Dr Steven Matthews is a senior lecturer in the School of Engineering and Advanced Technology at Massey University in Auckland. In this video, he demonstrates the basic components of a plasma gun and then explains how the gun operates. Given the extremely high temperatures developed within the plasma jet, Steven provides some startling statistics relevant to its operation.
Well this is our plasma gun that we’ll be using today, and there’s three key components within the plasma gun. There’s a cathode that looks something like this, and an anode nozzle like this and a gas injection system. And when the gun is operating, the cathode is installed within the anode here, and it strikes an arc between the end of the cathode and the interior surface of the anode. The gases that are introduced are introduced behind the cathode. They flow over it and through the gap between the cathode and the anode here and out the front of the anode nozzle. And as they pass through the high-energy arc, they get converted into a plasma, and that plasma expands and accelerates out the front of the nozzle here and out the front of the plasma gun.
Now, when the plasma is leaving the front of the gun, it’s got a temperature in the region of up to or exceeding 15,000 kelvin and velocities in excess of 900 metres per second. But it’s mixing with all the air as it is penetrating out into the atmosphere, and that mixing tends to slow down and cool down the plasma. So at a typical spray distance of 100 millimetres from the front of the gun, the plasma temperature may be only a few thousand kelvin and a couple of 100 metres per second.
The material that we want to spray as a coating is typically used in a powder form. In this gun here, the powder is introduced internally into the front of the gun here, fed from a powder feeder via a carrier gas along through this port here, and it gets injected into the front of the gun. And the powder particles penetrate into the high-temperature high-velocity plasma, they’re heated up to their melting point, and they’re also accelerated to the surface.
And in that process, we now have liquid droplets which hit the surface, they impact and spread, and they cool at cooling rates of about a million degrees Celsius per second. In doing that, they shrink onto the surface, and by having lots of different powder particles impacting and building up one on top of the other, we get to form the coating.
Dr Steven Matthews, School of Engineering and Advanced Technology, Massey University, Auckland
Holster Engineering Ltd, Tokoroa
Plasma spray gun schematic, courtesy of Rudolfensis, Creative Commons Licence 3.0