Rendering hydrophilic glass-ceramic enamel surfaces hydrophobic by acid etching and surface silanization for heat transfer applications
The study revealed a facile method to hydrophobize enamel coatings with a thermally stable hydrophobicity by using halogenic acid etching and perfluorooctyl triethoxysilane surface modification. The results showed that the modification was successfully achieved as evidenced by the presence of the organic modifier from the measurements by FTIR and XPS. SEM/EDS and 3D optical profilometer showed that hydrofluoric/hydrochloric acid exposed the distinct inner microstructures of glass-ceramic/glassy enamels, with corresponding structures of multi particles/island morphology in micro and sub-micro scales. The combination of these unique structures and fluoroalkyl silane modifier improved the contact angle of the enamel surfaces up to 134°, compared to 110° of the non-etched samples. The resulted hydrophobic film was thermally stable up to 400 °C and able to withstand a temperature of 200 °C in a period of 15 h. During a 60 min water condensation testing, the obtained hydrophobic enamel surfaces demonstrated an intriguing dropwise water condensation which is advantageous to the surface heat transfer. During mechanical sliding, the nano scaled multi-spike surface of crystalized enamel was more vulnerable than the micro island structured surface of glassy enamel. The former experienced a sharp decrease in hydrophobicity due to both the loss of modifier and the collapse of fragile nanostructure, and the latter still remained hydrophobic thanks to a mechanically stable micro structure. The study showed that the combination of the facile etching process and widely-used fluoroalkyl silanization could be utilized to introduce hydrophobicity on a large scale to improve the surface function of the conventional enamel coating for various heat transfer applications.