Microstructure and 3D microtomographic characterization of porosity of MAO surface layers formed on aluminium and 2214-T6 alloy

A fine characterization of the microstructure of the MAO coatings formed on aluminium (purity 99,999%) and its alloy 2214-T6 was observed using field emission gun scanning electron microscopy (FEG-SEM) with energy dispersive spectrometry (EDS) and X-ray microtomography (XMT). In addition, the influence of current frequency (pure aluminium) and the precipitates of substrate were investigated (case of 2214-T6 alloy).The MAO surface layers formed on aluminium and its alloy 2214-T6 can be divided into two parts: the outer layer with high porosity (so-call porous layer) and the inner layer with low porosity (so-call dense layer). However, the porosity of the inner layer increases toward the MAO layer/substrate interface. It is found that, both the thickness and the porosity increase when the current frequency decreases. The porosity of the MAO coating can be attributed to discharges formation in the vicinity of the MAO coating/substrate interface.In the particular case of 2214-T6 aluminium alloy (3.9% Cu), with coarse non-valve metal rich precipitates aligned perpendicular to the surface of the work electrode; the transformation of precipitates under discharge effect can provoke the formation of big channels, which go through the MAO surface layer. This last one induces local formation of a layer rich electrolyte species toward the substrate.