The influence of semiconducting properties of passive films on the cavitation erosion resistance of a NbN nanoceramic coating

• Published in: Ultrasonics sonochemistry Vol. 71; p. 105406
• Main Authors: Xu, Jiang, Peng, Shuang, Li, Zhengyang, Jiang, Shuyun, Xie, Zong-Han, Munroe, Paul
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.03.2021; Elsevier
• Subjects: First-principle; Mott–schottky; NbN nanoceramic coating; Original; Point defect model; Ultrasonic cavitation erosion; Point defect model; Ultrasonic cavitation erosion; Mott–schottky; First-principle; NbN nanoceramic coating
• ISSN: 1350-4177; 1873-2828
• DOI: 10.1016/j.ultsonch.2020.105406

[Display omitted] •The NbN coating exhibits high contact damage resistance and good adhesion to Ti6Al4V substrate.•The intermediate product of NbOxNy is further oxidized to Nb2O5 with increasing cavitation erosion time.•The passive film formed on the coating has lower concentration and diffusivity of oxygen vacancies than Ti6Al4V.•The coating has lower formation energy of oxygen vacancies and the Cl− adsorption energies than Ti6Al4V. To alleviate the cavitation damage of metallic engineering components in hydrodynamic systems operating in marine environments, a NbN nanoceramic coating was synthesized on to a Ti-6Al-4V substrate via a double cathode glow discharge technique. The microstructure of the coating consisted of a ~13 μm thick deposition layer of a hexagonal δ′-NbN phase and a diffusion layer ~2 μm in thickness composed of face-centered cubic (fcc) B1-NaCl–structured (Ti,Nb)N. The NbN coating not only exhibited higher values of H/E and H2/E than those measured from NbN coatings deposited by other techniques, but also possessed good adhesion to the substrate. The cavitation erosion resistance of the NbN coating in a 3.5 wt% NaCl solution was investigated using an ultrasonic cavitation-induced apparatus combined with a range of electrochemical test methods. Potentiodynamic polarization measurements demonstrated that the NbN coated specimens demonstrated both a higher corrosion potential (Ecorr) and lower corrosion current density (icorr) than the uncoated substrate. Mott-Schottky analysis, combined with the point defect model (PDM), revealed that, for a given cavitation time, the donor density (ND) of the passive film on the NbN coating was reduced by 1 ~ 2 orders of magnitude relative to the uncoated Ti-6Al-4V, and the diffusivity of the point defects (D0) in the passive film grown on the NbN coating was nearly one order of magnitude lower than that on the uncoated substrate. In order to better understand the experimental observations obtained from Mott-Schottky analysis and double-charge layer capacitance measurements, first-principles density-functional theory was employed to calculate the energy of vacancy formation and the adsorption energy for chloride ions for the passive films present on both the NbN coating and bare Ti-6Al-4V.