The influence of semiconducting properties of passive films on the cavitation erosion resistance of a NbN nanoceramic coating

To alleviate the cavitation damage of metallic engineering components in hydrodynamic systems operating in marine environments, a NbN nanoceramic coating was synthesized on to a Ti-6Al-4V substrate via a double cathode glow discharge technique. The microstructure of the coating consisted of a ~13 μm thick deposition layer of a hexagonal δ′-NbN phase and a diffusion layer ~2 μm in thickness composed of face-centered cubic (fcc) B1-NaCl–structured (Ti,Nb)N. The NbN coating not only exhibited higher values of H/E and H²/E than those measured from NbN coatings deposited by other techniques, but also possessed good adhesion to the substrate. The cavitation erosion resistance of the NbN coating in a 3.5 wt% NaCl solution was investigated using an ultrasonic cavitation-induced apparatus combined with a range of electrochemical test methods. Potentiodynamic polarization measurements demonstrated that the NbN coated specimens demonstrated both a higher corrosion potential (E_corr) and lower corrosion current density (i_corr) than the uncoated substrate. Mott-Schottky analysis, combined with the point defect model (PDM), revealed that, for a given cavitation time, the donor density (N_D) of the passive film on the NbN coating was reduced by 1 ~ 2 orders of magnitude relative to the uncoated Ti-6Al-4V, and the diffusivity of the point defects (D₀) in the passive film grown on the NbN coating was nearly one order of magnitude lower than that on the uncoated substrate. In order to better understand the experimental observations obtained from Mott-Schottky analysis and double-charge layer capacitance measurements, first-principles density-functional theory was employed to calculate the energy of vacancy formation and the adsorption energy for chloride ions for the passive films present on both the NbN coating and bare Ti-6Al-4V.     

In vitro corrosion behavior of Ti-O film deposited on fluoride-treated Mg-Zn-Y-Nd alloy

In this paper, a new composite coating was fabricated on magnesium alloy by a two-step approach, to improve the corrosion resistance and biocompatibility of Mg-Zn-Y-Nd alloy. First, fluoride conversion layer was synthesized on magnesium alloy surface by immersion treatment in hydrofluoric acid and then, Ti-O film was deposited on the preceding fluoride layer by magnetron sputtering. FE-SEM images revealed a smooth and uniform surface consisting of aggregated nano-particles with average size of 100 nm, and a total coating thickness of ∼1.5 μm, including an outer Ti-O film of ∼250 nm. The surface EDS and XRD data indicated that the composite coating was mainly composed of crystalline magnesium fluoride (MgF₂), and non-crystalline Ti-O. Potentiodynamic polarization tests revealed that the composite coated sample have a corrosion potential (E_corr) of −1.60 V and a corrosion current density (I_corr) of 0.17 μA/cm², which improved by 100 mV and reduced by two orders of magnitude, compared with the sample only coated by Ti-O. EIS results showed a polarization resistance of 3.98 kΩ cm² for the Ti-O coated sample and 0.42 kΩ cm² for the composite coated sample, giving an improvement of about 100 times. After 72 h immersion in SBF, widespread damage and deep corrosion holes were observed on the Ti-O coated sample surface, while the integrity of composite coating remained well after 7 d. In brief, the data suggested that single Ti-O film on degradable magnesium alloys was apt to become failure prematurely in corrosion environment. Ti-O film deposited on fluoride-treated magnesium alloys might potentially meet the requirements for future clinical magnesium alloy stent application.     

Microstructure and corrosion behavior of coated AZ91 alloy by microarc oxidation for biomedical application

Magnesium and its alloy currently are considered as the potential biodegradable implant materials, while the accelerated corrosion rate in intro environment leads to implant failure by losing the mechanical integrity before complete restoration. Dense oxide coatings formed in alkaline silicate electrolyte with and without titania sol addition were fabricated on magnesium alloy using microarc oxidation process. The microstructure, composition and degradation behavior in simulated body fluid (SBF) of the coated specimens were evaluated. It reveals that a small amount of TiO₂ is introduced into the as-deposited coating mainly composed of MgO and Mg₂SiO₄ by the addition of titania sol into based alkaline silicate electrolytic bath. With increasing concentration of titania sol from 0 to 10 vol.%, the coating thickness decreases from 22 to 18 μm. Electrochemical tests show that the E_corr of Mg substrate positively shifted about 300500 mV and i_corr lowers more than 100 times after microarc oxidation. However, the TiO₂ modified coatings formed in electrolyte containing 5 and 10 vol.% titania sol indicate an increasing worse corrosion resistance compared with that of the unmodified coating, which is possibly attributed to the increasing amorphous components caused by TiO₂ involvement. The long term immersing test in SBF is consistent with the electrochemical test, with the coated Mg alloy obviously slowing down the biodegradation rate, meanwhile accompanied by the increasing damage trends in the coatings modified by 5 and 10 vol.% titania sol.     

Ultrasonic-assisted electrodeposition of Ni/diamond composite coatings and its structure and electrochemical properties

Ni/diamond composite coatings have been synthesized by ultrasonic-assisted electrodeposition in a Ni electroplating bath containing diamond nanoparticles. The influences of current density and ultrasonic agitation on the coating composition, morphology, topography, phase structure, and electrochemical characteristics of the electrodeposits were evaluated. Ultrasonic agitation was provided using an external ultrasonic bath at a frequency of 40 kHz and acoustic power of 300 W. Coating samples were also prepared under magnetic stirring for comparison with the ultrasonic-assisted deposits. This work reveals that the diamonds have been incorporated and evenly distributed in the composites. The coatings exhibit dense, granular like morphology with pyramid-like grains. As current density increases, the diamond amount of ultrasonic-assisted electrodeposits first increased to maximum of 11.4 wt% at 3 A dm⁻² and then decreases to 9.9 wt% at 5 A dm⁻², and the RTC of the preferred orientation (2 0 0) plane increases from 76.3% up to 93.4%. The crystallite size was 60–80 nm and the R_a of the magnetic and ultrasonic agitations were 116 nm, 110 nm, respectively. The maximum R_p of 39.9, 50.3 kΩ cm² was obtained at 4 A dm⁻² when respectively immersed 30 min and 7 days, illustrating the best corrosion resistance of the coatings of 4 A dm⁻². The effects of mechanical and ultrasonic agitations on the mechanism of the co-electrodeposition process were both proposed. The incorporation of diamond particles enhances the hardness and wear-resisting property of the electrodeposits. The ultrasonic-assisted electrodeposited Ni/diamond coating has better corrosion resistance than that prepared under mechanical stirring conditions.     

Study of La0.8Sr0.2Co0.2Cr0.8O3 − δ as a candidate coating material for the positive current collector in Na/S battery

Perovskite La_0.8Sr_0.2Co_0.2Cr_0.8O_3 − δ (LSCC) ceramic synthesized by the conventional ceramic processing technique was studied as a novel coating material for the cathode current collector in Na/S battery. Its structure, electrical conductivity, density and thermal expansion coefficient (TEC) were investigated. The corrosion performance of LSCC was in particular evaluated by electrochemical techniques in combination with long-term dip-immersion tests. The results indicated that LSCC exhibited excellent corrosion resistance in molten sodium tetrasulfide at 350 °C. The corrosion current density i_corr (0.081 mA cm^− 2) was much lower than that of 316 L stainless steel by approximately two orders of magnitude. The corrosion rate of LSCC deduced from immersion test was as low as about 12 μm year^− 1.     

Preparation and properties of ceramic coating on Q235 carbon steel by plasma electrolytic oxidation

Ceramic coating was achieved on Q235 carbon steel by PEO (plasma electrolytic oxidation, PEO) without any pretreatment in sodium aluminate system. The discharge process as well as the accompanied surface morphology evolution was analyzed. The phase and elemental composition of the coatings were also investigated. The corrosion, mechanical and tribological properties of the ceramic coating were primarily studied. It is found that the coating surface was porous and the thickness of the coating was about 120 μm. The coating mainly consisted of FeAl₂O₄, Fe₃O₄ and a little γ-A1₂O₃. The corrosion current of the coated sample was 3.082 × 10⁻⁷ A/cm², which was decreased by two orders of magnitude compared with the uncoated one. The micro hardness of the ceramic coating was 1210 Hv, which was about three times as that of the uncoated sample. The friction coefficient of coated sample was also well improved. Investigations revealed that PEO provided a promising technique for preparation of protective ceramic coatings on steels.     

ZnO-treated TiO2 inverse opal electrodes for dye-sensitized solar cells

We report that the photovoltaic properties of inverse opal TiO₂ (io-TiO₂) electrodes in dye-sensitized solar cells can be enhanced by ZnO treatment of the inverse opal structures. ZnO was coated on the surface of io-TiO₂ via the sol–gel reaction of ZnO precursors. Energy dispersive X-ray spectroscopy (EDX) measurements showed that the amount of ZnO on the io-TiO₂ surface was measured to be 0.12–0.50 wt% of zinc, depending on the number of coatings. Compared to bare inverse opal electrodes, the energy conversion efficiency of cells increased for the 0.35 wt% ZnO-coated electrodes, and then decreased for the 0.50 wt% ZnO-coated electrodes. The maximum efficiency of 5.3% was achieved, corresponding to a 23% increase in efficiency compared with bare io-TiO₂ electrodes. The enhanced efficiency was mainly attributed to the improvement of the open-circuit voltage (V_OC). EIS and dark current measurements confirmed that this enhancement in V_OC was due to the movement of the conduction band edge in a negative direction after ZnO treatment, rather than the formation of a barrier layer for electron recombination.     

Electrochemical synthesis of polythiophene on nickel coated mild steel and corrosion performance

Electrochemical polymerization of polythiophene (PTh) was investigated on nickel coated mild steel (MS) electrode, in LiClO₄ containing acetonitrile medium (ACN-LiClO₄). Nickel layer (1 μm thick) was deposited galvanostatically, from a proper bath solution. Then, the synthesis of PTh film was achieved in 0.1 M thiophene containing ACN-LiClO₄, by using cyclic voltammetry technique. The corrosion performances of nickel coated samples with and without polymer top coats were investigated in 3.5% NaCl solution, by using electrochemical impedance spectroscopy (EIS) and anodic polarization curves. The nickel coating behaved like a physical barrier and provided some protection to MS against corrosion. But its barrier property diminished significantly with time and failed to protect MS. It was shown that PTh top coat improved the barrier efficiency remarkably, and excellent protection efficiency was obtained against MS corrosion, for considerable exposure time in such aggressive environment.     

Synergetic effects of ultrasound and sodium alginate coating on mass transfer and qualities of osmotic dehydrated pumpkin

This research studied the effects of combined ultrasound and 3% sodium alginate (SA) coating pretreatment (US + Coat) on mass transfer kinetics, quality aspects, and cell structure of osmotic dehydrated (OD) pumpkin. The results of the pretreatment were compared with the results of control (non-pretreated osmotic dehydration) and other three pretreatment methods, which were 1) ultrasound in distilled water for 10 min (USC), 2) ultrasound in 70% (w/w) sucrose solution (US) for 10, 20 and 30 min, and 3) coating with 1%, 2%, 3% (w/w) SA. The coating pretreatments with SA resulted in a higher water loss (WL) but lower water activity and solid gain (SG) than other treatments. US pretreatments resulted in the highest effective diffusion coefficients of water (D_w) and solid (D_s) but the cell structure of the product was deformed. The 3% SA coating treatment had the highest WL/SG (5.28) but with the longest OD time (12 h). Using the US + Coat pretreatment gave satisfactory high WL/SG (5.18), D_w (1.09 × 10⁻¹⁰ m²s⁻¹) and D_s (5.15 × 10⁻¹¹ m²s⁻¹), reduced the OD time to 9 h, and preserved the cell structure of the product. This research suggests that US + Coat pretreatment can be an effective processing step in the production of OD pumpkin.     

Improved moisture resistance of SrS:Eu phosphors with nanoscale SiO2 coating

Orange-emitting SrS:Eu²⁺ phosphors were coated with nanoscale SiO₂ and their photoluminescence (PL) degradation behavior in moist air was investigated. The SiO₂ coating was obtained by sol-gel process using diethoxydimethylsilane (DEDMS) and the coating content was varied from 0.5 to 2 wt%. The coatings were composed of a uniform, continuous, and amorphous SiO₂ layer of 30-50 nm thickness and the coating thickness was not varied significantly with the coating content. No peak shift and no decrease of PL intensity were observed after coating. The PL intensity of the coated phosphors decreased to ∼75% of the original value after 10 h exposure to moist air, while the uncoated phosphor decreased to ∼33%, which indicates the improved moisture resistance of the nanoscale SiO₂ coated SrS:Eu²⁺ phosphors.     

Effects of SnO2 surface coating on the degradation of ZnS thin film phosphor

Pulsed laser deposited ZnS bare and SnO₂ coated ultra thin films were subjected to prolonged electron beam bombardment with 2 keV energy and a steady 44 mA/cm² current density, in 1 × 10⁻⁶ Torr O₂ pressure backfilled from a base pressure of 3 × 10⁻⁹ Torr at room temperature. Auger electron spectroscopy (AES) was used to monitor changes of the surface chemical composition of both the bare and coated phosphor films during electron bombardment. Degradation was manifested by the decrease of sulphur and accumulation of oxygen on the surface of the bare phosphor. However, with the SnO₂ coating this phenomenon was delayed until the protective SnO₂ was depleted on the surface through dissociation and reduction.     

Molecular hydrogen storage in Al-doped bulk graphite with wider layer distances

Molecular hydrogen storage at room temperature in Al-doped bulk graphite with wider layer distances was studied using density functional theory calculation. Hydrogen storage capacity of 3.48 wt% or volume density of 51 kg/m³ was predicted at T=300 K and P=0.1 GPa with adsorption energy E_b=?0.264 eV/H₂. This is close to the target of volume density 62 kg/m³ and satisfies the requirement of immobilization hydrogen with binding strength of 0.2–0.7 eV/H₂ at ambient temperature and modest pressure for commercial applications specified by the U.S. Department of Energy.     

Electrical conductivity,optical properties and mechanical stability of 3, 4, 9, 10-perylenetetracarboxylic dianhidride based organic semiconductor

The 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) doped polymer films were prepared with Polypyrrole (PPy) and Polyvinyl alcohol (PVA) polymers by solution-casting. The change in structure and chemical composition of samples was identified by XRD and FTIR respectively. The UV–visible spectroscopy demonstrates the optical characteristics and band gap properties of sample. The homogeneous morphology of sample for higher wt% of PTCDA was examined by atomic force microscopy (AFM). The differential scanning calorimetry (DSC) results demonstrate the decrease in melting temperature (T_m) and degree of crystallinity (χ_c%) of polymeric organic semiconductor. The mechanical property demonstrates the high tensile strength and improved plasticity nature. Impedance spectroscopy was evaluated to determine the conductivity response of polymeric organic semiconductor. The highest DC conductivity (2.08×10⁻³ S/m) was obtained for 10 wt% of PTCDA at 140 °C. The decrease in activation energy (E_a) represents the non-Debye process and was evaluated from the slope of ln σ_dc vs. 10³/T plot.     

Oxidation and contact resistance of Sn-Ag coated superconducting strands for the Large Hadron Collider (LHC)

The oxides formed on the Sn-Ag coated Large Hadron Collider (LHC) superconducting cables during a 200 °C heat treatment in air are described and the oxide composition is compared with the interstrand contact resistance (R_C). The analysis of more than 250 interstrand contact areas shows that the higher the average Cu content with respect to the Sn content in the oxide, the higher is R_C. During the 200 °C heat treatment, Sn in the coating is transformed into a Cu₃Sn layer, on which an oxide grows that consists essentially of a thin outermost layer of CuO on top of Cu₂O, similar to the oxide structure formed on bare Cu. The underlying Cu₃Sn layer acts as an O diffusion barrier that prevents O diffusion into the Cu bulk during the subsequent cable heat treatment under high pressure. On contact zones where the Cu₃Sn layer is not formed during the 200 °C heat treatment mainly Sn oxide grows and R_C is comparatively low.     

Oxygen transport model for layered MIEC composite membranes

Surface exchange resistance can reduce the oxygen transport through dense mixed ionic-electronic conducting (MIEC) membranes. Addition of an MIEC surface layer to a base substrate can reduce the surface exchange resistance. Existing oxygen transport relations that consider bulk diffusion and surface exchange resistance are extended to treat coated membranes formed by depositing a highly conductive, thin layer of MIEC on the surface of a dissimilar MIEC substrate and accounting for the solid/solid interfacial resistance. The oxygen flux through the coated membrane may exceed that through the bare membrane only if: 1) the surface exchange coefficient of the added layer is larger than the surface exchange coefficient of the bare membrane; and 2) the solid/solid interfacial resistance is sufficiently small. In general, deposition of the surface layer on the membrane tube surface exposed to lean gas leads to a larger oxygen flux than deposition of the layer on the oxygen rich side. A La_0.5Sr_0.5Fe_0.8Ga_0.2O_3-δ/SrCo_0.8Fe_0.2O_3-δ membrane achieved an oxygen outwards flux of 0.45 mL/min^⁎cm² at 1000 °C from an air/helium gradient. This was a ∼ 50% increase over that obtained using an uncoated LSFG tube.     

Poly (3-aminobenzoic acid) @ MWCNTs hybrid conducting nanocomposite: preparation,characterization, and application as a coating for copper corrosion protection

Nowadays, corrosion is a widely observed phenomenon in metal and one of the most important reasons of degradation in industrial parts. As a result, developing and applying the methods to reduce corrosion costs in industry is essential. In this research, emulsion polymerization route was used to prepare poly (3-aminobenzoic acid) @ multi-walled carbon nanotubes (P3ABA@MWCNTs) hybrid conducting nanocomposite film. Different techniques like Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal gravimetric analysis (TGA) were used to evaluate structure and morphology of resultant materials. P3ABA@MWCNTs hybrid conducting nanocomposite demonstrated an improvement in the crystallinity as well as thermal stability. In this literature for the first time, P3ABA and P3ABA@MWCNTs hybrid conducting nanocomposite films were used as a coating for corrosion protection of copper metal. The corrosion inhibition applicability of the P3ABA and P3ABA@MWCNTs hybrid conducting nanocomposite films on copper immersed in salt solution (3.5 wt.%) was investigated by potentiodynamic polarization, electrochemical impedance spectroscopic (EIS), and open circuit potential (OCP) measurements. The results showed that the presence of MWCNTs in P3ABA matrix considerably improved the corrosion inhibition efficiency of copper metal where corrosion potential (E_corr) and corrosion current density (I_corr) were ?103 mV and 0.562 μA cm^?2, respectively. The good anti-corrosion activity of P3ABA@MWCNTs hybrid nanocomposite was most likely due to physical barrier effect, and anodic protection performance of P3ABA together enhances the overall compactness of the MWCNTs.     

Effect of Al on the electrochemical corrosion behaviour of Pb free Sn-8.5 Zn-0.5 Ag-XAl-0.5 Ga solder in 3.5% NaCl solution

The effect of Al on the electrochemical corrosion behaviour of Pb-free Sn-8.5 Zn-0.5 Ag-XAl-0.5 Ga solder in 3.5% NaCl solution was investigated by using potentiodynamic polarization techniques. The X content in the solder varied from 0.1 to 3 wt.%. Polarization studies revealed that an increase in Al content upto 1.5 wt.% decreased the corrosion current density (I_corr), corrosion rate of the solder and shifted the corrosion potential (E_corr) towards more noble values. However, higher content of Al, i.e. 3 (wt.%) in the five-element solder enhanced the corrosion rate and resulted in a significant increase in the E_corr towards more negative values. Passivation behaviour was noticed in all the solders having varying Al content, but the passive film formed at 1.5 wt.% Al was most stable due to its low passivation current density (i_p) and low critical current density (i_cc) value in comparison to the other solders. XPS and Auger depth profile results revealed that the passive film consisted of oxides/hydroxides of Al and Zn formed on the surface of the solder with Sn being formed in the subsequent layer. Considerable aluminium segregation occurred towards the surface principally as Al₂O₃/Al(OH)₃ with increase in Al content to 1.5 wt.% in the five element solder. The formation of Al₂O₃ seemed to prevent the oxidation of zinc on the surface of the solder.     

Surface treatment of iron by electrochemical oxidation and subsequent annealing for the improvement of anti-corrosive properties

Corrosion resistance of iron oxides on iron foils prepared by anodization, annealing or a combination of both was characterized by electrochemical methods. Even though iron oxide film with a thickness of more than 2 μm could be prepared by single anodization, corrosion resistance deteriorated because the oxide film was in the amorphous phase and contained many defects. Corrosion resistance of iron oxides was also not enhanced by single annealing. Conversely, combination of anodization and subsequent annealing led to a positive shift of the corrosion potential in the Tafel plot, indicating that corrosion resistance was improved. Formation of thicker oxide during anodization was associated with a more positive shift in corrosion potential after annealing. Electrochemical impedance spectroscopy showed that the slowest charge transfer was observed in oxide films grown by a combination of anodization and annealing. We found that the optimum annealing temperature of anodic films in terms of the most positive shift of E_corr was 500 °C.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.     

Effect of electrodeposition modes on surface characteristics and corrosion properties of fluorine-doped hydroxyapatite coatings on Mg-Zn-Ca alloy

The microstructure, morphology and composition highly determine the corrosion resistance and bioactivity of coating. In traditional cathodic electrodeposition process, because of the unfavorable effects of the polarization of concentration difference and H₂ evolution, fluorine-doped hydroxyapatite coating was loose and porous. This coating could not ensure the long-term stability of the Mg alloy implants. In order to improve the corrosion resistance and bioactivity of coating, pulse electrodeposition and H₂O₂ were introduced into the electrodeposition to deposit fluorine-doped hydroxyapatite coating. As a comparative study, microstructure, corrosion resistance properties and bioactivity of traditional cathodic electrodeposition coating and pulse electrodeposition coating were investigated, respectively. The results revealed that nano fluorine-doped hydroxyapatite coating could be prepared by pulse electrodeposition, and the coating was dense and uniform. The potentiodynamic polarization experiment indicated that the dense and uniform coating could effectively protect Mg alloy substrate from corrosion. Immersion testing was performed in simulated body fluid. It was found that pulse electrodeposition coating could more effectively induce the precipitation of Mg²⁺, Ca²⁺ and PO₄³⁻ in comparison with traditional cathodic electrodeposition coating, because the nano phase had comparatively high specific surface area. Thus magnesium alloy coated with fluorine-doped nano-hydroxyapatite coating may be a promising candidate as biodegradable bone implants, and was worthwhile to further investigate the in vivo degradation behavior.