Evolution of Oxide Scales on Aluminide Coatings under Isothermal and Cyclic Conditions

The oxide scale evolution with high temperature on CVD aluminide coatings deposited on a Directionally Solidified (DS) Ni-base superalloy is studied in this work. High temperature oxidation was carried out at 1100°C in air for 240 h under isothermal conditions and for 10 cycles (1 cycle = 24 h at 1100°C). The morphological and microstructural characterisation of the coatings has been performed using optical and electron microscopy as well as X-ray diffraction. Contrary to most of the results published in the literature, the rumpling phenomenon appears on the isothermally oxidised specimens whereas spallation, nodule formation and wrinkling of the oxide scale occur on the cyclically oxidised samples. The results are discussed in terms of the β-NiAl to γ’-Ni3Al phase transformation, the likely associated volume changes and of the growth stresses at high temperature.