Structure and Mechanical Properties of Reactive Sputter Deposited TiN/TaN Multilayered Films
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development of low-load indentation methods to measure the mechanical behaviors of thin films have made possible to investigate the plastic properties of multilayered films in more detail. Cammarata et al. [2], for example, have performed indentation tests on Cu/Ni system and also observed a less pronounced monotonic increase in hardness as bilayer thickness (A) decreased from 120 to 16 A. Much of the recent work on superlattice hardness has centered on ceramic systems. Barnett et al. [3] have observed a sharp peak in hardness of epitaxial TiN/VN superlattices, at a bilayer thickness of about 26 A. Subsequent work on TiN/V0.6 Nb0.4N also displays a peak in hardness, although it is less pronounced and occurs at approximately twice the bilayer thickness observed on TiNNN [4]. However, this work is complemented by observations that show no peak in hardness with grain size. These results for microhardness (which is a measure of compressive yield stress) can be understood in general terms as resulting from the Koehler mechanism. Substantial hardness enhancements have also been reported for epitaxial transition-metal nitride superlattices such as TiN/NbN [5]. Recently, this work has been extended to polycrystalline nitride multilayers [6]. The hardness dependence is quite similar to that observed for epitaxial TiN/NbN superlattices, showing a peak hardness of > 5000 kg/mm 2 at A = 50-80 A. At larger A, hardness values approached ruleof-mixtures values (2500 kg/mm 2). The similarity with the epitaxial film result suggests that grainsize effects play a relatively little role in the hardness enhancement. For ceramic multilayer materials, the spatial variation in elastic modulus, as well as the de463 Mat. Res. Soc. Symp. Proc. Vol. 505 ©1998Materials Research Society
velopment of coherent interfaces and coherency stresses at small multilayer thicknesses, may produce changes in hardness. In particular, Baral et al. [71 have proposed that loss of coherency and the concomitant increase in interfacial dislocation density may produce anomalous increases in hardness with A. In comparison, Barnett and Shinn [8] summarize experimental work on TiN/NbN superlattices with a 3.6% mismatch between the layers, and TiN/VN superlattices with a 2.4% lattice mismatch, and propose that a peak in hardness at a bilayer thickness of about 50 A may be due to the finite compositional gradients imposed by the interfaces. Shinn et al. [5][9] concluded that the effect of moduli on hardness is important by observing that the hardness of TiN/NbN, which has a large modulus difference from one layer to the next, was substantially larger than that of V0 .6Nb 0 .4 N/NbN which has a small modulus difference. This work also concluded that the effect of coherency stresses and misfit dislocations is not as important as the contribution from modulus differences. On the contrary, Helmersson et al. [3] suggested that the TiN/VN hardness peak was due, at least in part, to the supermodulus effect, i.e., increases by a factor of 2-4 of the elastic modulus at specif
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