Growth, structure, and microhardness of epitaxial TiN/NbN superlattices
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Epitaxial TiN/NbN superlattices with wavelengths A ranging from 1.6 to 450 nm have been grown by reactive magnetron sputtering on MgO(lOO). Cross-sectional transmission electron microscopy (XTEM) studies showed well-defined superlattice layers. Voided low-angle boundaries, aligned perpendicular to the film plane, were also present. High-resolution images showed misfit dislocations for A = 9.4 nm, but not A = 4.6 nm. Up to ninth-order superlattice reflections were observed in diffraction, indicating that the interfaces were relatively sharp. Analysis of the first-order x-ray superlattice reflection intensities indicated that the composition modulation amplitude increased and the coherency strains decreased for A increased from 2 to 10 nm. Vickers microhardness H was found to increase rapidly with increasing A, from 1700 kg/mm 2 for a TiN-NbN alloy (i.e., A = 0) to a maximum of 4900 kg/mm 2 at A = 4.6 nm. H decreased gradually for further increases in A above 4.6 nm, to H — 2500 kg/mm 2 at A = 450 nm. The hardness results are compared with theories for strengthening of multilayers.
I. INTRODUCTION While there have been numerous reports of enhanced strength and hardness in multilayered thin films and superlattices, the understanding of the strengthening mechanisms is incomplete. Much of the work to date has been on metal/metal layered structures with A > 0.1 /zm, including Cu/Ni, 1 ' 2 Cu/Fe,2-3 Al/Cu, 4 ' 5 and Al/Ag. 5 Ceramic-containing structures with A in this range, including TiC/TiB 2 , 6 Al/A10 x , 7 A1/A1NX,8 Ni/TiC, 9 and Ta/TaQ, 1 0 have also been reported. The strength and hardness generally increase with decreasing A below ~ 1 jtzm. Reported dependences are of a Hall-Petch type,11 i.e., A~0-5, or a A" 1 dependence predicted by Lehoczky.4-5 In at least one case, the presence of the composition modulation decreased the film grain size, and it was suggested that this was responsible for the strength enhancements.10 Superlattices with lower A values have also been reported.12"15 For example, maximum strength enhancements of 300 to 400% have been observed in polycrystalline Cu/Ni 12 superlattices at A w 24 nm. Vickers microhardness H for single-crystal transition-metal nitride superlattices has recently been reported.13'14 H increased rapidly with increasing A, to values 2000 to 3500 kg/mm 2 greater than pure nitrides or alloys, at A = 5-10 nm. H decreased gradually with further increases in A, although the largest A value a)
Permanent address: Department of Physics, Thin Film Division, Linkoping University, S-58183 Linkoping, Sweden. J. Mater. Res., Vol. 7, No. 4, Apr 1992
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reported was 30 nm. Nitride superlattices with two different lattice mismatch values, 2.4% for TiN/VN 13 and *-* t«- - .'I* • -r*.v f ? - >. "T _•
FIG. 3. Cross-sectional transmission electron micrographs, high resolution {200} lattice fringe images, and selected area diffraction patterns of TiN/NbN(100) superlattice films with wavelengths of (a) A = 4.6 nm and (b) A — 9.4 nm. High
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