Fabrication of novel sol-gel silica coatings reinforced with multi-walled carbon nanotubes

Novel sol-gel silica coatings reinforced with multi-walled carbon nanotubes (MWCNTs) have been prepared using the organic sol-gel route and the dip-coating technique on magnesium alloy substrates. Homogeneous and dense composite coatings with good reinforcement dispersion were fabricated using low temperature and atmospheric pressure fabrication conditions. The presence of nanotubes caused a substantial enhancement of silica coating fracture toughness on coatings deposited on grounded substrates but it was not as effective on polished substrates because of the low adhesion of the coating to the substrate. Bridging phenomena caused by the MWCNTs was observed, indicating that an effective load transfer between the silica matrix and the nanotube reinforcement was also achieved.     

Al/SiC composite coatings of steels by thermal spraying

Thermal spraying has been used to coat carbon steels (F112) and austenitic stainless steels (AISI 304) with aluminium matrix composites. Mixtures of aluminium powder and SiC particles were used as spraying material. A sol-gel silica coating was laid on SiC particles to reduce the porosity of the composite coatings and to inhibit the formation of aluminium carbide. The sol-gel silica coating acts as an active barrier enhancing the wettability of the reinforcement by molten aluminium. Coatings with a reinforcement volume fraction up to 30 vol.% were obtained with porosities of about 1.0 vol.%. The incorporation of sol-gel silica coated SiC particles reduces the coefficient of thermal expansion of the composite coating and enhances its adhesion to the substrates more than when uncoated SiC particles were used.     

Properties of Al2O3 nano-particle reinforced copper matrix composite coatings prepared by pulse and direct current electroplating

Cu–Al₂O₃ nano-composite coatings have high potential for use in applications in which high mechanical properties together with high corrosion resistance are required. In the present study it is intended to produce copper nano-alumina composite coatings with various nano-alumina contents in order to investigate the effect of alumina reinforcement particles on corrosion resistance and mechanical properties such as hardness and wear resistance. The composite coatings were deposited using direct current (DC) and pulse current (PC) plating. The microstructures of the coatings produced from both methods were examined via scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The wear behaviors, micro hardness, coating thickness, corrosion rate and coating porosity were examined using appropriate methods. Compared to DC deposition, PC plating facilitated higher amounts of particle incorporation with more uniform distribution. The results indicated that the mechanical properties of the applied coatings with incorporated nano-alumina reinforcement were far more superior as compared to its own matrix as well as non-composite copper coatings. It was also found out that increasing the amount of nano-alumina content in the coating, led to enhanced general properties of the coatings.     

Modelling of the scratch resistance of particle filled sol-gel coatings on aluminium

Sol-gel derived coatings with 12 nm and 0.5-10 μm silica fillers were prepared on aluminium to evaluate the effect of particle size and filler content on coating properties The measured maximum crack-free thickness and hardness strongly depended on the type of particles used and the filler volume fraction. The scratch resistance primarily depended on coating thickness and much less on the mechanical properties of the coating. This finding was interpreted via modelling of the stresses under the scratching stylus. It was established that the initial yield occurred in the substrate for most of the coatings and the load needed for this initial yield had a relatively small dependence on the coating properties but it was strongly influenced by the coating thickness.     

Suppression of active-oxidation of Si-C fibers (Hi-Nicalon) under reduced pressure by carbon or silica coating

The suppression of the active-oxidation of Si-C fibers (Hi-Nicalon) by carbon or silica coating were investigated at 1773 K under a reduced pressure of 1.32 Pa, through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile test. The coating of carbon and silica were conducted by heat-treating at 1773 K in CO-CO₂ gas mixtures. After exposure under a reduced pressure, the carbon coating formed in pure CO gas remained almost perfect and the retained strength of 1.7 GPa was nearly identical to that in the as-heat treated state. On the other hand, the carbon coatings formed under other conditions and all the silica coatings were completely lost during vacuum exposure. The retained strengths were in the range of 0 to 1.6 GPa. The silica film was found to be ineffective for suppressing the active-oxidation during vacuum exposure.     

Preparation and characterization of cross-linked phospholipid bilayer capillary coatings for protein separations

Analysis of protein and peptide mixtures via capillary electrophoresis is hindered by nonspecific adsorption of analytes to the capillary walls, resulting in poor separations and quantitative reproducibility. Phospholipid bilayer (PLB) coatings are very promising for improving protein and peptide separations due to the native resistance to nonspecific protein adsorption offered by PLBs; however, these coatings display limited chemical and temporal stability. Here, we show the preparation and characterization of a highly cross-linked, polymerized phospholipid capillary coating prepared using bis-SorbPC. Poly(bis-SorbPC) PLB coatings are prepared in situ within fully enclosed fused silica capillaries via self-assembly and radical polymerization. Polymerization of the PLB coating stabilizes the membrane against desorption from the surface and migration in an electric field, improves the temporal and chemical stability, and allows for the separation of both cationic and anionic proteins, while preserving the native resistance to nonspecific protein adsorption of natural PLBs.     

Fabrication of superhydrophilic and antireflective silica coatings on poly(methyl methacrylate) substrates

Silica nanoparticles of ca. 20 nm in size were synthesized, from which hierarchically porous silica coatings were fabricated on poly(methyl methacrylate) (PMMA) substrates via layer-by-layer (LbL) assembly followed by oxygen plasma treatment. These porous silica coatings were highly transparent and superhydrophilic. The maximum transmittance reached as high as 99%, whereas that of the PMMA substrate is only 92%. After oxygen plasma treatment, the time for a water droplet to spread to a contact angle of lower than 5° decreased to as short as 0.5 s. Scanning and transmission electron microscopy were used to observe the morphology and structure of nanoparticles and coating surfaces. Transmission and reflection spectra were recorded on UV–vis spectrophotometer. Surface wettability was studied by a contact angle/interface system. The influence of mesopores on the transmittance and wetting properties of coatings was discussed on the basis of experimental observations.     

疏水疏油二氧化硅增透膜的制备

以正硅酸乙酯为前驱体，氨水为催化剂，采用溶胶-凝胶法，结合全氟辛基癸烷三甲氧基硅烷（FAS）自组装对膜层表面改性，制备了疏水疏油二氧化硅增透膜．采用红外光谱仪，分光光度计，扫描探针显微镜，椭偏仪，静滴接触角测量仪，抗油污染能力测试等技术对膜层性质进行了分析．结果表明：疏水疏油增透膜的峰值透光率为99．8％；与水的接触角为118．0°，与二甲基硅油的接触角达到74．5°；在抗油污染能力测试中，疏水疏油增透膜的抗油污染能力较常规增透膜大大增强．     

Combined Langmuir-Blodgett and sol-gel coatings

Optical properties and in-depth structure of double-layer coatings on glass substrates were investigated. One of the layers was prepared by dip coating either from silica or titania sol, the other layer was made from ca. 130 nm Stöber silica particles by the Langmuir-Blodgett (LB) technique. Two different types of combined coatings were prepared: (1) nanoparticulate LB films coated with sol-gel (SG) films and (2) nanoparticulate LB films drawn onto SG films.Scanning electron microscopy and optical methods, i.e. UV-vis spectroscopy and scanning angle reflectometry were applied for analyzing the structure and thickness of coatings. These measurements revealed that the precursor sols could not penetrate into the particulate LB film completely in case of coating type (1). For coating type (2) very little overlap between the SG and LB layers was found resulting in significant improvement of light transmittance of combined coatings compared to single SG films.To show some possible advantages of the combination of these techniques additional studies were carried out. Surface morphology of combined coatings (1) was studied by atomic force microscopy. Surfaces with different roughness were developed depending on the thickness of the sol-gel film (titania: ca. 70 nm; silica: ca. 210 nm). The adhesive peel off test revealed improved mechanical stability of combined coatings (2), in comparison to LB films which makes them good candidates for further applications.     

Development and bio-electrochemical characterization of a novel TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> mixed oxide coating for titanium implants

Titanium and its alloys, the most commonly used materials for dental and orthopaedic implants are generally coated with bioactive materials such as sol-gel derived titania, silica and calcium phosphate in order to render these materials bioactive. In the present work a coating containing nanosized titania particles having anatase structure was developed on titanium substrate by thermal decomposition of titanium tetrachloride in isopropanol. A modified titania-silica mixed oxide coating was developed by incorporating the required amount of silica in the coating system. The presence of silica at small weight percentage caused improvement of adhesion and corrosion resistance of the coating. In vitro bioactivity tests were performed in 1.5 Kokubo's simulated body fluid after alkaline treatment of the titania/titania-silica coatings and the performance was compared with that of the titania coating developed by simple thermal oxidation. TF-XRD, FTIR and SEM-EDAX were used to investigate the microstructural morphology and crystallinity of the coatings. Elemental analysis of simulated body fluid was carried out using ICP-AES and spectrophotometry. Enhanced biogrowth was facilitated on the titania coating incorporated with low silica content.     

Microstructure of organic–inorganic composite coatings studied by TEM and XANES

Chromate coatings on Zn or Zn alloy coated steel sheets often include silica for the aim to improve corrosion resistance. In the case of dry-in-place chromate coatings containing acrylic resin (hereafter referred to as an organic–inorganic composite coating), an addition of silica, however, did not show an improvement in corrosion resistance. The microstructures of the organic–inorganic composite coatings were observed by transmission electron microscopy (TEM) and the chemical states of Cr were investigated by the total electron yield X-ray absorption near edge structure (TEY-XANES) method. TEM samples were successfully prepared by dry ultramicrotomy preventing water-soluble components in the coatings fromdissolving out. TEY-XANES revealed the chemical states of components even in the organic matrix. Using these methods, it was found that the addition of silica changed just the morphology of the chromium compound in the organic–inorganic composite coating but not the chemical state of Cr. This is a reason for the addition of silica being not effective at improving corrosion resistance. The combination of dry ultramicrotomy-TEM and TEY-XANES spectroscopy was proven to be a powerful tool for characterizing organic–inorganic composite coatings.     

Microstructure of organic–inorganic composite coatings studied by TEM and XANES

Chromate coatings on Zn or Zn alloy coated steel sheets often include silica for the aim to improve corrosion resistance. In the case of dry-in-place chromate coatings containing acrylic resin (hereafter referred to as an organic–inorganic composite coating), an addition of silica, however, did not show an improvement in corrosion resistance. The microstructures of the organic–inorganic composite coatings were observed by transmission electron microscopy (TEM) and the chemical states of Cr were investigated by the total electron yield X-ray absorption near edge structure (TEY-XANES) method. TEM samples were successfully prepared by dry ultramicrotomy preventing water-soluble components in the coatings from dissolving out. TEY-XANES revealed the chemical states of components even in the organic matrix. Using these methods, it was found that the addition of silica changed just the morphology of the chromium compound in the organic–inorganic composite coating but not the chemical state of Cr. This is a reason for the addition of silica being not effective at improving corrosion resistance. The combination of dry ultramicrotomy-TEM and TEY-XANES spectroscopy was proven to be a powerful tool for characterizing organic–inorganic composite coatings.     

Perhydropolysilazane derived silica coating protecting Kapton from atomic oxygen attack

By using surface sol-gel method with perhydropolysilazane (PHPS) as a precursor, a silica coating was prepared on a Kapton substrate as an atomic oxygen (AO) protective coating. The AO exposure tests were conducted in a ground-based simulator. It is found that the erosion yield of Kapton decreases by about three orders of magnitude after the superficial application of the coating. After AO exposure, the surface of the coating is smooth and uniform, no surface shrinkage induced cracks or undercutting erosion are observed. This is because that during AO exposure the PHPS is oxidized directly to form SiO₂ without through intermediate reaction processes, the surface shrinkage and cracking tendency are prohibited. Meanwhile, this PHPS derived silica coating also presents self-healing effect due to the oxidation of free Si. Compared with other kinds of silica or organic polymer coatings, this PHPS derived silica coating exhibits a superior AO erosion resistance.     

自组装法制备中空二氧化硅纳米粒子减反射薄膜

以正硅酸乙酯(TEOS)为壳层材料, 聚丙烯酸(PAA)为核材料, 以传统的Stöber水解法为基础制备得到结构规整的中空二氧化硅纳米粒子, 并采用自组装法制备单层减反射薄膜和宽波段双层减反射薄膜。主要研究中空二氧化硅纳米粒子的结构调控方法; 自组装次数和中空二氧化硅纳米粒子分散液的pH值对减反射薄膜透光率的影响规律, 以及具有渐变折射率的双层减反射薄膜的制备。研究结果表明: 通过调节PAA和TEOS的用量可精确调控中空二氧化硅纳米粒子的粒径和空腔体积分率, 进而可精确调控减反射薄膜的厚度和折射率; 通过酸洗工艺, 将自组装次数由10次减少为2次, 简化了涂膜的工艺条件, 在最佳工艺条件下所制备的单层减反射薄膜在350~800 nm波长范围内可显著提高玻璃的透光率, 在最佳波长(λ=520 nm)处将玻璃的透光率由91.6%提高至98.1%; 双层减反射薄膜可在更宽的波段范围内提高基材的透光率, 在400~1500 nm波长范围内将玻璃的透光率提高了5%以上。     

Fabrication of titania-coated silica fibers and effect of substrate shape on coating growth rate

Titania coating was fabricated on fused silica glass fibers of 4-6 μm in diameter by the hydrolysis of Ti-tetraethoxide in ethanol at 20 °C. Changes in the coating thickness with the deposition time were examined by scanning electron microscopy and compared with those on flat soda-lime glass substrate examined by atomic force microscopy. Uniform titania coating was obtained, but severe cracking occurred on coatings thicker than 150 nm due to isotropic shrinkage during the drying process. The faster growth rate of coating on fiber than that on flat substrate could be explained by employing an ‘adhesion growth model.’ X-ray diffractometry showed that titania coating on fused silica glass plate was transformed to anatase-phase TiO₂ by annealing in air at 300 °C. This indicates that the titania coating on a fused silica glass substrate becomes crystalline after annealing at that temperature.     

Factors affecting the behavior and effectiveness of phospholipid bilayer coatings for capillary electrophoretic separations of basic proteins

Phospholipid bilayer coatings can prevent adsorption of cationic proteins on the surface of fused silica capillaries used in capillary electrophoresis. However, the performance of such bilayer coatings is strongly dependent on solution conditions. The factors affecting the rate of formation of phospholipid bilayer coatings were investigated using the double-chained zwitterionic 1,2-dimyristoyl-sn-glycero-phosphocholine (DMPC, C(14)) as a model phospholipid. The effectiveness of these coatings for CE separations of model cationic lysozyme, cytochrome c, ribonuclease A, and alpha-chymotrypsinogen A was also assessed. Increasing the ionic strength of a 0.1 mM DMPC solution reduced capillary coat times from >2 hours in 2.5 mM Tris (pH 7.4) buffer to 3.4 min in 40 mM Tris and dramatically improved separation performance such that > or =1.4 x 10(5) plates/m were observed in capillaries coated for 5 min with 0.1 mM DMPC in 20 mM Tris-HCl (pH 7.4). The presence of Ca(2+) in the coating solution also increases the rate of formation of the phospholipid bilayer coating. The type of vesicle strongly affects its adsorption rate onto the silica surface. The time required to coat the capillary was 7.2 min for small unilamellar vesicles (SUVs) and 22.5 min for large unilamellar vesicles and excessively long for multilamellar vesicles. Highest efficiency protein separations were achieved with bilayer coatings prepared from SUVs. The coating rate was enhanced by using greater DMPC concentrations and unaffected by pH. The type of buffer present in the DMPC coating solution affects the coating behavior, with HEPES buffer yielding a faster coat time than either Tris or phosphate buffers. Histone H1 was separated on a 0.1 mM DMPC-coated capillary.     

Antireflective coatings based on SiO<Subscript>2</Subscript> nanoparticles

Single-layer antireflective coatings, produced by sol-gel method on soda-lime glass substrates, were studied. Optimal parameters of the silica sols synthesis and coating procedure of the antireflective coatings based on SiO₂ nanoparticles on soda-lime glasses for producing composite glasses with high optical transmittance within the visible range of wavelengths were obtained.     

Hardness recovery of ceramic coated aluminium matrix composites using thermal-shock resistant sol-gel silica coatings

Sol-gel silica coatings increase wear and corrosion resistance of aluminium matrix composites, but a heat treatment stage at temperatures as high as 500 °C is usually required to consolidate the ceramic coating. Under these thermal conditions, sol-gel ceramic coatings tend to crack and the aluminium matrix hardness strongly reduces. Present paper describes how the combination of a certain sol-gel procedure accompanied with quenching treatment in silicone oil allows recovering most of the hardness of the substrate, while keeping undamaged the sol-gel silica coating deposited.     

Reinforcement coatings and interfaces in aluminium metal matrix composites

The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites.The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of in situ hybridising and in situ alloying are described.     

Transparent anti-stain coatings with good thermal and mechanical properties based on polyimide-silica nanohybrids

In this work, we synthesized polyimide/silica hybrid materials via sol-gel method using a fluorinated poly(amic acid) silane precursor and a variety of perfluorosilane contents. We studied the influence of a hybrid coating film with the following characteristics; hydrophobicity, oleophobicity, optical transparency, and surface hardness of the coating films. The hybrid coatings with the fluorosilane contents up to 10 wt% are optically transparent and present good thermal stability with a degradation temperature of > 500 degrees C as well as a glass transition of > 300 degrees C. Both water contact angle and oil contact angle increase rapidly with introducing small amount of the fluorosilane in the hybrids and reaches the maximum of 115 degrees and 61 degrees, respectively. The hardness of the hybrid coatings increases up to 5H with an increase of the FTES content in the hybrids. These colorless, transparent, and thermally stable hybrid materials could be suitable for applications as anti-stain coatings.