Abstract

In-flight icing due to the presence of super-cooled water droplets is a major problem for aircraft operators. Accumulation of ice on the surface of wings, control parts and sensors can result in a range of problems including navigation issues, decreased efficiency, increasing fuel consumption, forced flight delays and cancelation and, if neglected or mismanaged, even fatal flight incidents. A significant potential solution for mitigating the icing problem is the use of superhydrophobic coatings i.e. coating that are extremely water repelling. Superhydrophobic coatings, by repelling the water droplets, can delay and in some cases prevent ice accumulation. Additionally, superhydrophobic coatings can facilitate ice removal by heating or vibration due to their non-stick properties. The superhydrophobicity of a surface is a result of the combination of the surface micro-texture and its surface energy which is determined by the chemistry of the surface. The major challenge facing the use of superhydrophobic coatings is the fact that low surface energy materials are mainly organic, polymeric compounds that suffer from poor durability, and in addition, micro-textured coatings are typically made by complex and expensive techniques. In this work atmospheric plasma spray (APS) and suspension plasma spray (SPS) which are flexible, scalable and efficient surface engineering techniques, are employed to develop micro-textured superhydrophobic coatings for anti-icing applications.
In this research, APS and SPS TiO2 micro-textured coatings are developed. After treatment by a stearic acid solution in order to lower their surface energy, these coatings demonstrate hydrophobicity and superhydrophobicity to different extents. APS coatings that are produced using 10-80 micron-sized particle feedstock, although highly hydrophobic, lack the extreme water repellency known as water mobility, due to their relatively coarse micro-texture. In the SPS process, submicron-sized TiO2 particles in the form of a suspension are used as feedstock. The SPS coatings typically show superhydrophobicity with water contact angles higher than 150°. The coatings produced using an ethanol-based suspension demonstrated extreme hydrophobicity and a water droplet impacting on their surface bounces back and detaches from them easily. The parameters influencing the SPS process are further studied and optimized to achieve coatings with hierarchical surface micro-texture i.e. a surface with a primary micron-sized and a secondary submicron-sized micro-texture. After optimization of the process, the SPS TiO2 coatings show extreme superhydrophobicity with water contact angles as high as 170°, water sliding angles as small as 1.3° and a contact angle hysteresis as small as 4°. These superhydrophobic SPS TiO2 coatings demonstrate promising results in terms icing performance and durability.