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

Abstract

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 (PFI) 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 remaining 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.

Experimental study on the characteristics of V‐type arc plasma torch with four electrodes

“…Due to its excellent characteristics, such as high temperature (>3000 K), increased enthalpy (∼1 GJ/kmol), and high energy density (∼10 GW/m 3 ), arc plasma is widely used in several fields including metal cutting, [1,2] material preparation, [3][4][5] powder spheroidization, [6,7] and thermal spraying. [8][9][10] To meet the application requirements, the design of large-scale, high-power, and high-enthalpy arc plasma torches has always been one of the most challenging tasks. So far, many scholars have developed various plasma torch configurations through protracted and unremitting efforts.…”

Characteristics of DC arcs in a multi-arc generator and their application in the spheroidization of SiO2 *

“…Arc thermal plasmas have drawn extensive attention for their unique features, such as high temperature, high-energy density and high activity. They have been widely used in the industrial field, such as metal cutting, [1,2] preparation of spherical powder, [3][4][5] thermal plasma spray, [6,7] solid hazardous wastes treatment, [8,9] and nanoparticles synthesis. [10,11] However, it should be noted that some of the characteristics of arcs may be positive for one specific application, while those may be negative for other applications.…”

“…For instance, due to self-induced magnetic compression and thermal-pinch effect, [12][13][14] the energy of an arc is mainly concentrated in the arc column, and its radial dimension is very small, which is very suitable for metal cutting. [1,2] However, this characteristic is disadvantageous to material processing, such as powder spheroidization [3][4][5] and thermal spray, [6,7] since it is difficult for particles to fully enter the arc column owing to its small size. Therefore, it is necessary to regulate arc plasmas to obtain the best performance according to specific applications, in order to meet the needs of actual industrial applications.…”