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Super hydrophobic and erosion resistant coating for turbine scroll and downstream pipe

Periodic Reporting for period 1 - ERICE (Super hydrophobic and erosion resistant coating for turbine scroll and downstream pipe)

Reporting period: 2018-10-01 to 2020-03-31

The deposition of ice on aircraft surfaces is a widespread problem that causes high costs and impairs functionality and safety, as it leads to higher energy consumption, increased drag, lower energy output, but also risk of damage and therefore risk of accidents. Current solutions for the air cycle machines use a heating system on the downstream pipe to heat the surface, requiring high energy, hence the need to reduce energy consumption through the development of passive energy-saving solutions.
The main objective of ERICE is to develop an improved eco-friendly and cost-effective hydrophobic/ice-phobic surface solution (i.e. a passive ice protection solution) able to resist ice erosion conditions in a turbine scroll and downstream pipe of a representative air cycle machine.
The first step is the implementation of a new experimental test set-up at Cranfield University, to reproduce the conditions of ice formation, accretion and erosion phenomena inside the turbine and the pipe of a representative air cycle machine.
Existing super-hydrophobic solutions applicable on the inner surfaces of turbines and pipes are screened for required properties besides using the innovative set-up and used as a baseline for new developments.
TECNALIA is in charge of developing new solutions for Al alloys AA2024 and AS7G06 and PEEK surfaces. They consist of super-hydrophobic surfaces achieved through innovative surface texturing techniques in combination with hybrid sol-gel technology.
Scale-up issues will be also addressed and the surface treatments will be applied on technological samples (4”x4” pipes) for validation and comparison. Super-hydrophobic performance and erosion resistance of the surfaces will be assessed with the experimental set-up developed within ERICE. The final tests will be also performed at the TM facilities on a complete Turbomachine.
In addition, an economic and ecological analysis for both existing and new coatings will be performed by QUANTIS.
WP1 has been completed successfully, with the implementation and test running of the ice test rig. The rig is capable of reproducing a range of conditions similar to the actual ACM, for which icing conditions develop at the turbine exhaust pipe. Various types of ice can be formed depending on the inlet conditions applied. Two bespoke ice adhesion measurement devices have also been developed and manufactured. Figure 1 and Figure 2 show full-scale rig assembly and an example of ice accreted inside the AA2024 exhaust pipe.
WP2 is very close to completion, pending some ice tests to be performed at CU (temporarily closed due to COVID19). Six commercial materials have been screened at TECNALIA in order to perform a benchmark reference for ERICE development: thin sulfuric acid anodizing (SAA) sealed with TCS-PACS, Thin SAA sealed with PTFE, plasma electrolytic oxidation (PEO), Wearlon paint Super F6M and Whitford paint Xylan 1514-G7122 Whit Al – all on Al substrate – and PEEK, with 40% carbon microfiber reinforcement. In agreement with the TM, the commercial products were applied on AA2024 Al alloy, while the thermoplastic substrate (PEEK) was characterized as is. The commercial samples were evaluated for coating thickness, roughness, adhesion (when applicable), hardness, static surface contact angle and surface free energy measurements, corrosion resistance (SST), erosion resistance and ice-phobicity through Mode I test. With the results obtained as of now (Figure 3), PEO treatment will be excluded from further study as it showed bad corrosion resistance and anti-icing performance. More defined conclusions will be drawn as soon as the latest ice tests are performed.
WP3 is proceeding as planned, with the development of an innovative eco-friendly solution combining a sol-gel coating and surface texturing on AA2024. Three different coatings with hydrophobic behaviour have been formulated in order to fulfil the requirements for corrosion and erosion resistance with anti-icing performance. The combination of chemical texturing with a thin hydrophobic coating did render the surface superhydrophobic and a hydrophobic sol-gel formulation with high viscosity has been successfully textured by nanoimprinting lithography, attaining water a contact angle of about 140 ⁰. Corrosion protection performance is promising and the weight loss after erosion test of sol-gel coatings was lower than any of the commercial solutions tested in WP2. The ice-phobicity estimated through Mode I test (performed at CU) gave also promising results. In some conditions, sol-gel coatings displayed the best anti-icing performance compared to thin SAA and commercial solutions.
The water contact angle of the surface of microfiber reinforced PEEK changed from ⁓ 90 ⁰ to ⁓ 140 ⁰ after hot embossing process, which is being optimized.
The expansion of CU background studies of icing into condensing flow (mixtures of droplets, ice particles) and the consideration of the flow regime (combined ice deposition and the erosion of the accreted ice) provides many new opportunities and challenges in experimental science. An area of novelty is how a combination of droplet and solid particle erosion data will be used to specify suitable impact conditions for erosion testing and map out the erosion resisting qualities of the finished coating.
Hence ERICE´s approach, based on an intimate coupling of experiment and ice mechanical modelling, is novel in the development of a methodology for predicting the correct conditions for ice shedding. In addition, to the best of partners´ knowledge, it will be first fracture mechanics-based ice adhesion test on the inside of a pipe.
The work is also novel in relation to the specific application.
New surface treatments have been developed based on the modification of hybrid sol-gel formulations and its combination with new processing methods such as nanoimprinting lithography. ERICE project is giving a great opportunity for studying the combination of the mentioned technologies, with the aim developing low surface free energy and super-hydrophobic surfaces and understanding the relationship to anti-icing and ice-phobicity properties. Thanks to the tailoring of an organic-inorganic material, which allows to balance mechanical properties such as hardness and flexibility, corrosion and erosion resistance in specific conditions are being demonstrated.
A patent has been already submitted (EP19383186, 23/12/2019) in the field of omniphobic hybrid sol-gel formulations.
ERICE novel technology will have projection in more electrical aircraft configurations, as part of electrically driven environmental control systems (E-ECS) and in bleed-less power configurations, thus contributing to the environmental, competitiveness and societal impacts. ERICE development is free of toxic materials that might have a detrimental impact on the environment, and it meets REACH regulations. In addition, ERICE solution will substitute the active anti-icing systems currently used and that are based on heating systems and are hence energy consuming. ERICE will provide a passive anti-icing answer, which will contribute to energy saving. The novel technology will be also transferrable to other ice-sensitive turbine environments, ice-sensitive laminar-flow environments (e.g. nacelle wings) and even to ice-mitigation in other industry sectors, such as wind turbines.
Full-scale rig front (left) and rear (right) assembly.
Example of ice accreted inside the AA2024 exhaust pipe.
Table with results of screening of commercial solutions.