Influence of voltage fluctuations related to plasma torch working conditions on zirconia particle thermal treatment

The microstructure of plasma sprayed coatings is strongly linked to particle trajectories depending among other parameters on arc root fluctuations. The latters depend on the torch anode-nozzle shape and internal diameter, the plasma forming gas composition and flow rate, and the anode erosion. Experiments were performed with yttria partially stabilized zirconia (8 wt% yttria) particles with a size distribution between 5 to 25 mu m and either a PTF4 or a 3MB GE Sulzer-Metco torch. The PTF4 torch was fed with an Ar-H-2 mixture and an arc current of 600 A, while the 3MB torch was fed with N-2-H-2 mixture with an arc current of 500 A. Particle mean temperatures and mean velocities were measured to evaluate the influence of voltage fluctuations on the treatment. This study shows that the torch working parameters, except the arc current, have nearly the same effects on the arc voltage fluctuations for the both torches. The 3MB torch voltage fluctuations are more important than those for a PTF4 torch, but they are less affected by the variations of spray parameters. Furthermore, the fluctuations of particle temperatures and velocities versus the voltage fluctuations are not the same.