Microstructure assessment at high temperature in NiCoCrAlY overlay coating obtained by laser metal deposition

Abstract

Laser metal deposition (LMD) and laser cladding (LC) could be innovative methods to the current thermal spray techniques for obtaining dense and high-quality bond coats with nickel and cobalt based superalloys in thermal barrier coatings (TBCs) systems. The microstructural evolution of the coatings when is subjected to high temperatures allows us to evaluate their oxidation behavior and integrity in the time, and is especially important in the development of bond coats for new TBCs systems. In this work, the microstructural evolution of a dense NiCoCrAlY overlay coating obtained by coaxial laser cladding and subjected to isothermal oxidation tests at 1100°C up to 200h was evaluated. The initial coating microstructure is composed of a β-NiAl phase within the γ-Ni matrix phase, and some Y inclusions, confirmed by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). At high temperature, the thermally-grown oxides (TGO) protect the underlying coating and substrate from oxidation, modifying the initial microstructure of the coating. The formation, morphology and evolution of oxides α-Al2O3, Y2O3, YAlO3 and (Ni,Co)(AlCr2)2O4-type spinel oxides on the cross section of the oxidized coating on different oxidation stages were evaluated using focused ion beam (FIB) and field emission scanning electron microscopy (FESEM), and indexed by XRD and TEM. After oxidation tests, no cracks or spallation on oxidized sample for NiCoCrAlY coating were observed, and the formation of Y-Al oxides in TGO scale are strongly dependent on the initial Y distribution of as-built condition in the coating.