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Forming high‐temperature α‐Al2O3 phase under soft sparking is an intriguing phenomenon in plasma electrolytic oxidation (PEO) of Al alloys, which contradicts the low energy input of the process. In this study, α‐Al2O3 doped with black CuO is formed beneath an amorphous white outer layer on Al–Cu–Li alloy by PEO in a dilute silicate electrolyte under soft sparking. In comparison, reddish coatings with dominating γ‐Al2O3 are formed under the conventional plasma discharges, although blackish inner layer with α‐Al2O3 can also be exposed by heavily polishing the samples. In order to know the underlying mechanism, temperatures at the coating surface and the underlying substrate have been monitored by a thermocouple under the conventional and soft sparking PEO regimes, respectively. Interestingly, high temperatures are detected in the case of soft sparking rather than PEO with strong discharges. The formation of CuO, quartz, and cristobalite within the soft sparking coating also supports the existence of high temperature. Hence, the formation of α‐Al2O3 under soft sparking can be resolved to the conventional thermal activation mechanism, without the need of seeking other plausible explanations. Thermal condition evaluation for soft sparking PEO suggests that values of the effective thermal conductivity during PEO process for the outer layer and the barrier layer at the coating/substrate interface might be lower than ∼0.05 and ∼0.0017 W m−1 K−1, respectively. It is believed that the amorphous structure of the outer and barrier layers effectively blocks the heat dissipation, facilitating the formation of a highly wear‐resistant inner layer with α‐Al2O3, CuO, and the other high‐temperature species under soft sparking. [ABSTRACT FROM AUTHOR]

In this study, the influence of cathodic polarization on the plasma electrolytic oxidation (PEO) behaviors of pure magnesium and AZ31 and AZ91 magnesium alloys with varied Al alloying contents was systematically examined in a dilute alkaline silicate electrolyte by adjusting the cathodic-to-anodic current density ratio (R = jc/ja) from 0 to 3.2. The results show that moderate cathodic polarization (R = 0.6) led to the thickest coatings on the Mg and Mg alloys, and the coatings grew in an outward-and-inward mode compared with the inward growth at R = 0. Excessive cathodic polarization (high R ratios) differently influenced the PEO behaviors of the magnesium alloys. For the pure magnesium and AZ31 alloy, the coatings blistered or peeled off when the R ≥ 0.9. However, the tolerance to cathodic polarization was significantly improved for the AZ91 Mg alloy. The coatings were undamaged even with the highest R ratio of 3.2, and their compactness was further improved as the R ratio increased to 0.9 and 1.2. An increase in cathodic polarization led to a reduction in the anodic potential and spark softening but did not result in an improvement in the coating quality. Optical emission spectroscopy identified two spectral lines at 559.79 and 570.11 nm, which are assigned to the Mg species but not found in databases or the literature. The corrosion and wear resistance of the PEO coatings were also investigated. The coating formed on the AZ91 magnesium alloy at R = 1.2 displayed the narrowest wear track due to its high compactness. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy under pulsed bipolar regimes has been carried out in an aluminate electrolyte with the addition of 0–25 g l −1 Na 2 WO 4 ·2H 2 O. Black coatings are formed with the addition of tungstate. Sequential anodizing has also been adopted to investigate the coating formation mechanisms by tracing the elemental distribution of W and Al in the coatings. The coatings develop an outer layer, inner layer and a barrier layer after a certain period of PEO. At the later stage of the PEO, the coating grows inwardly, which was accompanied by the strong penetrating discharges. The penetrating discharges have caused significant anion deposition, and the electrolyte species, such as W and Al, can be transported to the coating/substrate interface instantly. The anodic current density within the penetrating discharge channels is estimated to be ∼10 4 A cm −2 , which is high enough to melt the coating materials beneath the pancake structure and cause the direct thermal decomposition of water and hence the anomalous gas evolution reported for PEO. X-ray photoelectron spectroscopy (XPS) denies that free state W exists in PEO coatings. [ABSTRACT FROM AUTHOR]

[Display omitted] • Plasma electrolytic oxidation of copper is first reported in an aluminate electrolyte. • The additives of sodium phosphates/ hypophosphite are critical to the PEO behavior. • An initial layer of AlPO 4 and Al 2 O 3 (AAP) favors the stable discharges with NaH 2 PO 4 as additive. • Hypophosphite reduced original passive film on Cu, leading to deposition of Cu(OH) 2. • Cu(OH) 2 and AAP re-passivate Cu and coatings were ultimately formed in the case of NaH 2 PO 2. Plasma electrolytic oxidation of copper is first investigated in an aluminate-based electrolyte using trisodium phosphate (TP), sodium dihydrogen phosphate (DP) and sodium hypophosphite (HP), respectively, as additives. Black coatings can be successfully formed in the electrolytes of DP and HP. Detailed characterizations show that an initial thin layer of aluminum phosphate and alumina (AAP) was formed on cooper in DP electrolyte, which helps the fast establishment of plasma discharges, leading to successful coating formation. The AAP layer also formed in the electrolyte with TP as additive, but the AAP layer may be too thin to ensure stable plasma discharges. The coating formation mechanism is drastically different in HP: pitting corrosion along with loose Cu(OH) 2 deposition occurs at initial stage, which may be a dual effect of local acidification and the destruction of the passive layer by the reducing hypophosphite ions. However, the deposited Cu(OH) 2 and the subsequent formation of AAP layer may re-passivate the copper surface and provide an insulating layer for plasma discharging and coating formation. Optical emission spectroscopy (OES) and Mott-Schottky tests have also been used to investigate the features of plasma discharges and the semiconductor properties of the coatings, respectively. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation (PEO) is first reported for the non-valve metal of brass. The surface morphology of the PEO-treated brass has been examined, indicating the formation of a porous layer similar to that of PEO of valve metals. Abundant lines of Cu and Zn in the optical emission spectra (OES) of the PEO plasma confirm the participation of substrate in the oxide formation. Plasma electron temperature has been estimated by two-line method based on species of Cu, Zn and H, respectively, and also the Boltzmann plot with multiple lines of Cu I. The temperature lies between ~4393 and ~6911 K. However, the spectral lines of Cu I in ultraviolet region are found to be not suitable for temperature estimation. The coating contains crystalline phases of CuO, Cu 2 O and ZnO, as well as amorphous SiO 2. Tribological tests show that the wear resistance of the PEO-treated brass is nearly ~16 times that of the uncoated metal. The mechanism of the PEO of brass has also been discussed. • Plasma electrolytic oxidation is first reported for brass. • Abundant spectral lines from Cu and Zn are detected in the OES spectra of PEO plasma. • Spectral lines of Cu at ultraviolet region cannot be used for temperature calculation. • The coatings consist of CuO, Cu 2 O and ZnO and amorphous SiO 2. • The wear resistance of PEO-treated brass is ~16 times higher than uncoated metal. [ABSTRACT FROM AUTHOR]

The effect of cathodic polarization on plasma electrolytic oxidation (PEO) of the refractory metal of tantalum is investigated in 10 g/l NaAlO 2 +2 g/l KOH under 1000 and 100 Hz. Ceramic coatings are formed under various cathodic-to-anodic current densities ratios (R = jc/ja) of 0, 0.6, 1.3 and 2.3. For PEO under 1000 Hz, the plasma discharges intensify with the increase of R from 0 to 1.3, with optical emission from Ta being detected at later stage of PEO under cathodic polarization. However, excessive R ratio of 2.3 has caused early extinguishment of the plasma discharges. Correspondingly, thicker coatings are formed as the R ratio increases from 0 to 1.3, and the coating under R = 1.3 exhibits a most porous outer layer. A thin and compact coating is formed under R = 2.3. It is also found that the application of cathodic polarization increases the Na content in the resultant coatings, with a new crystalline phase NaTaO 3 being formed under R = 2.3. Cathodic polarization also decreases the corrosion resistance of the coatings. Surface wettability tests show that the coatings formed under R = 1.3 are the most hydrophilic. PEO under 100 Hz shows some differences in discharge and coating formation. Larger sized discharges are also observed at the later stage under R = 0.6, but Ta species are detected at a delayed time. For the R values of 1.3 and 2.3, patches of soft sparking occur on the sample surface, leading to non-uniform coatings. This study shows that both cathodic polarization and frequency significantly affect the PEO process. Moreover, compared with the PEO of Al, tantalum responds quite differently to cathodic polarization. Mechanisms of coating formation have been illustrated: Protons and other cations are attracted by cathodic electric field into the coatings, where they undergo a series of reactions to significantly affect the PEO behaviors. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation (PEO) is commonly known as a coating technique. However, the present study shows that PEO of pure Al in 10 g l −1 Na 2 WO 4 ·2H 2 O leads to the synthesis of micro and nano sized powders of WO 3 . The as-synthesized WO 3 powders have been characterized by SEM, TEM and photocatalytic tests. The plasma discharges during the PEO process have been investigated by real-time imaging and spectrographic method. The energetic features of the single discharge at different stages of PEO have been evaluated. It was suggested that the electrolyte species were directly decomposed into tungsten oxides within the discharge channels and then the oxides re-entered the electrolyte due to the lower melting and boiling points of WO 3 , and more importantly, its tendency of sublimation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel double-layer nanocomposite coating is prepared via a facile one-step PEO-SDSN technique. • The addition of nanoparticles creates interesting rising and falling changes in current. • A new fabrication strategy of multifunctional coatings by tailoring particles species is proposed. • The instant, dense discharges are induced by Highfield, high temperature and bubbles accumulation. • Such intense discharges significantly improve the growth rate of nanocomposite coating. A novel double-layer structural nanocomposite coating was prepared via one-step plasma electrolytic oxidation-synchronous deposition & sintering of nanoparticles (PEO-SDSN) technique under unique threshold conditions of concentration of added nanoparticles, electrolyte temperature, and applied voltage. The special discharges at the coating/electrolyte interface lead to the directional migration, deposition and sintering of organic (eg. PTFE) or inorganic (eg. SiC) nanoparticles, combined with the dynamic growth equilibrium of bottom oxide layer and outer nanoparticle deposition & sintering layer. Different from traditional PEO, the addition of nanoparticles creates interesting rising and falling changes in current and corresponding discharging characteristics. The underlined formation mechanism of the double-layer coating was detected by OES combined with discharging phenomena and coating structure evolution. Moreover, compared with the single PEO coating, the double-layer structural nanocomposite coating has better comprehensive performances, such as corrosion protection, and low and stable friction coefficient. The PEO-SDSN technique is energy saving, more importantly, it provides a brand-new fabrication strategy for a series of double-layer multifunctional coatings by tailoring the nanoparticles species. Eventually, the potential applications and guiding strategies of double-layer nanocomposite coating are presented. [ABSTRACT FROM AUTHOR]