Hydrophobic surfaces have drawn a lot of interest in both academia and industry due to their widespread applications, including self-cleaning, dropwise condensation and anti-corrosion. Many studies have focused on how to enhance the water repellency with different methods and materials. However, few of them have shown how these surfaces benefit the real industrial processes, the robustness of hydrophobic surfaces remains a challenge. One reason is that most hydrophobic surfaces rely on low-surface-energy modifiers, which are mostly polymer-based materials that are readily deteriorated when used in harsh industrial conditions, however, conventional durable materials such as metals and ceramics are generally hydrophilic. Therefore, the requirement for developing robust hydrophobic surfaces is highly desirable. In this study, the recently discovered hydrophobic rare-earth oxide ceramic – cerium oxide, is studied for its robustness in the form of coating material prepared by magnetron sputtering technique.
In this research, ceria coatings were first synthesized on silicon substrates to study the basic coating properties and performances. The effects of airborne hydrocarbon contamination and surface O/Ce ratio on the wettability were studied to understand the hydrophobicity of sputtered ceria coating. Influential deposition parameters, such as oxygen flow ratio (fO2) and bias voltage were then systematically examined. It was found that surface chemistry and microstructure of ceria coatings varied with different oxygen flow ratios. All deposited coatings exhibited similar wetting behaviors with the water contact angle around 100°, which was associated with surface lattice oxygen-to-cerium ratio. X-ray diffraction (XRD) peak intensity of CeO2 (111) orientation gradually decreased with increasing fO2 and disordered crystalline structure was observed among the coating when deposited at high fO2. Additionally, another important factor - bias voltage, which plays an important role in controlling the energy of impacting ions was studied as well. Results indicate that bias voltage strongly affected the surface morphology, microstructure and mechanical properties of as-deposited coatings. All ceria coatings showed a hydrophobic feature regardless of bias variation from –20 to –160 V. A dense structure with high hardness (~18.0 GPa) could be obtained with optimized bias voltage (–80 V), showing the best anti-wear performance. After 5000s wear tests against WC ball under a load of 10 N, the wear volumes were two orders of magnitude lower than that of 316 stainless steel (SS) and three orders of magnitude lower than that of Teflon. Additionally, the effect of UV irradiation on the wettability of ceria coatings on silicon substrates was investigated. Water contact angle measurements showed that after 3 hour UV irradiation, water contact angle decreased from 105° to 43° and then stabilized around 55°, the initial hydrophobic surface turned into a hydrophilic one. However, after 4-day storage in the dark, the hydrophobic character was regenerated and the water contact angle could recover back to ~95°. This may shed new light on the development of wettability driven microfluidics or other biochip applications.
Next, ceria films were deposited onto 316 SS substrates, which are one of the most commonly used engineering material in our daily life from biomedical devices, household to transport, architecture and industrial equipment. In order to obtain adherent ceria films on steel substrates, four coating systems with different interlayers were prepared. Rockwell C indentation tests demonstrated the best adhesion quality for the Ce/ceria coating system due to the good compatibility of metal Ce and ceramic ceria layers, thereby metal Ce layer was selected as the interlayer. The film properties were similar to that on silicon substrates. Subsequently, film performances exposed to high temperature, corrosive and abrasive conditions were investigated. Annealing treatment results revealed that the morphological, structural and wetting properties can be sustained under the temperature up to 700 °C, which is much better than the Teflon coating that is readily broken down at temperature around 300 °C. Dropwise condensation was demonstrated on ceria coated steel surfaces. The potentiodynamic polarization test results showed that the corrosion current density of ceria coated sample was one order of magnitude lower than that of bare 316 SS, indicating an excellent corrosion protection of deposited ceria film. Meanwhile, adherent ceria film also significantly improved the wear resistance of steel substrate, the wear volumes of ceria coated sample was two orders of magnitude lower than that of bare 316 SS.
Lastly, textured surfaces with different scale pillars were fabricated on steel and silicon substrates to enhance the surface hydrophobicity. Following that, ceria coatings were deposited onto textured surfaces with metal Ce interlayers. As a result, on textured steel surfaces, surface hydrophobicity was enhanced and water contact angle could reach 138°. Superhydrophobicity was achieved on textured silicon surfaces with nano-scale pillars, the water contact angle was around 156°. Subsequently, adhesive tape peel tests, water jet tests and sandpaper abrasion tests were carried out to evaluate the mechanical robustness of textured surfaces. Experimental results demonstrated the excellent mechanical robustness of textured steel surfaces, the water repellency showed almost no change after peeling with adhesive tape, water impacting or sandpaper abrasion. By contrast, the hydrophobicity of textured Si surfaces degraded rapidly when exposed to the mechanical attack, suggesting the vulnerability of textured Si surfaces.
In summary, successful fabrication of ceria coated hydrophobic surfaces with robustness against harsh environments, e.g. high temperature, corrosion and mechanical abrasion, demonstrating the high potential of ceria in the form of coating materials to be applied in far reaching technological applications, and will pave the way for the development of robust hydrophobic surfaces.