Analysis of the microstructural evolution and interface diffusion behavior of NiCoCrAlYTa coating in high temperature oxidation

Abstract

A NiCoCrAlYTa coating was synthesized successfully onto a Ni-based superalloy by arc ion plating (AIP) technology. After accomplishing the coating, a vacuum heat treatment was carried out and the coating was then subjected to isothermal oxidation in air at 1050 °C for up to 200 h. The microstructural evolution and interface diffusion behavior were studied systematically using transmission electron microscopy (TEM) and scanning transmission electron microscopy-energy dispersive X-ray microanalysis (STEM-EDX) combined with other characterization methods, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS). The results revealed that the thickness of the obtained NiCoCrAlYTa coating was 60–70 μm and the coating was primarily composed of γ-Ni and γ’-Ni3Al phases. During the oxidation process, the Al content was gradually reduced and the oxide scales eventually formed a multi-layer structure, consisting of an outer layer of mixed oxides, namely α-Al2O3, Cr2O3, and NiO, and an inner dense α-Al2O3 layer with columnar microstructure. More particularly, after 100 and 200 h of oxidation, a Ta-riched oxide and (Al0.9Cr0.1)2O3 were detected, respectively. The finding provides solid experimental evidence demonstrating that the external diffusion of cations during the growth of oxide scales plays a greater role in the oxidation of the coating compared with the internal diffusion of oxygen.