Reinforced sol–gel thermal barrier coatings and their cyclic oxidation life

Cyclic oxidation life enhancement of sol–gel thermal barrier coatings is obtained via the reinforcement of the controlled micro-crack network that forms during the initial sintering of the deposit. Two different sol–gel methods are used to fill in the process-induced cracks, namely dip-coating and spray-coating. Filling parameters, for instance the number of passes or the viscosity of the sol are adjusted, using various techniques such as profilometry and microstructural analysis, to optimise crack filling. Cyclic oxidation tests are implemented at both 1100 °C and 1150 °C to investigate the efficiency of the various reinforcement procedures developed and address the influence of the specific microstructure on the oxidation behaviour.     

Slow sintering in garnet-containing Y and Gd zirconate–aluminate mixtures for thermal barrier coatings

Mixtures of rare-earth zirconates and aluminates containing Y or Y + Gd that form a two-phase garnet–fluorite mixture exhibit much slower sintering than pure fluorite at 1400°C. An equivalent Y-free, Gd-containing composition that forms a perovskite aluminate instead of garnet showed faster densification after the metastable garnet decomposes. At 1500°C, the Y-free sample also showed the fastest initial sintering rate, whereas there was more divergence in the sintering rate for the samples containing Y + Gd. The zirconate–aluminate with equimolar Y + Gd shows the slowest densification at 1500°C and retains ∼25% porosity after 250 h. The results highlight possibilities for designing compliant thermal barrier coatings that can retain significant porosity at 1400°C or higher.     

Bond strength of hybrid sol–gel coatings with different additives

The hybrid sol–gel coating on Al 2024-T3 was modified by adding polyaniline, TiO₂, or γ-Al₂O₃ nanoparticles in the formulation separately. The coating was then used as an adhesive to bond Al 2024-T3 alloys, forming a single lap joint. The bond strength of the sol–gel coating was investigated using a universal tensile test machine. The lap shear strength of the original sol–gel coating was about 1.38 MPa and it was increased up to 2.26 MPa after the modification by adding 0.05 wt% PANI microparticles in the sol–gel coating. The small increase in strength was attributed to an improvement in its adhesive flexibility because of incorporation of the long-chain organic polymer in its structure. Furthermore, the addition of different amounts of TiO₂ nanoparticles in the unmodified sol–gel coating also led to an increase in shear strength compared to the undoped sol–gel coating. Typically, a sol–gel coating containing 2.0 wt% of TiO₂ recorded the highest adhesive strength of about 4.0 MPa. A similar increase in strength was observed when doping γ-Al₂O₃ nanoparticles into the original hybrid sol–gel coating. Adding 0.5 wt% of γ-Al₂O₃ in the sol–gel coating increased the adhesive bonding strength up to 4.48 MPa. The fracture surface of the specimen was separately observed by SEM and Optical Microscopy in order to examine potential evidences of mechanism and nature of failure. The reason why the adhesive strength increased after the modification of the sol–gel coating is discussed in this article.     

Composite anticorrosion coatings for AZ91D magnesium alloy with molybdate conversion coating and silicon sol–gel coatings

This study evaluated the corrosion resistance of AZ91D magnesium alloy coated by composite coatings which consisted of a molybdate conversion coating and three layers of silicon sol–gel coatings. For molybdate conversion treatment, various conditions including the pH of the molybdate baths, immersion time and bath temperature were investigated using electrochemical measurements. The corrosion resistance of the AZ91D magnesium alloy was improved to some extent by the conversion coating with the optimal conversion parameters (7.3 g/L (NH₄)₆Mo₇O₂₄·6H₂O solution with pH 5 for 30 min at 30 °C).     

Non-fluorinated,room temperature curable hydrophobic coatings by sol–gel process

Non-fluorinated hydrophobic silica surfaces were generated on soda lime glass (SLG) substrates using hexamethyldisilazane (HMDS) as a surface modifying agent. Silica coatings were fabricated by dip coating of a sol derived from base catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS). Two methodologies were adopted to generate the hydrophobic surface; one where the hydrophilic silica coated surface was treated by immersion into different concentrations of alcoholic solutions of HMDS varying from 2.5 wt% to 15 wt%. In the other method, HMDS was directly added to a mixture of TEOS, water, ethanol, and ammonium hydroxide and coatings were deposited using this sol by dip coating and spray coating. Water contact angles (WCA) were measured to study the effect of HMDS treatment times and concentrations on hydrophobicity in the first case, and in the second case, WCA were measured for dip and spray coated samples. UV–visible transmission, scratch resistance, and thermal stability of the coatings were determined. The WCA increased from 66 ± 2° to 125 ± 4° after the treatment of the silica coatings with HMDS. In case of coatings generated from direct addition of HMDS to silica sol, WCA varied from 145 ± 2° to 166 ± 4° for dip and spray coated surfaces respectively. Surface morphology was studied to explain the difference in hydrophobicity of coatings generated using the two methods.     

Biodegradable sol–gel coatings of waterborne polyurethane/gelatin chemical hybrids

Gelatin from cold fish skin has strong mechanical properties and biodegradability. Cold fish gelatin was introduced into waterborne polyurethane (WPU) by covalent bonding to reinforce and render biodegradability of WPU. For this, gelatin was chemically modified with vinyltrimethoxysilane (VTMS) via the sol–gel type reactions and incorporated into hydroxyl ethyl acrylate (HEA) termini of WPU by UV curing. Covalent incorporations provided the hybrids with enhanced water resistance, hardness, glassy and rubbery state moduli, yield strength, and thermal resistance of soft segment along with significantly enhanced biodegradability both in trypsin solution and soil.     

Preparation of scratch resistant superhydrophobic hybrid coatings by sol–gel process

Organic–inorganic hybrid coatings on glass substrates with superhydrophobic properties and with improved scratch resistance were obtained by means of applying a multilayer approach including multiple sol–gel processes. The coatings exhibited a water contact angle (WCA) higher than 150°. Ultraviolet (UV)-curable vinyl ester resins and vinyltriethoxysilane (VTEOS) as coupling agent were employed to increase the adhesion between substrate and the inorganic layers. The surfaces were characterized by means of dynamic contact angle and roughness measurements. Indeed, the occurrence of superhydrophobic behavior was observed. The scratch resistance of the hybrid coatings was tested to evaluate the adhesion of the coatings to the glass substrate. The proposed preparation method for scratch resistant, mechanically stable, superhydrophobic coatings is simple and can be applied on large areas of different kinds of substrates.     

Characterization of wollastonite coatings prepared by sol–gel on Ti substrate

Wollastonite coatings were prepared by sol–gel on Ti substrate and their microstructures have been studied. The phase compositions and the surface morphologies of these coatings were examined by X-ray diffraction and scanning electron microscopy. Thermal behavior of dried gel was examined by differential scanning calorimetry (DSC) and thermogravimetry (TG). There are many cracks among coatings and particles with size about 200–300 nm distributing inside cracks. DSC and TG results show that the glass transformation temperature of dried gel is about 850°C. After calcined at temperature 900°C, the phase of coatings consists of wollastonite, SiO₂, and CaSi₂O₅.     

Antireflective coatings prepared by sol–gel processing: Principles and applications

Sol–gel processing is a powerful tool to prepare antireflective (AR) coatings on optical surfaces. In this paper the different strategies to obtain antireflective properties are reviewed: porous λ/4 layers, multilayer interference-type films and index-gradient materials such as “moth eye” structures. The processing of the respective films is described and evaluated; references to respective commercial products on glass substrates are given.AR coatings may have a particularly high importance for transparent ceramics as their index of refraction is significantly higher than that of common glass types. Reflective losses therefore are higher which is especially unpleasant for materials with a yet improvable intrinsic transparency.Recent studies indicate that specific porous λ/4 layers may exhibit pronounced anti-soiling features. Laboratory experiments as well as outdoor exposure tests were used to demonstrate the dust-repellant properties.     

Hybrid sol–gel coatings for corrosion protection of galvanized steel in simulated concrete pore solution

The aim of this experimental research was to study the electrochemical behavior of organic–inorganic hybrid (OIH) coatings for corrosion protection of hot-dip galvanized steel (HDGS) in the first instants of immersion in simulated concrete pore solution (SCPS) (pH > 12.5). The electrochemical performance of the OIH coatings was assessed by electrochemical impedance spectroscopy, potentiodynamic polarization curves, macrocell current density, and polarization resistance. The OIH coatings were prepared via the sol–gel method and were deposited on HDGS surfaces by dip-coating using one or three dip steps. The electrochemical results obtained for HDGS samples coated with OIH matrices in SCPS showed higher corrosion resistance than bare HDGS; as the molecular weight (MW) of Jeffamine^® increased the barrier protection of the coating decreased. The lowest protection efficiency was found for HDGS samples synthesized with oligopolymers with an MW of 2000. Coatings produced with an oligopolymer of 230 MW conferred the highest protection. The surface morphology of the OIH coatings deposited on HDGS surfaces was studied by atomic force microscopy. The results show that the roughness of the OIH films depends on the MW of Jeffamine^® and on the number of dip-coating steps used. Thermogravimetry results show that the Jeffamine^® MW affected the thermal properties of the prepared OIH samples. The prepared OIH materials are thermally stable within the range of 20–80°C.     

Porous water repellent silica coatings on glass by sol–gel method

Development of the solid surfaces with water-repellent and self-cleaning ability has attracted much research interest in recent years. In the present research work, we have prepared water repellent silica coatings on glass at room temperature (~27 °C) by sol gel process and surface silylation technique. Coating sol was prepared by keeping the molar ratio of tetramethoxysilane (TMOS), methanol (MeOH) and water (H₂O) constant at 1:12.36:4.25, respectively, with 0.01 M NH₄F. The dip coated silica films were surface silylated using two different silylating agents namely hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDZ). The HMDSO and HMDZ in hexane solvent were varied from 0 to 1 vol.% and silylation period was varied from 1 to 3 h. The HMDSO and HMDZ modified films showed dense and porous surface morphology, respectively. The HMDSO modified silica films showed static water contact angle of 122° whereas HMDZ modified films showed 165°. The HMDZ modified films displayed the extreme water repellency comparing with that of lotus leaves. The silica films were characterized by surface profilometer, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared microscopy, thermal and chemical aging tests, optical transmission and static water contact angle measurements.     

Deposition of alumina coatings on monocrystalline diamonds by sol–gel techniques

Machining of steel or iron-based alloys with diamond tools leads to rapid tool failure — probably due to chemical wear. The use of monocrystalline diamond tools has, up to now, been obligatory for precision machining. Coating the diamonds with a thin but hard and chemically inert alumina film may overcome the problem. Alumina coatings were deposited by sol–gel techniques. It was shown that a very thin TiN intermediate layer, deposited by reactive sputtering, results in a good adhesion of the alumina coatings to the monocrystalline diamonds. The microstructure of the coatings was characterized by field-emission scanning electron microscopy (FE-SEM) and by transmission electron microscopy (TEM). The deposited coatings showed a nanocrystalline, dense microstructure. The hardness of the coatings was investigated by ultramicrohardness measurements (UMH).     

Eriochrome cyanine doped sol–gel coatings. Optical behavior against pH

Sol-gel silica coatings doped with eriochrome cyanine (EC) have been prepared upon soda lime glass substrates by dip-coating. Sols were obtained starting from a silicon alkoxide hydrolysed in acid medium. EC was added to the silica sol dissolved in water and methyl alcohol to reach 1% of the total weight. Dried coatings showed pH-sensitivity (change of optical absorption and luminescence) when dipped into liquids as well as when exposed to aqueous gases at different pH. On this basis, EC doped coatings could be used as optical sensors, for instance, in analytical probes. Coatings have been demonstrated to be stable enough to UV radiation, at least at handling temperature to be used for optical sensors in common analytical instrumentation. Additional tests pointed out the behaviour of coatings to different chemical media at temperatures below 100°C. Adherence of coatings was evaluated by scratching and peeling, as well as dipping the samples into a cleaner solution submitted to ultrasounds.     

Sol–gel based protective coatings for copper products

Corrosion, fouling, and wearing of metal surfaces are the most common problems faced in industry as well as in environmental use. In the recent study, sol–gel based protective coatings were developed and deposited on different copper substrates. Different chemical pretreatments were used to enhance the adhesion and spreadability of coatings on copper surfaces. Substantial improvement of the corrosion resistance of sol–gel coated copper surfaces was obtained by a salt spray test. In addition, the sol–gel coatings increased hydrophobic and easy-to-clean properties of the copper surface. Variation of the curing temperature of the coatings caused changes in the morphology, adhesion, and topography of the sol–gel coatings. On the basis of the important information obtained in this study, the protective properties of sol–gel coatings can be tailored for copper and copper alloy substrates.     

Investigation of the mechanical and thermal fatigue properties of hybrid sol–gel coatings applied to AA2024 substrates

This research article reports on the response of various hybrid sol–gel materials when applied as coatings to pre-treated bare AA2024 substrates, to mechanical indentation and cyclic thermal stimuli, in order to determine their usefulness in aeronautical applications. Three groups of hybrid sol–gel-coated samples were prepared using various organosilanes and transition metal oxides. The characterization of the materials revealed that the presence of the organic functionalities, especially the methacrylate group, has a noticeable effect on the mechanical response of the hybrid coatings, in particular their flexibility. The presence of methacrylate group in the cured material gives it ability to flex which influenced the thermal fatigue characteristics of the coatings which are able to withstand the cyclic temperature regimes of 82 ± 3 to ?37 ± 3°C over 25 2 h cycles. This capability to maintain substrate protection is reflected in the corrosion resistance of the coatings as measured using electrochemical impedance spectroscopy and accelerated exposure testing. This result is important, as it shows that hybrid sol–gel materials can be used in applications where protecting a metal or ally substrate is paramount, especially in thermally volatile environments.     

Mechanical properties and thermal stability of ambient-cured thick polysiloxane coatings prepared by a sol–gel process of organoalkoxysilanes

Ambient-curable polysiloxane coatings were prepared by pre-hydrolysis/condensation of phenyltrimethoxysilane (PTMS) and dimethyldimethoxysilane (DMDMS) in the presence of ammonia solution and subsequently mixing with aminopropyltriethoxysilane (APS). The mechanical properties of coatings were thoroughly examined at both macro- and micro-level and the thermal stability of coatings was characterized by thermogravimetic analysis, both of which were correlated with coating composition and the hydrolysis/condensation degree of polysiloxane oligomer. It was found that pro-hydrolysis step is essential for fabrication of thick crack-free coatings (18–35 μm). Higher DMDMS molar ratio, more APS dosage and lower hydrolysis/condensation degree of polysiloxane oligomer favor enhancing the hardness. Excellent impact resistance (50 cm kg) of coatings was obtained at 5% and 10% APS dosage, despite of the type and structure of polysiloxane oligomer. Whatever, the best scratch resistance of coatings was attained using the polysiloxane oligomer, prepared at PTMS-to-DMDMS molar ratio of 2:8 and water-to-precursor molar ratio of 1:1, and 5% APS dosage. The polysiloxane coatings exhibit high thermal stability, however, which strongly depends on the coating composition.     

Photoactive sol–gel hybrid coatings from modified fluorocarbon polymers and amorphous titania

A series of organic–inorganic hybrid coatings was prepared through sol–gel chemistry by combining silanized chlorotrifluoroethylene-vinylether (FEVE) binders with tetraalkoxy silicon and titania sols under acidic conditions. The best compositions to obtain highly transparent and homogeneous coatings after thermal curing were determined. All the hybrid coatings easily pass the MEK test and show high scratch hardness. The atomic force microscopy (AFM) shows the formation of very smooth surfaces (R_rms routinely <1 nm) without clear phase separation phenomena. The typical size of the “objects” which may be individuated is in the range of 40–80 nm. Wettability through contact angle measurements shows the formation of moderately hydrophobic surfaces with a low contact angle hysteresis (~20°) which is a further indication of very smooth, homogeneous and chemically stable surfaces. After irradiation with UV-B light only hybrid coatings containing titania phases show a significant switch to a superhydrophilic behavior with a contact angle against H₂O down to 6°, which is only partially recovered after storage of the material in the dark. Titania based hybrid coatings also showed a fast and efficient UV-induced discoloration of the resazurin ink. The formulation of the coatings with photostabilizers belonging to the class of radical scavengers and UV absorbers does not change the photoinduced surface properties while eliminating the yellowing of the coating after UV exposure. It is concluded that titania-fluoropolymer hybrid coating show photoactivity and UV-induced superhydrophylicity mostly through ionic mechanisms, which could be beneficial to develop high durability and self-cleaning protective coatings.     

Photocurable cyanate ester containing hybrid coatings by an anhydrous sol–gel technique

This work reports the synthesis and characterization of hybrid coatings obtained by UV curable anhydrous sol–gel process. Chemical structure of the products was confirmed using spectroscopic methods such as infrared and nuclear magnetic resonance spectroscopy. The properties of the hybrid coatings such as thermal and mechanical properties were investigated in detail by scanning electron microscope, thermogravimetric analysis, and mechanical measurement. Cure kinetics of the coating formulations was investigated by differential scanning photo-calorimetry (Photo-DSC). Cross cut adhesion, pendulum hardness, gloss, pencil hardness, MEK rub test were also performed to measure the coating performance of the hybrid coatings. The results indicate that the addition of the appropriate amount of CPM and sol–gel can effectively improve the thermal and mechanical properties.     

Study of the degradation of hybrid sol–gel coatings in aqueous medium

The design and development of suitable multilayered functional coatings for delaying corrosion advance in metals and become controlled-release vehicles requires that the properties of the coatings are known. Coatings prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces. This kind of coating can be prepared from pure chemical reagents at room temperature and atmospheric pressure, with compositions in a very wide range of environmentally non-aggressive precursors. Sol–gel coatings based on siloxane bonded units were prepared, starting with an organic–inorganic hybrid system. Synthesis procedures included acid-catalysed hydrolysis, sol preparation, and the subsequent gelation and drying. The alkoxide precursors used were methyl-triethoxysilane (MTMOS) and tetraethyl-orthosilicate (TEOS) in molar ratios of 10:0, 9:1, 8:2 and 7:3. After determination of the optimal synthesis parameters, the materials were characterised by solid ²⁹Si nuclear magnetic resonance (²⁹Si NMR), Fourier transform infrared spectroscopy (FTIR), contact angle measurement and electrochemical impedance spectroscopy (EIS) test. Finally, the materials were assayed by controlling their weight in contact with water, to determine their ability to degrade by hydrolysis. Electrochemical analysis reveals the formation of pores and water uptake during the degradation. The quantity of TEOS is one of the principal parameters that determine the kinetics of degradation. There is a correlation between the degradation process obtained for long periods and the electrochemical parameters obtained by EIS in short times. The study tries to incorporate knowledge that can be used for designing the degradation process of the functional coatings and to control their properties in short times.     

Anatase–rutile transformation of TiO2 sol–gel coatings deposited on different substrates

Titanium dioxide is widely used in a lot of applications. The properties of TiO₂ strongly depend on its phase composition. The transformation temperature between phases is influenced by a lot of factors. One of them is a type of substrate under the TiO₂ film. In presented work, thin films of TiO₂ were deposited by the sol–gel method on silicon, stainless steel (304 L) and Co–Cr–Mo alloy (Vitallium). The process of anatase–rutile phase transformation was investigated by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) studies of deposited coatings. The results were compared with anatase–rutile transformations temperature of TiO₂ powders obtained by analogous sol–gel process. The temperature of anatase–rutile phase transformation changed in the range of 700–1000 °C and strongly depends on a kind of substrate. It was found that anatase–rutile transformation of TiO₂ coating proceeded at a higher temperature than rutilization of titania powders.