Investigation of the process stability and effectiveness using a multi-arc spraying torch with axial suspension injection

The Axial III torch is a multi-arc plasma generator equipped with three single cathode-anode units. It is widely used for suspension plasma spraying because of its axial feedstock injection. Previous work has shown that the plasma jet is not completely homogeneous, due to power fluctuations and the triple distribution of the plasma enthalpy. This could affect the stability and efficiency of the spraying process. Therefore, state-of-the-art process diagnostics were used to further characterize the process. The plasma characteristics were studied at three different compositions while spraying a 30% Y₂O₃ suspension. High-speed imaging techniques, tomography and time-resolved measurements of voltages and currents were utilized to gain insight into the arc dynamics and the plasma-suspension interaction. The resulting in-flight particle temperatures and velocities were analyzed using the Accuraspray 4.0. The specific plasma gas composition affected the temperature and transport coefficients of the plasma and thus the particle characteristics. These, in turn, had a direct effect on the deposition efficiency and the microstructure of the resulting coatings as the experiments showed. This study provides the basis for deriving measures to improve the stability and efficiency of suspension plasma spraying.