Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys

Melhem, Amer; Henrion, G.; Czerwiec, T.; Briançon, Jean-Luc; Duchanoy, T.; Brochard, F.; Belmonte, T.

doi:10.1016/j.surfcoat.2011.01.046

Cited by 68 publications

(55 citation statements)

References 20 publications

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“…The trend of more frequent discharge around the edge remains similar before and after soft sparking, even though the overall intensity diminishes after soft sparking. Variations in coating thickness with respect to position have been well documented and compared between two PEOs, with and without soft sparking, by Melhem and co-workers [83], as illustrated in Figure 7. The thickness data along the vertical positions of central line, indicated in the inset, are plotted for R pn = 0.89 (soft sparking) and 1.57 (without).…”

Section: Coating Growth and Uniformitymentioning

confidence: 88%

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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: Coating Growth and Uniformitymentioning

confidence: 88%

“…Most of the samples are in the shape of a flat plate. The coating thickness is usually thinner at the center, and thicker around the edge of the sample [36,83]. The edge of higher thickness is attributed to more frequent discharge events around the perimeter of the sample.…”

Section: Coating Growth and Uniformitymentioning

confidence: 99%

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

Tsai

Chou

2018

Metals

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“…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).…”

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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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“…During the process, a high voltage between the anode and the sample was generated, inducing micro-discharges on the specimen surface and converting the alloy surface into a hard oxide layer. PEO layers on Mg alloys can be usually divided into two or three regions, according to the operative treatment conditions: (i) a thin barrier layer (generally hundreds of nanometres) at the interface with the base material; (ii) a compact intermediate inner layer, with a small amount of porosities and cavities; (iii) an outer region with a high number of pores and crater density [37][38][39].…”

Section: Microstructure Of the Peo-treated Alloymentioning

confidence: 99%

Fatigue Behavior of the Rare Earth Rich EV31A Mg Alloy: Influence of Plasma Electrolytic Oxidation

Ceschini¹,

Morri²,

Angelini³

et al. 2017

Preprint

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Rare earth rich magnesium alloys are used in aerospace and automotive fields because of their high specific strength and good castability. However, due to their low corrosion resistance, protective surface treatments, such as conversion coating or electroless plating are necessary, when they have to be used in humid or corrosive environments. The present study was aimed to evaluate the effect of Plasma Electrolytic Oxidation (PEO) and different surface roughness on the rotating bending fatigue of an innovative Mg alloy, with a high content of Nd (up to 3.1 wt%) and Gd (up to 1.7 wt %). Fatigue tests revealed a 15% decrease in the fatigue strength of the PEO treated alloy with respect to the bare alloy, probably due to the residual tensile stresses induced by the treatment. The effect of surface roughness on the bare alloy was, instead, negligible. The mechanisms of crack initiation were similar in the untreated and PEO treated alloy, with crack nucleation sites located in correspondence of large facets of the cleavage planes.

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Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys

Cited by 68 publications

References 20 publications

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

High-Frequency-Induced Cathodic Breakdown during Plasma Electrolytic Oxidation

Fatigue Behavior of the Rare Earth Rich EV31A Mg Alloy: Influence of Plasma Electrolytic Oxidation

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