Nanostructured coatings find application in transportation due to their superior characteristics, that are not typically found in conventional coatings in this sector. They are now routinely applied to a wide variety of substrate materials (plastics, glass, metals, ceramics and textiles) in the automotive, aviation and marine sectors. Interest in nanostructured materials for coatings in these markets is due to their remarkable mechanical, electrical, magnetic and optical properties and the possibilities of synthesizing materials with unique physical–chemical properties. Highly sophisticated surface related properties, such as optical, magnetic, electronic, catalytic, mechanical, tribological, chemical as well as magnetic, electronic and optical can be obtained by advanced nanostructured coatings, making them attractive for many modern transportation applications.
Properties
Due to the properties inherent at the nanoscale, nanostructured coatings are typically multifunctional, exhibiting one or combinations of the following properties: scratch and abrasion resistance, anti-static, oleophobic, easy-to-clean, anti-reflective, anti-microbial activity, sensor and catalytic activity. They are mainly used for the prevention of soiling (incrustation/clogging; protein adhesion/cell adhesion; biofilm formation); aging/degradation; and friction/wear, all areas of importance in transportation coatings. Advantages of nanocoatings include:
• Better surface appearance.
• Good chemical resistance.
• Decrease in permeability to corrosive environment and hence better corrosion properties.
• Optical clarity.
• Increase in modulus and thermal stability.
• Easy to clean surface.
• Anti-skid, anti-fogging, anti-fouling and anti-graffiti properties.
• Better thermal and electrical conductivity.
• Better retention of gloss and other mechanical properties like scratch resistance.
• Anti-reflective in nature.
• Chromate and lead free.
• Good adherence on different type of materials.
Nanoparticles allow for novel products such as:
• Easy-to-clean coatings.
• Effect coatings.
• Anti-bacterial coatings.
• Scratch-resistant coatings.
• Photocatalytic coatings.
• Paints with UV protection.
• Wall coatings as screens against high-frequency electromagnetic radiation.
• Switchable coatings.
• Electro-conductive coatings.
• Self-healing coatings.
• Nano-primers for anti-corrosive coatings and paints.
• Heat-insulating coatings.
Nanomaterials used in the coatings industry include:
• Titanium dioxide.
• Silicon dioxide.
• Iron oxide.
• Graphene.
• Carbon nanotubes.
• Zinc oxide.
• Silver.
Nanomaterials incorporated into hybrid coatings have been widely adopted in the automotive industry. Nanocoating allows for new coloration effects and greater hardness and durability. Coatings containing nanoscale carbides, nitrides, metals or ceramics play a key role in the performance of internal mechanical components of a vehicle, such as the engine. Other applications include anti-scratch and self-healing, self-cleaning, thermal barrier, conductive and anti-fingerprint coatings. There is also a high demand in the automotive industry for anti-fingerprint coatings, especially with the increasing incorporation of touch panel displays.
In aviation, nanocoatings are currently being commercialised to detect corrosion and mechanical damage to aircraft skin; react to chemical and physical damage, improve adhesion, and increase the life span of metal parts. Lightweight, high-strength, heat stable nanomaterials are also under development for aircraft engines.
Nanocoatings have been applied in military and commercial steel ship hulls and steel, aluminum or fiberglass boat hulls, making them non-susceptible to permanent adhesion of marine fouling organisms such as barnacles, sea-grasses etc. When organisms such as bacteria, barnacles and algae stick to the surfaces of ships it is a costly nuisance. These biofouling organisms mean that ships burn 40% more fuel at an annual cost of more than €5 million for businesses, as well as an incalculable cost to the environment.
AUTOMOTIVE
Surface protection is a key area in the automotive market both for protection from UV, wear, heat; promotion of adhesion; and reduction of engine friction. The overall world automotive paints and coatings market was estimated to be $7.75billion in 2010 and nanomaterials will play a key role in future growth. Many automotive parts have a protective coating applied to improve the appearance or provide additional durability to the substrate which can be enhanced by the incorporation of nanomaterials. Desirable functional properties for the automotive coatings industry afforded by nanomaterials include:
• Scratch resistance (alumina and slica nanoparticles).
• Anti-fingerprint.
• Self-cleaning (Nano-Tio2, nanosilica).
• Chemical resistance.
• UV resistance (zinc oxide, cerium oxide, titanium oxide, iron oxide nanoparticles).
• Abrasion resistance (silica and aluminium oxide nanoparticles).
By reducing wear and friction, nanostructured coatings increase the lifetime of the working material at the same time that they reduce the dissipation of energy as heat, thus increasing the efficiency of the vehicle. Nanocomposite coatings offer improved solvent, fuel and gas barriers, and have heightened flame resistance, stiffness, and other mechanical properties. These coatings can increase tool productivity (longer tool life, higher cycle frequencies, less work piece finishing), reduce manufacturing costs, improve the quality of products (due to smoother surfaces, better dimensional stability, higher degrees of metal deformation and fewer manufacturing steps) and reduce lubricant consumption.
Current applications in the automotive industry are for oxide scale protection and easy to clean coatings for automotive glass. Volkswagen, BMW, Toyota and Subaru all utilise nanomaterials in these areas. Mercedes-Benz have introduced ceramic scratch-resistant nanocoatings to automobiles. These nanoparticle clearcoats display significantly greater scratch resistance and enhanced paint gloss compared to vehicles with conventional paintwork.
Scratch-resistance
Consumers desire a permanent, scratch-free finish on all parts of automobiles, and scratch performance is the highest rated customer concern for automotive paint systems and displays. Nanocoatings provide protection against scratches caused by mechanical car-washes, for example ensure visibly enhanced gloss over an extended period of time. Aluminum oxide nanocomposite scratch resistance coatings have been applied as automotive finishes. When the additives are blended into resins and coatings at very low 1.5 to 6 percent concentrations, scratch resistance increases dramatically.
BASF (www.basf.com)has created a topcoat for automobile paint that combines two types of materials in a nanostructured hybrid. Between 90 and 95% of the coating is organic material, making the finish flexible,elastic and weather resistant, the company says, while the remaining 5 to 10% consists of nanoscale clusters of inorganic silicate that are particularly hard and scratch-resistant. The organic and inorganic components are covalently bound.
Nanovere Technologies (www.nanocoatings.com) has developed coatings for BMW to improve scratch resistance and gloss. PPG’s (www.ppg.com) CeramiClear is another product in this area and Bayer also develop nanoparticle coatings for scratch, mar, and etch resistance.
Altana’s NANOBYK® (www.byk.com/en/additives/additives-by-name/nanobyk.php) additives are utilized in scratch-resistant coating applications in automotive refinish. Nanophase (www.nanophase.com) produces NanoArc® Aluminum Oxide for surface coatings and films to provide long-term scratch resistance without significantly impacting optical clarity, gloss, color, or physical properties.
Hydrophobic and oleophic
Hydrophobic and oleophic glass allowing greatly improved visibility in the rain, and reduces the adherence of dirt and contaminants to a treated surface. Nanovere Technologies has developed a Wipe-On clear nanocoating, Vecdor Nano-Clear®. to restore original color, gloss and surface hardness back into oxidized textured plastics, highly oxidized fiberglass and highly oxidized paint surfaces while reducing surface maintenance by 60%.
Diamon-Fusion International (www.dfisolutions.com) flagship product Diamon-Fusion® provides multi-functional characteristics that include: water and oil repellency (hydrophobic and oleophobic), impact and scratch resistance, protection against graffiti, dirt and stains, finger print protection, UV stability, additional electrical insulation, protection against calcium and sodium deposits and increased brilliance and lubricity for application in the automotive industry.
ISTN, Inc.’s (www.istninc.com) UV FOGuard™ is hydrophobic and hydrophilic coating for preventing fog formation on automotive windows.
NGimat’s (www.ngimat.com) NanoSpraySM Combustion Process technology enables synthesis of thin films and nanoparticles for application of self-cleaning coatings for automotive glass.
Image 1: Nanocoated fog-free windscreen (Source: MIT).
AVIATION
Coatings are generally used in the aerospace industry for protecting the structures and surfaces of the aircraft from harsh environments. Stringent regulatory and technological requirements such as resistance to extreme temperatures, extreme climates, corrosion, abrasion and wear of engine parts have lead to an increased demand for more reliable high performance coatings.
Desirable functional properties for the aerospace and aviation industry afforded by nanomaterials in coatings include:
• Reduced weight and increased strength (carbon nanomaterials)
• High temperature control/resistance (SiC Nanoparticles in SiC-particle-reinforced alumina, Yittria stablized nanozirconia)
• Electrostatic discharge, EMI shielding and low friction (CNT, graphene, nanoaluminium, copper, iron, silver nanoparticles)
• Corrosion/Wear Resistance (silica nanoparticles, aluminium, Nanorystalline Carbide, Diamond like Carbide and metal dichalcogenide, TiN nanocrystallites embedded in amorphous Si3N4)
• Easy reparability & reusability
• Less maintenance & increased durability.
There is an increasing use of composite components in aircraft design such as radomes, propellers and aircraft structural components. Nanomaterials such as CNTs and graphene are seen as an alternative to the more traditional carbon black/graphite or metal particle additives; their high conductivity and high aspect ratio result in the formation of percolated conductive networks within coatings at very low loadings. Nanocoatings can also allow for new aerodynamic concepts designed to reduce the air resistance and thus the fuel consumption of aircraft.
De-icing
According to the Environmental Protection Agency, 25 million gallons of deicing agents are applied to aircraft at U.S. commercial airports each year. The aviation industry is also using energy-intensive pneumatic and electric anti-icing systems on aircraft to prevent ice formation on wings and other surfaces.
Graphene coatings are under development for numerous aerospace coatings applications. Promising areas include de-icing coatings. The EU-funded SANAD project is combining graphene with carbon nanotubes to make a coating for planes that can be connected to the electrical system and heated up to stop the build-up of ice. SAAB has also filed a patent for the development of de-icing coatings. The graphene additive could strengthen the acrylics and shield against EMIinterference.
GE are one of a number of companies developing anti-icing nanocoatings that reduce ice adhesion and have also been shown to delay the onset of ice formation. Easyjet has applied a nanocoating from tripleO (www.tripleops.com) for fuel savings and carbon footprint reduction.
Image 2: Aircraft wing coated with carbon nanotubes for de-icing (Source: www.lbf.fraunhofer.de/en/projects-products.html).
The coating reduces drag by up to 39%. SAAB (www.saab.com) has filed a patent to use graphene for de-icing airplanes. The graphene layer would be embedded in a heating jacket covering the aircraft.
CG2 NanoCoatings, Inc. (www.cg2nanocoatings.com) has developed a process to utilize nanoscale properties by first functionalizing nanoparticles and then incorporating them into a base material (polymers, metals, ceramics or composites) for anti-icing coatings.
Nanovere (www.nanovere.com) also produces nanocoatings to significantly reduce ice adhesion, de-icing maintenance costs, and reduce the coefficient of wind and water drag resistance, thereby decreasing the cost of jet fuel.
Thermal barrier
The use of nanomaterials greatly improves the thermal barrier properties in the manufacture of aircraft turbine engines. Nanocoatings allow for high thermal conductivity, high melting points and excellent adhesion to the underlying metallic substrates. the high thermal insulation properties of the coatings produced by this technique will also allow aircraft engines to be operated at higher temperatures, improving the engine’s burn efficiency for lower fuel consumption. Applied Thin Films, Inc. (www.atfinet.com) is developing a patented ceramic amorphous aluminum phosphate material (CERABLAK®) with excellent dielectric properties and thermal stability. It is suitable for various military and commercial aerospace applications, including next-generation radomes for hypersonic missiles such as the SM-6.
Image 3: Cerablak™ copting technology (Source: Applied Thin Films, Inc.).
Nanocoatings are also safer than the existing coatings, as they emit lesser volatile organic compounds (VOC) and employ thermal spray and diamond-like technologies, which are non-hazardous when compared to electroplating chrome technologies and organic paints. Inframat Corporation (www.inframat.com) produce nanocoatings to insulate hot section metallic components (turbine blades, turbine vanes, combustors) from the hot gas stream in all modern aircraft gas turbine engines.
Wear-resistance
Nanostructured metals can provide superhard coatings that are resistant to corrosion, for applications in aerospace components such as landing gear. Nanoparticles significantly reduce wear while maintaining low friction in tests. Super hard nanocomposite coatings can be widely applied on various kinds of cutting and forming tools, particularly for applications in high speed machining, high temperature wear components, moulds and dies.
Sensors
Multi-functional sensor coatings can achieve the dual requirements of higher turbine efficiency and lower gaseous emissions, and the monitoring of temperature, erosion and phase changes, are projected to reduce maintenance costs and improve safety standards. Carbon coated nanoparticles show potential in pressure/temperature sensing.
Anti-static
For aircraft applications there is an increasing need for conductive and anti-static coatings, for instance to protect safety-relevant structural elements made of fibre composites against lightning strikes. An aircraft can statistically expect a lightning strike to occur every 1,000- 10,000 flight hours, or at least one per year In terms of safety for aircrafts, lightning induced damages have emerged as an important issues. Currently, most of the external surfaces of composite fuselages are covered with wire mesh, expanded foil of aluminum to prevent and/or reduce damages from lightning strikes; however, it tends to increase the weight of fuselage by adding a thin metal layer in the fuselage and wings, and also it may induce galvanic corrosion that may become a concern after long service flights. Nanomaterials are used as nanocomposite coatings for carbon-fiber (CFRP) layers to enhance electrical conductivity of carbon-fiber reinforced plastics, without increasing the weight of structures. The use of CFRP materials on commercial aircraft has increased with the Boeing 787 “Dreamliner” and Airbus A350 XWB, incorporating over 50% composite materials by weight. Graphene and carbon nanotubes offers superior current carrying and heat dissipating qualities as is being developed for this application.
Powdermet (www.powdermetinc.com) has partnered with the U.S. Navy to provide a solution to contamination issues in spherical plain airframe bearings using advanced coatings that have already been commercialized through Abakan subsidiary MesoCoat. These nanocomposite cermet materials have applications across the transportation, energy, military, construction and other sectors for reducing friction and extending the life of, or eliminating the need for lubricants, in highly stressed systems.
Magnetic Shield (www.magnetic-shield.com) produces electromagnetic shield coatings. Vorbeck and BASF are collaboratively developing dispersions of highly conductive graphene for graphene/epoxy composites for use as EMI shielding materials.
Marine
Biofouling is a major problem in the marine industry. Biofouling negatively impacts maritime activities, creating biological films on different structures. Due to biofouling, regular maintenance and the use of biocides is needed, which interrupts maritime operations and contaminates water.
The ban on organic tin compounds for use as antifouling agents to prevent organic growth has necessitated relative frequent cleaning of the coating surface on the ship’s hull to ensure cost-effective transport. Due to the self-sacrificing (self-ablative) mode of operation, or “shedding” itself into the water, typical marine hull antifouling coatings are notorious for releasing environmental and health-compromising “anti-foulant” toxins/poisons into the water during routine underwater hull cleaning.
The increase in frictional drag caused by the development of fouling on hulls of ships can reduce speed in excess of 10% (Townsin, 2003). A vessel with a fouled hull burns 40% more fuel which has an impact on fuel costs and additional greenhouse gas production (estimated to be 20 million tonnes per annum).
There is a continual drive to develop improved anti-fouling coatings technologies that will have less environmental impact by allowing a reduction in fuel consumption and the release of biocides.
Marine coatings are in service to protect structures such as ships and platforms or sensors. By settling on ship hulls, mussels, barnacles and algae can substantially increase ship weight up to several tons within a short time thus leading to a higher resistance during transport, higher fuel consumption – even on long hauls. Typically, marine coatings are tributyltin self-polishing copolymer paints containing toxic molecules called biocides. They have been the most successful in combating biofouling on ships, but their widespread use has caused severe pollution in the marine ecosystem. Nanocoatings are an entirely non-toxic alternative, which reduces the adhesion strength of marine organisms, facilitating their hydrodynamic removal at high speeds.
Properties
Nanocoatings reduce the flow resistance between the ship’s hull and the water, thereby enabling a significant reduction in fuel consumption and carbon dioxide (CO2) emissions. The market will continue to grow, driven by environmental concerns and reducing maintenance costs. Desirable functional properties for the marine industry afforded by nanomaterial water based, anti-foul coatings include:
• super hydrophobicity (less fuel consumption).
• increase durability.
• water repellent.
• UV-resistant.
• fungus / algae / bacteria resistant.
• thermally insulating.
• elastomeric.
• anti-sticking and -corrosive (up to 1000 – 4000 hours of salt spray).
• high resistance to weathering (UV, fungus, water, dirt, etc.) and any blistering over standard marine coatings.
Another major advantage is the reduction of maintenance costs. The ban on organic tin compounds for use as antifouling agents to prevent organic growth has necessitated relative frequent cleaning of the coating surface on the ship’s hull to ensure cost-effective transport. The smoothness and greater hardness of nanoscale coatings provide better durability and extends the cleaning cycle. Since biofouling represents a substantial maintenance expense for hydro-kinetic systems, anti-fouling nanocoatings could potentially provide an important value-added solution for these systems. Areas in marine coatings that nanocoatings will impact include:
• Cargo holds: Abrasion resistance
• Cargo tanks: Chemical resistance
• Underwater hull: Fouling control
• Decks: Hard wearing, impact and chemical resistance
• Ballast tanks: Corrosion resistance.
Advanced Marine Coatings (www.amcoat.no/contact-amc.html) is a Norwegian paint company producing and marketing next generation marine coating with CNTs using a new unique patented technology.
Image 4: LNG carrier ‘Berge Arzew’ provided a floating test-bed for AMC’s Green Ocean Coatings.
BIOCYL™ is Nanocyl’s (www.nanocyl.com) trade name for a range of eco-friendly, fouling release paints designed for all major marine coating applications, such as ship hulls, oil rigs, and underwater intake valves.
Hempel’s (www.hempel.com) anti-fouling products, Globic 9000 and Globic 6000 are designed around patented nano-capsule binder technology. The nano-capsule acrylate copolymers are the main binder which in combination with a powerful bioactive mixture makes it suitable for protection on vessels operating in aggressive fouling waters.
Wetted surfaces coated with the Inframat Corporation’s (www.inframat.com) product on naval vessels that were coated with this material approximately five years ago still show virtually no signs of biofouling. NanoCover A/S (www.nanocover.dk) products include marine seals and marine glass. Nanogate Technologies GmbH has developed a non-toxic surface coating, Nano® FPU to ensure a biofouling-free ship’s hull.
Nanovere Technologies, Inc. (www.nanovere.com) specializes in the development of clear coatings ultra-scratch resistant and self-cleaning paint that can be applied to marine paint. Tesla NanoCoatings (www.teslanano.com) supplies corrosion control coatings incorporating carbon nanotubes for marine markets. Diamon-Fusion International, Inc. (www.diamonfusion.com) develops nanocoatings with water and oil repellency (hydrophobic and oleophobic) for marine glass.
PURETi (www.pureti.com) produces a water based solution that air dries to form an invisible, well adhered, ultra thin, long lasting coating that actively protects all surfaces to which it is applied from the buildup of any organic matter – including bio-film, bacteria, molds or fungi. PURETi products can be applied to virtually any surface, including buildings, signs, solar panels, sidewalks, outdoor furniture, holding tanks, boats, and planes. Reactive Surfaces (www.reactivesurfaces.com) develop exterior marine coatings functionalized with bio-based additives for submersed hull surfaces and stationary structures.
