What types of plasma spraying equipment are available?

Frequently Asked Questions

A plasma spraying gun usually comprises a copper anode, and a tungsten cathode. An electric arc, discharged across these electrodes, heats a working gas, typically Ar or a mixture of Ar+He, Ar+H₂, Ar+N₂, to produce a high temperature plasma (ionised gas). The temperature of the plasma can reach about 15000K and the expanding plasma jet leaving the nozzle exit can have velocities up to about 800ms^-1 or above. A coating material in powder form is introduced in to the plasma, usually in a radial direction at the end of the nozzle.

Generally, plasma spraying techniques can be classified according to the environment where the process is performed. These include:

• Atmospheric Plasma Spraying (APS) - spraying is performed in an air atmosphere.
• Vacuum Plasma Spraying (VPS) or Low Pressure Plasma Spraying (LPPS) - spraying is undertaken in a chamber at low pressure or in vacuum. It is used to meet specifications that require oxide-free coatings, for example MCrAlY coatings used with thermal barrier coating systems. These coatings usually exhibit densities and tensile bond strengths better than those achieved with the APS technique.
• Controlled Atmosphere Plasma Spraying (CAPS) or Inert Plasma Spraying (IPS) - plasma spraying is undertaken in a chamber containing an inert gas, usually Ar, He or N₂.
• Shrouded Plasma Spray (SPS) - the plasma jet is protected from contact with the surrounding atmosphere by a shielded nozzle attached to the anode of the plasma torch. This system is used to deposit metallic alloys with low oxide levels.

Traditional plasma torches inject powder through the side of the plasma stream and only particles within the optimum size range are fully entrained in the plasma and deposited on to the target.

More recently some plasma torches have become available that inject powder into the centre of the plasma stream. This promotes high deposition rates, high deposition efficiencies and less oxygen entrainment.

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