In this research, a metallic layer (NiCrAlY) was applied on the low-carbon Ni-based superalloy IN-738LC, as substrate. Subsequently, a gradient ceramic layer comprising mullite and yttria stabilized zirconia (YSZ) was coated on the NiCrAlY metallic layer. Finally, an outer ceramic layer, consisting of 100% mullite was sprayed as top coat. All coating layers were applied using the air plasma spraying (APS) method. The gradient coatings were subjected to isothermal oxidation at 1000 ℃ for 50 and 100 h to investigate their high-temperature properties. The findings revealed that the thickness of the thermally grown oxide (TGO) layer increases with the increasing oxidation time. The thicknesses of the TGO layers were 5 and 7 ± 0.3 μm at 50 and 100 h, respectively. According to energy dispersive spectroscopy (EDS) analysis, the oxides formed in the TGO layer mostly consist of aluminum oxide along with a few amount of spinel oxides. This indicates the superior resistance of the gradient coating to the oxygen penetration and forming harmful oxides. The hard-ness values of the gradient ceramic layer, comprising mullite and YSZ, were 880 and 995Hv before and after oxidation, respectively. However, the hardness values for the monolithic mullite and YSZ ceramic layers were lower. The porosity content of each layer was also examined. In the gradient layer, the porosity level was 16% which was significantly lower compared to those of the monolithic YSZ and mullite layers. This low porosity content in the gradient layer was a key factor contributing to its excellent performance under high-temperature oxidation, demonstrating a marked superiority over conventional hybrid coatings.

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