Electrochemical Oxidation of ZrN Hard (PVD) Coatings Studied by XPS

Abstract

Zirconium nitride (ZrN) belongs to the group of technologically important materials, namely hard tribological coatings. X-ray photoelectron spectroscopy (XPS) in combination with polarization experiments and electrochemical a.c. impedance spectroscopy has been used to study the composition, structure, thickness and electronic properties of layers formed by electrochemical oxidation of ZrN in phthalate buffer (pH 5.0). Results are compared to those obtained for layers formed by thermal oxidation at elevated temperature, i.e. ZrO2 , as well as by air oxidation at room temperature. Electrochemical oxidation of ZrN to ZrO2 proceeds via the formation of a mixed oxynitride/oxide layer, which then transforms into oxide at sufficiently positive potentials. Angle-resolved XPS measurements contribute to a better understanding of the in-depth layer structure. Exposure of ZrN to air at room temperature results in the formation of a thin oxynitride/oxide layer. The layer exhibits a gradually changing in-depth distribution of various species, i.e. nitride, oxynitride and oxide. Analysis of XPS valence band spectra offers valuable information concerning the electronic properties of investigated materials. The experimental valence band spectrum for ZrN correlates well with a theoretically calculated density-of-states diagram from the literature. In contrast to ZrN, which is a metallic conductor, layers formed by electrochemical oxidation, as well as by air oxidation, exhibit insulating properties. The insulating character of the formed layers is supported also by a decreased current density in subsequent cycles in cyclic voltammograms, as well as by the resistivity value (∽1012 Ω · cm) calculated from a.c. impedance measurements. Therefore, the oxide layer formed on ZrN is stable and prevents further oxidation of the ZrN substrate.