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Thermal Spray Process Brief Introduction
Nov 14, 2017

High Velocity Oxy Fuel (HVOF) Thermal spraying

High Velocity Oxy Fuel spraying is a high-energy spraying method that emerged in the 1980s. Its development is the most innovative progress in the thermal spraying industry after plasma spraying. Supersonic flame spraying is the use of rocket engine principle to obtain a lower temperature (3400K), but the speed is very high spray flame (as shown). The most prominent contribution of supersonic flame spraying is to significantly increase the bond strength (see photo), density, and hardness (see photo) of the coating, while reducing the oxide content of the coating and making the coating more pure. As a result, supersonic flame sprayed coatings have outstanding wear and corrosion resistance. In addition, the residual stress in supersonic flame sprayed coatings is compressive, so their coating thickness is virtually unlimited and can reach several millimeters.

Plasma Spraying (PS)

Plasma spraying technology was developed in the 1950s as a surface treatment process. Plasma spraying equipment and process is the use of plasma arc generator (spray gun) will be introduced into the nozzle gas (commonly used Ar, N and H and other gases) heating and ionization, the formation of high-temperature high-speed plasma jet, melting and atomization of metal or non-metallic materials , And it is sprayed at a high speed onto the pre-treated surface of the workpiece to form a coating

Plasma spraying process characteristics:

1) high heat source temperature (17000K), suitable for spraying refractory materials

2) plasma jet speed, up to tens to hundreds of meters / second, so the coating and the substrate has a high bonding strength, and the coating is more dense;

3) The thermal effect on the substrate during the spraying process is small, and the finished workpiece can be sprayed on the surface;

4) Spraying process is stable, the operation is relatively simple, spraying high efficiency.

Currently, plasma spray technology is used to prepare ceramic coatings and metal transition coatings for thermal barrier coatings. There are several processes for plasma spraying classified by spray environment: atmospheric plasma spray (APS), protective gas (argon) plasma spray (ASPS), low pressure (LPPS) or vacuum plasma spray (VPS).

Atmospheric plasma spray method is the most widely used conventional spraying method, with easy operation, high production efficiency, but the coating content of impurities, and the coating density and low bonding strength. Vacuum plasma spraying method can overcome the interaction between plasma jet and ambient atmosphere, thus obtaining a pure coating which is consistent with the original spraying material composition, and the plasma jet velocity is high (240-610m / s) The coating is dense and the bonding strength is higher than that of the APS coating. In general, the MCrAlY bonding layer is prepared by the LPPS or VPS process, while the YSZ ceramic layer is prepared by the APS method.

Explosion spray (D-GUN)

Explosion spraying was invented by United Carbide Company in the 1950s. It was monopolized by the company for many years after it was introduced. It does not sell technology and equipment to foreign countries. It only spray-processes the users in its service companies. The main application is spraying of ceramic materials Aero engine maintenance. Explosion spraying the principle shown in Figure, which is the use of oxygen and combustible gas mixture, the ignition after the explosion in the gun, the use of pulsed gas explosion of energy, will be sprayed powder material heating, accelerated bombardment to the workpiece surface and form coating. As the explosion wave propagation speed up to 3000m / s, the center temperature up to 3450 , the flying speed of powder particles up to 1200m / s, so the spray coating layer dense, high bond strength with the substrate.

Arc spraying

Arc spraying technology is based on two metal wire electrode for the short circuit at the gun port arc generated as a heat source to make the metal wire melting, and then use compressed air to melt metal atomized into micro-droplet, and the micro-droplet accelerated jet To the workpiece surface, and then deposition, cooling the formation of a coating process (Principle Xiang Figure). In addition to the above process, the process has lower residual stress and slower growth with coating thickness, as well as high production efficiency, high energy efficiency, good and stable coating quality, small heat input to the substrate, Less pollution, less equipment investment, easy to operate and easy to train and coating the advantages of low cost. Therefore, the arc spraying technology is widely used in aviation and other fields and accounts for 15% of the total global thermal spraying, second only to plasma spraying and HVOF.

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