Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - WINNER (smart WINg panels for Natural laminar flow with functional Erosion Resistant COATings)

Teaser

The increasing need to reduce weight in aircrafts in order to improve fuel efficiency has promoted the adoption of lightweight materials like Carbon Fiber Reinforced Polymers (CFRPs) which are intensively used in aerospace applications due to their strength/weight ratio...

Summary

The increasing need to reduce weight in aircrafts in order to improve fuel efficiency has promoted the adoption of lightweight materials like Carbon Fiber Reinforced Polymers (CFRPs) which are intensively used in aerospace applications due to their strength/weight ratio. Nevertheless, when subjected to erosion caused by sand, dust, volcanic ash, or rain, fibre reinforced polymers suffer wear with mass losses that can be several orders of magnitude higher than steel. In order to avoid severe damage and high maintenance costs, the CFRPs in erosion critical areas such as leading edges of airplanes, the material must be protected against erosion. To this purpose, protective coatings can effectively reduce the erosive wear of CFRP components.

Besides erosion-resistant coatings there are other functionalities that are critical to the overall performance of components such as the wing and the platform. Thus, additional functionalities required, especially in the wing leading edge area, include lightning strike protection, de-icing or sensing capabilities. In order to obtain these functionalities, the electrical properties of the surfaces are a key factor.
Physical Vapour Deposition (PVD) processes are used to deposit films with thicknesses in the range of a few nanometres to thousands of nanometres. PVD processes offer several advantages as they have already shown good erosion protection in other substrates as metallic components of compressor blades. Additionally, they have already shown their potential to achieve good adhesion on polymeric substrates using low temperature processes. On the other hand, PVD processes can provide with a wide selection of coatings having different electrical properties.
The overall objective of the WINNER project is to develop erosion-resistant functional coatings with tailored electrical properties for Natural Laminar Flow (NLF) wing skin by using PVD deposition technologies.
Especifically, three different multifunctional coatings are targeted:

- Electrically conductive erosion-resistant coating

- Electrically resistive erosion-resistant coating

- Non-conductive erosion-resistant coating

Work performed

During the first period the following WPs were active.
WP1 Specification of coatings and technologies
WP2 Development of coatings and technologies
WP3 Characterization and testing of candidates
WP1 was finished with the selection of coating candidates and processes. Combination of hard and ductile layers has been proposed with Ti/TiN layers as main initial candidate. On the other hand, sputtering processes were selected as PVD technology to be used as it has more possibilities in terms of the range of materials, conditions and quality of the film (as the same chamber includes HiPIMS technology) that can be coated in comparison with other PVD techniques such as cathodic arc.
WP2 started with the development of Ti and TiN coatings and a first batch of coatings was sent for rain erosion testing. From these initial testing Ti/TiN bilayer having 2.5 microns of total thickness showed the capability to protect against rain erosion to some extent but not enough to cover the aeronautics requirement in their hardest conditions. Thus, further development focused on obtaining thicker coatings to enlarge the duration of the erosion protection. Different multilayer stacks composed of Ti/TiN nanometric layers for total thicknesses ranging between 7 and 8 microns were developed for this purpose. Although a better rain erosion performance was expected due to the use of thicker coatings, testing from WP3 revealed that an adhesion problem arise when increasing coating thickness. Thus it is necessary to investigate pre-treatments that improve the adhesion of thicker coatings to meet the rain erosion requirements. Ion source pre-treatment has been selected as main potential pretreatment to improve the adhesion of thicker coatings and experiments are currently ongoing.
In WP3 the characterization of the coatings developed in WP2 was carried out. Although basic adhesion tests (cross-hatch) showed good adhesion of the coatings, the harsh conditions required for rain erosion protection showed that thin coatings cannot stand the required time under rain erosion. Thus, thicker coatings are required to increase the erosion protection time. Nevertheless, according to erosion testing with particles and water, when going to thicker coatings the protection is not improved as the failure occurs by a delamination mechanism instead of a smooth erosive wear.

Final results

Thin hard tribological coatings such as TiN, TiAlN, WC and nanostructured materials, such as hard nanocomposite (nc) systems based on transition metal nitrides embedded in an amorphous (a) matrix, such as nc-TiN/a-SiN1.3 or nc-TiCN/a-SiCN, can provide erosion protection for components operating in an erosive environment. Nevertheless, there is a lack of studies of such erosion protective coatings for the protection of polymeric materials or CFRP. Indeed, there are several difficulties to be addressed in order to obtain the erosion protection on that kind of substrates by means of hard coatings commonly used in other applications. One of the key problems to address is he effect of having a hard coating on a softer substrate that is under hard erosive conditions, as the particles or drops impinging on the surface may induce the deformation of the substrate leading to problem of delamination in a substrate driven erosive mechanism. In order to obtain a smooth erosive wear behavior it is necessary to grow thick coatings. The main issue is that as the film grows thicker the residual stresses of the coatings may also lead to adhesion problems. Thus, a combination of hard and ductile layers is necessary in order to reduce de residual stresses. Another problem with the adhesion is that typical strategies to improve the adhesion between substrate and coating, such as increasing processing temperature and applying a bias voltage to the substrate, do not meet the requirements for activating temperature-sensitive and electrically insulating materials such as CFRP substrates. So different pre-treatments need to be investigated. For this reason, the use of PVD coatings as erosion protective coatings on polymeric substrates has been poorly addressed in literature, and especifically never done for rain erosion protection. On the other hand, WINNER aims to obtain additional features as different electrical properties for the erosion protective coatings, that gives them the potential to be used as de-icing systems, lightning strike protection or sensing capabilities. These multifunctional coatings have never been addressed before.
By achieving the objectives of the project, three innovative multifunctional coatings will be obtained. Moreover the project aims to provide a proof-of-concept of the scalability of the solutions proposed by coating medium size wing panels. With the three different coatings a range applications can be covered as erosion protection combined with resistive heating de-icing, sensing capabilities, EMI shielding, etc, has the potential to be applied to areas beyond the aeronautics sector, such as wind energy or automotive industry.