Deposition of TiN/NbN Superlattice Hard Coatings by Ionised Magnetron Sputter Deposition
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Deposition of TiN/NbN Superlattice Hard Coatings by Ionised Magnetron Sputter Deposition Yi Long, Robert J. Stearn, Zoe H. Barber, Stephen J. Lloyd, William J. Clegg Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, CB2, 3QZ, UK. ABSTRACT We have investigated the effect of ion bombardment on the structure and hardness of thin coatings of TiN/NbN multilayered structures and monolithic films of both TiN and NbN. A radio frequency coil was used to generate an additional inductively coupled plasma between the substrate and the target enabling the sample to be bombarded by a high flux of relatively low energy ions under the appropriate conditions. It is shown that the effect of such bombardment in the case of the monolithic films is to reduce the porosity. This gave an increase in the hardness of both the TiN and the NbN films up to a power of 100 W (using a coil with a cross-sectional area of 2 × 103 mm2). Further increasing the power density led to a decrease in hardness. TiN/NbN multilayer coatings were made under the optimum deposition conditions for the monolithic materials and gave hardnesses greater than those observed in either TiN or NbN and approximately 50% greater than that predicted by a mixtures rule. INTRODUCTION It has been shown that substantial increases in hardness, sometimes up to a factor of 3, can be achieved by making layered structures in which the layers have a thickness of the order of 10 nm [1]. Some recent observations have also shown that layers of a TiN/NbN multilayer film embedded within a TiN matrix stay relatively intact during mechanical deformation with no large slip bands, whereas the deformation of the surrounding, monolithic TiN occurred by the movement of dislocations along persistent slip bands [2]. Theories proposed to explain this apparent strengthening include models based on differences in the elastic properties or in Burgers vectors of the layers [3]. However, there are many instances where no such hardening is observed, even though the structures appear to be the same, suggesting either that some other effect is leading to weakening or that the hardening effect does not arise as has been suggested [4,5]. Previous work using d.c. magnetron sputtering showed that substantial increases in hardness could be obtained by bombarding the growing film with more energetic ions, although there was no observation of any multilayer hardening [5]. In this paper we report the effect of using a radio frequency (r.f.) coil within the chamber to generate an additional plasma so that the surface of a biased substrate might be bombarded with relatively low energy ions, during the growth, and the resulting changes in structure and hardnesses of multilayer TiN/NbN and monolithic films has been studied. EXPERIMENTAL DETAILS Single TiN and NbN layers and epitaxial multilayer TiN/NbN films were deposited using ultrahigh-vacuum reactive ionised d.c. magnetron sputtering in an Ar–40% N2 gas mixture at a gas pressure of 2.3 Pa onto (001)-oriented MgO sing
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