Effect of time-dependent DC plasma flow on particle treatment.

The D.C. plasma spraying process leads to coatings with high-level properties. The control of these properties implies to well overcome the process and in particular the interactions between the particles and plasma plume and between particles and substrate. The plasma jet exhibits high fluctuations outside the torch due to the arc root movements on the anode wall inside the torch. These movements result in a fluctuating plasma jet outside the torch and air engulfment. In this paper, a simple numerical model based on gas heating by JOULE effect in the anode is proposed to reproduce the flow transient behaviour. The predictions of the model (centreline velocities and temperatures of the gas) are compared with experimental values. Also, zirconia particles are injected in the transient simulated flow. Depending on the plasma plume stretching outside the torch and particle size, the particle trajectories, velocities and temperatures present high variations, leading to particle thermocinetic states at impact quite different from one particle to the other.