Plasma Spray Process

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In the plasma spray coating process, a plasma plume is created by striking an arc between an anode and a cathode with gas flowing through. The arc rips off electrons from the primary gas flowing through making it highly unstable; its natural tendency is to replace the lost electrons and thus come back to its stable state. This distance can vary quite a bit depending upon the voltage, amperage as well as the properties of the gas used. Generally this is about 4 inches. A coating material in the form of a powder is fed into this plasma plume; the material gets highly energized and reaches a plastic state and reaches the substrate where it bonds to form the coating. This is the succint description of the process.
As one can deduce, if the pressure of the surrounding "atmosphere" is lowered, then the length of the plasma plume increases; this is the case in low-pressure-plasma-spray coating process, where the plasma is generated under reduced pressure conditions. The length of the plasma plume in such cases can be as high as 14 inches. If metallic coating materials are used, then they are prone to oxidation by the surrounding air. The level of oxidation can be controlled either by coating under low-pressure ( sometimes referred to as 'vacuum plasma' ) or under a shroud of inert gas such as argon, referred to as 'shrouded plasma'.
The primary gas used for standard air plasma sprayed coatings is generally nitrogen or agon. In most cases, a secondary gas is introduced into the stream to enhance the energy level. This gas, usually referred to as the secondary gas, is either hydrogen or helium. Hence the gas flow is either a nitrogen-hydrogen or argon-helium combination.
Proper introduction of the powder into the plasma plume is critical to coating quality. Improper introduction of the powder will result in unmelted particles in the coating aggregate. The powder is usually fed by a powder feeder with a carrier gas at a predetermined feed rate that is usually closely controlled. Too high a feedrate will also cause bad coating quality since the particles will not have sufficient energy.
There are several variations to gun/substrate movement -- the common methods being (a) the substrate to be coated is either held stationary and the plasma gun is moved relative to it either manually or attached to a mechansim such as a robot arm; OR (b) the substrate is installed on to a turntable and the plasma gun is moved in an up-down manner. In all instances, proper speeds for the moving equipment is important to ensure uniform high quality deposition. For example, turn-table rpm, traverse speeds (inches/minute), etc are critical. In more critical applications, advanced controls to turn off the powder feeder and the gun if the turntable stops, etc, can be instituted.
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