Plasma sprayed functionally graded thermal barrier coatings

Functionally graded thermal barrier coatings of the system yttria stabilized zirconia/NiCoCrAlY were fabricated through plasma spraying using pre-alloyed composite powders as feedstock. Composite powders with different compositions (75% NiCoCrAlY:25% YSZ; 50% NiCoCrAly:50% YSZ and 25% NiCoCrAlY:75% YSZ) were prepared by mechanical alloying and plasma powder spheroidization, and are subsequently sprayed successively in a single plasma torch to form the functionally graded coating. This method has ensured a homogeneous coating with satisfactory inter-layer formation, and a flexibility for depositing different numbers of inter-layers by changing the composition of the composite powders. In the present study, five-layered functionally graded coatings have been plasma sprayed. These coatings demonstrated attractive properties compared to the conventional duplex thermal barrier coatings. This is because of the unique microstructure formed in the functionally graded coating with this new approach which also results in reduced residual thermal stresses in the coatings.