A Study of the Influence of the Surrounding Gas on the Plasma Jet and Coating Quality During Plasma Spraying

Coating quality is affected by arc and plume instabilities during plasma spraying. In closed chamber plasma spraying, gradual drift is one of the intermediate instabilities, which is mainly due to the electrode erosion. This work focuses on the source of the gradual drift of the plasma jet and the influence on coating quality. The ambient state inside the chamber was controlled by a ventilation system and a vacuum system. The variation in the plasma jet was observed by a particle flux image device based on a CCD camera. The optical spectrum of the plasma plume was measured and analyzed through an optical spectrometer. The results indicated that the addition of hydrogen to plasma gas induced the change in the plasma jet length and width with changing rates depending on the chamber state and the ventilation power. With poor ventilation, the intensity of H_α emission was found to become gradually stronger while H_β and H_γ were found to become weaker. On closing the chamber and retaining enough ventilation power, it was observed that the ambient gas slowly turned red. Simultaneously, the coating weight and thickness were slightly decreased meanwhile the porosity ratio was obviously increased. The red ambient gas has been proved to be able to acidify the city water with pH value decreased from 7 to 1–3. Without hydrogen, the plasma jet was found to be stable without reddening and variation, but the plasma enthalpy was unfortunately low.