Influence Of The Fe22Cr5Al Substrate Surface Finish, On The Morphological Texture Of Al2O3 Growth, Studied By SEM-EDXS And XRD
Alumina may display a complex polymorphism depending on the production method. Starting from Al-solution, their amorphous or crystalline structures as well as its phase are strongly determined by the precursor, temperature, pressure, pH, etc. However, starting from Al containing solids, there is a lack of information about the procedure to obtain a determined phase and concerning the influence of its surface finish on the morphological texture of the alumina growth. In this work, Fe22Cr5Al plates were subjected to mirror polishing and mechanical erosion. The fractal dimension determined from optical images was 2.084 for mirror polished samples and 2.472 for the eroded ones; these values give a comparative element for the starting surface finishing, because a higher fractal dimension value means an increasing surface roughness. Thereafter, both samples were oxidized at 900 °C, 24 h in air. By SEM, the polished-oxidized samples show a passivation layer formed by agglomeration of relative small particles, of around 400 nm; whereas the eroded-oxidized samples show whiskers with 100 nm thickness. The fractal dimension determined from SEM images was 2.511 for the polished-oxidized samples and 2.582 for the eroded-oxidized ones. EDXS analysis detects Al and O enrichment at the surface and XRD identifies the θ and α phases of alumina, for both kind of surface finish. Results demonstrate that the difference found in the oxide morphology is directly related to the surface finish before oxidation. This in turn is attributed to the difference in the surface defects density acting as nucleation centers for alumina whiskers.