Spectroscopic study of plasma during electrolytic oxidation of magnesium- and aluminium-alloy

Jovović, Jovica; Stojadinović, Stevan; Šišović, N. M.; Konjević, N.

doi:10.1016/j.jqsrt.2012.06.008

Cited by 82 publications

(50 citation statements)

References 26 publications

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“…Researchers have employed OES in monitoring the electrolytic plasma on Al alloys. The following spectral lines are commonly analyzed; 309 nm (Al I), 396 nm (Al I), 656 nm (H α ) 486 nm (H β ), 589 nm (Na I), 777 nm (O I) [17,[53][54][55]. Under the assumption of local thermodynamic equilibrium, the intensity ratio (J ma /J nb ) of two spectral lines of the same atom can be used to extract the (plasma) electron temperature T e of a specific location, using Equation (1).…”

Section: Plasma State In Electron Temperaturementioning

confidence: 99%

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

Tsai

Chou

2018

Metals

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Abstract:A dense inner layer is highly valued among the surface coatings created through plasma electrolytic oxidation (PEO) treatment, because the PEO coating has been troubled by inherent porosity since its conception. To produce the favored structure, a proven technique is to prompt a soft sparking transition, which involves a sudden decrease in light and acoustic emissions, and a drop in anodic voltage under controlled current mode. Typically these phenomena occur in an electrolyte of sodium silicate and potassium hydroxide, when an Al-based sample is oxidized with an AC or DC (alternating or direct current) pulse current preset with the cathodic current exceeding the anodic counterpart. The dense inner layer feature is pronounced if a sufficient amount of oxide has been amassed on the surface before the transition begins. Tremendous efforts have been devoted to understand soft sparking at the metal-oxide-electrolyte interface. Studies on aluminum alloys reveal that the dense inner layer requires plasma softening to avoid discharge damages while maintaining a sufficient growth rate, a porous top layer to retain heat for sintering the amassed oxide, and proper timing to initiate the transition and end the surface processing after transition. Despite our understanding, efforts to replicate this structural feature in Mg-and Ti-based alloys have not been very successful. The soft sparking phenomena can be reproduced, but the acquired structures are inferior to those on aluminum alloys. An analogous quality of the dense inner layer is only achieved on Mg-and Ti-based alloys with aluminate anion in the electrolytic solution and a suitable cathodic current. These facts point out that the current soft sparking knowledge on Mg-and Ti-based alloys is insufficient. The superior inner layer on the two alloys still relies on rectification and densification of aluminum oxide.

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Section: Plasma State In Electron Temperaturementioning

confidence: 99%

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

Tsai

Chou

2018

Metals

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“…The majority of investigations have dealt with the study of micro-discharges by means of high-speed video imaging [13] and optical emission spectroscopy [27][28][29] treat actually a large number of events 10 5 per dm 2 per second). To overcome this difficulty, the Gordon Laboratory has developed a setup that makes it possible to study one individual discharge at a time [10,19,30] and to simultaneously record its current and image its shape [26,31].…”

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confidence: 99%

High-Frequency-Induced Cathodic Breakdown during Plasma Electrolytic Oxidation

et al. 2017

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The present communication shows the possibility of observing micro-discharges under cathodic polarization during Plasma Electrolytic Oxidation (PEO) at high frequency. Cathodic micro-discharges can ignite beyond a threshold frequency found close to 2 kHz. The presence (resp. absence) of an Electrical Double Layer (EDL) was put forward to explain how the applied voltage can be screened, which therefore prevents (resp. promotes) the ignition of a discharge. Interestingly, in the conditions of the present study, the EDL requires between 175 and 260 µs to form. This situates the expected threshold frequency between 1.92 and 2.86 kHz, which is in good agreement with the value obtained experimentally.Plasma Electrolytic Oxidation (PEO) is at a pivotal moment of its development. If this process has undeniable assets for surface treatments of light alloys (high growth rate, high ultimate coating thickness, ecofriendliness, wear and corrosion resistance, etc.), the high energy consumption and the lack of understanding of the fundamental processes underlying the breakdown and the oxidation mechanisms still limit a larger scale development.Improving the energetic efficiency of PEO is a major stake and several strategies can then be followed: dual treatments[1], addition of nanoparticles [2,3], electrolyte composition [4,5] and electric regimes [5][6][7][8]. Here, we focus on the influence of the electric regime as it has a direct and strong influence on both the space and time properties of micro-discharges (MD), which affect the coating growth. Interestingly, the best properties and the highest growth rates of coatings are obtained while using bipolar current waveform [5,9] even though MDs are only observed during the anodic (i.e. positive) half-period [10][11][12] in these conditions.To explain this observation, one should keep in mind that the growth of PEO coatings relies on a balance between the constructive effect of MDs (oxidation and coating growth) and their destructive effect (large craters and porosities). Several studies conducted either by high speed video imaging [5,6,[11][12][13][14][15][16][17][18] or by direct electrical measurements [10,19] have investigated this balance. They made it possible to correlate space and time parameters of MDs (number, life-time, size) and the properties of as-grown coatings (thickness, porosity). In short, large and long-lived discharges promote the growth of high temperature phases but create large porosities (several tens of µm), high roughness at limited growth rates (strong destructive effect of MDs). On the other hand, too small and short-lived MDs will create more homogeneous coating but with lower amount of high-temperature phases and also at limited growth rates (weak constructive effect).Nevertheless, all these studies support the same conclusion: adjusting the cathodic half period probably opens up the possibility to tune the behaviour of anodic MDs. Then, combining high-growth rates, low levels of large-scale porosity (bigger than 10 µm) and a significant proportio...

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“…The phenomenon in the whole process seem to be a symmetry inverse process compared with the constant current density set in the MAO process [20]. The current of the sample without ZrO 2 particles in electrolyte is obviously larger than that prepared with ZrO 2 particle during the MAO process, and holds at a higher current.…”

Section: Coating Growth Kinetics and Evolution Of The Coating Morphologymentioning

confidence: 85%