Structure and mechanical properties of epitaxial TiN/V 0.3 Nb 0.7 N(100) superlattices
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K. M. Hubbard and T. R. Jervis Los Alamos National Laboratory, Los Alamos, New Mexico 87545
L. Hultman Department of Physics, Linkoping University, S-581 83 Linkoping, Sweden (Received 25 October 1993; accepted 10 February 1994)
Epitaxial TiN/Vo.3Nbo.7N superlattices with a 1.7% lattice mismatch between the layers were grown by reactive magnetron sputtering on MgO(OOl) substrates. Superlattice structure, crystalline perfection, composition modulation amplitudes, and coherency strains were studied using transmission electron microscopy and x-ray diffraction. Hardness H and elastic modulus were measured by nanoindentation. H increased rapidly with increasing A, peaking at H values —75% greater than rule-of-mixtures values at A ~ 6 nm, before decreasing slightly with further increases in A. A comparison with previously studied lattice-matched TiN/Vo.6Nbo.4N superlattices, which had nearly identical composition modulation amplitudes, showed a similar H variation, but a smaller H enhancement of =50%. The results suggest that coherency strains, which were larger for the mismatched TiN/Vo.sNbojN superlattices, were responsible for the larger hardness enhancement. The results are discussed in terms of coherency strain theories developed for spinodally decomposed materials. Nanoindenter elastic modulus results showed no significant anomalies.
I. INTRODUCTION The growth and properties of epitaxial TiN/VN, 1 TiN/Vo.6Nbo.4N,2'3 and TiN/NbN 4 superlattices and polycrystalline TiN/NbN 5 superlattices have recently been reported. Vickers microhardness Hv values were 3=2 times those for comparable homogeneous nitrides for superlattice periods A of 5-10 nm. Polycrystalline nitride superlattices thus have potential advantages over conventional nitride protective coatings for applications where high hardness is important.6'7 Epitaxial nitride superlattices have been used for fundamental mechanical property studies since the possibility of grain boundary, orientation-dependent, and defect effects on properties is minimized. Several Bl-structure nitrides8-9 can be combined to investigate the effect of varying layer elastic moduli and lattice parameters on properties. Superlattice strength and hardness enhancements are often explained by the dislocation barriers provided by layers with differing dislocation line energies, as suggested by Koehler10 and Lehoczky.11 Nitride superlattice results suggest that layer coherency strains also play a
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address: Sandia National Laboratories, P.O. Box 969, Livermore, California 94551-0969.
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J. Mater. Res., Vol. 9, No. 6, Jun 1994
http://journals.cambridge.org
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role in the hardness enhancements. In particular, comparisons of TiN/Vo.6Nbo.4N (—0.3% mismatch) superlattices with TiN/VN (2.4% mismatch) and TiN/NbN (3.6% mismatch) superlattices show a higher hardness peak at A « 5 nm for the large mismatch systems. Based on preliminary x-ray diffraction and XTEM results4 along with relaxation theories,12 the TiN/NbN superlattice layers were coherent at A ~ 5 nm
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