Control of the hydrophobicity of rare earth oxide coatings deposited by solution precursor plasma spray by hydrocarbon adsorption

Abstract The basis of the hydrophobicity of lanthanide rare earth oxides (REOs) has been the subject of considerable debate. To explore this question, the wetting behaviors and surface compositions of hierarchically-structured Yb2O3 (one of the REOs) coatings and non-REO Al2O3 coatings deposited via solution precursor plasma spray process were investigated in this work. The Yb2O3 coatings were subjected to a number of post-deposition treatments including vacuum (1−15 Pa) treatment, Ar-plasma treatment, heat treatment (400 °C), long-time air exposure and ultra-high vacuum (1 × 10−7 Pa) treatment. Subsequent characterization showed that different post-deposition treatments resulted in different wetting behavior for the Yb2O3 coatings which correlated with the content of hydrocarbon on the surface. Yb2O3 coatings exhibited reversible transitions between superhydrophobicity after vacuum treatment and superhydrophilicity after Ar-plasma or heat treatment, linked to hydrocarbon adsorption onto and desorption from the surface. Yb2O3 coatings after long-time air exposure and ultra-high vacuum treatment both remained hydrophilic and showed a smaller hydrocarbon content than coatings after vacuum treatment. Al2O3 coatings with hierarchical surface structures similar to the Yb2O3 coatings showed an increase in WCA to only ∼17° after the same vacuum treatment, indicating the REO has a much higher affinity for hydrocarbon adsorption than Al2O3, and that the content of hydrocarbon adsorbed on the surface of the REO determined the wetting behavior.