Optimization of the injection with a twin-fluid atomizer for suspension plasma spray process using three non-intrusive diagnostic tools

Suspension injection is a crucial point for suspension plasma spray, an emerging coating process. It must satisfy several requirements such as stability, reproducibility, high yield of suspension particles penetrating the plasma core. Indeed, the particles should follow a proper trajectory inside the plasma in order to be well treated and accelerated. Thus, the quality of the injection has a direct effect on the coating microstructure, therefore on the properties. This study focuses on the three-step characterization of a twin-fluid atomizer, without and within the plasma. While varying gas or suspension flow rates, the spray was firstly observed thanks to a shadowgraphy system. This system uses a short-pulsed diode laser in a back illumination configuration and a robust image processing software, allowing the detection of suspension droplets, even within a high emissive plasma. Secondly, droplet size measurements were performed via laser diffraction. Thirdly, droplet velocities were measured using a particle image velocimetry system. The combination of the results obtained by these techniques helped us to understand how gas and suspension flow rates impact on droplets size and velocity distributions and finally how one could select the best injection conditions. The injector characterization was completed by coating manufacturing and microstructure analysis.