Smart coatings: next-generation supersonic aircraft, high-performance engine parts and gas turbines

With recent advancements in materials science, the definition of ‘smart coatings’ is continually evolving – although it would be preferable to have one simple, universal definition (ie, next-generation coatings which react dynamically and can change in response to their environment).

The technology has already penetrated various industries, such as the aerospace, marine, automotive, construction, communication, textile, biomedical, electronics, power and energy, and defence weaponry sectors.

In the aerospace industry, patenting activity has grown at an average rate of 9.5% per year. The patent filings show that the technology has attracted the Chinese scientific community (nearly 40% of such patents are filed in China), followed by the United States and Japan. In China, the patents are mainly filed by universities, research institutes and independent inventors. IP filings show that patenting activity has increased rapidly in this domain, with an average growth rate of approximately 24% per year. The grant rate in China is also significant compared with other countries. The major universities and research institutes in China that are filing patents in this technology domain are:

• Beihang University of Aeronautics and Astronautics;
• Jilin University;
• Southeast University Nanjing;
• Nanjing University of Aeronautics and Astronautics;
• Zhengzhou Institute of Aeronautical Industry Management;
• Shanghai Jiao Tong University;
• Harbin Institute of Technology;
• Lanzhou Institute of Physics of the Chinese Academy of Space Technology; and
• Zhejiang University.

In the aerospace sector, industries and organisations adopting this evolving technology are collaborating and strategising in order to engage in fast-growing aerospace markets, such as India. Pexa, a global supplier of surface coatings, paints and treatments for the aerospace industry, also partners with aerospace and defence manufacturing companies in India. Pexa is also marketing Socomore’s surface treatment products in India.

Key industry players in this domain are:

• Airbus;
• Boeing;
• PPG Industries;
• Rolls Royce;
• General Electric;
• European Aeronautic Defence and Space Company;
• Saint Gobain Glass;
• Mitsubishi Heavy Industries; and
• AVIC.

The patent and non-patent literature in this domain reveals that various types of coating material have been categorised under ‘smart coatings’. These include:

• anti-corrosion coatings;
• colour-shifting coatings;
• bioactive coatings, such as:
  • anti-microbial/anti-fungal coatings;
  • anti-fouling coatings;
  • biodecontamination coatings; and
  • biocatalytic coatings;
• self-stratifying/assembling coatings;
• electrically conducting coatings;
• super-insulating coatings;
• self-repair/self-healing coatings;
• super-hydrophobic coatings;
• self-lubricating coatings;
• optically active coatings;
• self-cleaning coatings;
• self-dimming coatings;
• electrochromic coatings;
• thermochromic coatings;
• super hydrophilic coatings;
• omniphobic coatings;
• piezoelectric coatings;
• photovoltaic coatings; and
• piezomagnetic coatings.

These patents generally focus on coating materials for aerospace parts and components, such as aircraft structural body parts, compressor or turbine blades, airfoils and discs, valves and heat exchangers. In most cases they include super-hydrophobic coatings for aircraft (particularly fixed-wing aircraft) skin, airfoil, structural components (eg, wings and stabilisers) and satellite antennae. The remaining patents focus on aircraft parts which are subjected to harsh environments (eg, high temperatures and pressures) resulting in corrosion and wear due to oxidation, hot corrosion or sulphidation.

Pre-2000
Before 2000, the major focus of inventions was on hydrophobic coatings for various aircraft components, such as the engine surface, wings, stabiliser systems and aircraft window glass and coatings – capable of withstanding severe conditions due to high temperature and pressure – for structural parts for next-generation supersonic aircraft, engine parts and gas turbines. Although limited, research also focused on satellite components. Several notable patent disclosures during this period are outlined below:

• The design and development of black paint for spacecraft elements, such as deployable antennae. The coating has low surface resistivity to dissipate electrical charges that develop on the external surface of the spacecraft, and this surface resistivity is maintained (with relatively minor change) over extended periods. The paint also has mechanical properties, including a high degree of flexibility which makes it suitable for use on structures that flex, bend or deploy and prevent it from cracking and flaking.
• To prevent the thermal degradation of aircraft structures – such as leading edges and engine exhaust nozzles that are exposed to high-speed environments resulting in heat generation, further causing thermal erosion – the plurality of microcapsules which contain heat-absorbing materials were designed.
• As silica is covered with a hydrophobic compound in water-repellent coatings, the hydrophobic fine particles are likely to peel off from the surface. To overcome this, a durable and ultra-high water-repellent film coating was created.
• Self-lubricating coatings, which are useful in aircraft fuel and oil pump housings where severe operating conditions can occur.
• Bayer filed four patents for the use of the ultraphobic surface as friction-reducing coating for the hulls of an aircraft, which also has self-cleaning properties:
  • for the production of a coating with ultraphobic properties, a smooth substrate surface is coated with nickel hydroxide (Ni(OH)₂) particles with a diameter of 0.5 to 20 micrometres. The substrate is then given a hydrophobic or oleophobic coating;
  • for the production of an ultrahydrophobic surface on aluminium as substrate material;
  • for the production of an ultrahydrophobic surface on tungsten carbide as substrate material. The ultraphobic material is deposited onto the tungsten carbide after its laser ablation; and
  • for a sand blasting method for depositing the ultraphobic coatings.
• High-temperature oxidation-resistant coating that can withstand the aerodynamic heating environment at the time a space craft re-enters the earth’s atmosphere. This coating also self-repairs cracks and has low catalytic properties.

Between 2000 and 2005
During this time patent literature focused on fixed-wing aircrafts. Most of these patents concerned electrically conductive, hydrophobic and self-cleaning structural aircraft parts. Others focused on the design and development of anti-corrosion and self-lubricating coatings for components designed for use in hostile thermal environments, such as the turbine, combustor and augmentor components of a gas turbine engine. Notable patents included the following:

• Coating for de-icing an aircraft, which has other potential applications in the aerospace industry – such as helicopter rotor blade coating and an aircraft service system – and can also be used for submarine periscope windows to enhance optical features.
• Self-cleaning super hydrophobic coating for aircraft structures (in particular, titanium structures) that are susceptible to build-ups of ice, water and other contaminants.
• A hard, wear-resistant and ice-phobic coating for an airfoil surface to enhance the de-icing properties via low-pressure plasma vapour deposition technologies, such as plasma-enhanced chemical vapor deposition, chemical vapor deposition, physical vapor deposition (‘sputtering’) and reactive sputtering. The coating includes a functional top layer which is harder than the air foil surface and has a high contact angle with water.
• Electrical conductor coating for aircraft windows comprising polymeric film composed of polycarbonate, acrylic, polypropylene and/or polyethylene terephthalate, and filler material such as metallic oxide (preferably antimony tin oxide).

Between 2005 and 2010
Although research mainly focused on super hydrophobic and electrical conductor coatings during this time, significant patents were filed for other smart coatings, such as anti-corrosion, self-cleaning, self-lubricating, electrochromic, hydrophilic and self-healing coatings for various aerospace parts and components. For example:

• a variable emissivity and variable reflectivity electrochromic intelligent thermal control coating for spacecraft;
• electrically conducting coating for electric and/or electronic systems typically, such as the avionics or the distribution network of power of an aircraft;
• self-lubricating bearing for use in aircraft;
• super-hydrophilic inorganic coating with improved light transmittance;
• corrosion-resistant, wear-resistant and self-lubricating coating materials;
• nano super-hydrophobic surface coating used for aeroplane anti-freezing and de-icing;
• anti-corrosion and low-friction coating;
• shape-memory polymers with self-healing properties suitable for making morphing skins for aircraft;
• hydrophobic coatings with anti-corrosion and self-cleaning properties for life support systems for spacecraft and self-cleaning of external walls; and
• coatings with anti-fouling, anti-fogging and stain-resistant properties, preferably for aeroplane windows.

Post-2010
After 2010, researchers began to develop coatings which can perform multiple functions; the term ‘multifunctional’ was therefore incorporated into the definition of ‘smart coatings’. Significant patents filed for such coatings and their preparation methods include:

• an anti-fog coating for aircraft housing, with anti-reflection, self-cleaning, anti-glare and hydrophobic properties;
• a corrosion-resistant coating with integrated hydrophobic properties;
• a super-hydrophobic coating with self-cleaning properties, which also protects aircraft against ice, soiling and erosion;
• a thermal insulation coating with anti-corrosion properties;
• transparent and insulating hydrophobic coatings;
• a self-cleaning coating with anti-microbial properties for use in aircraft cabins; and
• an ice-phobic coating made by bonding a hydrophobic entity to a hydrophilic moiety, in which the hydrophilic moiety can lower the freezing point of water.

Other patents still focus on aircraft and satellite structural parts, such as aircraft and spacecraft windshields, aeroplane wings and landing gears, and gas turbine engine components (including vanes, blades, blade outer air seals, fuel nozzle guides, combustor liners and exhaust liners), as well as augmentor liners using electrically conductive, hydrophobic, anti-corrosion, anti-microbial, self-healing, self-cleaning and corrosion-resistant coatings for greater efficiency. Polyurethane resin is the most commonly used polymer for various smart coatings.

Looking ahead
Smart coating technology has not successfully penetrated other industrial sectors due to the cost of its development and adoption; however, market pull and patenting activity in the aerospace sector indicate a bright future for this technology. There are several key industry players in the aerospace sector which supply smart or semi-smart coatings, but chemical firms must begin to focus on smart coatings. This can be achieved through investment in or acquisition of innovative smaller firms which specialise in smart coatings, or collaboration with research institutes and universities (especially in countries such as China where universities are supplying the majority of such inventions). Since the definition of ‘smart coatings’ is still expanding, researchers have the opportunity to contribute to the term.