13.7 Hybrid nanocoatings
The use of hybrid nanocoatings i.e., containing organic/organic, organic/inorganic, and inorganic/inorganic nanoparticle combinations can result in a combination having the advantages of both mixtures.
Momen at al. (2011) developed a one-step technique for manufacturing room temperature vulcanizing (RTV) silicone rubber and stearic acid icephobic coatings with micro/nano structure roughness. The coating was sprayed onto glass substrates to replicate the icing process for outdoor glass insulators. The increase in stearic acid concentration from 0% to 43% contributed to increasing CA from 115 to 160 degrees while enhancing droplet rolling-off properties.
Li et al. (2012) developed an icephobic nanocoating for power line insulators. The combination of materials consisted of nanosilica and PDMS. The average contact angle for the coatings was approximately 161 degrees i.e., a superhydrophobic surface. Insulators commonly form icicles due to their orientation and location. The presence of this hybrid coating resulted in two main advantages: firstly the concentration of the icicles and their lengths were both reduced, and the flashover voltage increased in comparison to the insulators with a standard silicone rubber coating.
Yan et al. (2012) developed a PDMS-modified nanostructured silica coating for glass slides, resulting in ice shear strength values 2149 and 139.6 times lower than bare and RTV coated substrates. Li et al. (2016b) modified this coating to add reactivated hydroxyl and carbon coating for glass slides as well as insulators. The FC-100/146 insulators were dip coated and cured in two consecutive steps before testing in an environmental chamber at −6°C, 3 m/s wind speed, 12 kV AC voltage, and 90 L/h water flow rate. The peak leakage current value was greatly reduced for the coated insulator (0.32–0.45 mA) as compared to the uncoated insulator (0.35–0.48 mA). Additionally, 30 min of glaze ice accretion in the climate chamber yielded sparse ice formation on the coated cylinder characterized by discrete droplets and minimal accretion.
Nosonovsky and Sobolev (2013) prepared a hydrophobic and icephobic emulsion containing polymethyl-hydrogen siloxane oil, metakaolin/silica fume (MK/SF), and nanostructured polyvinyl alcohol (PVA) fibers for Portland cement mortar tiles. Apart from low roll-off angles, shear stress tests with ice columns indicated good icephobicity along with hydrophobicity.
Liao et al. (2015a) used magnetic agitation to mix silica nanoparticles with fluorosilicone resin, liquid epoxy resin, and ethyl acetate. The entire mixture was dip coated onto glass slides and insulators respectively. After 80 min of 100 μm water droplet spray at −5°C, 90% of the coated glass slide remained ice free. Similar tests on the insulator exhibited icicle formation on only isolated points.
Wang et al. (2016a) spray coated a mixture of modified silica nanoparticles, PMMA and THF, followed by drying for 2 h onto substrates. Even at −20°C, the droplet easily slid off the lubricated surface, thereby indicating a good icephobic property.
Liao et al. (2017) used the RF magnetron sputtering technique to develop a nanostructured ZnO–silica–PTFE sandwich coating with CA 167.2 degrees and CAH equal to 1 degree. After a 2 h glaze ice spray, 82% of the surface remained uncoated. These coatings were manufactured on glass slides to mimic glass insulators for power transmission systems.
Qian et al. (2017) used a TiO2-modified PDMS icephobic coating with fluoro-additives on glass substrates for outdoor insulators and windows for buildings. An air spray gun was used to coat the substrates with the alumina nanoparticles and the resultant CAs measured were between 108.4 and 112.1 degree. In addition to weathering tests which indicated no change in the icephobicity despite 1000 h exposure to wet/dry cycles, ultraviolet (UV) exposure, and humidity between 50% and 100%, 20 μL water droplets slid straight off the coated surface at −20°C and a tilt angle of 30 degrees.
Wang et al. (2017) developed a mixture of silica nanoparticles, ultra-high molecular weight polyethylene (UHMWPE), and decahydronaphthalene to be poured on various substrates including copper, aluminum, titanium alloys, steel, and polyethylene terephthalate (PET) wallpaper. The coating was infused with kerosene, silicon oil, and perfluorinated fluid to develop a SLIPS surface. Durability was tested by 3 h bombardment of 16,200 water droplets at 2.42 m/s and dipping for 24 h in HCL (pH 2–6) or NaOH (pH 8–12). No impact was seen on hydrophobicity. To overcome the issue of humidity, the SLIPS surface showed good water droplet mobility even at −15°C.