Particle Temperature Fluctuations in Plasma Spraying

In direct current (d.c.) plasma spray torches, the dynamic behavior of the arc attachment at the anode nozzle results in arc voltage fluctuations and correspondingly power fluctuations. The resulting plasma jet instabilities affect the treatment (heat and momentum) of 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 online techniques are used to investigate, respectively, the time variation of particle temperatures and its correlation with voltage variations; the first technique makes it possible to analyze plasma voltage instabilities and the second one to investigate those of particle temperatures. Both allow determining the frequencies and amplitude variations of voltage and particle temperature. Experiments were carried out with three plasma torches (F4-type and two 3MB-type) using, respectively, argon-hydrogen (F4-type and 3MB) and nitrogen-hydrogen (3MB) mixtures (all with restrike mode for the voltage fluctuations) as plasma-forming gases. A good correlation between arc voltage and particle temperature fluctuations is observed when the plasma torch is operated with argon-hydrogen mixtures and high mass flow rate. However, it is not the case when the torch is operated with nitrogen-hydrogen mixtures even if the amplitudes of voltage fluctuations are two to three times higher than those obtained with Ar-H2.