In a DC plasma spray torch, the dynamic behaviour of the arc attachment at the anode nozzle results in fluctuations of arc voltage and the resulting plasma jet instabilities affect the treatment of the particles injected in the plasma flow, and thus, the coating quality. However it is not clear if the experimentally-observed fluctuations of particle temperatures are a major phenomena and if their frequencies are always in unison with those of voltage. In this study, two on-line techniques are used to investigate respectively the time-variation of particle temperatures and their correlations with voltage variations; the first technique makes it possible to analyse plasma voltage instabilities and the second one to investigate the instabilities of particle temperatures. Both allow determining the frequencies and amplitude variations of voltage and particle temperatures. The experiments are carried out with two plasma torches (F4-type and 3MB-type) using respectively argon-hydrogen or nitrogen–hydrogen mixtures as plasma-forming gases. A good correlation between arc voltage and particle temperature fluctuations is observed when the plasma torch is operating with argon-hydrogen while that's not the case when the torch is operating with nitrogen-hydrogen.

In plasma spray process, ceramic coatings can be sprayed by using either argon-hydrogen or nitrogen-hydrogen plasma gas mixtures. Starting from a given particle size distribution the question is what are the spray parameters allowing achieving similar coatings with these types of plasmas? The problem is made more complex because a torch working with Ar-H 2 is different from that using N 2 -H 2 as plasma forming gas. It is thus necessary to compare the gas mixture properties, the torches working conditions (mean voltage and thermal efficiency for given current), the arc column diameter relatively to the nozzle internal diameter and the spray parameters, the arc root fluctuations, the powder injection, the particles mean temperatures and velocities as well as their fluctuations linked to those of arc root, the splat formation, the coating porosity and deposition efficiency. This comparison has been achieved for ZrO 2 -Y 2 O 3 (7 vol%) powder with a size distribution between 5 and 25 µm and using the same plasma torch for the two plasma gas mixtures: a 3MB torch with a cylindrical anode nozzle and 5.5 mm in internal diameter.