The basic principle of plasma spraying is as follows: ignition between an anode and a cathode produces a high frequency arc. Gases (such as helium, hydrogen, nitrogen or their mixture) flow through the electrodes and are ionized, resulting in several centimeter-long plasma flames. The central temperature of plasma flame is as high as 16 000 K. The spraying material is injected into plasma flame by powder form outside the nozzle and melted. Finally, the compressed gas is injected into the substrate at high speed to form a coating.