Microstructure and oxidation behavior of conventional and pseudo graded NiCrAlY/YSZ thermal barrier coatings produced by supersonic air plasma spraying process

In this paper, the microstructure and oxidation behavior of conventional and pseudo graded NiCrAlY/yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) is reported. Both conventional and graded TBCs were produced by supersonic air plasma spraying process. In the pseudo graded TBCs, no interface between bond-coat, graded region and top-coat was observed. The ceramic (YSZ) top-coat in both conventional and graded TBCs was found to exhibit a single phase tetragonal-prime structure. Isothermal oxidation results confirmed the formation of thermally grown oxides (TGO) layer in both conventional and graded TBCs. In graded TBCs, the dispersed metallic phase was also found to be oxidized in the graded region. For oxidation time ≥ 50 h, the TGO thickness in the conventional TBCs was found to be lower than that of the graded TBCs. Further, in graded TBCs, the vertical and parallel cracks were formed in the YSZ coating during oxidation for 5 h. The cracks were enlarged during oxidation at 1200 °C for 200 h. After oxidation (at 1200 °C for 200 h), the parallel cracks in the graded TBC penetrated into the graded/layered region. The evolution of cracks in the graded TBCs led to a constant residual stress (0.62 ± 0.2 GPa) in the TGO. However, owing to the presence of few cracks in the conventional TBCs, the residual stress in the TGO increased with oxidation time. After oxidation at 1200 °C for 200 h, the conventional TBCs have higher residual stress (2.1 ± 0.3 GPa) in the TGO as compared to the graded TBCs.