Influence of air oxidation on the properties of decorative NbO_xN_y coatings prepared by reactive gas pulsing

In this work the oxidation resistance of DC reactive sputtered niobium oxynitrides and its influence on the properties of the films are studied. The depositions have been carried out by DC magnetron sputtering with a reactive gas pulsing process. The nitrogen flow was kept constant and the oxygen flow was pulsed. Pulse durations of 10 s produced multilayer coatings with a period of λ = 10 nm. Three sets of films with increasing duty cycle (= on-time of high oxygen flow / pulse duration) have been deposited. The films were subsequently annealed in air at 400, 500 and 600 °C, respectively. X-ray diffraction measurements showed a clear and progressive change from a roughly amorphous nature of the films to a crystalline oxide-type compound for those annealed at 600 °C, which was consistent with the composition analysis. For annealing temperatures of 500 and 600 °C, the coatings presented a significant reduction in hardness, approaching the values characteristic of Nb₂O₅-type films. Moreover, the residual stress measurements performed by using the deflection method revealed low values in all the coatings nearly independent on the annealing temperature. Color variation in the CIE - L ^*a^*b^* color space and the reflectance in the UV-visible spectrum range of these niobium oxynitrides were investigated and correlated to their chemical composition and structural features. For both properties, the variation tendencies are quite similar, showing the transition from a nitride-type alloy to an oxide-type one with increasing annealing temperature. © 2007 Elsevier B.V. All rights reserved.