Benefits of Hydrogen in a Segmented-Anode Plasma Torch in Suspension Plasma Spraying

Alice Dolmaire,Aurélien Joulia,Emilie Béchade,Pierre-Marie Geffroy,Michel Vardelle,Simon Goutier,Enni Hartikainen

doi:10.1007/s11666-020-01134-2

Abstract

Suspension plasma spraying (SPS) enables the production of various coating microstructures with unique mechanical and thermal properties. Aeronautical manufacturers have been working for fifty years to improve the thermal barrier coating (TBC) performances in gas turbines. Commercial plasma torches with a segmented anode that are characterized by stable plasma jets should enable a better control of the TBC microstructure. The addition of diatomic gases such as hydrogen in the plasma-forming gas affects the plasma jet formation and causes some instabilities. However, it enhances the thermal conductivity of the gas flow, the plasma mass enthalpy and the heat transfer to particles. This study aims to characterise and describe the coating microstructure changes of yttria-stabilised zirconia when gradually adding hydrogen with argon into the plasma gas mixture. The effect of hydrogen is weighted out due to the gas mass enthalpy, mean velocity at the nozzle exit and “hot zone” length of the plasma jet. The coating microstructures, which depend on these plasma jet parameters, will be mapped from feathery and porous to dense and cracked deposits depending on the spraying conditions.

Highlights

Hydrogen is the most used diatomic gas in spraying plasma torches
This study aims to characterise and describe the coating microstructure changes of yttria-stabilised zirconia when gradually adding hydrogen with argon into the plasma gas mixture
The fluctuation increase with H2 does not affect the coating microstructures of 8 mol.% yttriastabilised zirconia

Summary

Introduction

Hydrogen is the most used diatomic gas in spraying plasma torches It is frequently introduced as a secondary gas to enhance the net power of the torch and increase the temperature of the thermal treated material. The percentage of hydrogen in the plasma gas has been considered one of the most important parameters mastering plasma torches; the other important parameters being the arc current and primary gas flow rate. It is often used with argon and with nitrogen (Ref [2, 8]). Better deposition efficiencies (up to 75%) and denser coatings were obtained by increasing the hydrogen flow rate. An upstream restrike occurs between the arc and

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