Soft Sparking Research Articles

Probe the micro arc softening phenomenon with pulse transient analysis in plasma electrolytic oxidation

Soft sparking can be utilized to dismiss the intrinsic porosity problem of plasma electrolytic oxidation (PEO) coatings on aluminum alloy, since this type of surface treatment generates a dense inner layer. Researchers usually recognize soft sparking through the anodic voltage drop, along with the decline in light and acoustic emissions. Analysis of V-I transients in the anodic pulse of bipolar DC current reveals the decline in RC time constant is a forerunner, which reflects the transition in micro arc state prior to the conventional definition. In studying the negative current dependence and the current frequency dependence of soft sparking, the results suggest the time constant may serve as an auxiliary indicator, monitoring growth activity variations and further assuring the dense inner layer. The role of time constant is exemplified in the soft sparking experiments at 500 Hz and duty 40%. Microstructure analysis reveals three relatively dense stacks separated by two layers of pores and loose grains. The porous layer is attributed to the micro arc state of less intense microdischarges, marked as a substantial drop in the R2C2 value. Matching between stratified layers and R2C2 rises-and-falls is verified in two electrolytic solutions with and without chromia inclusions. Thus, we suggest time constant as a probing tool of microdischarges softening to help analyze the events prior to voltage drop.

The role of cathodic current in plasma electrolytic oxidation of aluminium: current density ‘scanning waves’ on complex-shape substrates

This paper focuses on the factors that influence the surface distribution of the current density during plasma electrolytic oxidation of 2024 Al alloy under alternating polarisation. It was found that employing the combined current mode, including relatively long (100–2000 ms) pulse trains, improved coating uniformity even when the electrolyser provided a severe nonuniform primary electric field. An experimental investigation employing a sectioned sample showed that the nonuniform distribution of the primary electric field could be compensated by the changes in the coating properties induced by previous cathodic polarisation. Temporary changes in the secondary distribution of current density across the sample surface (attributed to the coating properties) caused dynamic redistribution of the anodic current density during the following AC pulse train, resulting in the so-called ‘scanning wave’ effect, i.e. migration of the maximum current density along the sample surface. Factorial experimental design, finite element modelling and analysis of the transient current–voltage curves were applied. A mechanistic explanation underlying the considered phenomenon has been suggested. In addition, the increase in plasma electrolytic oxidation efficiency under soft sparking conditions is considered as ‘electrocatalysis’ of anodic oxidation by previous cathodic treatment.

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

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.

The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum: Phenomenological Concepts of the “Soft Sparking” Mode

A comprehensive analysis of experimental data relating to so-called "soft sparking" mode of plasma electrolytic oxidation (PEO) has been undertaken. The transition to the soft sparking mode is accompanied by a number of characteristic effects, such as a decrease in anodic voltage, acoustic and light emission, increase in hysteresis in transient current-voltage curves, improved uniformity of the discharge distribution on the surface, disappearance of atomic lines, and development of continuous radiation in the optical emission spectra. An explanation of the main features of PEO process operated under soft sparking conditions is proposed assuming the existence of a specific narrow region in the coating thickness, where the main anodic voltage drops. Because of high electric field in this "active zone", both anodic oxidation of the metal substrate and high-energy processes may take place. According to this assertion, the soft sparking mode of PEO is caused by cathodic polarization (a) eliminating the potential barrier at the oxide-electrolyte interface due to local acidification and (b) increasing electric field at the metal-oxide interface during subsequent anodic half-cycle due to narrowing of low-conductive part within the active zone. Based on this consideration, it is possible to account for the main characteristic phenomena accompanying the PEO process on aluminum under alternating polarization.

The effects of anion deposition and negative pulse on the behaviours of plasma electrolytic oxidation (PEO)—A systematic study of the PEO of a Zirlo alloy in aluminate electrolytes

Plasma electrolytic oxidation (PEO) of a Zirlo alloy has been carried out under pulsed bipolar and pulsed unipolar regimes in aluminate electrolytes with the aim to promote a better understanding of the coating formation mechanisms and the role of negative pulse. Real time imaging, optical emission spectroscopy (OES), gas collection, scanning electron microscopy (SEM) assisted with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) have been used to characterize the PEO processes and the resultant coatings. The results show that the anion deposition process has significantly affected the composition, microstructure of the coatings and the plasma discharge behaviour. For the coatings formed with less incorporation from the electrolyte anions, the predominant discharge type is the energetic penetrating discharges, usually forming the pancake structure and big internal pores. However, the formation of coatings by the fast deposition of anion species with the assistance of “soft sparking” or “sintering arcs” occurs when PEO was carried out in concentrated electrolytes. The occurrence of such discharges is correlated with the fact that less energy is required for the anion deposition process. The hydrogen evolution associated with the negative pulse during PEO can promoted the transportation of electrolyte species to the electrode surface, favoring the formation of homogenous coatings with the incorporation of more electrolyte species and a modified discharge types.

The role of cathodic current in PEO of aluminum: Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra

In this paper, the influence of cationic electrolytes composition on electrical and optical responses of plasma electrolytic oxidation process of A1050 aluminum alloy under alternating polarization is considered. The electrolytes consist of 0.1M boric acid with addition of one of the following hydroxides: LiOH, NaOH, KOH, tetraethylammonium hydroxide, Ca(OH)2 up to pH value 9.2. Coatings microstructure, elemental and phase compositions were studied by SEM, EDS and XRD. It was shown that the hysteresis of anodic current-voltage curve (specific feature of “Soft sparking” PEO) was clear observed in the presence of sodium and potassium cations. It was found that composition of microdischarges plasma is also affected by the nature of the cations. It was shown that there are a number of reciprocal processes, which take place under anodic and cathodic polarization.

PEO of rheocast A356 Al alloy: energy efficiency and corrosion properties

PEO coatings were developed under voltage‐controlled mode on a rheocast A356 aluminum alloy without and with pretreatment by conventional anodizing in sulphuric acid. Microstructure and composition of uncoated and coated materials were studied using SEM, EDS and XRD. The anodic precursor oxide layer preserves the microstructure of the alloy because of the presence of secondary phases having a different behavior relative to the matrix during conventional anodizing. After PEO treatment, the coatings consisted of a mixture of α‐Al2O3, γ‐Al2O3, SiO2 and mullite (3Al2O3·2SiO2). The effect of pre‐anodizing on energy efficiency was investigated revealing considerable energy savings with the precursor porous films, because of the faster establishment of the so‐called ‘soft sparking’ regime that promotes faster coating growth rate. The electrochemical corrosion properties were evaluated for uncoated and coated materials using electrochemical techniques (potentiodynamic curves and electrochemical impedance spectroscopy). PEO coated materials showed remarkably reduced susceptibility to corrosion. Copyright © 2015 John Wiley & Sons, Ltd.

Preparation and Characterization of Amorphous Soft Magnetic FeSiB Powders and Spark Plasma Sintered Compacts

Wet mechanical alloying (MA) were used to prepare amorphous soft magnetic Fe75Si20B5 (at.%) powders starting from elemental powders of Fe, Si and B. The structural, morphological and magnetic properties of the powders were investigated. It was found that wet MA leads to the amorphisation of the alloy after 40 hours of wet milling using benzene (C6H6) as process control agent (PCA). The influence of the wet MA process on the saturation magnetization of the powders was investigated. Amorphous powder of Fe75Si20B5 (at.%) obtained by wet MA route was used to prepare compacts by spark plasma sintering (SPS). The chosen sintering temperature was 800, 850 and 900 oC. Toroidal samples of Fe75Si20B5 (at.%) were investigated in DC and AC magnetization regime and their magnetic properties were correlated with sintering parameters, compacts density and phases evolution during sintering.

Growth Characteristics of the Oxide Layer on Aluminium in the Process of Plasma Electrolytic Oxidation

Abstract During the formation of the oxide layer by plasma electrolytic oxidation (PEO), the electrochemical processes are accompanied by the plasma micro-discharges, occurring uniformly over the coated electrode. Application of an alternating current with strictly controlled electrical parameters can affect the character of the discharges, and consequently the properties of the obtained layers. When the cathodic current density exceeds anodic, at some point, a sudden change in the appearance of micro-discharges and a decrease in the intensity of the acoustic emission can be observed - in literature this effect is called “soft sparking”. In the present work, the evolution of the electrical properties of the layers at various stages of their formation has been characterized, using electrochemical impedance spectroscopy. The study showed a significant decrease in charge transfer resistance and increase in capacitance of the oxide layer after reaching the ”soft sparking”. This indicates a significant change in the structure of the oxide layer, in the barrier and main part, which is additionally confirmed by measuring the breakdown voltage.