Silica Sol-gel Coatings Research Articles

Comportamiento a desgaste de recubrimientos de material compuesto de matriz de aluminio fabricados por proyección térmica

In this work, the wear behaviour of coatings made of aluminium matrix composites reinforced with 20% of SiC particles and manufactured by thermal spray method with oxyacetylene flame has been investigated. The wear behaviour between coatings with uncoated particles and sol-gel silica coated ones heat treated at 500 °C and 725 °C have been compared. The sprayed coatings with silica coated particles are more homogenous and less porous due to increase of wettability by molten aluminium that takes place on coated particles. The microstructure of the sprayed coatings, the wear surfaces and the wear debris have been anlysed using optical microscopy, scanning electron microscopy and microanalysis techniques (EDX). The results show a smaller wear rate, a lower friction coefficient and more reduced loss of mass for the coatings sprayed with particles with sol-gel silica coatings than those made with uncoated particles.

Wear resistance of multilayered sol–gel silica layers on aluminium matrix composites

Sol–gel silica coatings have been developed, using a dip-coating process to provide corrosion and wear resistance to aluminium composites reinforced with SiC particles. Pin-on-disk tests indicate that these ceramic coatings reduce the adhesive wear between the steel pin and composite. The coatings extracted at lower deposition speeds are thinner than those extracted at faster speeds, but they show better wear resistance which can be explained by their greater hardness caused by a better consolidated structure.

Chemical composition and barrier properties of Ag nanoparticle-containing sol–gel films in oxidizing and reducing low-temperature plasmas

Silver nanoparticle-containing silica sol–gel coatings were synthesised using tetraethylorthosilicate (TEOS), HNO 3 and AgNO 3 as the main precursors and 3-(2-aminoethylamino)propyl trimethoxysilane (DIAMO) as a stabiliser. The stability and the barrier properties of the coatings were studied by means of UV/Vis spectroscopy, FTIR reflection absorption spectroscopy (IRRAS), spectroscopic Ellipsometry and electrochemical impedance spectroscopy (EIS). Both the long-term stability and barrier properties of the coatings could be tailored by low-temperature O 2 and H 2-plasma treatments. The oxidative plasma treatment enabled the change of the matrix from organic–inorganic to silica-like and the reductive treatment led to a reduction of the formed passive film on the Ag nanoparticles within the matrix. Thereby, based on the combination of sol–gel chemistry with subsequent plasma treatment new thin film functionalities can be achieved.

Investigations into sol-gel silica and silica hybrid coatings for dielectromagnetic soft magnetic composite applications

A series of inorganic and organic–inorganic hybrid silica coatings were synthesised by a sol-gel process from tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMS) precursors, and used to investigate their effectiveness as insulation materials in dielectromagnetic soft magnetic composites. The coating materials and coated iron powders were analysed by FTIR, electron and atomic force microscopy. Results showed that introduction of the organic phase imparted desirable hydrophobicity and flexibility to the coating, whilst still providing effective electrical insulation at temperatures up to 500 °C. The hybrid coatings covered the iron particle surface very effectively and formed continuous coatings that could remain intact even after compaction at pressures up to 900 MPa, depending on the coating amount. Initial magnetic characterisations are also encouraging. As a result, such hybrid-coated iron powders can be considered suitable candidates as heat-treatable high-performance dielectromagnetics.

Cerium hybrid silica coatings on stainless steel AISI 304 substrate

AISI 304 Stainless Steel is widely used in different industrial fields because of its mechanical and corrosion properties. However, its tendency to corrosion in presence of halide ions limits the applications. One strategy to improve the corrosion resistance is the use of coatings barriers containing corrosion inhibitors in their formulation. The lanthanides present attractive green and corrosion properties for the substitution of chromates, which are the most common substances used as corrosion protection. However, these compounds are highly toxic, and an intense effort is being undertaken to replace them. Cerium is a good alternative because of its relatively low cost and abundance. It fulfils the basics requirements for being considered an alternative inhibitor: the ions form insoluble hydroxides and they present low toxicity. Inorganic and hybrid sol-gel coatings have been developed to increase the corrosion resistance of metals and they provide an excellent vehicle for the incorporation of secondary phases including particles and metal ions as cerium ions. The aim of this work was to study the influence of the incorporation of cerium ions in hybrid silica sol-gel coatings deposited on AISI 304 stainless steel as substrate as a potential replacement of chromate treatments. This system should combine the barrier protection effect of silica coating with the corrosion inhibitor effect of the cerium ions inside the coatings. After 7 days of immersion in NaCl, coated substrates showed lower current densities than the bare steel, although the coatings produced from Ce (III) salts experience a slight weakening in time and those obtained from Ce (IV) chemicals evidence an enhance in the coating performance, probably due to the plugging of corrosion products in the defective areas of the film.

Sol–gel coatings with phosphonate functionalities for surface modification of magnesium alloys

Hybrid organic–inorganic coatings with phosphonate functionalities have been synthesized and evaluated as prospective surface treatments for magnesium materials. These coatings have been processed via a sol–gel route by hydrolysis and condensation of a mixture of diethylphosphonatoethyltriethoxy-silane and tetraethoxy-silane with variable molar ratios. The coatings morphology and the surface chemistry at the coating/substrate interface have been characterized using scanning electron microscopy, X-ray photoelectron spectroscopy, 31P nuclear magnetic resonance spectroscopy, and time-of-flight secondary ion mass spectrometry. The corrosion protection performance of the coatings deposited on magnesium alloy AZ31B has been examined by a group of electrochemical techniques including potentiodynamic polarization and electrochemical impedance spectroscopy. In addition, a scanning Kelvin probe technique has been used to investigate interfacial properties of the coatings. The improved corrosion protection of phosphonate-containing coatings as compared to pure silica sol–gel coatings has been observed and explained by the favorable combination of barrier properties of the organo-silicate matrix with strong chemical bonding of phosphonate groups to the magnesium substrate.

Sol-gel coatings on carbon steel: Electrochemical evaluation

Degradation of carbon steel has always been a concern. The use of coatings is especially recommended in aggressive atmospheres at moderate temperatures. Ceramic films can be used to improve the resistance against high temperature oxidation and corrosion of metals. Amid the different options, a sol-gel process provides a low cost, simple and non-hazardous method for processing ceramic coating with controllable composition and microstructure. This work evaluates the electrochemical behaviour of carbon steel coated by sol-gel method. Hybrid organic–inorganic silica sol-gel coatings were obtained by dip coating of planar samples in an organically modified silica sol made from hydrolysis and polycondensation of tetra-orthosilicate (TEOS) and methyltriethoxisilane (MTES) by acidic catalysis. Coatings free of defects were obtained at a sintering temperature of 400 °C. The coated samples were inspected by optical and electron microscopy and coating thickness was measured by using a Talystep surface roughness tester. Electrochemical evaluation was made by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. A comparison of the corrosion resistance of the coated metal with the uncoated one is presented. The measurements show the improvement of the corrosion resistance of the coated carbon steel.

Synthesis of hybrid silica sol–gel coatings containing Zn particles on carbon steel and Al/Zn coated carbon steel

Owing to increasing interest of coatings applied on low alloy steel and galvanised metal plates and their applications, this work presents the study of the process requirements to be fulfilled in order to coat these metals with an hybrid silica coatings containing Zn particles. The main idea was to combine the “barrier” effect of the hybrid silica coatings with the “active protection” effect of the zinc particles inside the coatings. Sol–gel parameters, Zn particles content and suspension stability have been studied. EMPHOS ES 21, an electrostatic dispersant, has been used to obtain stabilised suspensions. Thick, adherent and crack-free coatings containing 10% Zn particles were prepared by dip-coating on carbon steel and Al/Zn coated carbon steel substrates. The applied coating seems to improve corrosion protection of the two metal substrates, and this effect is more noticeable in the case of the Al/Zn system because of the combination of the complementary corrosion protection properties of both coatings.

Hybrid silica sol–gel coatings on Austempered Ductile Iron (ADI)

Sol–gel organic–inorganic hybrid coatings are being increasingly used to protect metal surfaces. The presence of non-hydrolysable groups in their structure can lead to the formation of coatings with properties intermediate between glasses and organic polymers. Crack-free, homogeneous and adherent hybrid silica sol–gel coatings around 1 μm thick were obtained by dip-coating on glass and Austempered Ductile Iron (ADI). Homogeneous multilayer coatings up to 4 μm were obtained on glass substrates, but not on ADI substrates since coating cracked after the densification of a second layer. The hardness of the coating on ADI substrates was slightly higher than that of as received ADI. Preliminary electrochemical impedance polarization spectroscopy (EIS), potentiodynamic polarization and polarization resistance tests in NaCl solution have shown an improvement of corrosion resistance of ADI.

Electrochemical deposition of black nickel solar absorber coatings on stainless steel AISI316L for thermal solar cells

Abstract We report the electrochemical deposition of nanostructured nickel-based solar absorber coatings on stainless steel AISI type 316L. A sol–gel silica-based antireflection coating, from TEOS, was also applied to the solar surface by the dip-coating method. We report our initial results and analyze the influence of the stainless steel substrate on the final total reflectance properties of the solar absorber. The relation between surface morphology, observed by SEM and AFM, the composition of the electrodeposited surfaces analyzed by X-ray powder diffraction and the study of different electrodeposition conditions and silica sol–gel coatings is described. The best solar absorptance and thermal emittance values obtained on stainless steel substrates were 0.91and 0.1, respectively.

Preparation and characterization of cerium doped silica sol–gel coatings on glass and aluminum substrates

Chromates are among the most common substances used as corrosion inhibitors. However, these compounds are highly toxic, and an intense effort is being undertaken to replace them. Cerium compounds seem to fulfil the basic requirements for consideration as alternative corrosion inhibitors. The aim of this work was to study the effect of the incorporation of cerium ions in silica sol–gel coatings on aluminum alloys as potential replacement of chromate treatments. The main idea was to combine the ‘barrier’ effect of silica coatings with the ‘corrosion inhibitor’ effect of the cerium inside the coatings. Thin (below 1μm for a single layer) and transparent cerium doped silica sol–gel coatings were prepared by dipping 3005 aluminum alloys in sol–gel solutions. Ultra-violet–visible spectra (UV–vis) show that cerium ions, Ce3+ and Ce4+, always are present in the coatings, independently of the cerium salt or firing atmosphere used. Active protection with single and two layer coatings prepared with Ce (IV) salt seems to improve corrosion protection of the coated aluminum while coatings prepared with Ce(III) salt only entails a protection when applied as a two layer, possibly due to sealing of pre-existent defects in the first layer. The improvement of active protection with immersion time would imply that corrosion is inhibited by cerium ions that migrate through the coating to the site of the attack (a defect in the coatings) and then react to passivate the site.

Silica Sol-Gel Coatings on Metals Produced by EPD

The objective of this work has been to combine the sol-gel method and the electrophoretic deposition (EPD) process to prepare thick coatings onto metallic substrates. Two different routes were used for preparing the sol-gel silica suspensions. On one hand, silica particulate sols were obtained by basic catalysis of alkoxides and alkylalkoxides. On the other, silica suspensions were prepared by adding a commercial colloidal silica sol to an organic-inorganic acid catalysed silica sol. The properties of the suspension and the physical parameters associated to EPD (current density, potential, electric field and deposition time) were studied. Crack-free deposits up to 20 µm were obtained after drying and crack-free glass-like coatings of 12 µm after sintering at 500 ◦ C for 30 minutes. The electrochemical behaviour of these coatings was evaluated by potentiodynamic methods, showing an excellent behaviour against corrosion.

Incorporation and optical behaviour of 4-dimethylaminazobenzene in sol-gel silica thin coatings

Various percentages of 4-dimethylaminazobenzene (dimethyl yellow, DY) have been incorporated into silica sol-gel coatings prepared from tetraethylorthosilicate and methyltriethoxysilane. The dye was entraped in the sol-gel layer maintaining its acid-basic properties. Sensitivity of coatings against pH in aqueous media was evaluated by absorption spectrophotometry for each type of samples, including one pre-treated at low temperature. The response time of coatings from acid pH to neutral conditions and vice versa allows to propose the use of this system as a pH optical sensor. On the other hand, several tests oriented to point out the behaviour of coatings during chemical attack were performed: firstly by dipping the samples in distilled water for different lengths of time and, secondly, submitting them to reiterated cycles at 60 °C in a washing machine using conventional abrasive detergent. Results obtained in both cases indicated good resistance, especially for those hybrid films prepared from tetraethylorthosilicate and methyltriethoxysilane doped with 5 wt.% DY.

Preparation and characterisation of calcein-doped thin coatings

Silica sol-gel coatings doped with calcein (CA) were prepared and optically charecterised by means of UV-VIS spectroscopies (optical absorption and luminescence). In addition, the stability of samples against thermal treatment, UV exposure and alkali attack (chemical durability) were studied. Microstructure was verified using transmission electron microscopy and specific surface area was measured using the BET method. Results highlighted the ability of sol-gel coatings to react reversibly to pH changes when immersed in liquid media at different pH values. The limits for maximum optical absorption sensitivity were determined to be 5<pH<9, while the 8.5<pH<14.0 and 0<pH<2ranges could be detected using photoluminescence spectroscopy. Stability studies showed good behaviour against UV (maximum loss of 50% in peak intensity after 30 h of continuous exposure) and good chemical durability under alkaline conditions (80 h attack time was necessary to reduce the percentage absorption to 50%). Thermal stability of doped films was good up to 130 °C (in this range a reduction of about 25% in the intensity of the initial peak absorption is lost).

Luminescence time decay from Cu + ions in sol–gel silica coatings

UV photo-excited luminescence from Cu-doped sol–gel silica coatings has been investigated. The intense emission in the visible part of the spectrum is assigned to electronic transitions 3d 94s 1→3d 10 in Cu + ions. Photoluminescence time decay corresponding to this emission has been measured in the temperature range of 80–300 K. The curves resulted biexponential functions with two temperature-dependent time constants. The results have been analysed in terms of a three level model including the effect of non-radiative transitions. A comparison between these results and those obtained from Cu + ions embedded in crystalline matrices shows differences in the value of the parameters governing the luminescent process. These differences are interpreted in terms of the local environment of Cu + in the glassy network.

Corrosion resistance of stainless steel coated with sol–gel silica

Sol–gel silica films were deposited on AISI 304 stainless steel using tetraethyl ortosilicate (TEOS) as chemical precursor. The coated steels were heat-treated from room temperature to 600°C. The structural evaluation of the coated steel samples was made by scanning electron microscopy, infrared absorption spectroscopy, and chemical deep profiles of Si, O, and Fe obtained by Rutherford backscattering spectroscopy. Corrosion experiments were carried out on samples with and without coatings, in 1 N H 2SO 4 and 3.5% NaCl solutions to measure corrosion potential, current density and pitting potential. The obtained sol–gel silica coatings were homogeneous, free of cracks and had thickness from 50 to 120 nm. Polarization curves of the coated samples indicated improvement of the corrosion resistance. For tests carried out in a 1 N H 2SO 4 solution, the corrosion potential increased from −341 mV (stainless steel) to +383 mV for coated sample heat-treated at 80°C, and up to +508 mV for coated sample heat-treated at 200°C. The corrosion current density decreased from 3.65 μA/ cm 2 to 0.027 μA/ cm 2 and 0.021 μA/ cm 2 , for the 80°C and 200°C heat-treated silica coatings, respectively. For the corrosion experiment carried out in a 3.5% NaCl solution, the coated sample (heat-treated at 80°C) increased the corrosion potential c.a. 230 mV and increased by one order of magnitude the pitting potential.

Characteristic UV luminescence from sol–gel silica coatings

Photoluminescence (PL) from silica coatings prepared via sol–gel under different conditions and using different precursors are investigated. Upon excitation with 3.85 eV (322 nm) light, the coatings prepared in air show a PL spectrum dominated by a peak at 3.35 eV (370 nm) with a shoulder at about 2.95 eV (∼420 nm). After heat treatments under reducing conditions or when the coatings are prepared in an oxygen-controlled atmosphere, the PL spectrum at 3.35 eV exhibits a significant decrease in intensity. Results are discussed on the basis of silanol groups at superficial regions, oxygen-rich defects and the presence of residual organic matter in the coatings.

Formation and Depth Profiling Analysis of Multilayers Derived from Sol-Gel Processing Using Dynamic SIMS, RBS, and TEM

Silica sol-gel single layer AR coatings are used in high peak power pulsed lasers due to their high laser induced damage threshold (LIDT) and their low refractive index (1.22). We have used sol-gel processing to spin and dip coat multilayers of alternating high index (zirconia/hafnia) and low index (base catalysed silica) sol-gels on to fused silica substrates. When tailored at the correct thickness these stacks have shown >95% reflectivity at 355 nm and normal incidence whilst retaining a high LIDT. Depth profiling using Dynamic Secondary Ion Mass Spectrometry (DSIMS) and Rutherford back scattering (RBS) through these multilayer coatings has revealed the effect of increasing the number of layers in the stack. Results are presented for both spin and dip coated multilayers and a significant difference in the interfacial boundary is seen between the two coating processes. These differences are related to changes in the LIDT of the coatings. Individual layer thicknesses were estimated using this technique and compared to values gained from UV-Visible spectroscopy. TEM analysis of an ion-milled cross-section of the multilayers was performed showing the colloidal silica coatings and the depth of interpenetration of the interfaces.

Effect of the silica sol–Gel coatings on the properties of glass substrate

Strengthening of the glass substrate with silica sol-gel coatings was investigated. Sol–gel silica coating microstructure (is related to the degree of substrate strengthening), as it is known depends on the silica sol structure. In order to produce different microstructures of the silica coating, a variety of silica sols were prepared starting from TEOS and changing the water and solvent content, catalyst type and concentration, at room temperature. Carefully prepared glass surfaces were coated with silica sol using a dipping technique, with constant withdrawing rate (all physical parameters of the deposition were constant, since they can additionally change the coating microstructure). After specific heat treatment of the samples, the modulus of rupture was calculated using the measured flexural strength data. The degree of the substrate strengthening depends on the quantity of water used for the sol preparation, coating sol pH, state of the substrate surface and the degree of the coating bonding to the substrate (all mentioned parameters are discussed as to how they effect the silica sol structure). Results showed that strengthening of the glass substrate is effected by the microstructure of the silica coating.

Preparation and characterization of Nd3+ and Er3+-doped silica sol-gel coatings by Rutherford backscattering spectroscopy and spectroscopic ellipsometry

Neodymium- or erbium-doped silica films are deposited on single crystal silicon substrates using a sol-gel process and a spin-coating technique. These glasses are doped with neodymium or erbium in various Nd/Si or Er/Si atomic ratios up to 8% using neodymium nitrate or erbium nitrate as precursor. A preparation method of such films is described. Film rare earth concentration measured by Rutherford Backscattering Spectroscopy (RBS) is the same as in the initial liquid solution. Film thickness and refractive index are obtained by variable angle spectroscopic ellipsometry. We have shown that both RBS analysis and spectroscopic ellipsometry are powerful tools to control rare earth doping level and optical properties of the silica films.