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Various plasma electrolytic oxidation (PEO) ceramic coatings were fabricated on aluminum in the electrolytes with different concentrations of Na2SiO3·9H2O. The morphology of fractured cross-section and coating/substrate (C/S) interface, phase compositions, and corrosion behavior of these ceramic coatings were characterized employing field emission scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffractometer (XRD), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The results showed that adding Na2SiO3·9H2O in the electrolyte could reduce the decrease amplitude of the voltage-time curve, and it can affect the types of the anodic film before the beginning of plasma discharges. A novel phenomenon was recorded as PEO treatment was carried out in the electrolyte with 1.85 g l−1 Na2SiO3·9H2O that the substrate was first covered by a barrier-type anodic film, and then a porous-type anodic film appeared after the barrier-type anodic film was broken down. The anodic film continued to grow for a long duration in the PEO process after being broken down. Characteristics of PEO ceramic coatings with a similar thickness acquired in various electrolytes were also evaluated. XRD results indicated that these ceramic coatings of similar thickness were mainly composed of α-Al2O3 and γ-Al2O3, and the α-Al2O3 proportion decreased with increasing Na2SiO3·9H2O concentration. The electrochemical data showed that the corrosion resistance of these PEO ceramic coatings of similar thickness decreased by adding Na2SiO3·9H2O in the electrolyte.

Plasma electrolytic oxidation (PEO) of Al was carried out in dilute alkaline sodium tripolyphosphate (Na5P3O10) electrolyte. Surface and cross-section morphologies, elemental and phase compositions were characterized by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometer (XRD). The results revealed that a severe thinning of the anodic film resulted in the voltage decrease at the initial stage of the PEO process. Porous-type anodic film with uniform pore diameter were fabricated at the initial stage of PEO process. Subsequently, detachments and cracks took place on the anodic films, and served as weak regions occurring dielectric breakdown preferentially due to the concentration of electric current at these sites. Two origins of PEO coatings, the transformation of anodic films and the normal generation of PEO coatings through the plasma discharges, were proposed. The results also indicated that P element mainly participated in the anodic oxidation and existed as amorphous phase in the inner side areas of anodic films except the barrier layer. Based on the analyses of the microstructure evolution and the chemical composition, a schematic of coating growth mechanism was proposed in the end.

A plasma electrolytic oxidation (PEO) coating was fabricated on AA1060 alloy in an alkaline sodium hexametaphosphate ((NaPO3)6) electrolyte. The growth characteristic of the PEO coating was investigated by means of a substrate-detachment technique, Scanning electron microscope (SEM) equipped with energy dispersive spectrometry (EDS), X-ray photoelectron spectrometer (XPS), X-ray diffractometer (XRD) and Mott-Schottky analysis. The results showed that an ordered and porous anodic aluminum oxide (AAO) film was formed at the anodic oxidation stage by the influence of (NaPO3)6 in the electrolyte, and the microstructure of the AAO film induced the plasma discharge events to initiate inside the AAO film. The Mott-Schottky analysis showed that the PEO coated samples were all represented n-type semiconducting behavior. The microstructures of detached coatings showed that the coating/substrate interfaces formed at the anodic oxidation stage and sparking stage consist of numerous hemispherical cap structures, but with different sizes. It was also revealed that the inward growth of the hemispherical cap structures is dependent on the diffusion of oxygen from the electrolyte or plasma gases to substrate, and P atoms mainly exist in the amorphous phase of the PEO coating.

The structures of materials have great influence on their properties. For materials with low electron conductivity, fast electron transport pathway can be constructed through carbon structure design. Here we report a simple but effective method to improve the electrochemical performances of Na3V2(PO4)3. Polyvinyl Pyrrolidone (PVP) can improve the viscosity of the precursor solution, thus forming aggregate structured material. In Na3V2(PO4)3, primary particles with a diameter of approximately 300 nm are aggregated through a special carbon network to form micro-sized secondary particles. This kind of structure will provide easy access for electron transportation, thereby improving electrochemical performance of the material. As a cathode material for sodium-ion batteries, Na3V2(PO4)3 delivers excellent rate (86.6 mAh g−1 at 30 C) and cycling performance (capacity retention of 88.4% after 2000 cycles at 10 C). The material also exhibits a specific capacity of 100.2 mAh g−1 at 5 C under 55 °C. The above-mentioned performance is far better than the control sample without PVP. The special carbon network provides electron transport channels which improves the electrochemical performance of the material. This method may provide new ideas for the preparation of phosphate materials.

In the current investigation, plasma electrolytic oxidation (PEO) ceramic coatings on Al are galvanostatically synthesized at various processing stages in an alkaline silicate system. The resultant coatings are systematically surveyed in terms of the following respects: the applied voltage and surface sparking evolution over the studied course of PEO are recorded by the signal acquisition system and the real time imaging, respectively; the phase constitution, the surface morphology, polished cross section of the coated specimens, the fractured cross section and coating/substrate interface of the detached coatings are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) assisted with energy-dispersive X-ray spectrometer (EDS), respectively. In particular, the coatings detached by an in-house developed electrochemical way disclose the coating microstructure in a new angle, which allows the cross section and the coating/substrate interface to come clearly into focus. The microstructure evolution of the oxide ceramic coatings and the correspondences among the characteristic microstructures of the free surface, the fractured cross section and the coating/substrate interface are presented and discussed. Further, based on the present PEO coating microstructure, a model of the coating growth that evolves over time is proposed.

The plasma electrolytic oxidation (PEO) coatings were fabricated on AA1060 aluminum alloy at a constant current density of 4.4 A/dm2. The images of discharge sparks and voltage-time response were recorded during the PEO process. The characteristics of the two interfaces of coatings were investigated as a function of PEO processing time by using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). Hundreds of coatings were detached from the substrate by an electrochemical method and ground into homogeneous powders to carry out differential scanning calorimeter (DSC) and further XRD qualitative test. In addition, matrix-flushing method was employed to quantitatively measure the content evolution of phase compositions of Al-based detached PEO coatings. The distribution rule of amorphous phases in the PEO coatings was investigated by thickness-reduction method for the first time. Based on the experiments above, gaining an insight into the formation, distribution and evolution of the amorphous and crystalline phases in the PEO process.

Evolution processes of the corrosion behavior and structural characteristics of the plasma electrolytic oxidation (PEO) coated AZ31 magnesium alloy were investigated by using scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), potentio-dynamic polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Detached coating samples were fabricated by an electrochemical method and more details of the internal micro-structure of coatings were clearly observed on the fractured cross-section morphologies of the samples compared to general polished cross-section morphologies. Evolution mechanisms of the coating corrosion behavior in relation to the evolution of micro-structural characteristics were discussed in detail.

Ceramic coatings were performed on aluminum through plasma electrolytic oxidation (PEO) in a Na5P3O10 solution with various concentrations of NaOH. It was revealed that the concentration of NaOH affected the types of anodic films and the activity of plasma discharges. Emphatic investigation was performed on the coatings fabricated in the electrolyte with addition of 1 g L−1 NaOH, focusing on the coating microstructure evolution and coating corrosion resistance. Some coatings were detached from the substrate, showing that the oxide particles on the anodic film C/S interface underwent profile variations from hemisphere to polygon, and its interpore distance decreased over time. The formations of pores of anodic films and plasma discharge of the PEO coatings were both easy to occur around the existing pores and the existing discharge channels. Fractured cross-section showed that a pancake-like structure corresponded to a discharge channel, a cavity, and a barrier layer protrusion whose area was similar to that of the corresponding pancake-like structure. The data of potentiodynamic polarization measurements showed that the sample treated with PEO for 10 min possessed the lowest icorr value of 4.97E-8 A cm−2, the most positive Epit value of −0.63 V, and the highest Rp value of 2.66E5 Ω cm2. EIS data of 10 min sample exhibited the highest R2 value of 1.61E6 Ω cm2, demonstrating the best corrosion resistance among these samples.