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The exceptional properties of sol–gel derived pure inorganic and organic–inorganic hybrid nanocomposite coatings have been adequately realized. Therefore they are presently being commercially exploited for various applications such as antireflective, solar selective, solar control, corrosion protection (barrier and self-healing type), antibacterial, self-cleaning, easy-to-clean, photocatalytic, scratch resistance, abrasion resistance, etc. The most obvious advantage of inorganic–organic hybrids is that they can combine the divergent properties of inorganic and organic components in a single material. Other notable advantages of the sol–gel hybrid nanocomposite coatings are: (1) high coating thicknesses (≥1 µm) are possible in a single coating step; (2) possibility of generation of multifunctional coatings; (3) flexibility of coating and compatibility with organic paints; (4) suitable for deposition on wide variety of substrates; (5) amenable to automation; and (6) ease of coating deposition, for example, use of spray, dip, and spin coating techniques. This chapter will focus on the global scenario of the past, present, and future of the applications of sol–gel coatings with selected examples. A detailed patent landscape analysis showing the market potential of sol–gel coatings with regard to various applications will also be presented to substantiate the discussions.

This study emphasizes on the evaluation and comparison of the anticorrosive properties of sol–gel coatings with and without inhibitor loaded nanocontainers. In this case, naturally available clay nanotubes (halloysite) were loaded with cationic corrosion inhibitors Ce3+/Zr4+. These nanocontainers were dispersed in hybrid organic–inorganic sol–gel matrix sol. Coating was applied on magnesium alloy AZ91D using the sols containing modified and unmodified nanocontainers employing the dip coating method and cured at 130 °C for 1 h in air. Corrosion resistance of coated/uncoated substrates were analyzed using electrochemical impedance spectroscopy, potentiodynamic polarization and weight loss measurements after exposure to 3.5 wt% NaCl solution for varying time durations between 24 h to 120 h. Self-healing ability of coatings was evaluated by micro-Raman spectroscopy after 120 h exposure to 3.5 wt% NaCl solution. Coatings generated after dispersion of corrosion inhibitor loaded clay in hybrid sol–gel matrix have shown more promising corrosion resistance when compared to just the sol–gel matrix coatings, after prolonged exposure to corrosive environment. Keywords: Self-healing coating, Halloysite nanoclay, Cationic corrosion inhibitors, Magnesium alloy AZ91D, Micro-Raman spectroscopy, Corrosion protection

Sol–gel derived multilayered solar selective coatings were generated on AISI SS 321 substrates using Ag-TiO 2 as the cermet layer, titania and silica as the dielectric layers with high and low refractive indices respectively. The phase compositions of the individual layers were independently confirmed using grazing angle incidence X-ray diffraction, which was corroborated by X-ray photoelectron spectroscopic analysis. Thickness of the layers was measured using variable angle spectroscopic ellipsometry. The solar absorbance was measured over the UV–Vis-NIR wavelength range. Thermal emissivity was determined using FTIR spectroscopic analysis. The durability of the coatings was ascertained using accelerated corrosion testing methods as well as by measuring the optical properties after thermal cycling experiments. The promising nature of hexavalent chrome-free, environmental friendly, multilayered solar selective coating was ascertained with respect to amenability to scale-up.

Non-fluorinated, hydrophobic silica coatings were generated on soda lime glass substrates using dimethyldiethoxysilane (DMDEOS), tetraethoxysilane (TEOS) and a silylating agent hexamethyldisilazane (HMDS) using sol–gel process. To improve the mechanical properties of the coating, epoxy modified nanosilica particles were added to the sol of dimethyldiethoxysilane, tetraethoxysilane and hexamethyldisilazane followed by sonication prior to coating. Coatings were characterized for their water contact angles, thickness, UV–Vis transmission, pencil hardness and surface morphology. The concentrations of the nanosilica particles as well as the sonication times were optimized to obtain a transparent, hydrophobic coating with improved pencil scratch hardness. An optimized coating composition with 1: 0.5 M ratio of the nanosilica particles and sol synthesized from DMDEOS, TEOS and HMDS sonicated for 60 min yielded coatings that exhibited a water contact angle of 125±2° with a pencil scratch hardness of HB and average visible light transmittance of 89%.

Organic–inorganic hybrid sol–gel based silica coatings derived from hydrolysis and condensation of organically modified silane precursors like phenyltrimethoxysilane and methyltriethoxysilane along with tetraethoxysilane were deposited on different surface pre-treated (as-cleaned, plasma-treated, shot-blasted) SS 316 grade stainless steel substrate, using dip coating technique. The coatings were heat treated at 150 °C for 2 h in air. The pre-treated surfaces were characterized using X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy. The water content of the sols was determined by Karl Fischer titration to evaluate the degree of completion of hydrolysis and condensation reactions. Cured coatings were characterized to evaluate thickness, water contact angle, pencil scratch hardness, gloss, and shrinkage in coating thickness. Impact test was carried out on pigmented coatings derived from sols synthesized using the two silane precursors. The corrosion resistance and water durability tests were carried out to compare the coatings derived from using different precursors and different surface pre-treatments. The corrosion tests were carried out for 1 h and 24 h exposure to a 3.5% NaCl solution by electrochemical polarization measurements. It was found that coatings from methyl substituted organically modified alkoxysilane exhibited better hydrophobicity, scratch hardness, impact resistance and barrier properties with respect to corrosion, when compared to those derived from phenyl substituted trialkoxysilane. The difference in performance of coatings was explained on the basis of difference in hydrolysis and condensation rates between the two organically modified silane precursors used for the sol synthesis.

Adhesion of the coatings to the substrate plays a vital role in improving mechanical properties of sol-gel coatings especially when deposited on plastics. In the present study, an attempt was made to study the effect of atmospheric air plasma surface activation on the adhesion of sol-gel nanocomposite coatings on polycarbonate substrate. The sol was synthesized by the hydrolysis and condensation of an epoxy silane (3-Glycidoxypropyl trimethoxy silane) along with titanium tetraisopropoxide. Coatings were deposited on 100 × 100 mm substrates by spray coating and subsequently subjected to UV curing followed by thermal curing at 130 °C for 1 h. One set of the substrates was subjected to atmospheric air plasma surface pretreatment prior to coating deposition. Coating thickness, adhesion as well as mechanical properties like pencil hardness, scratch resistance and taber abrasion resistance were evaluated for coatings deposited over plasma-treated and -untreated surfaces. It was found that plasma surface pretrea...

Surface engineering approaches are being increasingly employed for enhancing the effective life of twist drills with a view to reduce machining costs. The electro-spark coating (ESC) technique provides a promising means of depositing wear resistant coatings that can potentially enhance the performance of these tools. However, it is often necessary to also optimize the machining conditions for coated tools to achieve an enhanced tool life. In the present investigation, varying spindle speeds were employed at a fixed vertical feed to evaluate the performance of WC–8Co ESC coated HSS drills in comparison to bare HSS drills. The number of holes drilled before reaching a preset average flank wear (0.5 mm), or catastrophic failure of the drill, was taken as the measure of tool life. The drill flank wear, monitored at regular intervals, as well as the cutting torque and thrust measured for all holes, were considered to be the key criteria for optimizing the cutting conditions. Results indicate that the WC–8Co coated drill tool life can be increased by a factor of more than 5, depending on the machining conditions selected. Furthermore, flank wear of the drill was found to increase rapidly at the end of drill life. Cutting torque data was also found to provide a useful indicator for predicting the end of tool life.

A novel process was used for the preparation of dense, thick, and stable silica-zirconia coatings on aluminum by an in situ anodization along with sol-gel deposition. Anodic electrophoretic deposition was carried on aluminum using a SiO2-ZrO2 sol that was synthesized from an epoxy modified silane and zirconium n-propoxide along with a cerium salt (Ce(NO3)3·6H2O). Current density and time were varied during the deposition. The optimal parameters that yielded uniform coatings were determined. Coatings were characterized for their crystallinity, scratch hardness, and microstructure. The barrier properties of the coatings were tested using potentiodynamic polarization studies, electrochemical impedance spectroscopy, and neutral salt spray tests. Grazing angle incidence X-ray diffraction studies revealed that the coating comprised crystalline Al2SiO5 along with an amorphous phase. The novelty of the process was that the crystalline aluminosilicate phase was formed even at room temperature and could be deposited on aluminum by a simultaneous anodization of aluminum and sol-gel deposition. The coated substrates withstood more than 400 hours of salt spray tests. Polarization measurements reveal that the composite layer of aluminosilicate along with the Ce3