Lack of hardening effect in TiN/NbN multilayers
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Lack of hardening effect in TiN/NbN multilayers Jon M. Molina-Aldareguia, Stephen J. Lloyd, Zoe H. Barber and William J. Clegg Dept of Materials Science and Metallurgy, University of Cambridge, CB2 3QZ, UK ABSTRACT There is evidence indicating that multilayer films can be harder than monolithic ones. To investigate this, TiN/NbN multilayers with bilayer thicknesses ranging from 4 nm to 30 nm have been grown on MgO (001) single crystals using reactive magnetron sputtering. The sharpness of the interface and the composition modulation, which would be expected to strongly influence dislocation motion, have been studied by X-ray diffraction (XRD). These experiments show that the interfaces remain reasonably sharp (interface thickness ~1 nm) and the composition modulation amplitude is maximum for multilayers with bilayer thicknesses greater than ~10 nm. With thinner bilayers, the composition modulation decreases but the layered structure remains. Despite this, the nanoindentation hardness of the multilayers is between 20 and 25 GPa, which is similar to that of TiN and NbN alone, and therefore, no hardening due to the layering is observed. The deformation mechanisms observed under the indent in the TEM are consistent with these results. INTRODUCTION It has been shown that the hardness of nitride multilayers can be greater than that of monolithic films of the components [1]. These results have been attributed to hardening due to the elastic mismatch between the layers [2], according to Koehler’s ideas [3]. However, discrepancies still exist between the predictions and the experimental results. For instance, this paper examines the nanoindentation behaviour of TiN/NbN multilayers that do not show the hardening effect, although they posses a strong composition modulation. EXPERIMENTAL Multilayers were deposited using ultra-high vacuum reactive d. c. magnetron sputtering in an Ar-40% N2 gas mixture on (001) oriented MgO single-crystals heated to 800 °C. Previous work [4] showed that the highest quality films were produced under conditions where the residual intrinsic stresses, due to bombardment by energetic species during deposition, are small and slightly compressive. Under deposition conditions that prevented bombardment of the films, open porous structures were observed. Where the induced compressive stresses where very high, it was difficult to maintain epitaxy in the films. In the current work, the multilayers were deposited at a sputtering gas pressure of 2.3 Pa and a substrate to target distance of 30 mm, to promote bombardment by energetic species during deposition. It was found that deposition rates of 0.35 nm s-1 for NbN and 0.05 nm s-1 for TiN allowed the deposition of 1 micron thickness epitaxial multilayers with different bilayer thicknesses, ranging from 4 nm to 25 nm. Under these conditions, the intrinsic stresses in monolithic TiN films were found to be slightly compressive (~1.6 GPa) and no stresses were found in NbN films. The hardness and elastic modulus of the films for a range of indentation depths rangi
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