Coatings act as an interface between a substrate material and the environment, helping to boost the material's performance and durability in many industrial applications. Thin films and functional surface coatings are widely used in the aerospace and aviation sectors to improve the properties of advanced construction materials. A growing number of aerospace and aviation companies are investigating the use of nanocoatings that promise significant performance advantages over traditional coatings and improve cost-efficiency.
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Coatings are an integral part of practically any product or structure. Apart from improving the product's esthetic appeal, coatings serve as protection from a wide range of external factors, such as mechanical damage (scratches or impacts), corrosion, weathering, and bio-fouling.
In addition, coatings can provide specialized functionality to the product, like electrical conductivity, electrical or thermal insulation, hydrophobicity, and light reflection. However, conventional coatings possess limitations, such as poor adhesion between the coating layer and the substrate, limited flexibility, inadequate abrasion resistance, and limited durability and strength.
Applying nanotechnology in coatings has shown exceptional growth in the last two decades. Such remarkable development results from the increased availability of nanomaterials, such as nanoparticles, carbon nanotubes, and others, and the advances in deposition processes permitting control of the coating structure at the nanoscale.
Besides, the potential of nanotechnology to address many performance challenges introduced by the expanding range of products that require advanced coatings further contributes to the ever-growing interest from the industry and academia towards the development of nanotechnology-based coatings.
Nanocoatings Offer Tailored Characteristics for Various Applications
Nanocoatings are typically single- or multi-phase solid structures deposited onto an undelaying surface, with a thickness of around 100 nm or less, adding a specific property or function to the substrate material. Various materials exhibit unique properties at the nanoscale. Common characteristics such as melting temperature, electrical conductivity, magnetic permeability, hydrophobicity, and chemical reactivity strongly depend on the size of the material particles.
The material properties on the nanoscale are governed by intermolecular forces like Van der Waals interactions, hydrogen bonding, and electrostatic forces. These intermolecular forces act on length scales ranging from a few Angstroms to a few nanometers.
When the dimensions of the material become of the same order of magnitude as the intermolecular force distances (i.e. nanoscale), the properties of the material change dramatically. Thus, nanocoatings composed of nanoparticles or multiple nanoscale layers offer the opportunity of exploring their enhanced physical and chemical properties in a wide range of novel applications.
The main industrial applications of nanocoatings include corrosion protection, wear-resistant coatings, thermal protection, and self-cleaning (non-stick) surfaces. Such applications are particularly relevant to the aviation industry, enabling modification of aircraft frames, interior and engine components for improved performance, fuel efficiency, and lower operational costs.
Invisible Coatings Can Protect the Next Generation of Aviation Engines
Titanium alloys are among the lightest materials currently used in modern aircraft, particularly for manufacturing gas turbine engine components. Improving the heat and wear resistance of titanium alloys will permit replacing expensive nickel alloys with more affordable and lightweight alternatives, thus significantly reducing the aircraft's weight
Researchers at Riga Technical University's Institute of Aeronautics have developed a multilayer metal-ceramic nanocoating, called McBLADE, that can be used to protect the compressor and turbine blades in modern jet engines made of titanium alloys.
Each layer of the coating is extremely thin and has a specific function. The first enhances the adhesion to the base material. The second layer protects against the oxidation of the titanium alloy. In contrast, the top-most layer provides thermal protection and the coating's abrasion resistance, allowing the components to operate at high temperatures.
The layer deposition is based on a magnetron sputtering physical vapor deposition process that allows different components to be evaporated/sputtered from a condensed phase and deposited as a thin film on the substrate. Extensive tests demonstrated that the innovative coating can provide long-term protection of titanium alloys operating at temperatures in the range 800-870°C.
A similar process, electron-beam physical vapor deposition (EB-PVD), is used by Honeywell Aerospace for the deposition of yttria-stabilized zirconia nanocoatings. The company envisages this to be the next generation of thermal barrier coatings (TBCs) that can be used in industrial and aircraft gas turbine engines. The company's research team managed to adapt the advanced EB-PVD chemistry to fabricate a range of high-performance TBCs for different applications.
How Can Nanocoatings Reduce the Environmental Impact of Aircraft?
Developing novel methods for passive drag reduction in aviation is one of the most viable approaches to reduce aircraft fuel consumption, CO2, and noise emissions. Riblet surfaces, consisting of very small (2-100 microns) parallel grooves, are regarded as one of the most promising systems for passive drag reduction in next-generation aircraft.
An interdisciplinary research project, called ReSiSTant (Large Riblet Surface with Super Hardness, Mechanical and Temperature Resistance by nanofunctionalization) funded by the EU's Horizon 2020 research and innovation program, aims to develop advanced nanocoatings and deposition methods to enhance riblets performance in harsh environments by providing abrasion and corrosion resistance.
The use of silica nanoparticles in the coatings improves the thermal and flame resistance of the riblets (to temperatures up to 1000°C), permitting not only to use riblets on the aircraft exterior but also to use them for the optimization of gas flow within the aircraft's jet engines. The researchers expect to be able to demonstrate production prototypes by the end of 2021. The technology can also be translated into other industrial sectors, such as wind turbine manufacturing and industrial gas compressors.
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Nanocoatings for Reduced Ice Formation and Better Aerodynamics
Aircraft de-icing in cold weather can be costly and time-consuming. Scientists from the nanotechnology research department at Glonatech, a nanotechnology company based in Athens, Greece, in collaboration with aviation industry partners, are developing nanocoatings that can repel water and ice, leading to significant maintenance costs savings.
In-flight testing on British Airways Airbus A320 aircraft demonstrated 20-40% improvement of the surface hydrophobicity compared to other commercially available conventional coatings. At the same time, the innovative coatings, based on nanostructured carbon materials (such as carbon nanotubes and graphene oxide), reduce wind drag on the aircraft's surface, thus reducing fuel consumption and CO2 emissions.
Opportunities to Advance the Aviation Industry
Researchers and engineers are investigating even more advanced nanomaterials with lower low thermal conductivity and superior mechanical properties to take full advantage of the nanocoating technology. For example, such advanced nanocoatings would enable jet engine parts to last up to 50% longer compared to the existing materials.
Designing a jet engine that can operate at temperatures 300°C higher than the temperatures in the currently existing engines could also result in 5-10% improvement in power output and 1-2% better fuel efficiency, saving the industry billions of dollars in fuel costs and lowering CO2 emissions over the entire life cycle of the aircraft.
References and Further Reading
N. R. Lopez (2020) RESISTANT PROJECT: How can nanotechnology reduce environmental impacts of aircrafts? [Online] www.openaccessgovernment.org Available at: https://www.openaccessgovernment.org/nanotechnology-reduce-environmental-impacts-of-aircrafts/85311
Labs of Latvia (2021) Nanocoatings for the New Generation of Aviation Engine Parts Developed by RTU. [Online] www.labsoflatvia.com Available at: https://labsoflatvia.com/en/news/nanocoatings-for-the-new-generation-of-aviation-engine-parts-developed-by-rtu
Bao, W., et al. (2019) Next-Generation Composite Coating System: Nanocoating. Front. Mater. 6, 72. Available at: https://doi.org/10.3389/fmats.2019.00072
Gu, Y., et al. (2020) Technical Characteristics and Wear-Resistant Mechanism of Nano Coatings: A Review. Coatings 10, 233. Available at: https://doi.org/10.3390/coatings10030233
Pathak, S., et al. (2021) Engineered Nanomaterials for Aviation Industry in COVID-19 Context: A Time-Sensitive Review. Coatings 11, 382. Available at: https://doi.org/10.3390/coatings11040382
L. Kiesel., (2021) Advanced Air Plasma Spray TBCs for Aerospace and Industrial Components [Online] www.aerospace.honeywell.com Available at: https://aerospace.honeywell.com/us/en/learn/about-us/blogs/advanced-air-plasma-spray-tbcs
Glonatech. (2016) Glonatech White Paper on Nanotechnology. [Online] www.glonatech.com Available at: https://www.glonatech.com/wp-content/uploads/Carbon-Nanotubes-Nanotechnology-Consulting-Glonatech-White-Paper.pdf
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