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.

Suspension plasma spraying has shown its capacity to deposit finely structured coatings with a wide range of microstructures including columnar microstructures that are generally sought in thermal barrier coating applications for gas turbines. However, some challenges are still to be taken up before the application of the technology at an industrial scale. One deals with the deposition of a uniform and reliable coating on a complex shape substrate. This work offers an experimental observation of submicron particle streams close to the substrate in order to understand mechanisms of deposition. Effects of the substrate shape and tilting were investigated on particle velocity, directions and coating growth. It was shown that particle velocities and directions are disrupted by the substrate presence up to 10mm upstream. When the substrate is a cylinder or in a tilted orientation to the plasma jet, particles kinetic behaviour is less affected. Finally, submicron particle velocity vectors orientation near impact greatly shape the coating morphology. When impacting with a 40° angle of incidence, columns appeared on beads, contrary to submicron particle streams impacting orthogonally to the substrate surface.

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