Wettability of Y₂O₃: A Relative Analysis of Thermally Oxidized, Reactively Sputtered and Template Assisted Nanostructured Coatings

The wettability of reactively sputtered Y₂O₃, thermally oxidized Y-Y₂O₃ and Cd-CdO template assisted Y₂O₃ coatings has been studied. The wettability of as-deposited Y₂O₃ coatings was determined by contact angle measurements. The water contact angles for reactively sputtered, thermally oxidized and template assisted Y₂O₃ nanostructured coatings were 99°, 117° and 155°, respectively. The average surface roughness values of reactively sputtered, thermally oxidized and template assisted Y₂O₃ coatings were determined by using atomic force microscopy and the corresponding values were 3, 11 and 180 nm, respectively. The low contact angle of the sputter deposited Y₂O₃ and thermally oxidized Y-Y₂O₃ coatings is attributed to a densely packed nano-grain like microstructure without any void space, leading to low surface roughness. A water droplet on such surfaces is mostly in contact with a solid surface relative to a void space, leading to a hydrophobic surface (low contact angle). Surface roughness is a crucial factor for the fabrication of a superhydrophobic surface. For Y₂O₃ coatings, the surface roughness was improved by depositing a thin film of Y₂O₃ on the Cd-CdO template (average roughness = 178 nm), which resulted in a contact angle greater than 150°. The work of adhesion of water was very high for the reactively sputtered Y₂O₃ (54 mJ/m²) and thermally oxidized Y-Y₂O₃ coatings (43 mJ/m²) compared to the Cd-CdO template assisted Y₂O₃ coating (7 mJ/m²).