High-Speed Spraying: By increasing the jet velocity, HPPS enables high-speed spraying, making it suitable for applications requiring fast coating.
–Large Advanced Printing Rolls: Surface coating with chromium oxide (Cr2O3) to enhance wear resistance and extend the lifespan of printing rolls.
–Turbine Over-Flow Components: Surface coating with ZrO2 ceramic layer to improve the high-temperature and wear resistance properties of turbine components.
4. Operating Costs:
Despite the advantages of high efficiency and deposition rate, HPPS is associated with relatively high operating costs. This may involve high energy consumption, maintenance costs, and the use of significant amounts of gases.
In summary, High Power Plasma Spraying technology addresses the requirements for higher coating quality and efficiency. However, its usage requires a balance between its advantages and operating costs to ensure optimal performance in specific applications.
Vacuum Plasma Spraying (VPS) or Low-Pressure Plasma Spraying (LPPS)
It is a plasma spraying method conducted in a vacuum chamber. Despite variations in nomenclature used by different researchers, most literature considers them to be the same spraying method. Here is some key information about this technology:
1. Origin and Applications:
The technology originated in the 1980s and was initially used to produce high-quality MCrAlY coatings. Due to its excellent process flexibility, high spraying quality, and efficiency, VPS/LPPS is currently the only thermal spraying method competing with Electron Beam-Physical Vapor Deposition (EB-PVD).
2. Operating Principle:
The spraying chamber is first evacuated to a pressure below 0.1 kPa, and then inert gas (such as argon) is introduced until the pressure reaches 5–40 kPa before spraying begins. To maintain stable gas pressure in the chamber, an efficient vacuum pump is used to remove continuously injected plasma. This method allows for the production of pollution-free dense coatings, and in 1990, German scientist Steffens. H.D. successfully used this method to prepare titanium coatings with extremely low oxygen content.
3. Functions and Applications:
The method features the use of reverse transfer arc technology to achieve a clean substrate surface. By reversing the transfer arc power between the substrate and the nozzle, utilizing the cathodic atomization effect of the arc, the remaining oxides on the substrate surface can be quickly vaporized. This method is applied in the preparation of thermal barrier coatings and bio-coating materials.
4. Advantages and Limitations:
The jet’s turbulence is significant at a considerable distance from the nozzle, extending the high-speed and high-temperature regions of the jet (up to 900 m/s). This benefits the heating and acceleration of sprayed particles, enhancing wetting and flattening of the coatings. However, the method requires sophisticated vacuum equipment and incurs higher operating costs. Residual powders in the spraying chamber need to be promptly removed, and compared to atmospheric plasma spraying, its convection and heat conduction performance is relatively poor.
5. Technical Parameters:
Typical spraying power ranges from 50 to 120 kW, using mixed gases such as argon+helium or argon+hydrogen. The spraying distance is generally between 250 and 300 mm. The process is characterized by minimal chemical element volatility during spraying, with coating thickness typically ranging from 100 to 400 μm and porosity below 1%.
By pre-evacuating the vacuum, the adverse effects of oxygen on coatings can be effectively controlled, making it suitable for materials highly sensitive to oxygen, such as titanium and certain refractory metals. However, this method requires advanced operational skills and higher operating costs.
As we navigate the diverse landscape of plasma arc spraying technologies, from the simplicity of APS to the high-energy realm of HPPS and the controlled environment of VPS/LPPS, it becomes evident that each method presents a unique set of advantages and challenges.
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