https://spaepen.seas.harvard.edu/files/spaepen/files/218.pdf

Tensile testing of free-standing Cu, Ag and Al thin films and Ag/Cu multilayers

A model is presented that explains the yield stress and hardness enhancements that have been observed in superlattice thin films. The stress required for dislocations to glide across layers with different shear moduli was calculated using an expression that accounts for core effects and all interfaces in trapezoidal or sawtooth composition modulations. The predicted strength/hardness enhancement increased with increasing superlattice period Λ, before reaching a saturation value that depended on interface widths. A second mechanism, where dislocations glide within individual layers, was important at large Λ and gave a decrease in strength/hardness with increasing Λ. The combination of these two mechanisms gives a strength/hardness maximum versus Λ in good quantitative agreement with experimental results for nitride and metal superlattices. The results indicate that superlattice strength/hardness depends strongly on interface widths and the difference in shear moduli of the two components for Λ values below the maximum, and on the average shear modulus for larger Λ.

Model of superlattice yield stress and hardness enhancements

The unloading process in an indentation experiment is usually modeled by considering the contact of a rigid punch with an elastically isotropic half space. Here we extend the analysis to elastically anisotropic solids. We review some of the basic formulae for describing the indentation of elastically anisotropic solids with axisymmetric indenters. We show how the indentation modulus can be calculated for arbitrary anisotropic solids and give results for solids with cubic crystal symmetry. We have calculated the contact stiffness for a flat triangular punch on a half space for various anisotropic materials. The indentation modulus for a triangular indenter is typically 5–6% higher than that for an axisymmetric indenter and varies only slightly with the orientation of the indenter in the plane of the indentation. We have conducted microindentation experiments to measure the indentation moduli of differently oriented surfaces of both cubic and hexagonal single crystals. For copper and β-brass, the (111) indentation moduli are approximately 10 and 25% larger than the {100} modulus. The (110) moduli are typically slightly smaller than the (111) moduli. The indentation modulus of zinc varies by as much as a factor of two, depending on the sample orientation. The hardnesses of the single crystals do not vary much with the orientation of the plane of indentation. For /gb-brass, the hardness of a {110} surface is only about 13% lower than the hardness of a {100} or {111} surface; for copper, the {110} hardness is 6% higher than for the other orientations. For zinc the maximum change in hardness with orientation is 20%.

Measuring the elastic properties of anisotropic materials by means of indentation experiments

Nanometer scale multilayered hard coatings

Epitaxial TiN/NbN superlattices with wavelengths/ranging from 1.6 to 450 nm have been grown by reactive magnetron sputtering on MgO(100). 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 Λ = 9.4 nm, but not Λ = 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 Λ increased from 2 to 10 nm. Vickers microhardness H was found to increase rapidly with increasing Λ, from 1700 kg/mm2 for a TiN–NbN alloy (i.e., Λ = 0) to a maximum of 4900 kg/mm2 at Λ = 4.6 nm. H decreased gradually for further increases in Λ above 4.6 nm, to H = 2500 kg/mm2 at Λ = 450 nm. The hardness results are compared with theories for strengthening of multilayers.

Growth, structure, and microhardness of epitaxial TiN/NbN superlattices

The mechanical properties of multilayered Cu‐Ni thin films with bilayer thicknesses of 1.6–12 nm were investigated by a nanoindentation technique. Force‐displacement curves generated during loading and unloading of a diamond tip indenter were used to determine the hardness and elastic properties of the films. No enhancement in the elastic properties (supermodulus effect) was seen, but an enhancement in the hardness was observed. It is suggested that the enhancement, which displayed a Hall–Petch‐type behavior, can be understood as owing to dislocation pinning at the interfaces analogous to the mechanism of grain boundary hardening.

Nanoindentation study of the mechanical properties of copper‐nickel multilayered thin films

Low-dimension structural properties and microindentation studies of ion-beam-sputtered multilayers of Ag/Al films

Multilayer films of Cu/Ni and Fe/Ni have been prepared by ion beam sputtering. The observed X-ray intensities of the satellites of the (111) reflection in the case of the Cu/Ni multilayers are in approximate agreement with calculated intensities for 24 A ⩽ λ ⩽ 194 A where λ is the wavelength of the multilayer. The iron in the Fe/Ni multilayers may be in the f.c.c. phase at short wavelengths.

Ion-beam-sputtered Cu/Ni and Fe/Ni multilayers

Al/Fe multilayers were deposited using an ion-assisted ion-beam sputtering technique. X-ray diffraction at high angles showed peaks corresponding to the aluminum f.c.c. phase with a lattice parameter equal to that of bulk aluminum. Peaks from the iron b.c.c. phase were observed and the lattice parameter based on these peaks was calculated to be 2.94 A. The absence of satellite peaks is attributed to the small grain size. The low angle diffraction pattern showed three orders of peaks. Investigation of the magnetic properties using a vibrating sample magnetometer and ferromagnetic resonance showed a moderately reduced moment for iron when compared with bulk iron.

C. Kim

Papers

Nanoindentation study of the mechanical properties of copper‐nickel multilayered thin films

Low-dimension structural properties and microindentation studies of ion-beam-sputtered multilayers of Ag/Al films

Ion-beam-sputtered Cu/Ni and Fe/Ni multilayers

Structural and magnetic properties of Al/Fe multilayers deposited by ion beam sputtering