Durability of superhydrophobic duplex coating systems for aerospace applications

In-flight icing, caused by the collision of supercooled water droplets with exposed aircraft surfaces, is an important safety hazard and a major issue in aviation. Superhydrophobic surfaces (SHS) have been shown to offer improvements to heating-based anti-icing and de-icing systems, reducing energy requirements for ice prevention and facilitating ice removal. A common SHS fabrication technique is to develop a surface with hierarchical roughness, and to then coat this surface with a hydrophobic topcoat. From this arises an issue: the durability of the entire system is fundamentally limited to the durability of this topcoat. In the present study, we develop a superhydrophobic duplex coating system with an emphasis on the environmental durability of the thin hydrophobic layer. The system consists of a thick TiO2 coating deposited by suspension plasma spraying, and a thin coating stack deposited by plasma enhanced chemical vapor deposition. The thin coating stack is based on DLC:SiOx-diamond-like carbon networked with silicon oxide-which exhibits a water contact angle of up to 95 degrees and a hardness of up to 11 GPa, while the full coating system offers a contact angle of 159 degrees and a contact angle hysteresis of 3.8 degrees. The coating system is exposed to icing/deicing cycling, as well as rain erosion and accelerated aging tests; the results are compared with TiO2 coatings using stearic acid and fluoropolymer hydrophobic layers, as well as a commercially available superhydrophobic spray. The duplex coating system is shown to maintain water droplet mobility after 170 icing/deicing cycles, is resistant to prolonged UV and high-temperature exposure and offers a 300-fold improvement over the stearic acid in rain erosion tests.