Variations in the reflectance of TiN, ZrN and HfN

Abstract The gold-like color and high hardness of the group IVB nitrides have allowed their use as scratch-resistant decorative coatings. The color can be varied with composition and mode of preparation. In the present work, TiN, ZrN and HfN films with different nitrogen-to-metal ratios were prepared on stainless steel by reactive sputtering. Half of each sample was vacuum annealed at 900°C for 1 h. The lattice parameters, reflectance and color were determined. The lattice parameters remained above equilibrium even after tempering. The experimental reflectance curves were analyzed in terms of the reflectance minima (between the screened Drude plasma edge and the onset of interband transitions) and the value of the reflectance at a photon energy of 1 eV (which is representative of the relaxation behavior). For all three nitrides the reflectance minimum is reduced to lower values of photon energy and reflectance as the nitrogen content is increased through the stoichiometric point. The reflectance at 1 eV increases concurrently. The degree of yellowness increases with nitrogen content, but no consistent effect of tempering could be defined. The behavior is described as the combined result of charge transfer from the metal to the metalloid atom (thus changing the effective density of conduction electrons) and of varying concentrations of vacancies and interstitials.