The aviation industry makes widespread use of carbon fibre reinforced polymers (CFRPs). The material is stiff and lightweight. It is also smooth. Therefore, aircraft surfaces, such as wing panels and leading edges, are often made of CFRPs to help achieve reductions in drag. Yet, CFRPs are prone to erosion. Airborne particles and rain act like sandpaper on aircraft wings. So, critical surfaces, such as wing leading edges, are coated to protect against erosion. Current coatings are inadequate for this purpose because they need constant reapplication. They also have poor heat and electrical conduction properties. This means they offer little protection against lightning strikes, and they have poor de-icing and anti-icing capability. The industry needs a new coating, which the EU-funded WINNER project has developed. The team advanced several erosion-protection coatings, for use with wing panels having various electrical and thermal properties. The goal was to provide protection against erosion as well as lightning strikes and icing.
The coatings are based on multiple layers of titanium and titanium nitride. Titanium is a tough but lightweight metal able to absorb impact energy. Titanium nitride is a hard but fragile ceramic, often used for tool coatings, but it cracks under impact. Using both materials together combines their advantages. “For this reason,” explains Borja Coto, project coordinator, “we had to design a coating stack architecture, using the materials in alternating layers like lasagne.” Applying the coatings to the CFRP substrate imposes special challenges. Since a CFRP is basically plastic, it needs low-temperature application.
In order to achieve this, WINNER researchers modified an existing coating technology, called physical vapour deposition (PVD), for use with the project’s new coatings. PVD vapourises the coating material in a vacuum chamber and then deposits it onto the surface. Key advantages are that PVD works with many materials, it can create single or multiple layers on many types of substrate (including CFRPs), and it allows precise control of thickness. The combination of protective titanium compounds and PVD had never been attempted. Results indicate that PVD application of the new coating provides good protection against erosion. “The level of protection for rain erosion still has some room for improvement,” states Coto. “In terms of electrical properties, we have obtained resistive coatings, but the main focus of the project was on erosion properties.” The most important outcome was the newly gained knowledge about the complexities of different types of erosion on PVD coatings on CFPRs. This is important for designing suitable coating strategies. Next, the team will be optimising the coatings using other material candidates such as chromium and chromium nitride. These could be used as intermediate layers with the titanium and titanium nitride, to improve adhesion and therefore rain protection. In future, adding still more compounds to modify the electrical properties might help with lightning protection and de-icing. All coatings will eventually need testing and verification according to aviation safety standards. The WINNER project paved the way for future application of coatings on aircraft wings. Such coatings are an integral part of Europe’s broad base of work on improving aviation’s energy efficiency and environmental impact.
WINNER, coating, erosion, CFRP, PVD, aircraft wing, aviation, carbon fibre reinforced polymers, erosion-protection, physical vapour deposition