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The growth kinetics and mechanisms of thin aluminum-oxide films formed by the dry, thermal oxidation of a bare Al(431) substrate at a partial oxygen pressure of 1.33 × 10[sup -4] Pa in the temperature range of 373-773 K were studied using x-ray photoelectron spectroscopy. The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species. At low temperatures (T ≤ 573 K), an amorphous oxide film develops that attains a limiting (uniform) thickness. At high temperatures (T>573 K), growth is not impeded at a limiting thickness. Kinetic analysis established the occurrences of two different oxide-film growth regimes: an initial regime of very fast oxide-film growth and a second, much slower oxidation stage that is observed only at T > 573 K. These results could be discussed in terms of electric-field controlled, interstitial, outward transport of Al cations through a close packing of O anions in the amorphous films, and inward diffusion of O along grain boundaries in the crystalline films, respectively. For the electric-field controlled Al cation motion, a value of 2.6 eV was determined for the rate-limiting energy barrier, which is located at the metal/oxide interface. This corresponds with a Mott potential of -1.6 V. [ABSTRACT FROM AUTHOR]

MCrAlY (M = Ni, Co) coatings are commonly used to protect the underlying superalloy component in industrial gas turbines and aeroengines from oxidation attack. They also function as bond coats for thermal barrier coatings (TBC). The chemical life time of the coated component is mainly governed by the depletion of the β-phase in the bond coat as a result of simultaneously occurring surface oxidation and interdiffusion between the coating and the substrate. A new computational approach to model the microstructural evolution in MCrAlY (M = Ni, Co) coatings on Ni base superalloys was undertaken in the present study. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and wavelength dispersive X-ray spectroscopy (WDX) was employed to characterise MCrAlY coated superalloy samples after exposure at 1000 and 1100°C. Phases were identified by electron backscatter diffraction (EBSD) and correlated with SEM/EDX/WDX analyses. Modelling of the microstructural evolution was carried out considering simultaneously occurring surface oxidation and interdiffusion processes. A flux based calculation of the concentration profiles and stable phases was performed, taking into account diffusion of all elements in the γ, γ' and β-NiAl phases. Good agreement was found between the observed and computed phase distributions after specific time intervals. The computed widths of the β-depletion zones showed satisfactory agreement with the measurements. Additionally, the model was able to predict the formation of a TCP-phase and its penetration into the substrate with increasing exposure time in one of the investigated coating systems. [ABSTRACT FROM AUTHOR]