Effect of High Temperature Calcination on the Phase Composition of Nanostructured and Conventional Zirconia Thermal Barrier Coatings

In this study, NiCoCrAlTaY bond coat on Ni substrate was deposited by low pressure plasma spray and nanostructured and conventional ZrO₂+7-8wt.%Y₂O₃ top coat was deposited by air plasma spray. Agglomerated nano-powder calcinated at 1250^oC was used as feedstock powder. Nanostructured and conventional TBCs samples were calcinated from 1050^oC to 1250^oC for 2-20 h. Experimental results indicate that monoclinic/tetragonal phases in the agglomerated nano-powder were transformed into cubic phase after calcination. The cubic phase content increases with increasing calcination temperature. High temperature calcination can make the yttria segregated at grain boundaries dissolve in zirconia. Different from the phase constituent of the as-sprayed conventional TBC which consists of diffusionless transformed tetragonal, the as-sprayed nanostrcutured TBC consists of cubic phase containing high yttria. No phase transformation in both nanostructured and conventional TBCs after calcination is found.