Microstructure and phase composition of microarc oxidation surface layers formed on aluminium and its alloys 2214-T6 and 7050-T74

The influence of substrate on the microstructure and phase composition of surface layers synthesised by microarc oxidation (MAO) on aluminium and its alloys 2214-T6 and 7050-T74 is studied using scanning (SEM) and transmission electron microscopy (TEM) as well as cross-sectional X-ray diffraction. MAO layers are composed of three layers and are mainly made of gamma-Al2O3 and alpha-Al2O3 phases. The proportion of each phase depends on the substrate. The external porous layer is mainly composed of the gamma-Al2O3 phase. The internal dense layer can present two aspects according to the percentage of the alpha-Al2O3 phase. The so-called granular aspect indicates a high proportion of “dendrite” defect which results from discharge formation and implies a high percentage of the alpha-Al2O3 phase. The so-called columnar aspect indicates a high proportion of “small channels” associated with a very weak percentage in the alpha-Al2O3 phase. In the latter, it is believed that a Zn alloying element can inhibit the growth of alpha-Al2O3. During the MAO process, discharges likely occur in the vicinity of the MAO layer/substrate interface, probably in the spherical porosities that result from oxygen generated in the thin layer localised at the interface.