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Coatings with Hydrophobic and/or Omniphobic Properties against INsect contamination.

Periodic Reporting for period 1 - CHOPIN (Coatings with Hydrophobic and/or Omniphobic Properties against INsect contamination.)

Reporting period: 2018-03-01 to 2019-08-31

The aeronautical industry has long been concerned about the increase in aerodynamic drag, which has a direct impact on aircraft fuel consumption. Several studies have shown that insect sticking causes 3-D disturbances in the laminar flow. To prevent this from happening, the use of coatings to reduce insect contamination has begun to show promising results. However, some critical issues remain unresolved, such as the lack of durability.

CHOPIN's objectives are to develop highly durable hydrophobic/omnifobic coatings that can be applied to micro-perforated surfaces typically used to reduce drag and to validate the technologies by using tests to clearly assess the reduction of insect contamination under realistic conditions.

The project envisages different coating technologies to obtain hydrophobic surfaces: wet-chemical deposition (UV-curable resins, sol gel, ionogel) and dry technologies (ion implantation and PTFE spray). The application process must be compatible with micro-perforated substrates (no hole clogging) and the efficiency of the HLFC leading edges.

The insect contamination and cleaning behavior of the best coatings will be demonstrated during wind tunnel and field tests under realistic conditions. Field tests will be carried out using large drones in order to evaluate the impact of insects and their contamination on typical leading edges of aircraft during take-off, cruising and landing.
- Formulation and application of anti-contamination coating solutions based on the selected technologies: sol gel, UV-resin, ionogel, anti-stick spray, ion implantation.

Nearly 200 different formulations or application conditions have been tested to achieve the anti-contamination objectives. The coatings were mainly applied to a stainless steel substrate in the first phase of the project. The screening was carried out on the basis of the wettability of the surface (water contact angle, slide angle) and cleanability on one hand and the basic properties such and basic properties as adherence, scratch and impact resistance, and bendability on cylindrical mandrel on the other. The coatings were also applied to micro-perforated titanium in order to verify their compatibility with this specific type of substrate without clogging the holes. Based on the results, the best or optimized solutions were submitted to laboratory and durability tests.

- Durability evaluation
In addition to the cleanability effects, the durability of the coatings is crucial in aerospace applications, especially for leading edges where the material is subject to erosion, impact, UV, etc. Several test sequences were proposed to evaluate and select the best candidate coating (including mechanical and chemical resistance and accelerated aging tests).
To evaluate the coatings' fluid susceptibility, the most representative aircraft fluids, such as water, phosphate ester hydraulic fluid (Skydrol), kerosene and de-icing fluid, were used for the test. Accelerated weather tests (a combination of UV and humidity) and thermal cycling (-40ºC to 80ºC) for 1 month were performed in a second phase. At the end of these tests, the integrity of the coating and the conservation of its functionality (anti-stick/easy-to-clean) were checked. Based on the different results obtained, 3 coatings are selected for further evaluation in simulated (wind tunnel, rain, and sand erosion) and real environmental conditions (drones).

- Development of a test stand for wind tunnel and on drones
Specific test specimens have been designed and manufactured to evaluate the selected coatings in the wind tunnel under different conditions (injection of insects, water, ice, etc.). The aim of these tests is to evaluate the performance of the coatings in terms of anti-contamination and their durability against various impacts. An insect injection system has been tested and optimized. After the first series of tests on small test coupons, the best coating out of three will be selected and used for further testing on a test specimen representative of a leading edge. The same test specimen will be used on drones to enable correlation between the different tests.
A modified design of the drone fuselage has been proposed in order to be able to carry the test samples on the nose of the aircraft. A drone instrumentation plan was developed and a flight campaign was planned in areas with a high insect concentration during the summer period.
- New coating formulations: based on existing coating systems from different partners (MANO, CID, BERTHIER), new formulations were developed and their application parameters were optimized to meet the requirements in terms of anti-sticking/cleanability and durability. This means that by the end of the project, new coating formulations will be available that fully or partially meet aviation requirements and provide an anti-contamination effect for leading edge applications. This type of coating may have a wider scope of application (including trains, automotive, etc.) and the knowledge acquired will be useful in understanding and minimizing contamination in general. For BERTHIER, this project is linked to the creation of a new business unit dedicated to ECO-FRIENDLY coatings (based on a dry coating process). This coating has promising properties and is part of the selected coatings. The impact for BERTHIER can be very important for employment, since in the 5 years following the end of the project it can represent about 20 new people working on coating applications and in robotized systems to apply the coating.

- Wind tunnel and drone testing protocol: new test coupons or devices are being developed to take into account anti-contamination, self-cleaning and impact resistance. This applies to both wind tunnel and drone tests. In parallel, new test protocols are being developed that will be validated and correlated with laboratory tests. The combination of test equipment and protocol also goes beyond the state of the art and will lead to new test possibilities for aerospace applications. In particular, the availability of reliable test protocols for wind tunnel testing may, at least in part, reduce the need for testing in real aircraft conditions. The knowledge associated with these developments will be made available (at least in part) to the scientific and academic community for education, etc.
Slippery effect
Impact of insect on test coupon of wind tunnel