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In-situ control of microdischarge characteristics in unipolar pulsed plasma electrolytic oxidation of aluminum

Microdischarges occurring during plasma electrolytic oxidation are the main mechanism promoting oxide growth compared to classical anodization. When the dissipated energy by microdischarges during the coating process gets too large, high-intensity discharges might occur, which are detrimental to the oxide layer. In bipolar pulsed plasma electrolytic oxidation a so called 'soft-sparking' mode limits microdischarge growth. This method is not available for unipolar pulsing and for all material combinations. In this work, the authors provide a method to control the size- and intensity distributions of microdischarges by utilizing a multivariable closed-loop control. In-situ detection of microdischarge properties by CCD-camera measurements and fast image processing algorithms are deployed. The visible size of microdischarges is controlled by adjusting the duty cycle in a closed-loop feedback scheme, utilizing a PI-controller. Uncontrolled measurements are compared to controlled cases. The microdischarge sizes are controlled to a mean value of A = 510^-3 mm^2 and A = 710^-3 mm^2, respectively. Results for controlled cases show, that size and intensity distributions remain constant over the processing time of 35 minutes. Larger, high-intensity discharges can be effectively prevented. Optical emission spectra reveal, that certain spectral lines can be influenced or controlled with this method. Calculated black body radiation fits with very good agreement to measured continuum emission spectra (T = 3200 K). Variance of microdischarge size, emission intensity and continuum radiation between consecutive measurements is reduced to a large extent, promoting uniform microdischarge and oxide layer properties. A reduced variance in surface defects can be seen in SEM measurements, after coating for 35 minutes, for controlled cases. Surface defect study shows increased number density of microdischarge impact regions, while at the same time reducing pancake diameters, implying reduced microdischarge energies compared to uncontrolled cases.

FieldValue
Publisher
Authors
Release Date
2021-01-28
Resources
Identifier
5842c0c6-90b7-46e7-883a-c54b2251544f
Permanent Identifier (URI)
Is supplementing
Plasma Source Name
Plasma Source Application
Plasma Source Specification
Plasma Source Properties
Generator: Magpuls MP2-30 Current: I = 0.5 A (mean) Voltage: V = 0-1000 V Frequency: f = 2.3-5 kHz rectangular ubipolr pulsed voltage waveform galvanostatic dielectric breakdown
Plasma Source Procedure
Two electrodes are placed in an electrolytic cell containing an electrolyte. The cathode consists of stainless steel. An oxide coating is grown on an aluminum sample (anode). The plasma is ignited after the local dielectric breakdown of the oxide layer. The current induces evaporation of water and a bubble grows on the surface of the oxide. The plasma is ignited inside of the growing water vapor bubble.
Plasma Medium Name
Plasma Medium Properties
Electrolyte: 2 liter distilled water + 1 g/l KOH The pressure inside of the bubble is around 1-2 bar. The bubble size is in the region between a few microns to hundreds of microns.
Plasma Medium Procedure
First, add distilled water to the electrolytic cell. Then add 1 g/l KOH. The electrolyte is stirred by a magnetic stirrer and cooled with an external cooling circulator at 300 K.
Plasma Target Name
Plasma Target Properties
6061 aluminum cut to a size of 1 mm (thickness) x 10 mm (length) x 20 mm (height)
Plasma Target Procedure
Polishing, degreasing with acetone and isopropanol, rinsing with distilled water, and hot air drying.
Plasma Diagnostic Properties
Spectrometer: OceanOptics QE65 000 spectrometer Oscilloscope: LeCroy Waverunner 8254 Differential voltage divider: Tektronix ADP305 Current probe: LeCroy CP030 SEM: Jeol JSM-6510
Plasma Diagnostic Name
Language
English (United States)
License
Public Access Level
Public
Funding Agency
Project
Subproject
Contact Name
Patrick Hermanns
Contact Email

Data and Resources

Project Contact Name: 
Vera Bracht