Plasma characteristics and coating microstructures of yttria stabilized zirconia during plasma spray physical vapor deposition

Plasma spray physical vapor deposition (PS-PVD) is a promising technique used to manufacture strain tolerant and non-line-of-sight thermal barrier coatings (TBC). It bridges the gap between conventional plasma spraying and electron beam physical vapor deposition (EB-PVD). In this study, the effect of different compositions of Ar/He plasma on particle characteristics and coating morphologies of 7~8 wt. % yttria partially stabilized zirconia (YSZ) have been investigated. Plasma characteristics at different gas compositions were calculated by minimizing Gibbs free energy under chemical equilibrium conditions. Momentum and heat transfer between plasma and YSZ powder have been computed by assuming free molecular flow conditions at low pressure between 18 ~ 33 kPa inside the torch nozzle. Computational results showed that drag force and enthalpy transferred to the YSZ powder increases with the increase of Ar/He ratio at constant net plasma flow. Experimental results showed that uniform columnar microstructure deposited out of vapor phase can be manufactured by using Ar:He ratio 1:1 and confirms the theoretical predictions. Although further increase in Ar/He gas ratio enhances both the momentum and enthalpy transferred to feedstock particle but the coating morphologies are not uniform and homogeneous. This is due to low gas viscosity and rutted broadening of plasma jet along its length to the substrate.