Phase transition and mechanical properties of ZrNxOy thin films on AISI 304 stainless steel

ZrN_xO_y thin films were deposited on AISI 304 stainless steel (304SS) substrates by reactive magnetron sputtering. The specimens were produced by sputtering a Zr target at 500 °C and the reactive gasses were N₂ and O₂ at various flow rates (ranging from 0 to 2 sccm). The purpose of this study was to investigate the effect of oxygen flow rate on the phase transition and accompanying mechanical properties of the ZrN_xO_y thin films. The oxygen contents of the thin films increased significantly with increasing oxygen flow rate. X-ray diffraction (XRD) revealed that the characteristics of the films can be divided into three zones according to the major phase with increasing oxygen content: Zone I (ZrN), Zone II (Zr₂ON₂) and Zone III (m-ZrO₂). The hardness of the ZrN_xO_y films decreased with increasing oxygen content due to the formation of the soft oxide phase. Modified XRD sin²ψ method was used to respectively measure the residual stresses of ZrN, Zr₂ON₂ and m-ZrO₂ phases. The results showed that the residual stress in ZrN was relieved as the oxygen content increased, and Zr₂ON₂ and m-ZrO₂ were the phases with lower residual stress. Compositional depth profiles indicated that there was a ZrO₂ interlayer near the film/substrates interface for all samples except the mononitride ZrN specimen. Contact angle was used as an index to assess the wettability of the film on substrate. The contact angles of ZrN, Zr₂ON₂ and m-ZrO₂ on stainless steel were indirectly measured using Owens-Wendt method. The results showed that ZrO₂ possessed the lowest wettability on 304SS among the three ZrN_xO_y phases, indicating that the ZrO₂ interlayer may account for the spallation of the ZrN_xO_y films after salt spray tests.