Structure and properties of hydrophobic CeO_2−x coatings synthesized by reactive magnetron sputtering for biomedical applications

Cerium oxide coatings were synthesized onto Si wafers by reactive magnetron sputtering in an Ar + O₂ gas mixture. The argon gas flow rate was varied from 35 to 15 sccm with a concurrent change in oxygen gas flow rate from 0 to 20 sccm. The influence of increasing the oxygen flow rate on the composition, microstructure, mechanical properties and scratch adhesion behavior, along with surface morphology and hydrophobic response of the coatings were investigated. X-ray photoelectron spectroscopy (XPS) analysis revealed the existence of Ce⁴⁺ and Ce³⁺ species in the coatings with decreasing Ce³⁺ species as the oxygen flow rate increased, which is in accordance with grazing incidence X-ray diffraction (GI-XRD) results. GI-XRD analysis also showed a strongly preferred (111) orientation in all of the coatings. Moreover, the CeO₂ (111) peak became broader as the oxygen flow rate increased, indicative of a reduction in grain size, which was also noticeable from high resolution TEM analysis. The mechanical properties of the coatings improved, along with the scratch resistance of the coatings, owing to the increase in elastic strain to failure (H/E) and plastic deformation resistance (H³/E²). A marginal increase in surface roughness (Ra) was noticed, which was accompanied by reduction in surface energy leading to an increase in hydrophobic performance of the coatings in the presence of both water and a 0.9 wt% NaCl solution.