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ICECOAT Report Summary

Project ID: 325917
Funded under: FP7-JTI
Country: United Kingdom

Final Report Summary - ICECOAT (Novel aircraft de-icing concept based on smart coatings with electro-thermal system)

Executive Summary:
Icing on aircraft wings can occur in clouds at mid-range heights where the conditions are conducive to formation of super-cooled microscopic water droplets in clouds. The current control over adverse effects of ice formation on the wing is usually heating of the leading edge, which can release the ice from point of formation, but which has a following risk of run-back icing, where the melted ice in a film runs back over the upper surface of the wing and refreezes. This project addressed the development and application of ice-phobic surface treatments to prevent run-back ice accumulation.
Dynamic wettability of surfaces was tested in a wind tunnel which could produce temperatures down to -10ºC, with increased temperature with increasing air flow rate due to fan work. Atomised spray can be delivered in tests up to 88 g/m3 covering the main zone of interest at 0.3 g/m3. A slot flow delivery to simulate runback wetting of the aerofoil provides the opportunity to control rate of flow over the test surface. Centrifuge tests demonstrate the ability of the treated surface to remove glaze ice once formed, and the 10 tested coatings demonstrated varying positive outcomes over the parameters tested for effect on the material piece, but showing a clear candidate from cyclic testing for a repeatably high adhesion reduction factor. Run back icing tests demonstrated the effect of the high latent heat of fusion of water on time to form ice on the surface in freezing conditions. At landing approach conditions, the time to freeze a water film 0.05mm thick is in the order of 0.3s, and given the speed of approach, icing on untreated surface will occur about half way down the aerofoil upper surface. The coatings demonstrated that the freezing time could be more than doubled, which would lead to no icing on the wing surface at all. The rivulet flow demonstrated in the wind tunnel tests indicates that although faster flow may lead to a thinner film, the thickening of that film into rivulet flow will mean that the risk of faster freezing is avoided.

Project Context and Objectives:
The aim of ICECOAT project is to minimise the accretion of run-back ice on aircraft wings by optimizing the configuration of novel ice-phobic coatings (with controlled surface chemistry and topography) and the electro-thermal de-icing system. Bottlenecks in current anti-icing coating and de-icing system were identified in the state-of-the-art review in deliverable D1, where the methods to advance the technology beyond the state-of-the-art are summarised. The life time for the currently available silicone and fluorocarbons type coatings, as well as their nanocomposite coatings, is limited for anti-icing application for aircraft since they are normally soft materials and thus have poor erosion resistance. In the ICECOAT project, we have developed new types of coating matrices, new nanoparticles for nanocomposites and new method for coating surface modification to produce anti-icing coating with higher hardness and erosion resistance. These are summarised in deliverables D4 and D7. The work was then carried out to advance our understanding of the heat transfer processes that determine run-back icing accretion, so that more sophisticated parameterisation of the run-back icing process with stronger links to the underlying physics is developed. The results of this investigation is summarised in deliverable D3, while the design of ice accretion and adhesion test instrument are described in deliverable D6. These new anti-icing coatings were tested in an icing wind tunnel (deliverable D2), where the recommended optimisation of coating application over the wing is discussed in deliverable D5.

State-of-the-art review of icephobic coatings
Icephobicity has been defined as the ability of a solid surface to repel ice or prevent ice formation due to a certain topographical structure of the surface. There are three approaches adopted for the characterization of icephobicity of surfaces. Firstly, the icephobicity implies low adhesion force between ice and the solid surface. In most cases, the critical shear stress is calculated, although the normal stress can be used as well. While no explicit quantitative definition for the icephobicity has been suggested thus far, researchers have characterized icephobic surfaces as those having the shear strength (maximum stress) in the region between 150 kPa and 500 kPa and even as low as 15.6 kPa. Secondly, the icephobicity implies the ability to prevent ice formation on the surface. Such ability is characterized by whether a droplet of super-cooled water (below the normal freezing temperature of 0ºC) freezes at the interface. The process of freezing can be characterized by time delay of heterogeneous ice nucleation. The mechanisms of droplet freezing are quite complex and can depend on the temperature level, as well as whether cooling down of the droplet is performed from the side of the solid substrate or from vapour and by other factors. Thirdly, the icephobic surfaces should repel incoming small droplets (e.g. rain or fog) at the temperatures below the freezing point. These definitions indicate that icephobic surfaces should have the characteristics, such as to prevent freezing of water condensing on the surface, to prevent freezing of incoming water and to have weak adhesion strength with the solid, so that it can be easily removed. Anti-icing properties of coatings may depend on such circumstances as whether the solid surface is colder than the air/vapour, how large is the temperature gradient, and whether a thin film of water tends to form on the solid surface due to capillary effects, disjoining pressure etc. Icephobicity is different from the hydrophobicity. Hydrophobicity is a property which is characterized by the water contact angle (CA) and interfacial energies of the solid-water, solid-vapour, and water-vapour interfaces, thus it is a thermodynamic property and quantitatively defined as CA > 90°.
The presence of reinforced materials such as nanoparticles, nanowires and nanotubes in a matrix (typically polymer) would increase the surface roughness of the nanocomposites, thus resulting in higher contact angles with water droplet. Furthermore, the reinforced polymer could absorb and dissipate the higher impact energy which produces higher erosion resistance. For example, silica reinforced acrylic polymer composites can exhibit superhydrophobic surfaces and anti-icing capability upon the impact of supercooled water. The coating consisted of acrylic polymer resin as binder, and organosilane-modified silica particles with different diameters (e.g. 20-100 nm and 1-20 μm). The critical particle sizes that determined the superhydrophobicity and the anti-icing property were in two different length scales, i.e. aspect ratio. The results also confirmed that anti-icing property of a surface was not directly correlated with the superhydrophobicity. Thus, it is not clear whether the surface is anti-icing without detailed knowledge of the surface morphology. The surface morphology of the coating also strongly influence the anti-icing capability. Nanocomposites with different surface roughness have been prepared by varying the amount of ZrO2/fluoropolymer nanopowder suspension and mixing with perfluoroalkyl methacrylic copolymer have also been fabricated using spraying or spin-coating, followed by heat treatment to remove residual solvents. The results showed that the ice adhesion strength on rough hydrophobic surfaces had no relationship with the water contact angle value. However, good correlation with wetting hysteresis was observed, which could be due to the ice-solid contact area.
Nanocomposite coatings consisting of inorganic fullerence (IF) WS2 nanoparticles in PTFE have been fabricated recently using an aerosol assisted deposition method for the investigation of the hydrophobicity of nanocomposites. The content of IF-WS2 would influence the surface roughness of the coating. A higher content of IF-WS2 would increase the surface roughness and thus improve the water contact angle. Such approach can also be used to fabricate ice-phobic coating using nanoparticles with different size and shapes. Silica sol and fluorinated acrylate copolymer have been mixed to form low-cost hydrophobic and icephobic coatings on glass by a casting method. The test at -5.6°C indicated that ice formation has been delayed by 90 min as compared with the uncoated glass surface. On the other hand, silicon-epoxy based resins have also claimed to be ice-phobic, where the silicon part provides the desired low ice adhesion characteristics and epoxy part could provide the erosion and impact properties. However, their resistance to erosion and fouling has yet to be established. Nanocomposites incorporated with emerging nanomaterials (e.g. carbon nanotubes and graphene) are therefore potential anti-icing coatings for aircraft. In order to improve the anti-icing performance of aircrafts, the coating with superhydrophobic property should be deposited on the aluminium and copper substrate effectively. It has been reported that the anti-icing coating composition can be composed of the following components; an agent from glyoxal, polysiloxane, dimeric alkylketen, diethanolamine, a surfactant, an ether alcohol and a matrix forming polymer. Another composition of the anti-icing coating can be fluorosilane with microscopic texture, exhibiting superhydrophobic properties.
Development of smart anti-icing coatings
Ten coatings with potential anti-icing capability have been developed and tested during the ICECOAT project. These coatings were applied to an Aluminum 6028, with T651 temper, substrate plate with thickness 1 mm and total surface area 1000 mm2, on which ice could be grown and the sample plate fixed to the rotor of the centrifuge to measure adhesion strength. Of a total of 30 substrate plates, 20 were left uncoated for the sensitivity tests. The remaining 10 plates were coated with the ten coatings to be tested and labelled – A, B, C, D, E, F, G, H, I and J.
Our studies regarding texturing of the Al-alloy surfaces revealed that the increase of roughness alone is not sufficient to decrease the wettability. On the contrary, it could lead to super-hydrophilicity and even in full wetting of the surfaces in some cases. Therefore, the chemical modification of the surfaces has also been used in order to lower the surface energy further. The lowering of the surface energy was achieved by the deposition of hydrophobic and superhydrophobic nanocomposite coatings which have also increased the robustness and adhesion to Al-alloy substrates. The deposition of an interlayer between the substrate and the coating was the optimal processing method in order to increase the adhesion of the coating.
Among ice-phobic coatings that have been developed, Coating E exhibited very good performance as can see from the ice adhesion strength data and from consecutive test results. Coating J performed quite well and has stabilized towards the end of the test cycle at a very good icephobicity despite initial degradation. Coating D and F have not changed surface characteristics much, however, the ant-icing performance was inferior to E and J coatings. The Coatings D and F are extremely durable since the surface properties have not changed with attachment to ice multiple times and ice shearing off a couple of times. This surface strength and attachment to the aluminium substrate is suggested. Coatings G, H and I did not degrade much during the tests. This can be noted from the relative stability in contact angle measurements. Furthermore, the latter coatings are conducting.
The contact angle hysteresis appears to be a crucial factor determining the adhesion strength. Simply, hydrophobic coatings cannot be classified as icephobic coatings. High Static Contact Angle and High Advancing Contact Angle are very important parameters in order to prevent the wetting of the surface due to runback water before it eventually freezes. High Receding Contact Angle and Low Contact Angle Hysteresis are crucial for anti-icing applications: This reduces the contact area between ice and the coating and reduces adhesion strength. Among these coatings, coating E seems to be the best anti-icing coating meeting these criteria.

Understanding of run-back icing process
Ice accretion and adhesion tests were carried out using a centrifuge instrument. When the centripetal acceleration on the ice sample, induced by the rotor's rotation, overcomes the adhesion strength of the ice to the substrate, the ice detaches from the substrate. Thus, the adhesion strength of ice to the substrate can be estimated as F = m r ω2 where m is the mass of ice, r is the rotor length and ω is the speed of rotation at detachment in rad.s-1. Here, the adhesion stress is given by τ = F/A, where A is the ice/substrate contact area. This permits relative measurements of coated sample adhesion shear stress τc, compared with a reference sample τu, quantifying the performance of coatings in reducing ice adhesion. This term is defined as the adhesion reduction factor (ARF) = τu/τc. A coating leading to lower rotation speed at detachment for a specified ice type, will thus be more icephobic than a corresponding coating with detachment at a higher rotation speed. The adhesion strength of a glaze ice sample, grown as described above, on 20 uncoated Al 6082-T651 substrates was measured under nominally identical ice preparation and adhesion test conditions (-5ºC) to evaluate the repeatability of the preparation and test method.
To characterize the hydrophobicity and surface properties of the coatings static contact angle was measured using sessile drop technique on NAVITAR 1x adapter 1-6015 goniometer. Images were processed using FTÅ200 software. Drop volume was kept constant at 10 μL. We expect some link between hydrophobicity and icephobicity, but there seems to be no clear correlation between them. Advancing Contact Angle was measured as the average of three consecutive values when drop volume was increased at 2 μL/s. Receding Contact Angle was measured as the average of three consecutive values when drop volume was decreased and base diameter decreased at 2 μL/sec. All tests were carried out at temperature 25 ± 0.3°C with humidity 26%. Surface roughness of the 20 uncoated substrates was measured using a Mitutoyo Surftest SV-622 profilometer, while that of of coatings were obtained through the average of three optical microscope images (at 20x magnification) using Alicona InfiniteFocus Optical Microscope., with focus area 2 x 2 mm and characteristic wavelength LC = 250 μm. As a result of these measurements, we have obtained the following conclusions. To prevent runback icing, high static contact angle, high advancing contact angle, high receding contact angle and low Contact Angle Hysteresis (CAH) are required. In particular, CAH seems to be the key parameter determining icephobicity. Coating E appears to be the best coating based on the current analysis and with the second lowest average shear stress for ice removal. Highest Adhesion Reduction Factor (ARF) of 60.55 was obtained for Coating J which has also appeared to stabilise before the exponential asymptote with cycle numbers 5, 6, 8, 9 and 10 yielding consistent ARF values of 4.24, 5.45, 4.31, 3.78 and 4.5. The average from these 5 cycle numbers works out to an ARF of 4.46 ± 13.81% which is close to standard deviation error of ARF in general (13.16%). This can lead us to believe that the long term performance of Coating J is also highly desirable as it also has the lowest average shear stress of all the coatings under identical conditions.
Runback icing tests of anti-icing coatings has been carried out in an icing wind tunnel at the University of Nottingham. We have simulated the runback water (melted ice water at the leading edge by electro-thermal heating) by directly injecting the water from a slot located just downstream of the leading edge of NACA 0015 aerofoil. This way, we can bypass the convoluted and often unrepeatable process of (a) icing the leading edge of an aerofoil by water droplet spray, and (b) de-icing by electro-thermal de-icing unit. By using slot injection of water, therefore, we can control tests with a good repeatability. Different temperatures of runback water, between the freezing temperature (0°C) and 14°C, have been tested. The flow rate of the injected water was set to 156 ml/min, which has not been affected by the air speed of the icing wind tunnel or by the angle of attack of an aerofoil. The injected water flow rate was later adjusted to see its effect on the runback icing. The air speed was set between 5.2 and 20 m/s. The air temperature in the icing wind tunnel was as low as -10°C, but it usually takes nearly 8 hours realise this temperature. We, therefore set the chillers overnight for testing in the following morning. However, the air temperature could increase as soon as the air speed in the icing wind tunnel is increased, resulting in carrying out icing tests between -0.9 and 5.1°C. The air temperature was also affected by outside (atmospheric) temperature. The air temperature of the icing wind tunnel is, therefore, higher during the summer period as compared to winter. We placed four 5 mm x 5 mm square plates, 0.5 mm thick over the upper surface of an aerofoil, one of which is an anti-icing coating sample to be tested. The injected water did not freeze over the non-coated AL samples, running down the aerofoil in streaks. Meanwhile, water formed small droplets as it goes down the icephobic coated surfaces. We have carried out these runback icing tests over different anti-icing coating surfaces at different water and air temperatures. Unfortunately, none of them formed ice on any part of aerofoil surfaces.
In order to evaluate the capability in delaying icing of runback water over an aerofoil, we have carried out a laboratory test where icing time of a water droplet was measured over different anti-icing coatings A to J as well as the baseline material (AL 6028). Here, sample plates with anti-icing coating were placed over a cold plate for at least a few minutes before a water droplet was dropped. We were able to control the volume of water droplet, therefore the dimeter of the droplet by using a motorized syringe. Here, the average volume of deionized water droplet used was 6.2 μl, corresponding to 2.28 mm in diameter. The temperature of water droplet was 24°C, while the cold plate was kept at -5°C in the first test and at -10°C in the second. The rank order based on the longer time to freeze is also given, showing that Coating E, F and J are among the top three. Referring to the contact angle measurements, these coatings have indeed greatest static contact angle. This suggests that the best anti-icing coatings for runback water are hydrophobic coatings. Among these three, the coating E has the least contact angle hysteresis (CAH). It should be noted that Coating E is among the best anti-icing coating, having the least shear stress during the ice adhesion tests. It is also the best coating in terms of the long-term durability, having the least coating wear rate.

Optimisation and validation of mixed strategies
Since the latent heat of fusion required for water-to-ice phase transition is so great (333 J/cm3), icing of runback water does not take place so easily. For example, it takes about 0.3 second for the icing of runback water takes place when the water film is 0.05 mm thick. For an aircraft during the approach phase at a flight speed of 90 m/s, the water film slides downstream by nearly 4.5 m in 0.3 second. This sliding distance corresponds to nearly half the chord length at root of a large aircraft (e.g. Airbus A340). If we use anti-icing coatings (such as Coatings E) in the downstream of the electro-thermal deicing unit, the freezing time could be more than doubled, so the runback icing does not take place over the entire wing. Theoretically speaking, the thickness of runback water can become much thinner than 0.05 mm with an increase in the flow (flight) speed. However, thin water film will soon become streaky rivulet flow through flow instability mechanism, increasing the film thickness quickly. This will reduce the risk of runback icing much further.
As an optimised de-icing strategy that we recommend is to use an electro-thermal de-icing system at the leading edge only aim to make deicing water to “run wet”. However, the de-icing system must be followed by anti-icing coatings with high hydrophobicity in the downstream of the wing in to prolong the icing time. This will delay or completely eliminate the runback icing over the wing surface.
The energy required to elevate the water temperature from freezing point (0°C) to 100°C is 419 J/cm3, while the latent heat of water vaporization is 2,260 J/cm3, which is nearly 7 times greater than that of fusion. Therefore, additional 2,679 J/cm3 of energy is needed to evaporate the water completely in addition to 333 J/cm3 of de-icing energy. Considering this and in the light of our findings, it is not recommended or necessary to completely evaporate the deiced water as a part of de-icing strategy, as long as anti-icing coatings are used in conjunction with an electro-thermal system at the leading edge of aircraft wing.

Project Results:
The main scientific and technical outcomes from this work were development of ice-phobic, hydrophobic coatings on aerospace grade aluminium surfaces. The close scrutiny of the behaviour of ice on newly developed ice-phobic surfaces shows that there are a range of test outcomes (static, advancing and receding contact angles, contact angle hysteresis, roughness, detachment speed from centrifuge test, adhesion strength) and the durability tests identify that there are suitable nano-structured surface treatments which are capable of producing ice-formation-retarding contact with run back ice. Tests in the freezing wind tunnel show that the runback icing tends increasingly to rivulet pattern flow over the upper surface of the aerofoil, increasing the thickness of the flow into localised regions. Together with the high latent energy of fusion, this leads to a situation where the icing can only occur distant from the leading edge and the character of the flow over the hydrophobic surface indicates that this can be delayed until trailing edge departure of the water.
Potential Impact:
Potential impact on environment
The Intergovernmental Panel on Climate Change (IPCC) concluded in its most recent report that it is very likely that the global warming that we are experiencing is due to the increase in man-made green house gases. Among those gases, CO2 is the most dominant man-made greenhouse gas, which remains in the atmosphere for over a hundred years. To limit the global temperature rise to 2.6°C above the pre-industrial level by 2050, it is necessary to reduce global CO2 emissions by 45%. At the same time, disturbance from aircraft noise remains the single most important issue around airports. This led to a number of noise-related restrictions by airports to limit the flight capacity, seriously impacting on the local economy, competitiveness and employment. ACARE Vision for 2020 targets to reduce CO2, NOx and perceived aircraft noise by 50%, 80% and 50%, respectively by 2020. The results from ICECOAT project can contribute to achieve these objectives by helping to implement laminar flow control over the laminar wing surface.

Potential impact on EU competitiveness
Aerospace is one of the most research-intensive sectors in Europe, where more than 12% of the turnover is invested in research and development. European Air Transport must continuously innovative to remain competitive against strong competitions from North America. Emerging countries such as Brazil, Russia, India and China are also threatening the market share with a strong government support. It is therefore important that European industry must commit to the technology evolution and breakthrough. Laminar flow control is one of such evolutional technologies to help maintain the competiveness of European aerospace companies by making aircraft fuel efficient and environmentally friendly. The results obtained within the ICECOAT project can help develop this evolutional technology by maintaining LFC wing surface free from icing, thereby strengthen European Aerospace industry against our competitors.

Potential impact on EU employment
By applying NLF over aircraft wings, we can reduce skin-friction drag by up to 4%. A similar amount reduction in fuel burn and CO2 emissions is obtained through this technology. The flow noise emitted from the aircraft will also be reduced as the result by maintaining the laminar flow. The results obtained by the ICECOAT project can help develop this evolutional technology by maintaining the NLF wing free from icing, thereby, help strengthen the competitiveness of European Aerospace industry against North American as well as other emerging countries. This will secure the employment within European Union beyond 2020.

Potential impact on education and training
ICECOAT project has been run by the University of Nottingham (UNOTT) and the University College London (UCL). Both are the top 10 universities in the UK with more than 40,000 students. PhD students involved in this project have benefit from it for the accumulation of knowledge and experience through the execution of project. Also, post-doctoral researchers have been trained as a part of this project to improve their skills and to develop their expertise further. Academics have been engage in the dissemination of the knowledge and information obtained from this project through academic and professional publications.

Potential impact on quality of life and health
By maintaining the aircraft surface free from icing, ICECOAT project can help realise the potential of laminar flow control over aircraft wings. This reduces the impact of global warming through a reduction of CO2 emissions. The aircraft noise can also be reduced by NLF as a result of reduction in turbulent flow coverage over the aircraft wings. The improvement in our environment together with a better employment prospect will improve the quality of life and health for EU citizen.

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