Desarrollo de recubrimientos superhidrofóbicos y anti-hielo sobre materiales metálicos para aplicaciones aeronáuticas
Authors:   , Vicente Gómara Adrián 
Published: March 2024

The formation and accreation of ice is a safety risk for aircraft during all phases of flight, especially when the aircraft is at medium-low altitude and flying at low speeds (take off-ascent and approach-landing). Currently, most aircraft are equipped with active de-icing and/or anti-icing systems. However, severe conditions will inevitably lead to ice formation in these systems. Therefore, a combination of passive and active strategies will be necessary to prevent ice formation and its subsequent removal. In this way, passive systems such as the development of icephobic coatings will contribute to lower energy consumption in active systems. In this study, icephobic coatings have been developed through a novel strategy based on surfaces composed of fibres or particles with low surface energy polymers. Both random fibre distribution and particle coalescence result in micro/nano level structures that are suitable for the optimisation of superhydrophobic surfaces. These structures will contribute to reduce the wettability of water droplets and achieve high mobility (α < 5°), key to prevent ice formation (anti-icing). In addition, these surfaces will act as a protective barrier against corrosion in aerospace metallic alloys. On the one hand, by reducing the surface interaction with the corrosive agent and on the other hand, by encapsulating corrosion inhibitors such as metal oxides (ZnO NPs) in the fibres. Nevertheless, severe conditions will lead to ice formation due to the presence of micro droplets within the cavities of the rough structure, creating ice nucleation. Once ice has formed, the surfaces will exhibit high ice adhesion (65 kPa), generated by the anchorage or interlocking of the ice layer with the surface textures. This will result in Surface delamination due to the cohesive failure of the fibers after multiple ice adhesion test cycles. For this reason, the superhydrophobic surfaces that delay or prevent ice nucleation within the cavities of their structures, will provide a better anti-icing performance and reduced ice adhesion (improved de-icing). Therefore, in order to optimize the icephobic behavior, these fibrous surfaces are ideal for the development of porous surfaces infused with slippery liquids, known as SLIPS. Where the layer of air trapped between the surface textures and water droplets is replaced by a lubricant based on organic liquids with low surface energy. This interface results in high droplet mobility (α < 10°) and hinders nucleation, contributing to delay freezing and ultra-low ice adhesion (4.5 kPa). However, after several cycles without re-infusion, the ice adhesion will increase (30 kPa) due to the loss of the lubricant layer and leading to material delamination caused by ice nucleation in the cavities. For this reason, the icephobic properties of these surfaces will depend on the ability to retain the lubricant, as well as a good affinity and chemical stability of the lubricant with the fibres. The electrospinning technique has been used to fabricate the fibres, where a design of experiments (DoE) was applied to understand the relationship between the input and output parameters of the system, as well as to create a model that can be extrapolated to other polymer systems, in order to optimize the surface morphology, water contact angle (WCA) and corrosion rate. Furthermore, the electrospinning technique has been used in combination with spin coating and electrospraying techniques to obtain surfaces with structures based on particle coalescence while ensuring good substrate adhesion after various heat treatments. In addition, fibers composed of non-electrospinnable polymer nanoparticles have been obtained, resulting in fibers of new materials for the first time. In order to characterize the icephobic performance of the developed surfaces, various methodologies have been used to analyze ice adhesion and the amount of ice acreation under different icing conditions and to compare their performance with different antiicing materials, including commercial solutions and current trends. For this purpose, real icing conditions in flight have been reproduced within an icing wind tunnel, generating both glaze and rime ice, as well as testing the running wet icing mechanisms and direct impingement of supercooled droplets. Finally, the ice adhesion has been studied and compared using static ice formed in mould, using different ice adhesion test facilities and representing a step forward in the standarization efforts of icephobic surfaces designed along this Thesis.