Plasma Spray is typically found in two variations, Conventional or APS (Air Plasma Spray) and the purer coating yielding VPS (Vacuum Plasma Spray).
Plasma is often referred to as the fourth state of matter. Plasma, like the other three states of matter (Solid, Liquid and Gas) has it's own unique properties. Just as most substrates will become solid if cooled enough, any substance will become a plasma if heated enough. In a plasma the electrons are stripped from the atoms creating a substance that resembles a gas but that conducts electricity. Plasmas occur naturally on the earth in lightning bolts, flames, electrical discharges and the Northern Lights (aurora borealis).
In all applications the Plasma is created by passing an electric current (typically 500 to 1000A) through a primary gas such as argon or nitrogen. The resultant exiting Plasma plume provides an energy heat source of around 15,000°C under high pressure which heats and propels the coating material onto the substrate. The powder feedstock is normally injected via an inert carrier gas into the arc (at a position and angle suited to the material being sprayed) externally, however there are internal injection arrangements. Plasma temperatures in the powder heating region range from about 6000 to 15,000 °C (11,000 to 27,000 °F), significantly above the melting point of any known material. Commercial plasma spray guns operate in the range of 20 to 200 kW.
VPS uses modified plasma spray torches inside a sealed chamber at pressures in the range of 13–65 Pa. The gas or mixture of gases is energised by an electrical field from DC to microwave frequencies, typically 1–500W at 50V. The treated components are usually electrically isolated. At low pressures the plasma becomes larger in diameter and length, and, through the use of convergent/divergent nozzles, has a higher gas speed. The absence of oxygen and the ability to operate with higher substrate temperatures produce denser, more adherent coatings with much lower oxide contents.
VPS is used for coating medical implants with Hydroxyapatite (HA), titanium or titanium alloys. HA is “osteoconductive” in that it promotes new bone growth along the materials surface. VPS is also used for spraying high-temperature components for fuel cells.
Almost any metallic or ceramic material can be Plasma Sprayed. Sprayed coatings exhibit tremendous bond strengths and are deposited without causing distortion to the substrate.
TAFA® / PRAXAIR®