The manufacturing technique known as plasma spraying involves applying melted or heated plasma molecules onto a clean surface to coat an object. A fine powdery substance is introduced to an extremely hot plasma flame which is then further heated and increased to a high velocity. This intensely hot substance hits the object’s surface at up to 450 metres per second, cooling quickly to produce a perfect coating.
The technical bit
The spray gun is made up of a water cooled tungsten cathode and copper anode, plasma gas (hydrogen, argon, helium, nitrogen) which then moves through the cathode and the constricted nozzle-shaped anode. The plasma is created as a result of the high voltage discharge, resulting in ionisation.
The process causes the gas to reach exceptionally high temperatures, and it then dissociates and ionises to form a plasma. When the plasma comes out of the nozzle, it is not carrying any electrical current and so is known as a neutral plasma flame.
Powder particles are injected into the hot plasma and melted. The powder is heated so forcibly that the distance of the spray can be anything from 25 to 150 mm away from the surface to be coated. This particular process takes place in normal atmospheric conditions, known as APS.
Why plasma spraying?
One of the main advantages of the plasma spray techniques is their ability to adhere to extremely high melting point materials, including refractory metals such as tungsten and other materials such as zirconia.
The disadvantages of the plasma spray process is that it is relatively expensive and difficult to do. Of all thermal coating techniques, the plasma spray covers the broadest spectrum and its effective adherence also makes it the most adaptable. It is truly a technique which needs to be mastered, respected and understood. If you wish to know more about the benefits of this thermal spraying process, you may like to visit the Thermal Spraying and Surface Engineering Association website. That organisation holds regular conferences on a wide range of thermal finishing techniques.
Of all of the thermal spraying techniques, plasma sprayed coatings tend to be deeper, harder and sharper, making them ideal for high traffic or heavy use areas. This, of course, does not include some of the wider cold spraying processes which use a different technique entirely.