The role of torch instabilities in the suspension plasma spraying process

In spite of the fact that plasma spraying is a commonly used technique for coating elaboration, it remains important to follow in time the characteristics of the arc jet delivered by the plasma torch. Arc voltage and heat loss measurement could be used, together with the control parameters such as arc current and plasma gas flow rate, to investigate the global behavior of the torch in association with a simple analytical model. It is shown that the specific enthalpy and the isentropic coefficient of the plasma gas have a strong influence on the jet velocity, this latter being also significantly modified by the drift of the torch performance. This work is completed by the analysis of the plasma jet instabilities that are responsible for the discrepancy in particle thermal histories. The rear part of the torch is involved in self sustained oscillations, so that the torch shows the characteristics of a Helmholtz resonator, giving rise to periodic variations of the torch voltage, higher in amplitude than the commonly admitted restrike mode. It is also evoked, that the generated acoustic waves are interacting with the solvent vaporization.