Morphologic investigation and growth of the alumina scale on magnetron-sputtered CoCrAl NCs with and without yttrium

Nanostructured coatings exhibit superior oxidation resistance due to preferable formation of pure alpha-Al2O3 scale. In this work, the morphology and growth of the scale formed on magnetron-sputtered CoCrAl nanocoatings (NCs) with and without yttrium at 1000, 1100 and 1200 degreesC have been investigated in detail, by scanning electron microscopy observations of the time-dependent evolution of the scale surface and cross-sectional features. There were several observations: (1) intrinsic oxide ridges appeared on the surface scale on both NCs but their appearance was delayed on CoCrAlY, (2) CoCrAl coating tended to form a scale which was composed of multi-layered equiaxed crystals, especially at 1100 and 1200 degreesC, while the scale on CoCrAlY appeared double-layered and with inner columnar-like grains, (3) large interface cavities appeared after oxidation of CoCrAl at 1100 and 1200 degreesC but it was not be observed for oxidation of CoCrAlY. The results strongly suggest that the contribution of outward aluminum diffusion to the growth of the alpha-Al2O3 scale on both NCs is increased with an increase of time and temperature, and that it was reduced in the presence of yttrium. Additionally, compared to the scale on the normal commercial MCrAl (M = Fe, Ni, Co) alloys, the scale formed on the NCs is finer-grained and wrinkle-resistant. Based on the morphological investigation, a growth mechanism of the alumina scale on both NCs was proposed. (C) 2003 Published by Elsevier Ltd.