Plasma Spraying

Abstract

As previously stated, both the gas-sheath stabilized plasma jet and the vortex-stabilized plasma jet are used in the plasma spray process. The gases that have been most commonly used in the generation of a plasma arc are hydrogen, helium, nitrogen, argon and air. Gas selection is based primarily on gas energy, reactivity and cost. The relationship between plasma temperature and gas energy content is shown in Fig. 6 [9]. From Fig. 6, it can be seen that the energy content of nitrogen and hydrogen is considerably higher than that for argon or helium, due to the dissociation reactions in the nitrogen and hydrogen prior to ionization. The low cost and high internal energy of nitrogen makes it the most commonly used arc gas. Generally about 10% hydrogen is mixed with the nitrogen to increase the heat content and improve the heat transfer characteristics of the arc gas. Hydrogen also acts as a reducing agent in the plasma. Reactions of the nitrogen gas with the material being sprayed (nitride formation) somewhat limits the use of nitrogen gas. If a completely inert atmosphere is required, argon is usually selected as the arc gas. The use of air is very limited because of the excessive electrode oxidation. Excessive electrode wear, encountered when pure nitrogen is the arc gas, has been reduced by winding a coil of copper tubing concentrically about the nozzle axis in series with the arc power. This introduces a spinning component to the electric field, which maintains a fully annular arc, resulting in uniform arc impingement around the front nozzle. The copper coil does not alter the spray characteristics but greatly increases nozzle life [10].