Pulsed Plasma Electrolytic Oxidation Research Articles

(Invited) Industrial Relevance of Pulsed Electrochemical Methods

Industrial demands are the driving force on any optimization and development process for electrochemical plating processes and coatings. The classical demands focused on the mechanical properties such as wear resistance and hardness as well as the stability against corrosion.The galvanic coatings were tailored for meeting these requirements, defining the electrochemical systems that are currently standard in galvanic protection of industrial parts.With the introduction of new applications and production methods over the last 10 to 15 years additional functionality has been seeked by the industry, including chemical stability, bio-compatibility, microstructured surfaces, lubrication effects, magnetic properties and the ability of shock absorbance. Coating had to be adjusted, leading to the industrial realisation of dispersion and compound coatings, alloy deposition on a wider scale and Pulse (reverse) plating. While these, from an industrial perspective, relatively new plating processes already required some major rethinking of the traditional galvanic industry, the industrial requirements got even more demanding over the last few years. Especially with the origin of new industrial production methods, the homogenous treatment and coating of e.g. complex bionical structures became relevant.In order to follow the changing demands, the transfer from the classical thermodynamically controlled direct current or constant potential based surface treatment towards a kinetically controlled pulsed surface finishing has to be executed.While pulse plating is the most popular representation of these categories of finishing processes, pulsed electrochemical processes comprise a much larger variety of methods. Among these, pulse anodization, pulsed plasma electrolytic oxidation (PEO), pulsed electropolishing, electrochemical machining (ECM) and the recently introduced Hirtisation process are industrially used.The presenation will give an overview of possibilities for electrochemical engineering of surfaces for high-end technical applications by pulsed electrochemical techniques.

Acoustic emission characteristics of micro-discharges in initial pulsed PEO process on 60% SiCp/Al composite

The initial discharge process of pulsed plasma electrolytic oxidation (PEO) on the 60% SiCp/2009 aluminum metal matrix composite (Al MMC) in silicate solution was monitored by acoustic emission (AE) technique. Parameters and correlations of AE signals on the Al MMC sample and under water were analyzed, and their generation mechanism was discussed. It was found that the peak amplitudes of AE signals and AE hits during the pulse time quickly increased with the increase of micro-discharge intensity, and the absolute energy of AE signals improved several orders of magnitude. Moreover, different from the peak amplitude, duration and rise time, the duration and count had a strong correlation. Elastic stress waves resulted from the microjet of plasma bubble collapse, the inner-surface friction inside discharge channel, the expansion-shrinkage process of plasma bubbles and micro-crack propagation during rapid solidification of melt are sources of AE signals on the Al MMC sample during the pulse time. However, the expansion-shrinkage process of plasma bubbles plays a key role in the generation of underwater AE signals. In the pause time of one pulse period, the bursting and moving of vapor bubbles result in weak AE signals. It is demonstrated that the AE technique can effectively characterize the features of micro-discharges within a pulse period.

Development of Titanium Surfaces Oxidized by Electrolytic Plasma, for Biomedical Application

Local drug delivery systems used in dental implants need to have uniform pore size distribution, adequate wettability, chemical composition, and biocompatibility. In the present work, titanium was treated by pulsed plasma electrolytic oxidation (PPEO), using 0.025 M sodium dihydrogen phosphate and 0.25 M calcium acetate as electrolytes, aiming at use in drug delivery systems. Pulse with Ton/Toff (width pulse/repetition time) of 50 μs/100 μs or 100 μs/50 μs, duty cycle 0,33, and 0.67, respectively, were used. After treatment, Ca/P ratio, wettability, crystalline phase, pore size and distribution were determined. The average pore size ranged from 1.5 μm to 2.3 μm according to the increase of energy supplied to the system. Pore distributions with lower dispersion were verified for the Ton/Toff condition of 50 μs /100 μs, using a current density of 30 mA/dm2. On the other hand, the 100 μs/50 μs conditions produce larger pores, but with greater dispersion. In general, conditions with lower currents (30 mA/dm2 and 38 mA/dm2) and Ton/Toff ratio = 50/100 were the most appropriate for use in drug delivery systems, due to their size and distribution of uniform pores, greater hydrophilicity, and Ca/P ratio close to desirable (1.67) was obtained.

Micro-second range pulsed DC plasma electrolytic oxidation on Ti and Nb

The presented research reports on the formation of oxide coatings on pure Ti and Ni substrates by micro-second pulsed plasma electrolytic oxidation in sodium phosphate electrolyte. The formation process is monitored by real-time imaging and optical emission spectroscopy. At the beginning of the process, micro-discharges appear randomly over the sample's surface and migrate towards the edges with prolonged processing. Emission lines originating from Ti substrate are well resolved, while appearance of high-resolution emission lines originating from Nb substrate in the visible range is reported for the first time. Subsequent characterization of formed oxide coatings on Nb shows that its roughness increases with PEO processing time, contrasting the case of Ti. It is observed that longer processing time of Ti substrate results in the anatase-to-rutile phase transformation, while pseudohexagonal TT-Nb2O5 phase is present throughout the processing of Nb substrate. Photoluminescent and photocatalytic properties of obtained coatings are dependent on their morphology and phase composition i.e., on processing time.

Pulsed Plasma Electrolytic Oxidation as Strategy to Produce Optical Selective Surfaces

Porous alumina (PA) films have been investigated for selective optical surface application. In this work, we use pulsed plasma electrolytic oxidation (PPEO) to produce porous surfaces in aluminum. During the PPEO processing, image of plasma on the samples and active species present in micro-discharges were documented using CCD camera and standard optical emission spectroscopy (OES) technique. Porosity was controlled by varying the pulsewidth ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm {on}}$ </tex-math></inline-formula> ) and treatment time, keeping the pulse repetition time ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm {off}}$ </tex-math></inline-formula> ) constant. The optical reflectance and thermal efficiency of the samples were investigated for samples with different microstructure, crystalline phase and chemical composition. It is shown that coatings produced by shorter pulses ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm {on}}$ </tex-math></inline-formula> ) with smaller time of treatment resulted in smallest pore size and more uniform distribution. These results positively influenced the reflectance and thermal efficiency of the surfaces.

An insight into the evolution of corrosion resistant coatings on titanium during bipolar plasma electrolytic oxidation in sulfuric acid

This work describes recent progress related to bipolar pulsed plasma electrolytic oxidation (PEO) applied to grade 2 titanium in 0.5 M H2SO4. Corrosion resistance of oxide coatings was characterized by anodic polarization, immersion tests in acidic environment and electrochemical impedance spectroscopy (EIS). Scanning electron microscope (SEM), X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GD-OES) and visual observation were employed to describe oxide microstructure and composition. The influence of frequency and amount of cathodic polarization were rationalized in terms of modification of the electrical double layer (edl) formed at the oxide-electrolyte interface. Depending on its characteristics, different plasma regimes are established, leading to the growth of oxides optimized for corrosion resistance in different environments. It is here shown how the selection of high frequency (1000 Hz) and a negative-to-positive charge ratio (R) of 0.78 favors the establishment of a strong plasma regime, promoting the growth of a coating optimized for corrosion resistance in concentrated H2SO4. It is also reported that low frequency (20 Hz) and cathodic polarization (R = 0.07) should be used to enhance localized corrosion resistance.

Investigation of the frequency dependent spatio-temporal dynamics and controllability of microdischarges in unipolar pulsed plasma electrolytic oxidation

The unipolar pulsed-plasma electrolytic oxidation (PEO) of aluminum has been replaced by bipolar pulsed methods that use a so-called ‘soft-sparking’mode. This method results in an effective reduction of intense microdischarges, which are detrimental to the oxide layer. In a previous publication, we developed an in-situ multivariable microdischarge control scheme using unipolar pulsing. Using this method, it is possible to restrict the mean microdischarge size to well-defined limits, while at the same time influencing the mean microdischarge energy, number density or spectral emission behaviour. This method operates well inside a frequency range of . Although this method shows highly desirable plasma controlproperties, the mechanisms defining this frequency-dependent controllability are unclear. The aim of this study is to visualize the spatio-temporal behavior of microdischarges in higher frequency ranges.First, a wavelet transform was performed to estimate the temporal evolution of microdischarge lifetimes. Ceramic coatings were then deposited on aluminum alloy substrates in an aqueous solution using unipolar pulsed galvanostatic PEO. The aluminum samples were coated for 30 min at frequencies of , and . High-speed imaging was carried out utilizing four synchronized intensified charge-coupled device (ICCD) cameras, each with a 500 ns exposure time. At , the microdischarges were still able to follow the electrical pulses. In this regime, the process can be divided into two stages, an initial charging of the substrate surface without plasma emission and a subsequent slower evolution of microdischarges. Equivalent circuit model descriptions are given for both processes. At , microdischarges were not able to follow the pulse frequency, as the lifetimes and risetimes of the microdischarge characteristics were longer than the pulse length. Reignition at the same spatial location, clustering and permanent ignition through pulse periods were observed.

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 and , 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 (). 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.

Influence of Applied Frequency on Microstructural and Electrochemical Characteristics of Ceramic Coating Produced on Al-Mg Alloy by Pulsed Plasma Electrolytic Oxidation

Influence of Applied Frequency on Microstructural and Electrochemical Characteristics of Ceramic Coating Produced on Al-Mg Alloy by Pulsed Plasma Electrolytic Oxidation J.H. Lee, S.J. Lee and S.J. Kimc,∗ Korea Testing & Research Institute, Key Industry Division, Jongga-ro 15, Ulsan, 44412, Korea Kunsan National University, Department of Power System Engineering, Daehak-ro 558, Kunsan-si, Jeonbuk, Korea Division of Marine Engineering, Mokpo National Maritime University, Haeyangdaehak-Ro 91 Mokpo-si, Jeonnam, 530-729 Korea

Pulsed plasma electrolytic oxidation processes for aeronautical applications and their technical application

A range of aluminium alloys have been treated by plasma electrolytic oxidation in different alkaline electrolytes containing silicate, aluminate, vanadate and other additives, under pulsed current conditions and the dependence of oxide thickness growth rates on variation in electrolyte and operating conditions measured. The oxide morphology was examined by SEM, and coating phase and crystal size analysis was carried out by XRD. Coating hardness varied with anodising current type and electrolyte composition, and the incorporation of dispersed nano silica particles.