# Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft
## 1 Introduction  
Aircraft facing by super-cooled water droplets poses a major hazard for aviation [1]. For aircraft flying in cold regions, ice may form and accrete quickly on certain surfaces, particularly on the leading edge of a wing, the tail and the engine intakes, causing increased drag and weight and decreased thrust and lift, all of which are detrimental to aircraft performance. To mitigate the risks associated with icing, in addition to navigating away from the hazardous regions or grounding the aircraft [2, 3], which is not always possible due to difficulties in accurately forecasting the hazardous freezing layers, commercial jets are commonly equipped with active anti-icing technologies, such as passing hot bleeding air from engine compressors through icing prone areas, forcing freezing point depressant fluid out of porous panels, and using electro-thermal ice protection systems or pneumatic de-ice boots [4]. However, despite the effectiveness of these systems, they are not applicableto most small and light aircraft, such as helicopters, remotely piloted aircraft systems (RPAS) and next generation air taxis, due to system complexity, added weight and power requirement. As a result, intensive efforts have been made around the world to develop prospective icing protective measures, especially passive icephobic coatings that can minimize in-flight icing and ice adhesion and/or facilitate ice removal with minimal externally applied forces. In the pursuit of effective icephobic coatings for aircraft ice protection, various coating designs have been explored, including superhydrophobicity, lubricant or freezing point depressant infusion, hydrated surfaces and low shear modulus coatings [5; 6]. However, although numerous coatings have been developed to show substantially reduced ice adhesion strength (IAS), they all suffer critical shortcomings that limit their practical application. For example, superhydrophobic coatings are known to have low work of adhesion to liquid water; many studies [7; 8] have shown that a superhydrophobic surface could delay water freezing, reduce IAS and facilitate thermal de-icing. However, their downsides are significant, including low resistance to condensation and frosting, poor mechanical integrity and difficulty in quality control and repair [9; 10; 11]. For lubricant- or freezing point depressant-infused surfaces, a very low IAS of \(<\)50 kPa has been achieved [12; 13; 14]. Nevertheless, such coatings have not shown the high mechanical robustness and fluids resistance required for aircraft applications. More importantly, the infused lubricant or freezing point depressant is susceptible to depletion due to the high-speed impact of water droplets as well as due to repeated icing/de-icing cycles [15]. The hydratable coatings take advantage of the strong coating-water interactions that keep a portion of water non-freezing at sub-zero temperatures, thereby providing a water-lubricated slippery surface layer to depress ice formation and accretion [16; 17; 18; 19]. However, the water up-take may swell and plasticize the coating, leading to reduced mechanical strength. Low shear modulus coatings (e.g., soft silicone elastomers) have recently drawn great attention due to their ability to afford extremely low IAS of less than 5 kPa [20; 21; 22]. Such low IAS values are highly attractive, but the high softness typically relates to high dust pick-up, deformation under air pressure and low erosion resistance against high-speed solid particles and rain droplets, making it difficult to use these types of materials for aircraft icing protection. For a protective coating to be used on an aircraft, in addition to high icephobicity, it must meet comprehensive property requirements, such as workability, heat and low temperature resistance, weatherability, erosion durability and reparability.