In the atmosphere, as soon as the plasma flame leaves the nozzle, a large amount of air is drawn in from the surroundings. When the spray distance is 100mm, the air intake can account for more than 90% of the plasma. In the atmospheric plasma spraying process, the metal powder is severely oxidized. In addition, some toxic substances (such as beryllium and beryllium oxide) cannot be sprayed into the atmosphere. In order to solve the above problems, low-pressure plasma spraying has been proposed. Vacuum plasma spraying, also known as low-pressure plasma spraying, is plasma spraying (spraying technology in a controlled environment) in a closed space with a low vacuum below atmospheric pressure.
Controllable environmental spraying technology takes our understanding of plasma spraying one step forward. Operate the spray gun in the spray booth to keep the environment under complete control. The characteristics of the coating produced in this way are impossible to produce in a standard atmospheric environment. The environment can be changed in the range of close to vacuum (as low as 50 mPa) and increased pressure (up to 4 Pa). Choosing to spray in the spray booth can prevent contamination of the paint and/or substrate, or because the spray material needs to react with certain substances added.
The principle of controllable atmosphere plasma spraying is to place the plasma spray gun in a sealed cabin and operate it by a manipulator. Pumping the cabin to a vacuum state is vacuum plasma spraying (VPS). When the cabin is in a low pressure state, it becomes low pressure vacuum plasma spraying (LPPS). The atmosphere of the engine room can be an inert atmosphere or other protective atmosphere. Due to the low pressure of the environment or the controllable atmosphere, the plasma flame flow becomes longer, the particles are heated more fully, the oxidation is reduced, and the coating quality is significantly improved. It can be used to prepare deposited diamond films and superconductor oxide coatings.
The coating produced by this process has many advantages. The coating has good compactness, strong adhesion, will not be polluted, and there is no oxide in the metal coating. The ceramic and other non-metallic coatings sprayed in a spray booth filled with a non-reactive atmosphere have high purity. Moreover, the application of high melting point metal coatings such as tungsten is also very successful. Since the distance between the spray gun and the workpiece is not as important as in atmospheric spraying conditions, component control becomes simple. Due to the uniform plasma plume profile, the "focal spot diameter" can be large, so the spraying process time can be greatly shortened. In addition, the absence of atmospheric cooling of the coating particles means that the coating curing process is relatively slow. Atmospheric pressure plasma coating has almost no interlayer "flakes", and the crystal structure of the coating is close to that of the casting material.
In a low vacuum environment, since the non-transferred plasma arc jet becomes thicker and elongated, it has already contacted the surface of the workpiece to form a conductive channel, so the transfer arc can be superimposed on it. The transfer arc is used to sputter the surface of the workpiece, remove the surface oxide layer and pollution, and can heat the workpiece to a higher temperature, so that the coating is combined on the smooth surface and diffused at the interface, thereby improving the bonding strength. The thickness of the coating can also be unlimited. When spraying in a closed room, the pollution of noise and dust to the environment is also solved accordingly.
Compared with atmospheric plasma spraying, low-vacuum environment spraying has the following remarkable features:
1. The speed and temperature of the plasma jet are significantly higher than that of atmospheric pressure plasma spraying. The lower the pressure, the higher the jet velocity and temperature.
2. The residence time of the powder in the high temperature zone of the plasma jet is increased, the heating is more uniform, and the flying speed is faster.
3. The preheating temperature of the substrate surface can be greatly increased; the substrate can also be sputtered and cleaned with a reverse transfer arc to remove oxides and dirt, thereby improving the bonding between the coating and the substrate.
4. The powder and substrate surface completely avoid oxidation, and various active metal material coatings can be prepared.
5. Due to the above reasons, the bonding strength of the coating is greatly improved, the porosity is greatly reduced, the residual stress of the coating is reduced, and the quality of the coating is significantly improved.
6. Vacuum plasma spraying equipment is complicated and expensive, making it very difficult to promote and apply.