ELECTROCHEMICAL OXIDATION OF ZRN HARD (PVD) COATINGS STUDIED BY XPS

Zirconium nitride (ZrN) belongs to the group of technologically import ant materials, namely hard tribological coatings. X-ray photoelectron spectroscopy (XPS) in combination with polarization experiments and el ectrochemical a.c. impedance spectroscopy has been used to study the c omposition, structure, thickness and electronic properties of layers f ormed 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 oxidat ion at room temperature. Electrochemical oxidation of ZrN to ZrO2 proc eeds via the formation of a mixed oxynitride/oxide layer, which then t ransforms into oxide at sufficiently positive potentials. Angle-resolv ed XPS measurements contribute to a better understanding of the in-dep th 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. n itride, oxynitride and oxide. Analysis of XPS valence band spectra off ers valuable information concerning the electronic properties of inves tigated materials. The experimental valence band spectrum for ZrN corr elates web with a theoretically calculated density-of-states diagram f rom the literature. In contrast to ZrN, which is a metallic conductor, layers formed by electrochemical oxidation, as well as by air oxidati on, exhibit insulating properties. The insulating character of the for med layers is supported also by a decreased current density in subsequ ent cycles in cyclic voltammograms, as well as by the resistivity valu e (similar to 10(12) Ohm . cm) calculated from a.c. impedance measurem ents. Therefore, the oxide layer formed on ZrN is stable and prevents further oxidation of the ZrN substrate.