Process characteristics: high-end process
Process temperature: hot
Process speed: slow
Processable additive materials: powder
Process gases: argon, hydrogen, helium,
Plasma spraying processes are usually operated at normal atmospheres and are thus known as Atmospheric Plasma Spraying (APS). In addition, when handling oxidation-critical materials, the process can also be used in a vacuum chamber as vacuum plasma spraying (VPS) or low-pressure plasma spraying (LPPS). In vacuum plasma spraying, the vacuum chamber is first vacuumized down to approx. 10-2 mbar, or better still 10-3 mbar, to remove residual gas impurities. The pressure is then increased to a range of approx. 20 to 800 mbar by introducing an inert gas. During spraying, the chamber pressure is kept constant using a pump system.
On the one hand, this process group enables the processing of materials with an extremely high affinity for oxygen or nitrogen (refractory metals, MCrAIY alloys) while avoiding oxidation processes as far as possible and thus achieving high phase stability and purity. On the other hand, the production of very low-porosity coatings with high adhesion is possible. This results from the higher particle velocity due to the low pressure and the possibilities for component cleaning and preheating or even activation during spraying using a so-called transferred arc. For this purpose, another voltage source is connected between the plasma nozzle and the potential-free component holder. The cleaning, preheating or power booster effect is achieved by reversing the polarity accordingly.
In addition, the spraying process can be lowered to a range of less than 1mbar. Voraussetzung hierfür ist ein ausreichenddimensionierter Pumpenstand. Prerequisite for this is a sufficiently dimensioned pump stand. At about 1000A, a plasma cloud forms from the spray jet. This is then referred to as the thin-film process. However, only the finest powder particles in the range < 5 µm can be processed in this case.