Optical Insitu-Monitoring of Plasma-Spraying Processes by Means of Emission Spectroscopy

Thermal spray technologies are widely applied in different branches of industry and production. Especially plasma spraying is commonly used due to its versatile application for the production of ceramic and metallic coatings to guarantee heat isolation, wear resistance or corrosion protection properties of functional surfaces. In modern equipments, process parameters are monitored and controlled at the input-side of the system. It is implicitly assumed that stable input parameters result in stable plasma parameters and coating qualities. In practice, this assumption may lead to production losses because typical wear parts (cathode, anode nozzle etc.) cause a change in plasma parameters during their operating time. So, the monitoring of the output parameters such as spray particle temperature and velocity is necessary to achieve constant coating qualities.

The present work uses emission spectroscopy for monitoring the plasma-torch output parameters. Optical emission spectrograms are taken from different places of the plasma plume. These different states of interaction of plasma and spray powder particles are analysed concerning characteristically emitted wave lengths and their intensity. The intensities of different atomic transitions allow conclusions regarding the plasma and powder particle energies and temperatures. Investigations are done with respect to typically used metallic, ceramic and cermet feedstock powders as well as to the influence of changing plasma input parameters such as plasma gas composition, process current and powder feed rate.

The paper displays the defined conditions for the application of optical emission spectroscopy for the monitoring of plasma processes. Furthermore, the investigation results are demonstrated for the processing of Al₂O₃/TiO₂ powder. Finally, the application of optical spectrographs as insitu-process monitors and the possible extension to online control and regulation of process parameters are evaluated.