Sol-gel Coatings Research Articles

A Review of Coating Methods and Their Applications in Compression and Spark-Ignition Engines for Enhanced Performance

Coating of metallic surfaces such as cylinder heads, cylinder liners, piston heads and exhaust-valves in Internal Combustion Engines (ICEs) has improved engine performance in areas of brake-thermal-efficiency including brake-specific-fuel consumption, etc. This review showed different methods of coating in compression and spark-ignition engines for improved performance using different coating thicknesses on the metallic surfaces. These coating thicknesses may be either minimum or maximum depending on the coating method used. The coating methods discussed in this paper were Thermal Spraying (TS), Physical-Vapour Deposition (PVD), Chemical-Vapour Deposition (CVD) and Ion Implantation methods. Minimum thicknesses were achieved using the PVD, CVD and Ion-Implantation methods. In addition, Sol-Gel and Slurry coatings were also discussed. Having considered these methods of coating, thermal-spraying method was recommended for compression and spark-ignition engines because it produces temperature ranging from 8000 K-14000 K on surfaces with thick coating thicknesses.

Investigations of the Thermal Parameters of Hybrid Sol–Gel Coatings Using Nondestructive Photothermal Techniques

This article presents the results of comparative investigations of thermal parameters of hybrid sol–gel coatings (named TMZ) prepared from tetraethyl orthosilicate and organically modified 3-methacryloxypropyltrimethoxysilane. The coatings were prepared with the addition of zirconium(IV) tetrapropoxide chelated with methacrylic acid. Two series of samples were investigated: the first series, TMZ-I, TMZ-II and TMZ-III, with different amounts of zirconium, and the second series, TMZ-I/Ce, TMZ-II/Ce and TMZ-III/Ce, with the addition of cerium nitrate. The influence of the amount of zirconium and cerium on the thermal parameters of the sol–gel coatings was next analyzed. Two non-destructive and photothermal techniques were used for this purpose: photothermal radiometry (PTR) and beam deflection spectroscopy (BDS). The thermal diffusivity and conductivity of the coatings were extracted from the frequency experiments and are presented and discussed. The two-layer model was applied to interpret the photothermal spectra. The results obtained using these two techniques are compared and discussed.

Complex effects of Mg-biomaterials on the osteoblast cell machinery: A proteomic study.

The cell-biomaterial interface is highly complex; thousands of molecules and many processes participate in its formation. Growing demand for improved biomaterials has highlighted the need to understand the structure and functions of this interface. Proteomic methods offer a viable alternative to the traditional in vitro techniques for analyzing such systems. Magnesium is a promoter of cell adhesion and osteogenesis. Here, we used the LC-MS/MS to compare the protein expression profiles of human osteoblasts (HOb) exposed to sol-gel coatings without (MT) and with Mg (MT1.5Mg) for 1, 3, and 7days. PANTHER, DAVID, and IPA databases were employed for protein identification and data analysis. Confocal microscopy and gene expression analysis were used for further characterization. Exposure to MT1.5Mg increased the HOb cell area and the expression of SP7, RUNX2, IBP3, COL3A1, MXRA8, and FBN1 genes. Proteomic analysis showed that MT1.5Mg affected the early osteoblast maturation (PI3/AKT, mTOR, ERK/MAPK), insulin metabolism, cell adhesion (integrin, FAK, actin cytoskeleton regulation) and oxidative stress pathways. Thus, the effects of Mg on cell adhesion and osteogenesis are rather complex, affecting several pathways rather than single processes. Our analysis also confirms the potential of proteomics in biomaterial characterization, showing a good correlation with in vitro results.

Non-resonant luminescence enhancement in sol-gel coatings for broadband UV-light conversion on side-emitting optical fibers

Photoluminescence (PL) is a key mechanism for many light conversion applications but often provides only low yield and poor efficiency due to a small interaction cross-section and a relatively narrow range of spectral activity. Here, we present a simple technique to enhance the PL of europium(III) (Eu3+) from sol-gel coatings on the surface of a side-emitting optical fiber. We use small clusters of Ag to boost the emission of Eu3+ by an energy transfer mechanism. The coating's performance is studied as a function of Ag concentration and annealing time. We report a substantial enhancement in Eu3+ emission under resonant and non-resonant excitation with UV light. However, this enhancement vanishes when the Ag clusters grow into larger nanoparticles with plasmon activity: we demonstrate that silver clusters produce stronger amplification with a broader excitation range than metallic nanoparticles. Moreover, these clusters are easily generated and stabilized in a coating using standard sol-gel techniques, suitable for deposition on the surface of side-emitting fibers. Such fibers can then be employed as line-shaped emitters, for example, for structured illumination purposes.

The development of phosphorus-doped hybrid silica sol coating for silk with durable flame retardancy

The fabrication of phosphorus-doped sol-gel coatings for durable flame retardant (FR) modification of textiles are of interest. This study facilely synthesized a novel phosphorus-doped silica sol system using 3-aminopropyltriethoxysilane, glutaraldehyde, diethyl phosphite by simultaneous Kabachnik-Fields and sol-gel reactions. The morphology and chemical structures of hybrid silica particles were studied by TEM, FT-IR and NMR. Then, the hybrid silica sol was applied as durable FR coating to reduce the flammability of silk. The FR property and washing durability, smoke and heat generation capacity, thermal stability, and FR mechanism of coated silk fabrics were explored. The hybrid silica coating displayed good FR performance because of the synergistic action among Si, P and N. The coated fabrics self-extinguished after 25 launderings and had a high LOI value of 28.0% thanks to the covalent grafting of hybrid silica coating on silk fabrics. The explorations on silk residues confirmed the intumescent FR mechanism of coated silk. The hybrid silica coating is promising to functional as feasible and durable FR strategy for silk fabric.

Anticorrosion Properties of a Novel Hybrid Sol-Gel Coating on Aluminum 3003 Alloy.

In this study, a novel hybrid sol–gel coating on AA3003 substrate was developed and the effects of various waste material additives on the reinforcement of the sol–gel coating and the anticorrosion properties in the saline medium were investigated. Egg shell, crumb rubber, activated carbon obtained for pyrolysis of waste rubber tire, waste rubber tire, cement kiln dust, and ST100 additives were tested as reinforcement materials. The AFM characterization results of the coating formulations on AA3003 alloy revealed enhanced roughness values for the modified coatings as compared to the base coating. Similarly, no significant changes were detected in the Fourier transform infrared spectroscopy (FTIR) absorption peaks of the hybrid polymeric material upon loading it with the waste additives, while slight changes in the hydrophobic properties of the final modified coatings were observed as a result of the modification process. Electrochemical impedance spectroscopy (EIS) results revealed that the hybrid sol–gel coating had a promising potential for the protection of the AA3003 substrate against corrosion in the saline medium. However, the loaded additives negatively affected the corrosion resistance properties of the parent hybrid sol–gel coating. For instance, the egg shell additive had the least negative effect on the barrier properties, whereas the cured coating layer of the sample loaded with cement and clay additives showed some disintegration, inhomogeneity, and low barrier properties on the metal surface.

Design and construction of the dipole magnet of the space plasma environment research facility (SPERF) for simulating the earth magnetosphere

A dipole magnet is designed in the SPERF to create the magnetic field for simulating the large-scale magnetosphere of the earth. The design and construction process of the dipole magnet is presented here to settle the issues in regard to electricity, vacuum, and support, etc. Several measures are taken to improve the magnet's insulation performance so as to meet the challenge imposed by a high voltage for achieving an intensive pulsed current. Meanwhile, to avoid influencing the plasma quality, a layer of Inconel is sprayed on the magnet's outer surface covered with a sawtooth-shaped sol-gel coat in advance. The designed magnet itself is validated by the measurement with respect to electrical properties, magnetic field, and cooling. In addition, a transition structure is proposed to transfer the intensive current from two coaxial cables to the leads of the magnet reliably. A supporting and motion mechanism is designed for the magnet to increase the flexibility of the magnetic field, and its strength is evaluated under the most severe condition. The dipole magnet design lays a foundation for the SPERF operation.

Hybrid Epoxy-Alkyl Sol-Gel Coatings Reinforced with SiO2 Nanoparticles for Corrosion Protection of Anodized AZ31B Mg Alloy.

AZ31B Mg alloys were anodized at different potentials using an alkaline electrolyte. Then, an epoxy-alkyl silane sol reinforced with SiO2 nanoparticles was prepared by sol–gel and deposited on top of the optimized anodic layers. 1-Methyl imidazole was added to the sol to promote a partial epoxy ring aperture and improve the condensation degree of the inorganic network. The results showed the curing temperature affects the inorganic polycondensation of the organic-inorganic network; this effect was analyzed by 29Si and 13C solid-state NMR spectroscopy. Electrochemical impedance spectroscopy in 3.5 wt% NaCl solution revealed that the corrosion resistance is enhanced by the anodized process obtained for Mg alloy anodized at 100 V/2 min. However, a quick deterioration of the oxide film with immersion time was evident, showing a reduction of the protection efficiency (ηE%) of 76.5% after 16 h/immersion. The deposition of an epoxy-alkyl coating improved the ηE% up to 98.6% after 72 h/immersion. The proposed hybrid coating used for post-sealing the porous anodized Mg alloy looks like a good alternative protective barrier to control the corrosion process of Mg alloys. A suitable compromise between cross-linking network and curing temperature is necessary to obtain a good barrier coating.

Review of Cr-Free Coatings for the Corrosion Protection of Aluminum Aerospace Alloys

Aluminum alloys are known to have many advantages (e.g., light weight and low cost) but they are not immune to corrosion. So, it is important to assess their corrosion behavior, in particular under atmospheric conditions. To protect aluminum alloys against corrosion, paints are generally applied onto the materials. Corrosion protection in the aerospace industry consists of a conversion or anodized coating, an inhibited primer, and a top-coat. Chromate conversion coating (CCC) and primers containing chromate pigments have been widely used in the aerospace industry over the last decades. However, new environmental regulations have led to major changes for aluminum corrosion protection. By limiting or prohibiting some chemicals, for instance Cr(VI), the European regulation REACH (Regulation on Registration Evaluation, Authorization and Restriction of Chemicals) has induced major changes to some of the finishing processes of aluminum alloys (e.g., chromate conversion, chromic acid anodizing, and chromate sealing). Interesting results have been obtained while seeking replacements for Cr(VI), for example, with the incorporation of cerium, lithium salt, or nanocontainers loaded with corrosion inhibitors in organic coatings. For several years, hybrid sol–gel coatings able to replace the pre-treatment and primer steps have been under development, showing interesting results. New prospects for the future involve the use of photopolymerization to reduce the energy-intensive heat treatment needed in sol–gel technology. It will also be necessary to test these new technologies in service conditions or in accelerated corrosion tests before being able to conclude on the real effectiveness of these coatings. This review summarizes the recent developments in Cr-free coatings for aluminum alloys. Their advantages and drawbacks are also discussed.

Validation of adhesion characterization methods for antistick coatings applied in cooking systems

This study aims to validate an effective methodological analysis for the characterization of the adhesion values of antistick coatings employed in cooking systems. The morphology and chemistry of four different industrial products were investigated by roughness measurements, electron microscopy observations and infrared spectroscopy analysis. The adhesion of the coatings was evaluated comparing various techniques, such as the surface contact angle measurements, the pull-off test and the single lap-joint tensile shear method, in synergy with degradation phenomena simulated by P.E.I. abrasion and dishwashing tests. The single lap-joint tensile shear test was able to evaluate evident differences between all the proposed coatings, regardless of their state of deterioration, featuring the best selectivity among the used methods. However, the pull-off test proved to be a viable and more economical alternative for fast evaluation techniques, considering the overall instrument costs. While all the polytetrafluoroethylene-based systems turned out to be totally uninfluenced by the chemical attacks performed during the dishwasher cycles, the sol–gel coating exhibited lower performances of the applied releasing agent, before and after the mechanical degradation tests. Therefore, sol–gel coatings, despite being proposed as valid alternatives to PTFE ones, do not achieve the same quality standards in terms of releasing properties, being in addition much more sensible to degradation and loss in performances.

Performance and durability of thin film solar cells via testing the abrasion resistance of broadband anti-reflection coatings

Reflection from the front glass of solar modules causes over 4% optical loss leading to a significant decrease in module efficiency. Single layer solution gelation (sol-gel) anti-reflective (AR) coatings are effective over a narrow range of wavelengths, whereas reflection losses can be reduced over a broader wavelength when multilayer broadband AR coatings are applied. In this work, three different multilayer AR coatings including 4-layer SiO2/ZrO2, 4-layer SiO2/ITO, and 6-layer SiO2/ZrO2 were deposited using magnetron sputtering. The abrasion resistance is important because the coatings will be subject to regular cleaning cycles. A variety of abraders including Felt pad, CS-10 and CS-8 under different loads are used. The optical performance and durability of these coatings were analyzed using a spectrophotometer, optical microscope, scanning electron microscope, and scanning white light interferometer. No damage was observed after abrasion of the coatings with a felt pad under 1 and 2 N loads. However, there was a slight increase in Weighted Average Reflection. When coatings were tested with CS-10 and CS-8 abraders, coatings with ZrO2 resulted in higher scratch resistance in comparison to coating with ITO. However, all-dielectric broadband AR coatings are more durable and have better optical performance compared to single layer sol-gel coatings.

Formation of Composite Coatings on Galvanized Steel from Organosilane Solutions Using Electrophoresis and Sol-Gel Technology.

New hybrid composite coatings (HCCs) on hot-dip galvanized steel (HDGS) were obtained using electrophoresis (cathodic polarization (CP)) and sol–gel technology. For this purpose, a technique for the preparation of a cationic precursor based on TiCl4 and aminopropyltriethoxysilane was developed. Electrophoresis of the charged particles of the precursor and organosilanes promotes the production of denser sol–gel coatings with improved adhesion. Using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) methods, the formation mechanism and protective properties of HCC on galvanized steel were investigated.

Influence of the Micro-Nanostructuring of Titanium Dioxide Films on the Photocatalytic Degradation of Formic Acid under UV Illumination.

Surface micro-nanostructuring can provide new functionalities and properties to coatings. For example, it can improve the absorption efficiency, hydrophobicity and/or tribology properties. In this context, we studied the influence of micro-nanostructuring on the photocatalytic efficiency of sol-gel TiO2 coatings during formic acid degradation under UV illumination. The micro-nanostructuring was performed using the UV illumination of microspheres deposited on a photopatternable sol-gel layer, leading to a hexagonal arrangement of micropillars after development. The structures and coatings were characterized using Raman spectroscopy, ellipsometry, atomic force microscopy and scanning electron microscopy. When the sol-gel TiO2 films were unstructured and untreated at 500 °C, their effect on formic acid’s degradation under UV light was negligible. However, when the films were annealed at 500 °C, they crystallized in the anatase phase and affected the degradation of formic acid under UV light, also depending on the thickness of the layer. Finally, we demonstrated that surface micro-nanostructuring in the form of nanopillars can significantly increase the photocatalytic efficiency of a coating during the degradation of formic acid under UV light.

Improvement of the environmental stability of sol-gel silica anti-reflection coatings

Improvement of the environmental stability of sol-gel silica anti-reflection coatings

Tannic acid reinforced sol-gel silica coatings for corrosion protection of zinc substrates

Silica (SiO2) coatings are a widely discussed inorganic coating alternative, offering a feasible and simple method of passive corrosion protection, with the option of enhancement by incorporating different inhibitors into the matrix. The novelty of the present study lies in the reinforcement of the sol-gel SiO2 coating system, by the addition of different tannic acid (TA) amounts in the sol phase and producing nano scale coatings by dip-coating technique, on Zn. Tannic acid is a non-toxic substance, which acts as anodic corrosion inhibitor. Long term corrosion measurements were carried out to check the durability and corrosion resistance of the thin coatings. They were characterized by Electrochemical Impedance Spectroscopy and Potentiodynamic polarization curves, as well as by physical methods (SEM, contact angle and adhesion measurements). The pseudo-porosity of the coatings was also evaluated. It has been concluded that SiO2 coatings containing TA in an optimal concentration show lower corrosion current densities than simple SiO2 coatings, good adhesion to the zinc substrate, and lower porosity. The interactions occurring between the SiO2 and TA put in evidence by FT-IR, Raman spectroscopy offer an explanation of the better corrosion behavior of the SiO2+TA coating compared to the pure SiO2.

Towards AquaSun practical utilization: Strong adhesion and lack of ecotoxicity of solar-driven antifouling sol-gel coating

The outcomes of adhesion and ecotoxicity tests carried out on metal specimens faithfully representing the surface of real ships, including the primer and tie coat layers typically applied on ship hull prior to deposition of the antifouling paint, show the practical applicability of “AquaSun” antifouling sol-gel coatings. Newly developed AquaSun coatings share hydrophobicity (contact angle >115°) and exceptionally high scratch resistance (ASTM 5B). Coupled to the ecofriendly antifouling mechanism based on continuous H2O2 formation upon exposure to solar light and foul release due to low surface energy, these results open the route to the practical utilization of these novel marine coatings.

Matrix compatibility of typical sol-gel solid-phase microextraction coatings in undiluted plasma and whole blood for the analysis of phthalic acid esters.

Sol–gel materials have been widely used for solid-phase microextraction (SPME) coatings due to their outstanding performance; in contrast, sol–gel SPME coatings have seldom been used for in vivo sampling. The main reason is that their matrix compatibility is unclear. In order to promote the application of this type of coating and accelerate the development of in vivo SPME, in this study, the matrix compatibility of several typical sol–gel coatings was assessed in plasma and whole blood using phthalic acid esters as analytes. The service life of five kinds of sol–gel coatings was among 20–35 times in undiluted plasma, while it was 27 times for a homemade commercial polydimethylsiloxane coating, which indicates good matrix compatibility of sol–gel coatings in untreated plasma. The sol–gel hydroxy-terminated silicone oil/methacrylic acid fiber achieved the highest extraction ability among all of the fibers, and it was tested in pig whole blood. It could be continuously used for at least 22 times, demonstrating good potential for in vivo sampling. Subsequently, a direct-immersion SPME/gas chromatography-flame ionization detection method was established for the determination of 5 phthalic acid esters in blood. Compared with other methods reported in the literature, this method is rapid, simple, sensitive, and accurate, and does not need expensive instruments or tedious procedures. A simulation system of animal blood circulation was constructed to verify the practicability of sol–gel SPME coatings in animal vein sampling. The result illustrated the feasibility of that coating for in vivo blood sampling, but a more accurate quantification calibration approach needs to be explored.Graphical abstract Supplementary InformationThe online version contains supplementary material available at 10.1007/s00216-022-03890-2.

Novel Hydrophobic Nanostructured Antibacterial Coatings for Metallic Surface Protection

A simple and cost-efficient method to modify different surfaces in order to improve their bioactivity, corrosion and wear resistance proved to be sol-gel coatings. The silane layers have been shown to be effective in the protection of steel, aluminum or magnesium alloys and copper and copper alloys. Moreover, it has been found that the adding of different inorganic nanoparticles into silica films leads to increasing their performance regarding corrosion protection. In this study, we fabricated, a simple sol-gel method, transparent mono- and bi-layered hydrophobic coatings with simultaneous antibacterial, hydrophobic and anti-corrosive properties for the protection of metallic surfaces against the action of air pollutants or from biological attacks of pathogens. The first layer (the base) of the coating contains silver (Ag) or zinc oxide (ZnO) nanoparticles with an antibacterial effect. The second layer includes zinc oxide nanoparticles with flower-like morphology to increase the hydrophobicity of the coating and to improve corrosion-resistant properties. The second layer of the coating contains a fluorinated silica derivative, 1H,1H,2H,2H-perfluorooctyl triethoxysilane (PFOTES), which contributes to the hydrophobic properties of the final coating by means of its hydrophobic groups. The mono- and bi-layered coatings with micro/nano rough structures have been applied by brushing on various substrates, including metallic surfaces (copper, brass and mild steel) and glass (microscope slides). The as-prepared coatings showed improved hydrophobic properties (water CA > 90°) when compared with the untreated substrates while maintaining the transparent aspect. The corrosion resistance tests revealed significantly lower values of the corrosion rates recorded for all the protected metallic surfaces, with the lowest values being measured for the bi-layered coatings containing ZnO particles, both in the first and in the second layers of the coating. Considering the antibacterial activity, the most effective were the AOAg-II and AOZnO-II coatings, which exhibited the highest reduction of microbial growth.

Investigation of the Effect of Pretreatment of the Carbon Fiber Surface on the Continuity of the Sol-Gel Coating

The paper considers the dependence of the continuity of the sol-gel coating of carbon fiber on the preliminary anodic oxidation of the surface in a sodium hydroxide solution. The effect of the treatment time, the voltage between the cathode and the anode, NaOH concentration in the electrolyte, and the temperature of the electrolyte was investigated. It is shown that with an increase in NaOH concentration in the electrolyte, the area of uncoated fiber surface sections increases, the continuity of the coating decreases, while the coating obtained without preliminary surface treatment has sufficient continuity of at least 93%. Changes in other parameters (temperature, time, voltage) of preliminary electrochemical surface treatment in the ranges under study did not allow us to fix regularities other than the given one.

UV resistance of sol-gel hydrophobic silica antireflective coatings

UV resistance of sol-gel hydrophobic silica antireflective coatings