Anita Madan

Enhanced mechanical hardness in epitaxial nonisostructural Mo/NbN and W/NbN superlattices

Epitaxial Mo/NbN and W/NbN superlattices with modulation wavelengths Λ ranging from 1.3 to 120 nm were grown on MgO (001) substrates by dc reactive magnetron sputtering in Ar/N2 mixtures. The superlattices were shown to be epitaxial with nearly planar layers using high- and low-angle x-ray diffraction and transmission electron microscopy. Computer simulation fits of the x-ray data indicated that interface widths were ⩽0.3 nm. The epitaxial relationship between the layers was (001)metal∥(001)NbN and [110]metal∥[100]NbN. The nanoindenter microhardness values from W/NbN and Mo/NbN superlattices with 50 vol % metal were nearly identical. The largest hardnesses were 30 GPa, observed at superlattice periods Λ=2–3 nm, compared to rule-of-mixtures values of 10 GPa. The hardness decreased with increasing Λ above ≈3 nm, following the dependences H=10.3+26.70Λ−0.38 GPa for Mo/NbN and H=12.88+22.1Λ−0.3 GPa for W/NbN. Hardness versus metal volume fraction with Λ≈5 nm showed a flat-topped dependence. Brillouin scatteri...

An X-ray diffraction study of epitaxial TiN/NbN superlattices

Abstract X-ray diffraction measurements and simulations were carried out to characterize the composition modulation and structure of TiN/NbN superlattices. A trapezoidal/sawtooth form of the composition wave was assumed. Random d -spacing fluctuations with Gaussian width approximately 0.002 nm and layer thickness variations of 0.1–0.5 nm were incorporated to match the observed broadening of the peaks. Best fits to the experimental data showed that considerable interdiffusion was present, with up to 15 at.% metal substitution within the layers and interface widths of 0.4–2.0 nm. The interface width values agreed well with those used in hardness enhancement calculations for epitaxial TiN/NbN superlattices.

Structure, stability, and mechanical properties of epitaxial W/NbN superlattices

Epitaxial W/NbN superlattices with modulation wavelengths Λ ranging from 1.3 to 25 nm were grown on MgO(001) substrates by dc reactive magnetron sputtering in Ar/N2 mixtures. The epitaxial relationship between the layers is given by W(001)‖NbN(001) and W[110]‖NbN[100]. X-ray diffraction and Rutherford backscattering results fitted using simulations showed that the superlattices had well-defined planar layers with interface widths of ≈0.2 nm. Nanoindentation measurements showed superlattice hardnesses as high as 33 GPa compared to 8 for W and 20 for NbN. The superlattices showed little change in x-ray superlattice reflections or nanoindentation hardness after vacuum annealing up to the highest temperature tested, 1000 °C for 6 h. Thus, the layers remained intact during annealing, allowing the superlattice hardness enhancement to be retained.

Effect of process variables on the structure, residual stress, and hardness of sputtered nanocrystalline nickel films

Nanocrystalline nickel films of about 0.1 mm thickness grown by sputtering with andwithout substrate bias possessed average grain sizes of 9–25 nm. Variation in substratebias at room and liquid nitrogen temperature of deposition strongly affected grainstructure and size distribution. Qualitative studies of film surfaces showed variation inroughness and porosity level with substrate bias and film thickness (maximum of8 mm). The films had tensile residual stress, which varied with deposition conditions.The hardness values were much higher than those of coarse-grained nickel butdecreased with an increase in the film thickness because of grain growth.I. INTRODUCTIONNanocrystalline metals have generated a great deal ofinterest in recent years because they demonstrate impres-sive mechanical behavior characterized by very highstrengths.

Methods of producing plasma enhanced chemical vapor deposition silicon nitride thin films with high compressive and tensile stress

Various methods of generating high stress in thin plasma enhanced chemical vapor deposition (PECVD) silicon nitride (SiN) films are reported. Besides the mainstream variation of plasma power and other process parameters, novel techniques such as creation of high density layers in multilayer PECVD structures or exposure of SiN films to ultraviolet radiation are shown to increase intrinsic film stress. Thin PECVD SiN films have been analyzed by a variety of analytical techniques including Fourier transform infrared spectroscopy, x-ray reflectivity (XRR), time of flight secondary ion mass spectrometry, and transmission electron microscopy to collect data on bonding, density, chemical composition, and film thickness. The level of bonded hydrogen as well as film density has been found to correlate with film stress. Creation of multilayer structures and high density layers help to build up more stress compared to a standard single layer film deposition. Both the density and number of layers in a film, character...

Mechanical properties and thermal stability of TiN∕TiB2 nanolayered thin films

This article describes a study of TiN∕TiB2 nanolayered coatings on sapphire and M2 tool steel substrates. Residual stress in as-deposited TiN∕TiB2 varied from tensile to compressive with increasing substrate bias. Increasing the density of nanolayer interfaces (i.e., decreasing bi-layer period) decreased the compressive stress; this effect was explained by diffusion of point defects to interfaces and/or an indirect effect of interfaces on stress via film structure. A thin TiN buffer layer substantially reduced the stress and improved adhesion on steel. Nanolayer film adhesion on steel was generally intermediate between that of monolithic TiN coatings, which was good, and TiB2 coatings, which was poor. As-deposited nanolayers showed no hardness enhancement relative to rule of mixtures. X-ray diffraction results showed that the boride layers tended to be amorphous, especially for small layer thicknesses. After annealing at 1000°C, nanolayer structure was retained, thin boride layers were at least partially ...

High-temperature stability of epitaxial, non-isostructural Mo/NbN superlattices

The effect of 1000 degrees C vacuum annealing on the structure and hardness of epitaxial Mo/NbN superlattice thin films was studied. The intensity of superlattice satellite peaks, measured by x-ray ...

Stabilization of zinc-blende cubic AlN in AlN/W superlattices

AlN/W superlattices with bilayer periods of 3.5–7 nm were grown on MgO (001) by magnetron sputtering. For AlN thicknesses lAlN⩽1.5 nm, a cubic phase of AlN was observed using high-resolution cross-sectional transmission electron microscopy and x-ray diffraction (XRD). Pole-figure XRD scans showed a reflection that matched the theoretically predicted interplanar spacing of zinc-blende phase (Zb-AlN), and that could not be explained by either the wurtzite or rocksalt structures. The stabilization of zb–AlN is explained as a result of good interfacial matching between W(100) and zb–AlN(011). When lAlN was increased above 1.5 nm, XRD scans showed a rapid decrease in satellite peak intensities, indicating a degradation of the layered structure, and the appearance of a wurtzite structure.

Super hardening and deformability in epitaxially grown W/NbN nanolayers under shallow and deep nanoindentations

Epitaxially grown W/NbN nanolayers are superlattice materials that exhibit a hardness at small bilayer repeat periods which exceed the hardness predicted by the rule of mixtures for normal composites. Interfaces in the bilayered superlattice play a critical role in the superhardening process. The objective of this investigation was to examine the behavior of the superlattice material, W/NbN. Nanoindentations and in situ surface imaging were conducted over a range of applied loads on samples of W/NbN with two different bilayer periods (Λ=5.6 and 10.4 nm), and monolithic samples of the niobium nitride (NbN) ceramic and the tungsten (W) metal which comprise the superlattice material. Shallow nanoindentations were made to a depth equal to the individual layer thicknesses of the epitaxially grown W/NbN nanolayers in order to investigate the individual interface effect. The mechanical properties were determined using the Oliver and Pharr method and compared for all the samples. The energies of indentation are calculated. The characteristics of the material pileup resulting from the nanoindentations are determined from the scanned surface images. An increase in hardness is observed in the superlattice materials at deeper indentation depths. The results indicate that this increase in hardness is related to the nature of the interface between the layers in the superlattice materials.Epitaxially grown W/NbN nanolayers are superlattice materials that exhibit a hardness at small bilayer repeat periods which exceed the hardness predicted by the rule of mixtures for normal composites. Interfaces in the bilayered superlattice play a critical role in the superhardening process. The objective of this investigation was to examine the behavior of the superlattice material, W/NbN. Nanoindentations and in situ surface imaging were conducted over a range of applied loads on samples of W/NbN with two different bilayer periods (Λ=5.6 and 10.4 nm), and monolithic samples of the niobium nitride (NbN) ceramic and the tungsten (W) metal which comprise the superlattice material. Shallow nanoindentations were made to a depth equal to the individual layer thicknesses of the epitaxially grown W/NbN nanolayers in order to investigate the individual interface effect. The mechanical properties were determined using the Oliver and Pharr method and compared for all the samples. The energies of indentation are c...

Integration of modeling and acoustic microscopy measurements for thin films

A model for measuring the V(z) curve by line-focus acoustic microscopy contains the reflectance function of the specimen as a principal component. In this paper the reflectance function has been analyzed for multilayered thin films on a substrate for both fast-on-slow and slow-on-fast systems. The phase velocities of modes of surface acoustic wave propagation and their associated mode reflection coefficients can be obtained from the reflectance function. This information can be used together with estimates of the elastic constants to determine suitable frequency ranges for measuring the V(z) curve. Minimization of the difference between phase velocities obtained from measured and calculated V(z) curves is used to determine the elastic constants. Results are presented for TiN films on M2 high-speed steel substrates.