Wettability of hierarchically-textured ceramic coatings produced by suspension HVOF spraying

A novel but simple path for the preparation of superhydrophobic and superhydrophilic coatings has been demonstrated via a recently developed technology, namely suspension high velocity oxy-fuel spraying. Potential uses for robust superhydrophobic coatings include antifouling applications such as aeroplane wings, ship hulls, offshore wind turbine blades, or the above-deck structures on ice breaker vessels. Several fabrication techniques have been reported for preparing inorganic superhydrophobic surfaces, but existing coatings either lack the necessary robustness for engineering applications and/or their deposition methods are not suitable for industrial scale-up. In this work, the industrially established HVOF coating process was adapted to use a liquid suspension of commercially available nano-particles (titania-TiO2, and hexagonal boron nitride-h-BN) as feedstock to produce nanostructured suspension HVOF TiO2/h-BN coatings for the first time on stainless steel. Results indicate that agglomerates in the nano-feedstock can be dispersed by h-BN due to poor mutual wettability between h-BN and molten TiO2. It also inhibits the anatase-to-rutile transformation of TiO2 during coating deposition by inhibiting sintering of TiO2 in the HVOF flame. The resultant coating becomes superhydrophobic when the addition of h-BN reaches 10 wt% due to the presence of hierarchical nano-texture on the surface. The superhydrophobicity (contact angle of 163-170 degrees) is maintained over a long period of time (>13 months, test still ongoing) and remains stable after exposure to light and tape test. A potential route for industrial preparation of robust water-repellent coatings is therefore highlighted by the study.