Structure and properties of reactive DC magnetron sputtered TiN/NbN hard superlattices

Nanostructured TiN/NbN superlattices at various modulation wavelengths (Λ) were deposited on silicon (111) and tool steel substrates using a reactive DC magnetron sputtering process. Structural characterization of the coatings was done using X-ray diffraction (XRD) in Bragg-Brentano θ–2θ geometry. All the multilayer coatings exhibited (111) preferred orientation in the XRD data. Appearance of satellite reflections (SR) along (111) principal reflection (PR) was used to check the quality of the superlattice coatings. The coatings exhibited superlattice structure for 106 Å≥Λ≥30 Å. The mechanical properties of the coatings were measured using a nanoindentation technique. The multilayer coatings exhibited hardness as high as 4000 kg/mm2 at a modulation wavelength of 48 Å, which was ∼2.4 times the value of the rule-of-mixture. Thermal stability of the TiN/NbN multilayer coating was studied by vacuum annealing the coating for 30 min in the temperature range of 100–850 °C. Subsequent structural changes in the coating were measured by an in situ high-temperature X-ray diffractometer. Coating exhibited thermal stability up to 700 °C and subsequently showed reduction in the superlattice character. The corrosion behavior of 1.5 μm thick single layer TiN and NbN, and multilayer TiN/NbN (Λ=50 Å) coatings deposited on tool steel substrates were investigated using potentiodynamic polarization in 0.5 M NaCl and 0.5 M HCl solutions under non-deaerated conditions. The microstructural changes, as a result of corrosion, were investigated by atomic force microscopy (AFM). The results indicated that the multilayer coatings exhibited superior corrosion resistance as compared to the single layer coatings.