Improved Aviation Coatings Promise Greater Protection, Sustainability

Coatings and surface treatments are integral to the functionality, reliability and longevity of airframe components. Recent advances in coatings technology are providing greater protection and sustainability alike.

Dominic Shore, a specialist in surface technology at Airbus, explains that reducing use of environmentally harmful chemicals has been a “major activity within surface coating technology” at the airframer. In recent years, he says, that has included replacing hexavalent chromium compounds, which help inhibit erosion in surface treatments and are key in such process treatments as chrome plating and anodizing. Cadmium, widely used for its anti-corrosion properties, is also among the coatings being replaced.

“Airbus has qualified zinc-nickel coatings as a less environmentally impactful alternative to cadmium plating,” Shore says. “Airbus is also adopting hexavalent chromium-free anodizing processes and adopting new coating technologies to replace hard chrome plating, which was applied to many steel airframe components.”

Hard chrome helped increase the surface hardness on top of the softer steel substrate, he explains. Its microstructure also enabled retention of greases and lubricants, improving its tribological suitability for many applications. “Hard chrome was also very important for corrosion prevention as it provided a robust, wear-resistant environmental barrier on corrodible low-alloy steel materials,” Shore says.

Paul Brooks, head of vertical sales at Oerlikon Surface Solutions, reports that the Swiss chemical company has developed a wide range of coating products with applications to compressor and turbine blades, vanes, combustors and shrouds.

“As an example, Oerlikon Balzers . . . offers physical vapor deposition coatings, which are mainly applied to metal substrates,” Brooks says. “Their application is to compressor blades for erosion and corrosion protection of their surface finish and improved fuel efficiency.” He notes that the division recently launched a cathodic arc coating that offers oxidation and hot corrosion protection, and “can be used as a cost-efficient bond coat for high-pressure turbines or stand alone for low-pressure turbine applications, outperforming other market solutions such as platinum aluminide or aluminizing.”

Environmental barrier coatings (EBC) developed for ceramic matrix composite (CMC) components have improved durability under extreme conditions, Brooks adds.

“EBCs are designed to protect silicon-based, lightweight CMCs, which can operate at higher temperatures than traditional superalloy substrates,” he explains. “EBCs also protect CMCs from water vapor attack at high temperatures.” He adds that Oerlikon is also developing wear- and erosion-resistant coatings for polymer materials such as carbon-fiber-reinforced polymer.

Noting that shifting to more sustainable technologies has often run up against cost considerations, Brooks cites Oerlikon’s recently introduced flash tungsten carbide coating, available as a high-velocity oxygen fuel or high-velocity air fuel product. The coating, which provides wear and corrosion resistance, is available as a powder or thermal spray. The product is designed for engine components including blades, vanes, blisks, combustion liners, transition ducts, fuel injectors, shrouds, actuators and bearings. “It provides a more compelling business case for transitioning to new technologies,” Brooks says.

Celine Dorignac, global segment manager of structures and cabins at AkzoNobel, says the Dutch paints and coatings provider is focused on spray-applied chrome-free coatings designed to protect all internal structural airframe parts, as well as fuel tanks, landing gear and the aircraft’s exterior skins. One example is the lithium-based Aerolith P27CF structural primer, which Dorignac says has been tested and qualified to industry standards on aircraft fuel tanks, demonstrating that it works under harsh conditions and is chemically resistant. “It has also been successfully tested and monitored on the landing gear and accessible structural parts of commercial aircraft during normal corrosion management programs,” she explains.

Dorignac also notes that the one-coat, chrome-free systems can be applied directly to untreated aluminum. “This single-coat technology will work well with robotic technology working alongside spray painters and may, in the future, enable chrome-free anaphoresis dipping—a painting process that uses an electric current to apply a protective layer to a metal surface,” she says.

As turbine engines burn hotter, manufacturers have responded with thermal barrier coatings (TBC) to insulate metal parts in the hot section. Wil Baker, director of offering management at Honeywell Aerospace Technologies, explains that while jet fuel burns at temperatures in excess of 3,800F, many of the metals used in the hot section melt at around 2,500F.

“TBCs are a crucial part of several cooling technologies used to not only prevent parts from melting, but also [keep them] on wing for 15,000 hr. or longer,” Baker notes. “They are applied to nearly every hot-section part which is exposed to burning jet fuel, with rotating turbine blades and stationary nozzles the most common.” He points out that the most common offering TBC Honeywell provides is 7YSZ, referring to the product’s composition—7% yttria plus 93% zirconia.

Baker says gas turbines become more fuel-efficient as core pressure and temperatures increase. “The result is that the engine OEMs have been increasing temperatures by 50C (122F) every 10 years,” he says, adding that Honeywell expects that trend to continue for at least another 40 years. With that in mind, it is developing a TBC with about 33% lower thermal conductivity. “It is more chemically stable, enabling it to operate at temperatures 300C hotter,” he adds.

However, Baker cautions that airborne dust and pollution limit TBC life. “In some cases, that has been reduced to 1,500-3,000 hr., which isn’t affordable given the time and expense to overhaul an engine,” he says. “This will get worse as engine temperatures continue to increase.” To address this, Honeywell is testing a coating that has so far lengthened time on wing by 4-5 times in laboratory testing. Engine trials are planned for later this year.

Another new coating technology is AeroSHARK, a joint project of BASF and Lufthansa Technik (LHT). Designed as a surface film, AeroSHARK incorporates microscopic riblets to reduce aerodynamic drag and improve fuel efficiency. BASF developed and tested the material, which it is producing for global rollout under the NovaFlex SharkSkin brand. LHT specified the material requirements and designed the riblet structure at micro- and macroscopic levels, and the company is conducting inflight testing and overseeing application of the coating to aircraft as well as establishing application and repair procedures.

The joint team also defined a comprehensive laboratory test regime for AeroSHARK, says Soenke Burger, head of avionics and flight ops solutions at LHT. Burger explains that this included resilience tests “involving a broad spectrum of exposure to harsh liquids” such as deicing fluid, as well as reaction to temperature, humidity and ultraviolet lighting, along with the capability to withstand external cleaning. “Tests for adhesion to the aircraft’s surface, icing and flammability tests, and the stability of the riblet film, itself, were also included,” he notes. “We also conducted combinations of tests to create more complex failure scenarios.”

Burger says the riblet film is extremely resilient, withstanding large temperature shifts, pressure differentials and ultraviolet radiation at high flight levels. “Once applied, there are no special requirements for cleaning,” he adds.

LHT to date has applied AeroSHARKto 23 aircraft at seven different airlines, starting with the first application on a Boeing 747-400 in 2019. Other aircraft have included the 777-200ER, 777-F and 777-300ER. As of mid-January, AeroSHARK has accumulated 160,000 flight hours, a fuel savings of 9,900 metric tons and a reduction of 31,000 metric tons of CO₂. Fuel savings have averaged 1% per aircraft for the current modification, although LHT estimates this increasing to as much as 2.5-3% for larger, future modification stages.