Physical and electrochemical characterizations of Ni-SiO2 nanocomposite coatings

Advances in materials performance often require the development of composite system. In the present investigation, SiO₂-reinforced nickel composite coatings were deposited on a mild steel substrate using direct current electrodeposition process employing a nickel acetate bath. Surface morphology, composition, microstructure and crystal orientation of the Ni and Ni-SiO₂ nanocomposite coatings were investigated by scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffraction analysis, respectively. The effect of incorporation of SiO₂ particles in the Ni nanocomposite coating on the microhardness and corrosion behaviour has been evaluated. Smooth composite deposits containing well-distributed silicon oxide particles were obtained. The preferred growth process of the nickel matrix in crystallographic directions <111>, <200> and <220> is strongly influenced by SiO₂ nanoparticles. The average crystallite size was calculated by using X-ray diffraction analysis and it was ~23 nm for electrodeposited nickel and ~21 nm for Ni-SiO₂ nanocomposite coatings. The crystallite structure was fcc for electrodeposited nickel and Ni-SiO₂ nanocomposite coatings. The incorporation of SiO₂ particles into the Ni matrices was found to improve corrosion resistance of pure Ni coatings. The corrosion potential (E _corr) in the case of Ni-SiO₂ nanocomposite coatings had shown a negative shift, confirming the cathodic protective nature of the coating. The Ni-SiO₂ composite coatings have exhibited significantly improved microhardness (615 HV) compared to pure nickel coatings (265 HV)     

Effect of the number of iron oxide nanoparticle layers on the magnetic properties of nanocomposite LbL assemblies

Aqueous colloidal suspension of iron oxide nanoparticles has been synthesized. Z-potential of iron oxide nanoparticles stabilized by citric acid was −35±3 mV. Iron oxide nanoparticles have been characterized by the light scattering method and transmission electron microscopy. The polyelectrolyte/iron oxide nanoparticle thin films with different numbers of iron oxide nanoparticle layers have been prepared on the surface of silicon substrates via the layer-by-layer assembly technique. The physical properties and chemical composition of nanocomposite thin films have been studied by atomic force microscopy, magnetic force microscopy, magnetization measurements, Raman spectroscopy. Using the analysis of experimental data it was established, that the magnetic properties of nanocomposite films depended on the number of iron oxide nanoparticle layers, the size of iron oxide nanoparticle aggregates, the distance between aggregates, and the chemical composition of iron oxide nanoparticles embedded into the nanocomposite films. The magnetic permeability of nanocomposite coatings has been calculated. The magnetic permeability values depend on the number of iron oxide nanoparticle layers in nanocomposite film.     

Influence of embedded particles on microstructure, corrosion resistance and thermal conductivity of CuO/SiO2 and NiO/SiO2 nanocomposite coatings

Homogeneous CuO/SiO₂ and NiO/SiO₂ nanocomposite coatings containing CuO and NiO nanoparticles in silica matrix were successfully synthesized by sol–gel process on an aluminum alloy substrate, respectively. The evolution of phase and morphology of both nanocomposites was characterized by XRD, SEM, TEM and FTIR. The effect of incorporating various nanoparticles on the corrosion behavior and the thermal conductivity of nanocomposite coatings was investigated by potentiodynamic polarization curve and comparative exponential method. The thermal conductivity as well as the corrosion resistance of nanocomposite coatings was significantly improved by the introduction of metal oxide particles. In comparison with NiO/SiO₂ nanocomposite coatings, CuO/SiO₂ composite coatings displayed lower protective behavior as well as higher thermal conductivity. Experimental results revealed that those improvements can directly be related to the nanocomposite effect and the nature of added nanoparticles.     

Study of the Interaction Between Apis mellifera Venom and Micro-Heterogeneous Systems

The bee venom, used in treatment of inflammatory and articular diseases, is a complex mixture of peptides and enzymes and the presence of tryptophan allows the investigation by fluorescence techniques. Steady state and time-resolved fluorescence spectroscopy were used to study the interaction between bee venom extracted from Apis mellifera and three micro heterogeneous systems: sodium dodecylsulphate (SDS) micelles, sodium dodecylsulphate-poly(ethylene oxide) (SDS-PEO) aggregates, and the polymeric micelles LUTROL^® F127, formed by poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide). Fluorescence parameters in buffer solution were typical of peptides containing tryptophan exposed to the aqueous medium, and they gradually changed upon the addition of surfactant and polymeric micelles, demonstrating the interaction of the peptides with the micro heterogeneous systems. Quenching experiments were carried out using the N-alkylpyridinium ions (ethyl, hexyl, and dodecyl) as quenchers. In buffer solution the quenching has low efficiency and is independent of the alkyl chain length of the quencher. In the presence of the micro heterogeneous systems the extent of static and dynamic quenching enhanced, showing that both fluorophore and quenchers reside in the microvolume of the aggregates. The more hydrophobic quencher (dodecyl pyridinium ion) provides higher values for K _SV and dynamic quenching constants, and SDS-PEO aggregates are most efficient to promote interaction between peptides and alkyl pyridinium ions. The results proved that bee venon interacts with drug delivery micelles of the copolymer LUTROL^® F127.     

Process and performance of hot dip zinc coatings containing ZnO and Ni-P under layers as barrier protection

A new coating system of under layer for hot dip zinc coating was explored as an effective coating for steel especially for application in relatively high aggressive environments. The influence of different barrier layers formed prior to hot dip galvanization was investigated to optimize high performance protective galvanic coatings. The deposition of ZnO and Ni-P inner layers and characteristics of hotdip zinc coatings were explored in this study. The coating morphology was characterized by scanning electron microscope (SEM) analysis. The hot dip zinc coatings containing under layer showed substantial improvement in their properties such as good adhesion, and high hardness. In addition, a decrease in the thickness of the coating layer and an enhancement of the corrosion resistance were found. Open circuit potential (OCP) of different galvanized layers in different corrosive media viz. 5% NaCl and 0.5 M H₂SO₄ solutions at 25 ± 1 °C was measured as a function of time. A nobler OCP was exhibited for samples treated with ZnO and Ni than sample of pure Zn; this indicates a dissolution process followed by passivation due to the surface oxide formation. The high negative OCP can be attributed to the better alloying reaction between Zn and Fe and to the sacrificial nature of the top pure zinc layer.     

A novel electrodeposited Cu-Zn-Bi film with increased corrosion resistance in a 0.05 M K2SO4 solution

A single phase Cu-Zn-Bi film is fabricated on the steel wire by electrodeposition. Bi addition (∼1 wt.%) greatly increases the corrosion resistance of brass (Cu−36 wt.% Zn) film in a 0.05 M K₂SO₄ solution as shown by potentiodynamic polarization and electrochemical impendence spectroscopy (EIS) experiments. It is proposed that the main reason for the improvement in the corrosion resistance by the Bi addition is that it greatly increased the crack resistance, which thus prevents crack-induced galvanic corrosion occurring between the brass film and the steel substrate.     

Zn-ZrO2 nanocomposite coatings: Elecrodeposition and evaluation of corrosion resistance

The Zn and Zn-ZrO₂ composite coatings were produced by electrodeposition technique using sulphate bath. ZrO₂ particles were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The ZrO₂ particle size distribution in the plating bath and Zeta potential and the ZrO₂ were measured using dynamic light scattering technique (DLS). The corrosion resistance properties of Zn and Zn-ZrO₂ composite coatings were compared by examining the experimental data acquired through polarization, open circuit potential (OCP) and Tafel measurements. The corrosion environment was 3.5 wt% NaCl solution. The variation of amount of ZrO₂ in the solution on their % wt inclusion in the composite and on composite microhardness was investigated. XRD patterns were recorded for Zn and Zn-ZrO₂ coatings to compare their grain size. The SEM images of coatings before and after corrosion under chemical and electrochemical conditions were presented. The results were analyzed to establish the superiority of Zn-ZrO₂ composite over Zn coating.     

Alleviation of thermal corrosion caused by molten ash on heat-exchange tubes in MSW incinerators: Effects of Ni-, Co-, Fe-based HVOF coatings

Thermal corrosion on heat-exchange tubes had an adverse effect on the economical and safe operation of municipal solid waste (MSW) incinerators. In order to alleviate the corrosion caused by molten ash, high velocity oxygen fuel (HVOF) thermal spraying technology was considered for its ability of enhancing corrosion resistance. In this study, 10 typical Ni-, Co-, Fe-based coatings were employed to find the relationship between material composition and high temperature corrosion resistance. The MSW incineration ash adhesion tests at 873 K were performed to specially simulate the corrosion caused by molten ash on tubes. Besides, mass gain measurements under the exposure of Na/K chlorides and sulfates were also performed to assess the failure of tubes. From experimental results, the strong positive correlation between MSW incineration ash adhesion force and mass gain rate proved the feasibility of using the former as an in-furnace corrosion indicator. Fe-based coatings with Fe accounting for up to 80 wt.%, rather than common coatings with Fe content of ca.50 wt.%, had better performance against thermal corrosion. For Ni/Co-based coatings, the optimal content of ternary components, including body metal (Ni/Co), eutectic metal (Cr) and the other strengthening elements (Mo+W+Si+B, et al.), seemed to be 60–20–20 wt.%. The mutual allocation of three kinds of elements, rather than the content of one specific element, affected its MSW incineration ash adhesion tendency. The microstructure characterization indicated the prevented permeation of corrosive species containing Na, K, S, Cl by the presence of strengthening elements such as Mo. Further comparison of NiCr and NiCrMo coatings explored the effect of Mo reinforcing the coating microstructure free from cracking, besides the widely known effect of forming protective oxidation film. This investigation provided insight for the corrosion resistance in MSW incinerators of harsh operating conditions.     

Mesostructures and properties of transparent block copolymer/silica nanocomposite monoliths

In this work, three different block copolymer/silica hybrid nanocomposite monoliths that possess mesostructured domains (hexagonal, cubic, and disordered) were prepared through the micellization of the block copolymer during the sol-gel process of a silica precursor. Transparent block copolymer/silica nanocomposite monoliths were obtained from the amphiphilic triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (EO₁₀₆PO₇₀EO₁₀₆, Pluronic F127), which we used to organize the polymerizing silica networks; the ratio between the block copolymer and silica was fixed at 60:40 (wt%). Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to observe the mesostructural ordering. Temperature-dependent SAXS patterns of the cubic structured nanocomposites showed that the calcination process takes place at 210°C. The transmittances of the nanocomposite monoliths over the range of wavelengths from 400 to 800 nm was >85%. From rheological measurements at low frequency, it was found that the hexagonally structured monoliths had higher storage and loss moduli relative to the monoliths possessing cubic and disordered structures.     

Effect of alkaline cleaning and activation on aluminum alloy 7075-T6

The effect of alkaline cleaning and activation on the composition and thickness of the oxide layer on aluminum alloy 7075-T6 was studied. E-pH diagrams were developed to predict the effect of alkaline cleaning and activation solutions on the stability of the oxide surface layers. The thickness of the native oxide layer was determined to be ∼30 nm by Auger electron spectroscopy depth profiling analysis. The outer ∼20 nm was rich in magnesium while the remaining ∼10 nm was rich in aluminum. Cleaning in a 9.1 pH alkaline solution was found to remove the magnesium-rich layer and leave behind an aluminum-rich oxide layer ∼10 nm thick. Activation in alkaline solutions of NaOH (pH > 12.9) or Na₂CO₃ (pH > 11.5) produced an oxide that was ∼20 to 60 nm thick and rich in magnesium. Alkaline cleaning and activation altered the oxide composition and thickness making it possible for deposition of thicker cerium-based conversion coatings (∼100 to 250 nm) compared to only alkaline cleaning (∼30 nm), with application of one spray cycle of deposition solution.     

Mechanical properties and wear and corrosion resistance of electrodeposited Ni-Co/SiC nanocomposite coating

Ni-Co/SiC nanocomposite coatings with various contents of SiC nano-particulates were prepared by electrodeposition in a Ni-Co plating bath containing SiC nano-particulates to be co-deposited. The influences of the nanoparticulates concentration, current density, stirring rate and temperature of the plating bath on the composition of the coatings were investigated. The shape and size of the SiC nano-particulates were observed and determined using a transmission electron microscope. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The wear behavior of the Ni-Co/SiC nanocomposite coatings was evaluated on a ball-on-disk UMT-2MT test rig. The worn surface morphologies of the Ni-Co/SiC nanocomposite coatings were observed using a scanning electron microscope. The corrosion behavior of the nanocomposite coatings was evaluated by charting the Tafel curves of the solution of 0.5 mol L⁻¹ NaCl at room temperature. It was found that the cathodic polarization potential of the composite electrolyte increased with increasing SiC concentration in the plating bath. The microhardness and wear and corrosion resistance of the nanocomposite coatings also increased with increasing content of the nano-SiC in the plating bath, and the morphologies of the nanocomposite coatings varied with varying SiC concentration in the plating bath as well. Moreover, the co-deposited SiC nano-particulates were uniformly distributed in the Ni-Co matrix and contributed to greatly increase the microhardness and wear resistance of the Ni-Co alloy coating.     

Magnetic properties and structure of electrodeposited Zn-Co alloys granular thin films

We present several results concerning the preparation by means of electrolysis and characterization of Zn-Co alloys thin films. Films of Zn, Co and Zn-Co with various compositions (8-16 at% Co) were prepared in sulfate baths, using potentiostatic control, envisaging applications in the domain of corrosion resistant magnetic sensors. The effects of applied voltage on the magnetic properties, microstructure and phase content of the electrodeposited Zn-Co films were investigated. The applied voltage significantly influenced the film composition and their magnetic properties. These films, when deposited at an applied voltage of 4.5 V exhibited multiphase behavior due to the inclusion of new phases (cobalt hydroxide), whereas at 3.0 V, only Zn-Co alloys were deposited. The structure and morphology of the samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).     

Surface characterization of the Zn-Ni-Al2O3 nanocomposite coating fabricated under ultrasound condition

Zn-Ni-Al₂O₃ nanocomposite coating, which was fabricated by eletrodeposition technique with the aid of ultrasound, was investigated by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). The results reveal that 7.2 wt.% nano-alumina particles uniformly dispersed in the matrix of the composite coating. The XPS analyses demonstrate that the outermost layer of Zn-Ni-Al₂O₃ coating was composed of nano-alumina and Zn(OH)₂, while the transition layer between the outermost layer and the Zn-Ni matrix consisted of nano-alumina, metallic Zn, ZnO and metallic Ni. In order to investigate the influences of ultrasonic agitation and the incorporation of nano-alumina on the composition and surface structure of Zn-Ni matrix, the comparison studies of Zn-Ni-Al₂O₃ nanocomposite coating with Zn-Ni coatings fabricated with and without ultrasound were conducted. The results indicate that ultrasonic agitation resulted in a decrease of Ni content in the Zn-Ni matrix and an increase of the thickness of surface oxide layer; while the incorporation of nano-α-Al₂O₃ increased the Ni content in the Zn-Ni matrix.     

Preparation and annealing study of TaNx coatings on WC-Co substrates

To prevent Co diffusion from cemented carbides at high temperatures, we fabricated TaN_x coatings by reactive direct current (d.c.) magnetron sputtering onto 6 wt.% cobalt cemented carbide substrates, to form diffusion barrier layers. Varying the nitrogen flow ratio, N₂/(Ar + N₂), from 0.05 to 0.4 during the sputtering process had a significant effect on coating structure and content. Deposition rate reduced as the nitrogen flow ratio increased. The effects of nitrogen flow ratio on the crystalline characteristics of the TaN_x coatings were examined by X-ray diffraction. The TaN_x coatings annealing conditions were 500, 600, 700, and 800 °C for 4 h in air. We evaluated the performance of the diffusion barrier using both Auger electron spectroscopy depth-profiles and X-ray diffraction techniques. We also investigated oxidation resistance of the TaN_x coatings annealed in air, and under a 50 ppm O₂-N₂ atmosphere, to evaluate the fabricated layers effectiveness as a protective coating for glass molding dies.     

Effect of Ni-to-Fe ratio on structure and properties of Ni-Fe-B-Si-Nb coatings fabricated by laser processing

Ni-Fe-B-Si-Nb coatings have been deposited on mild steel substrates using high power laser cladding process followed by laser remelting. The influence of Ni-to-Fe concentration ratio in (Ni_100−xFe_x)₆₂B₁₈Si₁₈Nb₂ (x = 55, 50, 45 and 40) powders on the phase composition and microstructure is analyzed by X-ray diffraction, scanning- and transmission-electron microscopies. The microhardness and corrosion resistance properties of the coatings are also measured. The results reveal that amorphous matrix layers are obtained for all coatings. The increase of the Ni-to-Fe ratio can promote the formation of γ(Fe-Ni) phase and decrease the formation of Fe₂B phase and α-Fe phase. The coating with 1:1 ratio of Ni-to-Fe exhibits the highest microhardness of 1200 HV_0.5 and superior corrosion resistance property due to its largest volume fraction of amorphous phase in the coating. Higher or lower than 1:1 ratio of Ni-to-Fe may result in lower amorphous forming ability. However, even that the coating with ratio of 3:2, shows a minimum of microhardness, it shows a better corrosion resistance than other two coatings.     

Microphase separation behavior on the surfaces of PEG-MDI-PDMS multiblock copolymer coatings

A series of poly(ethylene glycol)(PEG)-4,4′-diphenylmethanediisocyanate(MDI)-poly(dimethylsiloxane) (PDMS) multiblock copolymers were synthesized by employing two-step growth polymerization technique. Atomic force microscopy (AFM) observed nanoscopically well-organized phase-separated surfaces consisting of hydrophilic domain from PEG and MDI segments and hydrophobic domain from PDMS segments even with 50 wt.% PDMS in the copolymer, and the multiblock copolymer coatings presented a surface free energy of as low as 6-8 mN m⁻¹.     

Adsorption studies of thermal stability of SBA-16 mesoporous silicas

Cage-like ordered mesoporous silicas, SBA-16, and ethane-silicas with cubic (Im3m) and (Fm3m) symmetry groups were synthesized with addition of sodium chloride by using tetraethyl orthosilicate (TEOS) as silica precursor, 1,2-bis(triethoxysilyl)ethane (BTESE) as bridged silsesquioxane and poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer Pluronic F127 (EO₁₀₆PO₇₀EO₁₀₆) as template at low acid concentrations. The resulting samples were subjected to extraction in order to remove the polymeric template. The as-synthesized and extracted materials were calcined in the range of 350-900 °C to determine their thermal stability. Based on the XRD analysis and nitrogen adsorption data such as the BET specific surface area, volume of primary mesopores, pore wall thickness and pore size distributions, the SBA-16 silicas exhibit relatively high thermal stability because their mesostructural ordering was retained even up to 900 °C. However, an increase in the calcination temperature tended to decrease significantly the BET surface area, volumes of primary and complementary pores, and to less extent the pore size and pore wall thickness due to the structural shrinkage. Furthermore, the as synthesized samples subjected to a short extraction with acidic ethanol solution possessed even better thermal stability. On the other hand, calcination at 550 °C of ethane-silicas caused a complete removal of the ethane bridging groups from the periodic mesoporous organosilicas and their calcination above 800 °C led to the partial collapse of the structure.     

Corrosion behavior of sputtered Cr-Si-Ni and Cr-Si-Ni-Al resistive films in 0.1 M NaOH

The corrosion behavior of magnetron sputtered Cr-Si-Ni and Cr-Si-Ni-Al resistive films had been investigated by means of the relative resistance change (ΔR/R), polarization measurements, AES, and SEM in 0.1 M NaOH solution at 25 °C and 50 °C, which simulated an alkaline environment. The results revealed that both the annealed Cr-Si-Ni and Cr-Si-Ni-Al films in Ar ambient exhibited good corrosion resistance and long-term reliability in 0.1 M NaOH solution at 25 °C, due to the formation of a protective oxide layer on the surface of two types of the films during corrosion. However, the corrosion properties of two types of the films became degraded rapidly with the solution temperature at 50 °C. The studies showed that the pro-formation of a protective oxide layer on the surface of two types of the films by annealing in air had an enhancing effect on the corrosion properties of the films in 0.1 M NaOH solution at 50 °C, and that Cr-Si-Ni-Al films by annealing in air had more improving effect on the corrosion resistance and long-term reliability than Cr-Si-Ni films.     

Microstructure, microhardness, composition, and corrosive properties of stainless steel 304 I. Laser surface alloying with silicon by beam-oscillating method.

Laser surface alloying (LSA) with silicon was conducted on austenitic stainless steel 304. Silicon slurry composed of silicon particle of 5 μm in average diameter was made and a uniform layer was supplied on the substrate stainless steel. The surface was melted with beam-oscillated carbon dioxide laser and then LSA layers of 0.4–1.2 mm in thickness were obtained. When an impinged energy density was adjusted to be equal to or lower than 100 W mm⁻², LSA layers retained rapidly solidified microstructure with dispersed cracks. In these samples, Fe₃Si was detected and the concentration of Si in LSA layer was estimated to be 10.5 wt.% maximum. When the energy density was equal to or greater than 147 W mm⁻², cellular grained structure with no crack was formed. No iron silicate was observed and alpha iron content in LSA layers increased. Si concentration within LSA layers was estimated to be 5 to 9 wt.% on average. Crack-free as-deposited samples exhibited no distinct corrosion resistance. The segregation of Si was confirmed along the grain boundaries and inside the grains. The microstructure of these samples changed with solution-annealing and the corrosion resistance was fairly improved with the time period of solution-annealing. Received: 2 September 1999 / Accepted: 6 September 1999 / Published online: 1 March 2000     

Coating formation by plasma electrolytic oxidation on ZC71/SiC/12p-T6 magnesium metal matrix composite

Plasma electrolytic oxidation (PEO) of a ZC71/SiC/12p-T6 magnesium metal matrix composite (MMC) is investigated in relation to coating growth and corrosion behaviour. PEO treatment was undertaken at 350 mA cm⁻² (rms) and 50 Hz with a square waveform in stirred 0.05 M Na₂SiO₃.5H₂O/0.1 M KOH electrolyte. The findings revealed thick, dense oxide coatings, with an average hardness of 3.4 GPa, formed at an average rate of ∼1 μm min⁻¹ for treatment times up to 100 min and ∼0.2 μm min⁻¹ for later times. The coatings are composed mainly of MgO and Mg₂SiO₄, with an increased silicon content in the outer regions, constituting <10% of the coating thickness. SiC particles are incorporated into the coating, with formation of a silicon-rich layer at the particle/coating interface due to exposure to high temperatures during coating formation. The distribution of the particles in the coating indicated growth of new oxide at the metal/coating interface. The corrosion rate of the MMC in 3.5% NaCl is reduced by approximately two orders of magnitude by the PEO treatment.