Plasma flow optimization for 7YSZ quasi-columnar coating by plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition is an emerging coating technique, developed to produce vapor deposited thermal barrier coatings with large area coverage. The degree of feedstock evaporation and resulting coating microstructures are dependent upon plasma spray process parameters like plasma flow, torch power, plasma gas composition, carrier gas flow and powder feed rate. In the present work, influence of total plasma flow rate on plasma-feedstock interaction inside plasma torch nozzle and corresponding deposition mechanism has been investigated. Theoretical calculations show that maximum energy (11276 kJ/kg) is transferred to injected powder feedstock at lowest plasma flow of 85 L/min (25Ar/60He). With the increase of total flow rate from 85 L/min to 100 L/min by incrementing argon (Ar) 5 L/min, the enthalpy transferred to representative 1 micron spherical YSZ particle reduced by 5%, 23% and 36% respectively. The drop in energy is due to decrease of plasma temperature and increase in momentum gained by the feedstock material with the increase of total plasma flow. 7~8 weight % agglomerated YSZ powder was used to manufacture the coating microstructures at total plasma flow of 90 L/min (30Ar/60He), 95 L/min (35Ar/60He) and 100 L/min (40Ar/60He). The morphology of produced coatings has shown good correspondence with the theoretical prediction. Quasi-columnar microstructure was observed with the former plasma flow confirming that majority deposition is from vapor phase. For the latter case, dense-layered microstructure was obtained mainly from molten droplets. Based on micro-structural observations, a phase deposition model for quasi-columnar coatings was proposed.