Plasma species and coating compositions in aluminum treated by PEO using shot square pulse

Up to now, plasma electrolytic oxidation (PEO) has been either produced direct current or ultra-low frequency (< 1 kHz) pulsed discharge using high concentration alkaline-silicate electrolytic (Na₂ SiO₃ > 8 g.L^-1 with presence of KOH). In order to contribute to these studies, the effect of current pulse width and time duration was investigated using diluted silicate electrolytic (Na₂ SiO₃ 2 g.L^-1) and high pulse frequencies (> 1 kHz). The PEO process was performed on pure aluminum to try to explain how the phases composing the coatings are formed and distributed over the treatment time. For this, was made in situ monitoring using optical emission spectroscopy (OES) coupled with CCD camera. The crystalline phases evolution in the sample surface was investigated using grazing incidence X-ray diffraction (GIXRD). Regarding the evolution of ceramic phases, it is possible to verify that, internally, the predominant phase is rhombohedral α-Al₂O₃ but, superficially, the predominant phase is cubic γ-Al₂O₃ . It was verified the presence of Si on the borders of the pores or in proximity to cracks, especially in the treatments with higher pulse width. SEM analysis shows a reduction of the superficial porosity and an increase in coating thickness with pulse width and treatment time. (author)