Mullite/YSZ hybrid thermal barrier coatings plasma sprayed on IN-738LC: Microstructure, phase analysis and oxidation resistance

Abstract

In the current study, the low carbon Ni-based superalloy Inconel-738 (IN-738LC) was used as the substrate and a metallic bond coat (NiCrAlY) was deposited on the substrate by air plasma spray (APS) method. Subsequently, the layer was coated by a ceramic interlayer Yttria-stabilized zirconia (YSZ) by APS. Finally, as the last ceramic layer, a hybrid ceramic top layer containing mullite and YSZ was plasma deposited on the YSZ interlayer in various compositions such as: 25 wt% mullite/75 wt% YSZ, 50 wt% mullite/50 wt% YSZ, 75 wt% mullite/25 wt% YSZ, and 100 wt% mullite. In order to investigate the oxidation resistance of the produced TBCs, they were heated at 1000 °C for 50 and 100 h. The results showed that the presence of mullite phase is advantageous to hybrid coatings, so that in the 50 wt% mullite/50 wt% YSZ hybrid coating, the percentage of destructive spinel oxides forming the thermally grown oxide (TGO) layer reduced compared to the 25 wt% mullite/75 wt% YSZ coating. Moreover, aluminum oxide is the dominant oxide in the 50 wt% mullite/50 wt% YSZ hybrid coating. However, the further increase in the mullite percentage had no noticeable effect on the reduction of spinel oxides. In addition, the presence of mullite alongside YSZ led to an increase in the hardness of the 25 wt% mullite/75 wt% YSZ hybrid coating (845 HV) compared to the conventional coating including a YSZ layer (680 HV) or a mullite layer (655 HV). The X-ray diffraction (XRD) analysis revealed that the presence of mullite beside YSZ in the coating increases the stability of the coating after oxidation test so that, in the XRD pattern, monoclinic phase was not detected in the hybrid coatings. In general, 50 wt% mullite/50 wt% YSZ hybrid coating provided the best results compared to other coatings. Moreover, with increasing thickness of the hybrid ceramic layer containing 50 wt% mullite to 350 μm, the thickness of the TGO layer decreased to approximately 7 μm. Additionally, it was shown that due to the decrease of chromium and nickel elements in the TGO layer, the level of the spinel oxides in the thick coatings decreases. This led to an increase in the stability of the TGO layer and an extension of the thermal system's lifespan.