How Hydrogen Admixture Changes Plasma Jet Characteristics in Spray Processes at Low Pressure

In plasma spraying, hydrogen is used as a secondary working gas because it enhances the heat transfer to the feedstock particles. Furthermore, hydrogen permits to increase the voltage and therefore the plasma power since the plasma arc adjusts itself to a size which minimizes the heat flux from the core to the cooled anode wall. This arc constriction is controlled mainly by the thermal conductivity of the plasma gas and thus by the amount of hydrogen admixture. As a consequence, the arc resistance which is inversely proportional to the effective arc cross section is increased.

An essential characteristic of some novel plasma spray processes like low or very low pressure plasma spraying (LPPS, VLPPS) and plasma spray-physical vapor deposition (PS-PVD) is the small plasma density in the jet implying a disposition to thermal non-equilibrium. Under such conditions, hydrogen affects fundamental mechanisms like diffusion and recombination while this is not relevant at equilibrium under atmospheric conditions.

In this work, optical emission spectroscopy was applied to investigate such phenomena in low pressure plasmas. The results show that even small admixtures of hydrogen can affect concentration and temperature distributions in the plasma jet considerably. Possible effects on the plasma particle interaction are discussed.