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

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 3.3 nm was obtained at a bias voltage of − 60 V. 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 − 80 V bias. Cross-sectional scanning electron microscope (SEM) images displayed typical columnar-type structures with a relatively large column grain for the coating deposited at − 20 V and a dense, fine-grained microstructure obtained at − 80 V. The microstructure improvement at the optimized bias (− 80 V) contributed further to the enhancement in the hardness, reaching a maximum value of ~ 18.0 GPa. The average friction coefficients of as-deposited coatings prepared between − 40 and − 120 V were measured to be around ~ 0.40, whereas the coating prepared at − 80 V 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.