High-Frequency-Induced Cathodic Breakdown during Plasma Electrolytic Oxidation

Abstract

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...

A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals

“…It is certainly the case that, at least under most conditions, discharges predominantly occur during the anodic half-cycle, although current does flow in both half-cycles. However, discharges have been observed in the cathodic half-cycle under some conditions, particularly when the electrolyte is strongly alkaline [172,[196][197][198] and for relatively thick coatings. It is also worth noting that PEO coatings have been produced under conditions in which cathodic discharges predominate.…”

Sequential run of the PEO process with various pulsed bipolar current waveforms

“…the presence of an outer porous sublayer filled with electrolyte and an inner dense sublayer. More recently, mechanisms involved in PEO treatments conducted under the specific "soft" sparking mode have been the subject of deeper investigations with a particular focus on the influence of the cathodic half-period in the ignition of MDs and coating growth [19,[33][34][35]. Indeed, if all studies agree with the fact the cathodic polarization is needed to prevent large and detrimental MDs, few works explain why this happens.…”

“…Considering a bipolar current pulse with 50% duty cycle, this corresponds to a frequency of ~2.5 kHz. [33]. At the same time, Rogov et al suggested that the cathodic polarization is responsible of a reduction in the potential barrier through the EDL due to a local acidification of the oxideelectrolyte interface that in turn reinforces the potential drop and the electric field at the metal / oxide interface during the subsequent anodic polarization.…”

Study of the Effect of the Field Emission on the Breakdown Voltage Characteristic of Direct Current Nitrogen Microdischarges

“…In direct current microdischarges formed between the microscale interelectrode separations, field emission can act as a significant source of primary electrons [10][11][12]. The generation of microplasmas usually involves breakdown of the gas, where electrons accelerated in a high electric field trigger avalanche ionization.…”