Epitaxial growth and oxidation behavior of an overlay coating on a Ni-base single-crystal superalloy by laser cladding

An overlay coating material was deposited on a single crystal superalloy SRR99 by laser cladding. The microstructure and oxidation behavior of this coating was investigated through scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that although the composition of the coating was chosen based on the γ’ composition in René N5 superalloy, the primary solidification phase of this coating during laser cladding was γ-Ni. Furthermore, under the laser cladding condition, fine parallel dendrites grew epitaxially in the coating from the substrate, indicating the single crystal structure of the substrate was reproduced. When the single crystal MCrAlY coating was oxidized at 1000℃ both θ-Al₂O₃ and α-Al₂O₃ formed during initial oxidation process. As the oxidation time proceeded, the presence of θ-Al₂O₃ facilitated the formation of NiAl₂O₄ spinel oxide. Once the spinel was observed, it flourished and induced some porosity in the scale. When the scale thickness increased to 6–7 μm, large area spallation of the scale began.