Effect of spray parameters on porosity and lifetime of suspension plasma sprayed thermal barrier coatings

Columnar microstructures produced by Suspension Plasma Spraying (SPS) are of significant commercial interest as SPS offers a cheaper alternative to the conventionally used electron beam – physical vapour deposition process. Additionally, Thermal Barrier Coatings (TBCs) produced by SPS have shown lower thermal conductivity as compared to conventional systems due to their very fine porous microstructure. However, improvements in reliability and life expectancy of SPS coatings are still desired for their wide scale commercialisation.

The objective of this work was to study the effect of changing topcoat spray parameters on microstructure and lifetime in SPS TBC systems. Commercial NiCoCrAlY bondcoat powder was deposited by high velocity air fuel spraying on Hastelloy-X substrates using the same spray parameters for all samples. SPS was used to deposit yttria stabilised zirconia topcoats by changing the spray parameters such as spray gas mixture resulting in different power and enthalpy, suspension feed rate, surface velocity, and spray distance. Porosity was measured by image analysis and thermal conductivity was measured by laser flash technique. The lifetime was examined by thermal cyclic fatigue and thermal shock testing. The failure mechanism in each case has been discussed and the optimal topcoat microstructure for high TBC lifetime has been proposed.