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-O-2 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 degrees, 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 similar to 18.0 GPa. The average friction coefficients of as-deposited coatings prepared between -40 and -120 V were measured to be around similar to 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. (C) 2016 Elsevier B.V. All rights reserved.