Abstract
Abstract Up to now, plasma electrolytic oxidation (PEO) has been either produced direct current or ultra-low frequency ( 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 (Na2SiO3 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.
Highlights
Plasma electrolytic oxidation (PEO) is an electrochemical plasma-assisted process used to produce an oxide ceramic on metal surface[1]
Recent studies[5,6,7,8,9,10,11,12] have utilized the optical emission spectroscopy technique (OES) to investigate the complex phenomena involved in this process
It was observed that the beginning of the curvature on the graphic is the moment in which the dielectric breakdown of the film is reached, and stage II begins
Summary
Plasma electrolytic oxidation (PEO) is an electrochemical plasma-assisted process used to produce an oxide ceramic on metal surface[1]. High voltages cause the dielectric breakdown of the double layer due to excessive production of electrons and ions These particles are accelerated towards the anode producing an avalanche of electrons that lead to the formation of several micro-discharges on the electrode/electrolyte interface. Besides that, studying the phase distribution of the PEO treatment on aluminum 6061 alloys with pulsed current, Dehnavi et al.[13] found that an increase of the Ton pulse in low frequencies caused an increase of the γ → α-Al2O3 phase transformation, while a low Ton could result in more porous and thicker coatings Based on this knowledge, plasma electrolytic oxidation (PEO) was performed using shorter square current pulses (frequency > 1KHz) than traditionally used so far. The influence of the pulse’s width and treatment time over the formation of the coatings was investigated
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