Silica Sol-gel Coatings Research Articles

Sol-Gel SiO2 Coatings with Curcumin and Thymol on 3D Printouts Manufactured from Ti6Al4V ELI

Bacterial biofilm on implants may cause inflammation, which disturbs the process of the implant’s integration with the surrounding tissues. Such problems are becoming critical for patients’ health, especially in connection with the presence of antibiotic-resistant bacterial strains. Among the existing alternatives for drug treatments are natural-based substances. This study focused on the examination of silica coatings with curcumin and thymol, which were deposited using the sol-gel method on 3D printouts made of Ti6Al4V ELI. This substrate material is commonly used in medicine. The selective laser melting technique used for the manufacturing of samples was in line with the existing procedures applied for individual orthopedic implants. The examination involved the assessment of the coatings’ morphology, chemical composition, and biological effect. The antibacterial properties were tested using a flow cytometer using Escherichia coli, and the cytotoxicity on Saos-2 cells was assessed using the LIVE/DEAD test. The obtained results showed that it is possible to produce silica sol-gel coatings with the addition of specific natural substances in concentrations assuring a bacteriostatic effect. The produced coatings did not show any cytotoxic effect, which confirms the possibility of using both curcumin and thymol as additives to coatings used in medicine, e.g., for orthopedic implants.

Dominant factors for moisture resistance of sol–gel silica coatings: Surface chemical composition or inner microstructure?

Moisture-proof coating is essential to protect humidity-sensitive optical or electronic components. Generally, the moisture resistance of sol–gel coating depends primarily on the surface chemical composition and the internal microstructure of the coating. In this work, experimental studies were conducted on sol–gel silica coatings with different surface free energy (SFE) and pore volumes to elucidate the effect of the two contributing factors on moisture resistance. The results show that these two factors are both independent and synergistic. As the SFE or pore volume of the coating decreases, the moisture resistance is improved. However, when the SFE is reduced to around 10 mJ/m2, the silica coatings exhibit superior moisture resistance and are independent of the microstructure. Furthermore, once the pore volume of coatings is reduced to approximately 0.5 cc/g, the coatings exhibit notable moisture resistance, regardless of their surface chemical composition. This work provides fundamental insights and valuable recommendations for the design and fabrication of sol–gel moisture-proof coatings, with broad applicability in moisture-sensitive electronic components, photovoltaic cells, and cultural relics protection, among others.

Sol–Gel Silica Coatings for Corrosion Protection of Aluminum Parts Manufactured by Selective Laser Melting (SLM) Technology

Metal additive manufacturing is a rapidly growing field based on the fabrication of complex parts with improved performance. The advantages of using this technology include the production of shapes that cannot be produced by traditional machining technologies, the possibility of using trabecular reinforcing structures, and the ability to make parts with topological optimization that allow for increased performance and decreased mass of the parts produced. Metal parts produced by selective laser melting technology exhibit high surface roughness, which limits their direct implementation. Corrosion protection of these surfaces is difficult, especially for galvanic processes. This paper analyzes the possibility of using sol–gel silica (silicon oxide) coatings to effectively protect various surfaces of aluminum alloys produced by selective laser melting technology. Silicon oxide sol–gel protective coatings have demonstrated excellent chemical stability and corrosion resistance, being able to be applied in very thin layers. These properties make them excellent candidates for protecting additive-manufactured metal parts, especially as-built surfaces with a high surface roughness. Nanostructured silica sol–gel protective coatings have demonstrated excellent corrosion resistance and have the potential to replace the highly toxic chromium-based galvanic treatments. Using nanostructured silica sol–gel coatings, aluminum parts can be seamlessly integrated into circular-economy cycles.

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.

Improving corrosion protection of Mg alloys (AZ31B) using graphene‐based hybrid coatings

AbstractIn the present work, a study was carried out to compare the morphology and electrochemical properties of pure and graphene‐modified hybrid silica sol–gel coatings deposited on anodized AZ31B magnesium alloys. The precursor solution was prepared using tetraethoxysilane (TEOS) and 3‐glycidoxypropyl‐trimethoxysilane (GPTMS), with the addition of chemically modified graphene nanosheets (GN‐chem). Homogeneous and adherent coatings were obtained using GPTMS‐TEOS (GT) and GPTMS‐TEOS‐GN‐chem (GT/GN‐chem) solutions with uniform thickness around 4 µm. Raman, SEM images and EDS analyses confirmed the presence of graphene nanosheets in the coatings. Potentiodynamic polarization results using 0.05 M NaCl electrolyte solution showed that graphene containing silica coatings significantly improve the corrosion resistance of AZ31B alloys. In addition, a significant passive region was observed for GT/GN‐chem coatings.

Influence of Zirconia and Organic Additives on Mechanical and Electrochemical Properties of Silica Sol-Gel Coatings.

This paper presents the result of the investigation of organically modified silica (ORMOSIL)-zirconia coatings used to enhance their protective properties, namely corrosion and scratch resistance. Two different materials, i.e., SiO2/ZrO2 and SiO2/GPTMS/ZrO2, were synthesized, measured, and analyzed to find the difference in the used organosilane precursor (dimethyldiethoxysilane and (3-glycidoxypropyl)trimethoxysilane, respectively). SiO2/ZrO2 coatings showed higher hardness than SiO2/GPTMS/ZrO2. Moreover, the value of polarization resistance (Rp) for SiO2/GPTMS/ZrO2 coated 316L steel relative to the uncoated one was obtained. It was nearly 84 times higher. The coating delamination was observed with load 16N. Additionally, the corrosion mitigation for 316L coated by SiO2/GPTMS/ZrO2 was observed even after extended exposure to corrosion agents.

Preparation and Characterization of Some Sol-Gel Modified Silica Coatings Deposited on Polyvinyl Chloride (PVC) Substrates

Transparent and antireflective coatings were prepared by deposition of modified silica materials onto polyvinyl chloride (PVC) substrates. These materials were obtained by the sol-gel route in acidic medium, at room temperature (25 °C), using different alkoxysilanes with various functional groups (methyl, vinyl, octyl or hexadecyl). Physicochemical and microstructural properties of resulted silica materials and of thin coatings were investigated through Fourier Transforms Infrared Spectroscopy (FTIR), UV-Vis spectroscopy, Thermal Gravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA), Atomic Force Microscopy (AFM) and ellipsometric measurements. Wetting behaviors of the silica coatings were evaluated by measurement of static contact angle against water. FTIR spectra of materials confirmed the high degree of cross-linking that result from the formation of the inorganic backbone through the hydrolysis and polycondensation reactions together with the formation of the organic network. These sol-gel silica coatings showed a reduction in the reflectance (10%) compared with uncoated PVC substrate. AFM reveals that the films are uniform, and adherent to the substrate, but their morphology is strongly influenced by the chemical composition of the coating matrices. These silica coatings can be useful for potential electronic and optical devices.

Tuning refractive indices of sol-gel silica coatings by ammonia treatment for broadband antireflection applications

Broadband antireflection coatings via sol-gel processes have find wide applications in a variety of optical areas. However, to adjust refractive indices of coatings, most sol-gel methods involved complicated processes that modify the microstructures of sol particles. Here, we find that ammonia treatment of the sol-gel silica coatings can significantly tailor their refractive indices. Based on the findings, we created broadband antireflection coatings through preparing two-layer coatings. The coatings comprised two layers: a bottom layer with a high refractive index via ammonia treatment of the sol-gel silica coating, and a top layer with a low refractive index from the normal sol-gel coating. The two-layer coatings demonstrated broadband antireflection characteristics, with transmittance higher than 98 % from 350 to 1400 nm. Further hexamethyldisilazane treatment led to formation of hydrophobic coatings, which exhibited high environmental stability. The study demonstrates a new and simple method to create broadband antireflection coatings, and the broadband antireflection coatings could be attractive in many areas, such as solar cells and high-powered laser system.

Enhancing mechanical stability of sol-gel silica antireflection coatings via ammonia treatment at low temperature

Silica antireflection coatings from sol-gel processes are of great importance in optical areas because of their capability to reduce light reflection and hence to increase transmission. However, it retained a challenge to enhance the mechanical durability of the sol-gel antireflection coatings. One option is to introduce strong chemical bonding between neighboring silica nanoparticles in the coatings. Here, we demonstrate that ammonia treatment is an effective approach to enhance the mechanical durability of the silica antireflection coatings. The coatings after ammonia treatment exhibited excellent mechanical properties, with tensile hardness of 4.4 GPa, pencil hardness of 4H, and good abrasion resistance, which is noticeably better than the coatings without ammonia treatment. The ammonia treatment did not impair the optical property of the coatings. The coatings after ammonia treatment remained their good transmission, with peak transmittance as high as 99.8%. Mechanism analysis suggests that the enhancement of the mechanical durability is likely due to the condensation of silanol groups between the neighboring silica nanoparticles under ammonia catalysis. The ammonia treatment provides a promising alternative to current approaches for improving the mechanical durability of the sol-gel silica coatings.

Heat transfer and droplet population during dropwise condensation on durable coatings

Long-term sustainability of dropwise condensation (DWC) on treated surfaces is a key point for the exploitation of this heat transfer mechanism in industrial applications. A viable solution to achieve DWC, consisting of hybrid organic-inorganic sol-gel silica coatings containing hydrophobic moieties (methyl or phenyl group) is here presented. Different sol-gel coatings for DWC promotion were tested during condensation of steam in saturated conditions exhibiting heat transfer coefficient (HTC) around 100 kW m−2 K−1 in the heat flux range 100–500 kW m−2. Endurance tests have been performed at 400 kW m−2; an optimized sol-gel coating deposited on an aluminum substrate is shown to sustain DWC for more than 100 h without transition to filmwise condensation (FWC), which is an excellent result among those achieved on aluminum substrates. A comparison between the different coatings is done by surface characterization (contact angles measurements and Scanning Electron Microscopy) performed before and after condensation tests. Video analyses are carried out looking at droplet departing radius, droplets population and surface time renewal using a home-made software to detect the dimensions of the droplets. The present data are used to assess the expression proposed by Le Fevre and Rose (1966) for the droplet population, the equation by Kim and Kim (2011) for the departing radius and the model proposed by Chavan et al. (2016) for the heat transfer coefficient.

Hybrid Sol-Gel Silica Coatings Containing Graphene Nanosheets for Improving the Corrosion Protection of AA2024-T3.

In the present work, nanostructured graphene nanosheets were added to hybrid silica sols and deposited on aluminium alloy A2024-T3 to study the effect on the corrosion behaviour. Sols were prepared using tetraethyl-orthosilicate (TEOS), 3-glycidoxypropyl-trimethoxysilane (GPTMS) and a colloidal silica suspension (LUDOX) as silica precursors with adding chemically modified graphene nanosheets (GN-chem). The graphene nanosheets were modified through a straightforward and simple hydrothermal approach and then, dispersed into a silica sol (SiO2/GN-chem). ATR-FTIR was used to optimize the silica sol–gel synthesis and to confirm the cross-linking of the silica network. The corrosion behaviour of the SiO2/GN-chem coatings was also analysed by electrochemical measurement (potentiodynamic polarization) in 0.05 M NaCl solution. The results showed that the incorporation of modified graphene nanosheets into hybrid silica sol–gel coatings affected the corrosion properties of the substrates. An improvement in the corrosion resistance was observed likely due to the enhanced barrier property and hydrophobic behaviour obtained by incorporation of GN-chem and colloidal silica nanoparticles.

Durability of microporous hybrid silica coatings: Optical and wetting properties

Bifunctional hybrid silica sol-gel coatings were synthesized, which showed both increased light transmittance and water-repellence. The as-prepared coatings of variable thickness (100–175 nm) were found to have low refractive index (1.250–1.275), high porosity (36%–42%), and maximum light transmittance up to 99.8%. Water-repellence of the surface was due to high water contact angle (90°) and low contact angle hysteresis (1–2°). Ellipsometric porosimetry and transmission electron microscopy measurements confirmed the microporous structure of the coatings. A systematic investigation for studying the durability and the deterioration of optical and surface properties was studied during aqueous soaking tests at different pH. During these experiments atomic force microscopy showed that the morphology of the surface did not change. The changes in the wetting properties were also confirmed by surface free energy measurements and an increase of the polar component was observed. The optical properties could be regenerated completely or partially after the soaking via heat treatment at 400 °C. The light transmittance of the samples remained high after the soaking, most likely due to the swelling of the solid matrix. The light transmittance and the water-repellent properties of the coatings showed long-term stability up to a year of storage.

Optimization of Hybrid Sol-Gel Coating for Dropwise Condensation of Pure Steam.

We developed hybrid organic–inorganic sol–gel silica coatings with good durability in harsh environment (high temperatures, high vapor velocities) and with slightly hydrophobic behavior, sufficient to promote dropwise condensation (DWC) of pure steam. DWC is a very promising mechanism in new trends of thermal management and power generation systems to enhance the heat transfer during condensation as compared to film-wise condensation (FWC). The sol–gel coatings have been prepared from methyl triethoxy silane (MTES) and tetraethyl-orthosilicate (TEOS) and deposited on an aluminum substrate. The coatings were optimized in terms of precursor ratio and annealing temperature highlighting potentials and limits of such mixtures. A comprehensive surface characterization before and after saturated steam condensation tests has been performed and related to the thermal measurements for evaluating the heat transfer augmentation as compared to FWC obtained on untreated aluminum surfaces. The results showed that the developed hybrid organic-inorganic sol–gel silica coatings are promising DWC promoters.

Sol-gel based coatings for the protection of cultural heritage textiles

In this paper, the procedure for the development of sol-gel silica coatings for the protection of cultural heritage textiles, using a multistep approach, is described. With this aim, Tetraethoxysilane precursor was used at two different concentrations to realize coatings onto textile samples, using Dibutyltindiacetate as polycondensation catalyst for sol-gel reactions. The relationship between the presence of the catalyst, the number of coating layers deposited on cotton fabrics, and the influence of such architecture on several properties of treated textiles, were investigated. In particular, the chemical structure of thin films and their morphology were studied by infrared spectroscopy and scanning electron microscopy, respectively. The thermo-oxidative properties and the abrasion resistance of sol-gel treated cotton fabrics were studied, as well as the durability of the coatings after different washing cycles. Both silica coatings provided high adhesion onto cotton textiles, while the higher silica concentration resulted in higher thermal stability and washing fastness of treated fabrics. An enhancement in the washing fastness and abrasion resistance for the two different concentrations of precursor in the presence of the catalyst was also observed. All experimental findings demonstrated the efficiency of sol-gel based materials specifically tailored to the protection and preservation of cultural heritage textiles.

Dual Role of Lithium on the Structure and Self-Healing Ability of PMMA-Silica Coatings on AA7075 Alloy.

In this work, structural and active corrosion inhibition effects induced by lithium ion addition in organic-inorganic coatings based on poly(methyl methacrylate) (PMMA)-silica sol-gel coatings have been investigated. The addition of increasing amounts of lithium carbonate (0, 500, 1000, and 2000 ppm), yielded homogeneous hybrid coatings with increased connectivity of nanometric silica cross-link nodes, covalently linked to the PMMA matrix, and improved adhesion to the aluminum substrate (AA7075). Electrochemical impedance spectroscopy (EIS), performed in 3.5% NaCl aqueous solution, showed that the improved structural properties of coatings with higher lithium loadings result in an increased corrosion resistance, with an impedance modulus up to 50 GΩ cm2, and revealed that the lithium induced self-healing ability significantly improves their durability. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) suggest that the regeneration process occurs by means of lithium ions leaching from the adjacent coating toward the corrosion spot, which is restored by a protective layer of precipitated Li rich aluminum hydroxide species. An analogue mechanism has been proposed for artificially scratched coatings presenting an increase of the impedance modulus after salt spray test compared to the lithium free coating. These results evidence the active role of lithium ions in improving the passive barrier of the PMMA-silica coating and in providing through the self-restoring ability a significantly extended service life of AA7075 alloy exposed to saline environment.

Control of degradation rate of Mg alloys using silica sol–gel coatings for biodegradable implant materials

Silica sol–gel coatings were deposited on AZ31B and AZ91D magnesium substrates with the aim of controlling the degradation rate, corrosion attack and further dissolution of magnesium alloys for temporal implants. Two different silica sols were prepared with and without colloidal silica particles (MTL and TG sols) and deposited by dip-coating on both alloys. The coatings were characterised in SBF using three different in vitro tests; hydrogen evolution, pH variation and potentiodynamic polarization curves. The results show that the corrosion behaviour depends on the alloy and on the coating composition. Coatings on AZ91D-MTL were able to block the degradation of the alloy during 8 days of immersion in SBF, whereas coatings AZ91D-TG only maintain their stability during 3 days. Mg(OH)2 and hydroxyapatite (HAp) were identified as the compounds precipitated on AZ31B samples, not appearing on AZ91D samples. The electrochemical tests confirm the promising corrosion results obtained for coated AZ91D samples.

Waterborne epoxy-thiol decorated silica sol-gel coatings: impact of crosslinking on corrosion prevention

Organic–inorganic hybrid coatings of crosslinked epoxy and thiol silicates (TGST) were synthesized for the corrosion protection of low carbon steel via a waterborne sol-gel route. The coatings were synthesized in two stages; in the first step two separate aqueous formulations of epoxy and thiol decorated silica colloids were prepared and subsequently crosslinked in a 1:1 ratio in the second stage. The structure and rate of crosslinking was monitored using Raman spectroscopy. Formulation stability was measured based on viscosity changes and pot-life assessment. The novel material was characterized for its particle size using dynamic light scattering. Viscosity average molecular weight was calculated using Fikentscher’s equation and using polyethylene oxide molecular weights as a calibration standard. The covalently bonded epoxy-thiol silicate thin-film coating was uniform and defect free. Accelerated corrosion resistance was tested by exposing the panels to standard salt solution tests. TGST showed a 96% improvement in corrosion inhibition over the non-crosslinked formulations in a corrosive environment. The synthesis and characterization of TGST using a simple approach, revealed key synthetic parameters for generating optimized sol-gel corrosion prevention materials.

Preparation of Antireflective Silica Coating by the Sol-Gel Method for Heliothermic Power Plants

This work deals with the deposition of a thin layer of porous silica antireflective coating onto glass substrates. The films were deposited with different withdrawal speeds and heat-treated at 425°C for 30 minutes. The effects of heat treatment and film deposition rate on the films reflectance were evaluated. The diffuse reflectance was measured using ultraviolet-visible (UV-Vis) spectroscopy. Scanning electron microscopy (SEM) was used for microstructural evaluation of the films. The water contact angle upon the films surface was evaluated using a tensiometer and was based on the sessile drop technique. The mechanical characteristics of the films were evaluated by tape test and pencil hardness. The obtained sol-gel silica coatings were homogeneous and free of cracks. UV-Vis analysis of the glass substrate revealed a reflectance value of 3.86%, whereas the lowest reflectance value obtained for antireflective coatings was 2.72%. The contact angle measurement showed that, for all films, there was wetting of the film by water, characterizing them as hydrophilic. The adhesion of the films were 4B and the pencil hardness were 3H.

Hybrid sol–gel coatings based on GPTMS/TEOS containing colloidal SiO2 and cerium nitrate for increasing corrosion protection of aluminium alloy 7075-T6

One of the promising candidates to replace the chromate conversion coatings for corrosion protection of aluminium alloy AA7075 are the hybrid sol–gel coatings. In the present work hybrid silica sol–gel coatings doped with cerium nitrate were prepared and characterized. Tetraethoxysilane (TEOS) and 3-glycidoxypropyl-trimethoxysilane (GPTMS) were used as precursors. Silica SiO2 (Ludox) particles were added to achieve a barrier properties of coating, while Ce(NO3)3·6H2O was added in order to obtain an active corrosion protection. Optimization of sol synthesis was based on the results of ATR-FTIR spectroscopy and UV–vis–NIR spectroscopy. Opening of epoxy rings and completion of hydrolysis and the condensation reactions during the synthesis process were confirmed. Coatings were characterized through thickness, water contact angle, roughness, adhesion, electrochemical properties (potentiodynamic and electrochemical impedance spectroscopy) and the response to prolonged immersion time in 0.1 M NaCl. The high degree of cross-linking of Si–O–Si network structure and high density was achieved during the synthesis of the sol. Moreover, the results showed that the curing process and the incorporation of cerium nitrate into the hybrid sol–gel coating affected to the corrosion properties of the coating. The observed enhancement in corrosion protection properties is attributed to the combination of the barrier properties of the silica matrix with the active protection of the cerium nitrate.

Enhanced visible transmittance and reduced transition temperature for VO2 thin films modulated by index-tunable SiO2 anti-reflection coatings.

The low visible transmission is one of the bottleneck problems for the application of vanadium dioxide films since the high refractive index (RI) of VO2 films results in strong reflection in the visible wavelength. To address this problem, in this paper, high-purity VO2 films were deposited on fused silica by DC reactive magnetron sputtering at low temperature of 320 °C. Silica sol–gel coatings with tunable refractive index (RI) coated onto VO2 films have been fabricated to enhance visible transmittance with the potential application in the field of smart windows. SiO2 coatings with tunable RI (1.16–1.42 at λ = 700 nm) were prepared by sol–gel dip-coating technique. The double structure SiO2/VO2 films were characterized through several techniques, including X-ray diffraction, UV-VIS-NIR spectrophotometry and scanning electron microscopy. Compared with the single-layer VO2 film (ΔTsol of 6.25% and Tlum of 38.58%), the three kinds of SiO2/VO2 bilayer films had higher Tlum (41.93–50.44%) and larger ΔTsol (8.15–8.51%) simultaneously due to significantly decreased reflectance. Moreover, the crystallization properties of VO2 films are essentially unchanged by applying a SiO2 top layer, while the phase transition temperature and thermal hysteresis width of sample S116 are lower than those of pure VO2 film. The presented RI-tunable SiO2 coatings, can regulate optical properties continuously for various VO2 substrates, paving the way for practical applications of VO2 films in the field of smart windows or others.