International Conference on Advances in Surface Treatment: Research & Applications (SMT XVII & IFHTSE) EDITED BY T.S. SUDARSHAN, S.V. JOSHI and G. TOTTEN Copyright © 2003 by Society for Advancement of Heat Treatment & Surface Engineering (SAHTSE), c/o ARCI, Hyderabad, India 1 Nanolayered TiN/NbN Multilayers as New Superhard Materials Paper No.: p129 Author's Corrections: Received on: Mailed back on: Initial temperatures with excellent adhesion to most of the engineering substrates. Superlattice coatings are not only important for technological applications but also very important scientific issues related to the enhancement in their physical properties need to be addressed. TiN/NbN multilayer system is technologically important since it is expected to have lower residual stresses apart from the superior mechanical properties. 5 TiN/NbN coatings are also thermally stable at higher temperatures. 3 Deposition of multilayer coatings of TiN/NbN is difficult because the heat of formation of NbN (56.2 kcal/mol) is considerably less than that of TiN (80.8 kcal/mol). Different partial pressures of nitrogen are, thus, required in the same deposition chamber to deposit stoichiometric TiN and NbN coatings. Another problem is that niobium nitride exists in cubic (c-NbN) and hexagonal (β-Nb 2 N) phases. 6 In a reactive sputtering process judicious control of process parameters, such as - nitrogen partial pressure, target power, energy and flux of ions, is essential to achieve high performance coatings. In this article, we briefly describe the growth of TiN/NbN superlattices. X-ray diffraction (XRD) has been used to study 1.0 Introduction Multilayer coatings have attained a considerable interest because of their exotic mechanical properties. Nanostructured multilayer coatings of ceramic materials (especially those of transition metal nitrides) are an emerging class of superhard materials. 1–4 In these coatings, alternate layers of two materials (e.g., TiN and NbN) are deposited onto a substrate; the thicknesses of individual layers being on the atomic scale. The thickness of each successive pair of layers is commonly known as modulation wavelength (Λ), which critically affects the properties of the multilayers. Nanostructured multilayer coatings are also commonly known as superlattices. Because of very small layer thicknesses and presence of a large number of interfaces these materials exhibit exotic mechanical properties. For example, hardness as high as 4000–5000 kg/mm 2 has been reported for TiN/NbN multilayer coatings, which is very high as compared to rule-of-mixtures value. 3, 5 Layering of the two materials not only improves mechanical properties but also the chemical and thermal stability of the coatings. Furthermore, these coatings can be deposited at low Harish C. Barshilia and K.S. Rajam Surface Engineering Division, National Aerospace Laboratories Post Bag No. 1779, Bangalore - 560 017 India E-mail: harish@css.cmmacs.ernet.in and rajam@css.cmmacs.ernet.in Abstract Nanolayered multilayer coatings of TiN/NbN on silicon and tool steel substrates were deposited using a reactive DC magnetron sputtering process. Judicious control of process parameters enabled the deposition of stoichiometric TiN/NbN coatings. The bilayer thickness, also known as modulation wavelength (L), critically affected the properties of the coatings. Coatings deposited at 150 Å ≥ L ≥ 30 Å exhibited superlattice structure as evidenced by XRD. Nanoindentation data indicated that polycrystalline TiN/NbN multilayers exhibited hardness as high as 4000 kg/mm 2 , which was ~2 times the rule-of-mixtures value. Annealing of the coatings in vacuum indicated that coatings retained superlattice structure even up to 850°C. Furthermore, the multilayer coatings showed a tremendous improvement in the corrosion resistance in 0.5 M HCl solution as compared to the single layer TiN and NbN coatings.