Plasma Species and Coating Compositions in Aluminum Treated by PEO Using Shot Square Pulse

Abstract

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 (Na 2 SiO 3 > 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 (Na 2 SiO 3 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-Al 2 O 3 but, superficially, the predominant phase is cubic γ-Al 2 O 3 . 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.

Unipolar plasma electrolytic oxidation: Waveform optimisation for corrosion resistance of commercially pure titanium

“…The composition of the region of interest (at%; Figure 9e) was as follows: O (48.8%), Ti (25.2%), S (15.6%; Figure 9c) and Ca (10.4%; Figure 9c). These results suggest TiO 2 as the main constituent, whereas S and Ca are mainly concentrated along the border line of the discharge site, generally called “nodule,” [ 21,40,41 ] favouring the hypothesis of a strong ejection of material towards the outer region adjacent to the discharge event, confirming the proposed mechanism. Thus, it is plausible to infer the occurrence of a plasma regime transition from surface spark to a mix of surface sparks and dielectric discharges requiring a higher amount of current to sustain the momentaneous oxide breakdown.…”

Comparison of 2D and 3D Plasma Electrolytic Oxidation (PEO)-Based Coating Porosity Data Obtained by X-ray Tomography Rendering and a Classical Metallographic Approach

“…The higher heat generated by the discharges also forms a thicker layer [34]. Moreover, the relatively high melting temperature of In [30], the authors described the surface morphology of P pancakes, where a pancake structure is more typical for strong ure 3, one can see that these pancakes are more representative o other hand, a combination nodule and pancake structure is mor alloy. These nodules have many micro-pores and, according to and pores full of electrolytes in the initial stages of the PEO for parable energy input during PEO processing, the differences i the coatings on the Al and Mg substrates can most likely be as melting temperatures (TM) of the formed phases on the respecti Al2O3, 2852 °C for MgO, 1710 °C for SiO2, and 1890 °C for M forsterite, a main PEO phase on the Mg substrates, has a low comparison with MgO and ɤ-Al2O3.…”

“…A typical PEO morphology can be observed, where there are randomly distributed pores and cracks in the coating surface, which indicate a non-uniform defect distribution in the coating volume. In [30], the authors described the surface morphology of PEO layers as nodules and pancakes, where a pancake structure is more typical for strong micro-discharges. In Figure 3, one can see that these pancakes are more representative of Mg-based alloys.…”

“…-Al 2 O 3 , 2852 • C for MgO, 1710 • C for SiO 2 , and 1890 • C for Mg 2 SiO 4 ). One can see that forsterite, a main PEO phase on the Mg substrates, has a lower melting temperature in comparison with MgO and In [30], the authors described the surface morphology of PEO layers as nodules and pancakes, where a pancake structure is more typical for strong micro-discharges. In Figure 3, one can see that these pancakes are more representative of Mg-based alloys.…”

The Plasma Electrolytic Oxidation of Aluminum Using Microsecond-Range DC Pulsing

“…This can be achieved by varying electrical parameters such as the voltage and current density, as well as by setting the duration of pulses (t on ) and the time between two consecutive pulses (t off ), which allows the system to return to its equilibrium. By controlling the duration of pulses and intercepting pauses, one can control the duration of micro-discharges, and through this, control the morphology, chemical and phase composition of obtained PEO coatings as well [4][5][6].…”