smart WINg panels for Natural laminar flow with functional Erosion Resistant COATings

Periodic Reporting for period 3 - WINNER (smart WINg panels for Natural laminar flow with functional Erosion Resistant COATings)

Berichtszeitraum: 2019-07-01 bis 2019-12-31

The increasing need to reduce weight in aircrafts in order to improve fuel efficiency has promoted the adoption of lightweight materials like 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. Polymeric coatings are widely used in aerospace industry to enhance the erosion resistance of composite materials but their lifetime is limited. Physical Vapour Deposition (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.
(PVD) coatings are well known as coatings for tribological applications, including erosion protection on different substrates. Although PVD coatings can be applied on polymer-based substrates to provide metallic aesthetics, there are critical issues to address in order to enable their use as protective coatings on soft substrates. While metallic aesthetic coatings on polymers are typically below 1 micron thick, tribological coatings on metallic substrates are in the range of 2-3 microns thick and temperature and bias voltage is typically applied during processing to increase adhesion.
In this context the overall objective of the WINNER project is to develop erosion-resistant functional coatings for Natural Laminar Flow (NLF) wing skin by using PVD deposition technologies on Carbon Fiber Reinforced Polymers substrates.

After results obtained in previous periods on the sand and rain erosion testing, the work in WP2 during the third period was limited to produce samples with and without lacquers for final testing in WP3. Thus, coatings ranging from 1.5 microns up to 11 microns on bare substrates as well as with three lacquered variations were produced and sent.
In WP3 the characterization of the coatings developed in WP2 was carried out. The results obtained from the final characterization have finally consolidated previous results and allow to the obtain following conclusions:
-The coatings behave in a different way when tested against particle erosion and rain erosion.
-In particle erosion tests, the thicker the coating the better protection performance even if adhesion is worst when depositing thick coatings. Thus, it was found that a qualitive improvement of the particle erosion protection performance occurs when applying coatings thicker than 10 m.
-The surface finishing of the CFRP substrate plays an in an important role in the nucleation of failure of the coatings.
-By using a lacquer as base coat, a more homogeneous surface finishing is obtained, and a steep increase of the sand erosion protection is achieved.
-In rain erosion testing the best result was obtained for a bilayer of Ti/TiN having an overall thickness of 1,5 m. As thickness increases, rain erosion protection is reduced which is also aligned with the adhesion of the coatings.
-The use of lacquers does not bring any improvement on rain erosion protection. Although the lacquer show good adhesion to the substrate, the adhesion of the PVD coating to the lacquer is not better than its adhesion to the bare substrates.

The results of the project have been disseminated on oral contributions on 3 conferences and a scientific article is expected to be published.

Thin hard tribological coatings 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. 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. 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.

Considering the objectives of the project the following outputs have been obtained

-Erosion resistant coating: PVD coatings having different thicknesses have been developed. Effective particle erosion protection have been obtained by combining lacquers and PVD thick coatings which is an interesting advance beyond the state of the art. On the other hand, the rain erosion protection level required for aeronautics standards have not been reached, although the strategy to obtain a better erosion protection have been identified. For these reasons the objective is considered partially achieved.

- Erosion mechanisms on PVD coatings protecting soft substrates: Achieved. By analysing the coatings developed during the project different mechanisms of erosion of coatings on CFRP substrates have been identified depending on the type of erosion (particle vs rain) revealing the different strategies required to provide erosion protection on soft substrates. This result paves the way towards future developments on protective coatings on soft substrates.

- Pretreatments to improve adhesion of PVD coatings on polymeric substrates under mechanical loads: Achieved. Polymeric substrates are non-conductive and temperature sensitive, so typical strategies (processing temperature and bias voltage) used in PVD coatings to improve adhesion can not be applied on polymeric substrates. WINNER has explored the use of lacquers and ion source pre-treatment in order to improve adhesion. Ion source pre-treatment has shown its potential to improve adhesion on polymeric substrates while lacquers have proven to be effective to achieve an adequate surface finish which allowed to boost particle erosion protection. Both strategies, are considered as possible exploitable results.

- Rain erosion resistant coating application method: PVD coatings having different thicknesses have been directly deposited on bare and on lacquered CFRP substrates. The use of a lacquer does not provide any improvement on rain erosion. Thin coatings having good adhesion showed the best performance on rain erosion protection. Thus, sputtering processes on bare substrates having moderate residual stresses (from process parameters) with short deposition times to obtain thin well adhered films on the surface have been identified as the best application method for rain erosion protection.

The level of achievement of the objectives obtained require of further developments in order to exploit the expected outcomes (medium term) and impacts (long term) foreseen regarding to the aeronautics sector. Nevertheless, the results allowed to identified different exploitable results on sectors beyond the aeronautics.

11 microns thick multilayer based on Ti/TiN

Time to failure of the different stacks tested under sand erosion conditions

Periodic Reporting for period 3 - WINNER (smart WINg panels for Natural laminar flow with functional Erosion Resistant COATings)

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