Effect of bias voltage on the properties of CeO2−x coatings prepared by magnetron sputtering

Abstract

In this study, cerium oxide (CeO2−x) coatings were deposited onto silicon substrates by reactive unbalanced magnetron sputtering in an Ar-O2 gas mixture. The effect of substrate negative bias voltage on the surface morphology, wetting behavior, microstructure, mechanical and tribological properties of CeO2−x coatings were systematically investigated. It was found that the surface morphology and the root mean square (RMS) roughness of the coatings exhibited a non-linear evolution. The smoothest surface with a RMS roughness of about 3.3nm was obtained at a bias voltage of −60V. All deposited coatings exhibited a hydrophobic feature with the water contact angle around 100°, which was close to the value obtained from Teflon surface. X-ray diffraction (XRD) analyses revealed the best crystallinity of CeO2−x coatings in cubic CeO2 phase at a critical value of −80V bias. Cross-sectional scanning electron microscope (SEM) images displayed typical columnar-type structures with a relatively large column grain for the coating deposited at −20V and a dense, fine-grained microstructure obtained at −80V. The microstructure improvement at the optimized bias (−80V) contributed further to the enhancement in the hardness, reaching a maximum value of ~18.0GPa. The average friction coefficients of as-deposited coatings prepared between −40 and −120V were measured to be around ~0.40, whereas the coating prepared at −80V showed the best anti-wear performance. 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.