Fabrication and characterization of bioactive composite coatings on Mg–Zn–Ca alloy by MAO/sol–gel

High corrosion rate and accumulation of hydrogen gas upon degradation impede magnesium alloys’ clinical application as implants. In this work, micro-arc oxidation (MAO) was used to fabricate a porous coating on magnesium alloys as an intermediate layer to enhance the bonding strength of propolis layer. Then the composite coatings were fabricated using sol–gel method by dipping sample into the solution containing propolis and polylactic acid at 40°C. The corrosion resistance of the samples was determined based on potentiodynamic polarization experiments and immersion tests. Biocompatibility was designed by observing the attachment and growth of wharton’s jelly-derived mesenchymal stem cells (WJCs) on substrates with MAO coating and substrates with composite coatings. The results showed that, compared with that of Mg–Zn–Ca alloy, the corrosion current density of the samples with composite coatings decreased from 5.37 × 10⁻⁵ to 1.10 × 10⁻⁶ A/cm² and the corrosion potential increased by 240 mV. Composite coatings exhibit homogeneous corrosion behavior and can promote WJCs cell adhesion and proliferation. In the meantime, pH value was relatively stable during the immersion tests, which may be significant for cellular survival. In conclusion, our results indicate that composite coatings on Mg–Zn–Ca alloy fabricated by MAO/sol–gel method provide a new type bioactive material.     

Hardness and wear resistance improvement of surface composite layer on Ti–6Al–4V substrate fabricated by powder sintering

A wear resistant surface composite layer on Ti–6Al–4V substrate was fabricated using powder sintering method. The surface composite layer consisted of Ti–6Al–4V matrix and different fractions of TiN particles as reinforcement phase. The surface layer and the substrate were directly bonded together while the powders were cold formed and then sintered at an elevated temperature. The two layers showed good metallurgical bond. In this study, 5%, 10% and 15% TiN weight fractions were adopted to fabricate the surface composite layer. Effects of TiN addition on the microstructure, hardness and wear resistance were investigated. It was found that the wear resistance of the surface composite layer was improved due to the addition of TiN compared to that of pure Ti–6Al–4V.     

Densification of mullite–SiC nanocomposite sol–gel precursors by pressureless sintering

Mullite–SiC nanocomposite has been synthesised based on a nanoprecursor route and sintered to high density through pressureless sintering technique. The approach has been first to evolve a method to obtain high density, fine-grained mullite matrix phase through a sol–gel seeded route. Nanosize SiC particles (∼200 nm) were dispersed in a seeded mullite precursor sol to obtain mullite-coated SiC particles, which were further compacted and sintered to hybrid composites, resulting in distribution of SiC particles at the inter- and intra-mullite grain positions.     

Effect of solution pH on the surface morphology of sol–gel derived silica gel

We have examined the effect of solution acidity on the textural characteristics of silica gels prepared by sol–gel synthesis using tetraethyl orthosilicate (TEOS) as a silica precursor and cetyltrimethylammonium bromide (CTAB) as a template. Using IR spectroscopy, we have studied micellar TEOS solutions and the synthesized silica gel samples. The results demonstrate that, in an alkaline medium in a water–ethanol solution, SiO₂ experiences short-range ordering on the surface of micelles formed by CTAB molecules, whereas in an acid medium the process is not influenced by the presence of CTAB. Nitrogen porosimetry and electron microscopy data indicate that the silica gel obtained at pH 2 is microporous, with an average pore size of 2 nm. In an alkaline medium at pH 10, we obtained mesoporous SiO₂ (18 nm) with a narrow pore size distribution and a specific surface area of 110 m²/g.     

Preparation of DNA–cyclodextrin–silica composite by sol–gel method and its utilization as an environmental material

A DNA–cyclodextrin–silica composite was prepared by the sol–gel method. This composite possessed the bi-functions of double-stranded DNA, such as intercalation into DNA, and cyclodextrin, such as inclusion into its intramolecular cavity. Therefore, we demonstrated the accumulation of harmful compounds from an aqueous multi-component solution using a DNA–cyclodextrin–silica composite column. As a result, the DNA–cyclodextrin–silica composite column can effectively accumulate not only planar structure-containing harmful compounds, such as dioxin and polychlorobiphenyl (PCB) derivatives, but also non-planar structure containing compounds, such as bisphenol A and diethylstilbestrol, from an aqueous multi-component solution. The accumulated amount of these harmful compounds was more than 90%. Additionally, the DNA–cyclodextrin–silica composite column was recycled by the application of methanol. Therefore, the DNA–cyclodextrin–silica composite may have the potential to be used as an environmental material for the accumulation of harmful compounds from industrial or experimental waste.     

Preparation of nanoporous silica–zirconia layers by in situ sol–gel method

Abstract

Silica–zirconia membranes were prepared using a sol–gel process, never before used on this system. Thin layers of gel were grown on the outside of porous alumina support tubes using the permeation of water through pores of the tube to control the rate of hydrolysis with a reactive silica–zirconia alkoxide solution. A recipe was developed and optimum coating conditions were found to be the ratios of tetraethylorthosilicate (TEOS) to Zr propoxide to 1-propanol molar ratios (1:0·5:17), with a coating time of 300 s. The method allowed the formation of a membrane in a single step compared with repeated coating and firing. The coatings could be prepared adherent and without cracks when properly prepared.     

Structure and biological response of polymer/silica nanocomposites prepared by sol–gel technique

Structure of P(EMA-co-HEA)/SiO₂ nanocomposites with silica content in the range from 0 to 30 wt.% was correlated with cell behavior on substrates of those compositions by making use of two different populations of primary human cells: articular cartilage chondrocytes and dental pulp cells. Substrates were prepared by the simultaneous copolymerization of the organic monomers and the sol–gel reaction of the silica precursor in different proportions, which led to weight fractions of the silica phase in the materials closely matching the stoichiometric ratios employed during the preparation, both in the bulk and at the material surface. The silica nanophase increases surface wettability and improves the mechanical properties of the base materials. Both chondrocytes and dental pulp cells were cultured on serum-coated nanocomposite substrates in the same conditions, but very different cellular responses were obtained. While chondrocytes adhered and proliferated, dental pulp cells formed viable aggregates weakly adhered on the sample that were viable up to 11 days. The results suggest that these sol–gel derived nancomposites may be used as culture surfaces maintaining the dental pulp cell phenotype in vitro.     

Role of particle size of alumina on the formation of aluminium titanate as well as on sintering and microstructure development in sol–gel alumina–aluminium titanate composites

With a view to develop low temperature fine grained alumina–aluminium titanate composite, influence of alumina particle size on the temperature of formation of the aluminium titanate, sintering behaviour and microstructure development of alumina–aluminium titanate composite prepared through a sol–gel core shell approach is reported. The alumina matrix composite containing 20 wt% aluminium titanate has been prepared from alumina powders having different average particle size in the range 300–600 nm. The alumina particle size appears to have no significant influence on the formation temperature of in situ formed aluminium titanate. However, the microstructural analysis of the dense ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increase in the alumina particle size. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. Smaller starting alumina particle size led to finer grain size composites. The present study therefore shows that although the starting particle size of alumina has no significant role on the lowering of formation temperature of aluminium titanate, it does influence the microstructure of the composite.     

Corrosion resistance of electrodeposited Ni–Al composite coatings on the aluminum substrate

Ni matrix–Al particle composite coating was adopted via sediment co-deposition (SCD) method on the zincate coated aluminum substrate. Surface morphology was investigated by scanning electron microscopy (SEM). The electrochemical behavior of the coatings was studied by polarization potentiodynamic test in 3.5 wt.% sodium chloride using a three electrode open cell. The effect of the electroplating parameters on the Al co-deposition was studied. Maximum of 22 wt.% Al particles were deposited in the coating. It was found that the zincate coating plays an important role in improving the nickel layer adherent. Furthermore, incorporation of aluminum particles in Ni matrix refined the Ni crystal coatings. However, polarization curves shifted to negative potentials and corrosion rate is decreased.     

Adsorptive properties of sol–gel derived hybrid organic/inorganic coatings

Hybrid organic-inorganic sol–gel derived coatings have been prepared by dip coating on glass substrates from alcoholic solutions of tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), 3-aminopropyltriethoxysilane (APTES), 3-glycidoxypropyltrimethoxysilane (GPTMS) and phenyltriethoxysilane (PhTES). The hybrid materials have been fully characterized by means of solid state NMR spectroscopy and X-ray diffraction. The degree of cross-linking and the extent of interaction between silica and silsesquioxane phases appear dependent on the ratio between TEOS and organotrialkoxysilane and on the chemical features of the organic function linked to silicon, and influence the sorption ability towards aromatic compounds of hybrid films. The hybrid coatings have been put into an optical-grade quartz chamber placed into a UV–VIS–NIR spectrophotometer and the organic compounds have been allowed flowing through the chamber recording of molecule absorption spectra vs. time. Absorbance curves vs. time have also been collected at a fixed wavelength for different molecule-coating couples and simple kinetic models have been used for comparing the adsorption capability of the different films, which has been related to the chemical interactions between molecules and coatings.     

Studies in structural characterization of silica–heteropolyacids composites prepared by sol–gel method

Heteropolyacids (HPAs) which are included in mesoporous silica, H₃PMo₁₂O₄₀/SiO₂ and H₄PMo₁₁VO₄₀/SiO₂ have been synthesized by a sol–gel technique that involves hydrolysis of ethyl orthosilicate. The effect of incorporation of heteropolyacids species on silica matrix was studied by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Raman spectroscopy, thermo gravimetric analysis (TGA) and differential thermal analysis (DTA), N₂ adsorption–desorption, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).     

Optical and morphological characterization of Si nanocrystals/silica composites prepared by sol–gel processing

Linear (visible photoluminescence) and non-linear (third-order susceptibility) optical properties have been measured for laser-synthesized crystalline Si nanoparticles. The observed luminescence is compared to similar measurements on nanocomposite aerogels prepared integrating the Si-nanoparticles into a continuous silica phase by sol–gel processing. The results show that incorporation of the Si-nanoparticles into a silica matrix by sol–gel technology is not detrimental to the PL (photoluminescence) emission intensity. The effect on the nanocomposites of annealing in air (at temperatures in the range of 300–500°C) has been investigated by means of PL and Raman spectroscopy. The obtained results indicate that the oxidation treatment induces the bleaching of the smaller luminescent particles and is not effective in reducing the average size. Micro-Raman mapping and TEM analysis of the nanocomposites have shown the presence of agglomerated nanoparticles (in the sub-micrometer range), as confirmed by dynamic light scattering (DLS) measurements on colloidal suspensions of the as-synthesized powders.     

Preparation of mullite fibres by sol–gel processes using aluminium carboxylates and tetraethylorthosilicate

Abstract

Mullite fibres were prepared using aluminium carboxylates (ACs) and tetraethylorthosilicate by sol–gel process. ACs were synthesised from dissolution aluminium in a mixture of formic acid and glacial acetic acid using aluminium chloride hexahydrate as catalyst. The optimum condition for obtaining ACs is as follows: the molar ratio of aluminium, formic acid and acetic acid was 1∶3∶2·26 and aluminium chloride hexahydrate was 10 wt-%. All the Al and Si components were mixed at the molecular level and linear molecules were formed in the precursor sol. The dried gel fibres completely transformed to mullite fibres at 1200°C and the calcinated fibres had a smooth surface and uniform diameter.     

Fabrication of silica glass containing yellow oxynitride phosphor by the sol–gel process

Abstract

We have prepared silica glass by the sol–gel method and studied its ability to disperse the Ca-α-SiAlON:Eu²⁺ phosphor for application in white light emitting diodes (LEDs). The emission color generated by irradiating doped glass with a blue LED at 450 nm depended on the concentration of SiAlON and the glass thickness, resulting in nearly white light. The luminescence efficiency of 1-mm-thick glass depended on the SiAlON concentration, and was highest at 4 wt% SiAlON.     

Fabrication of silica glass containing yellow oxynitride phosphor by the sol–gel process

We have prepared silica glass by the sol–gel method and studied its ability to disperse the Ca-α-SiAlON:Eu²⁺ phosphor for application in white light emitting diodes (LEDs). The emission color generated by irradiating doped glass with a blue LED at 450 nm depended on the concentration of SiAlON and the glass thickness, resulting in nearly white light. The luminescence efficiency of 1-mm-thick glass depended on the SiAlON concentration, and was highest at 4 wt% SiAlON.     

Preparation of polycrystalline YAG/alumina composite fibers and YAG fiber by sol–gel method

Polycrystalline yttrium–aluminum garnet, Y₃Al₅O₁₂ (YAG) fiber and α-alumina and YAG matrix composite fiber were prepared by the sol–gel method. α-Alumina and YAG matrix composite fiber with fine and homogeneous microstructure could be successfully fabricated by interpenetrating YAG in alumina matrix and adding α-alumina of seed particles to fibers. Effect of α-alumina seed particles and YAG on crystallization and microstructure of composite fiber were discussed. The size of alumina matrix of the composite fibers heated at 1600°C for 4 h was below 2 μm. The tensile of strength alumina fiber heat-treated at 1500°C was 0.2 GPa, while that of the composite fiber was 1.1 GPa.     

Measuring mechanical properties of coatings: a methodology applied to nano-particle-filled sol–gel coatings on glass

The main aim of this paper is to demonstrate the practical use of nano-indentation and scratch testing in determining mechanical properties of thin coatings. We place our emphasis on how information obtained using both techniques can be combined to give a more complete representation of the properties of a coating–substrate system. Part I of the paper gives an overview of approaches to determine mechanical properties of thin coatings that have been proposed in the literature, and develops them further to be useful tools in the analysis of coatings. This results in methods for measuring the mechanical properties of thin coatings. We particularly emphasise the determination of the elastic modulus, hardness, coating and interfacial fracture toughness and residual stress using indentation and scratch testing. Part II of the paper illustrates the application of these methods to relatively soft coatings of methyltrimethoxysilane (MTMS) filled with colloidal silica or alumina particles on glass. The coatings were prepared using a sol–gel process. We report results of the dependence of the mechanical properties on the filler particle content, illustrating that microstructural changes can also be tracked using these techniques. The effects of the nature and volume fraction of the filler particles are discussed.     

In-situ synthesis and characterization of CeO2–TiO2 composite coatings on titanium substrate by micro-arc oxidation

Journal of Materials Science: Materials in Electronics - Ceramic coatings were fabricated on titanium substrate by the micro-arc oxidation (MAO) treatment in compound electrolyte with different...     

Characteristics of CeOx–VO2 composite thin films synthesized by sol–gel process

Pure vanadium dioxide (VO₂) and CeOx–VO₂ (1.5 < x < 2) composite thin films were grown on muscovite substrate by inorganic sol–gel process using vanadium pentaoxide and cerium(III) nitrate hexahydrate powder as precursor. The crystalline structure, morphology and phase transition properties of the thin films were systematically investigated by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, FE-SEM and optical transmission measurements. High quality of the VO₂ and CeOx–VO₂ composite films were obtained, in which the relative fractions of +4 valence state vanadium were above 70 % though the concentrations of cerium reached 9.77 at %. However, much of cerium compounds were formed at the edge of grains and the addition of cerium resulted in more clearly defined grain boundaries as shown in SEM images. Meanwhile, the composite films exhibited excellent phase transition properties and the infrared transmittance decreased from about 70 to 10 % at λ = 4 μm bellow and above the metal–insulator phase transition temperature. The metal–insulator phase transition temperatures were quite similar with about 66 °C of the pure VO₂ and CeOx–VO₂ composite thin films. But hysteresis widths increased with more addition of cerium, due to the limiting effect of grain boundaries on the propagation of the phase transition. Particularly, the CeOx–VO₂ composite film with an addition of 7.82 at % Ce showed a largest hysteresis width with about 20.6 °C. In addition, the thermochromic performance of visible transmittance did not change obviously with more addition of cerium.     

Preliminary investigation on joining performance of SiCp-reinforced aluminium metal matrix composite (Al/SiCp–MMC) by vacuum brazing

The present study has investigated the joining performance of SiC particulate reinforced aluminium metal matrix composite (Al/SiC_p–MMC) by vacuum brazing process. After the joints brazed with Al–Si–Mg foil brazing filler metal at different brazing batches, both the mechanical properties and the microstructures of brazed joints were estimated. Moreover, the influence of SiC_p size, SiC_p volume percentage and the brazing parameters on bonding quality of the joints have also been discussed in detail. The results have revealed that the bonding quality either in SiC_p/Al interface or in SiC_p/SiC_p interface belongs to weak bonding, and the results also show that under the same brazing parameters condition, the strength of brazed joints decreases along with increasing the SiC_p volume percentage. In addition, the results also indicate that for a constant SiC_p volume percentage the strength of brazed joints decreases when SiC_p size increases. These results are very useful for the joining design of discontinuously reinforced metal matrix composites and further for the optimum design of composition of composites.