Nanoscale Multilayers Research Articles

Effect of Interfaces in the Work Hardening of Nanoscale Multilayer Metallic Composites During Nanoindentation: A Molecular Dynamics Investigation

To study the strain hardening in nanoscale multilayer metallic (NMM) composites, atomistic simulations of nanoindentation are performed on CuNi, CuNb, and CuNiNb multilayers. The load-depth data were converted to hardness-strain data that were then modeled using power law. The plastic deformation of the multilayers is closely examined. It is found that the strain hardening in the incoherent CuNb and NiNb interfaces is stronger than the coherent CuNi interface. The hardening parameters are discovered to be closely related to the density of the dislocations in the incoherent interfaces, which in turn is found to have power law dependence on two length scales: indentation depth and layer thickness. Based on these results, a constitutive law for extracting strain hardening in NMM from nanoindentation data is developed.

Structure Transformation and Coherent Interface in Large Lattice‐Mismatched Nanoscale Multilayers

Nanoscale Al/W multilayers were fabricated by DC magnetron sputtering and characterized by transmission electron microscopy and high‐resolution electron microscopy. Despite the large lattice mismatch and significantly different lattice structures between Al and W, a structural transition from face‐centered cubic to body‐centered cubic in Al layers was observed when the individual layer thickness was reduced from 5 nm to 1 nm, forming coherent Al/W interfaces. For potential mechanisms underlying the observed structure transition and forming of coherent interfaces, it was suggested that the reduction of interfacial energy and high stresses induced by large lattice‐mismatch play a crucial role.

Microstructure, Oxidation and Tribological Properties of TiAlCN/VCN Coatings Deposited by Reactive HIPIMS

2.5 μm thick TiAlCN/VCN coatings were deposited by reactive HIPIMS process. XTEM showed gradual evolution of the structure of the coating with thickness. The initial structure is nanoscale multilayer with sharp interlayer interfaces. This transforms to nanocomposite of TiAlCN and VCN nanocrystalline grains surrounded by C-rich tissue phase and finally changes to an amorphous carbon rich Me-C phase. Energy-resolved mass spectrometry revealed that HIPIMS plasma is a factor of 10 richer in C1+ ions, and therefore more reactive, as compared to the plasma generated by standard magnetron discharge at the same conditions. The peculiar structure evolution in HIPIMS is is attributed to target poisoning effect.

Cooling effect of nanoscale Bi2Te3/Sb2Te3 multilayered thermoelectric thin films

Managing high heat flux is one of the greatest technical challenges the integrated circuit (IC) industry is facing because the rising temperature limits device minimization and decreases its lifetime. In this paper, we report the characterization of the cooling effect of nanoscale Bi2Te3/Sb2Te3 multilayered thin films. The multilayer thin film was prepared with e-beam evaporation, and had 21 layers (5-nm-thick each layer and 105-nm-thick total). A thermoelectric device of the multilayer film, which is sandwiched between a diode temperature sensor and a platinum temperature sensor, was fabricated to measure the cooling effect. A maximum cooling temperature difference of about 3 K was obtained from the film at an applied dc electrical current of 5 mA. The nanoscale multilayer film could be integrated in the IC devices for the application of high-efficiency thermoelectric solid-state cooling.

Chiral magnetization configurations in magnetic nanostructures in the presence of Dzyaloshinskii-Moriya interactions

Many low-dimensional systems, such as nanoscale islands, thin films, and multilayers, as well as bulk systems, such as multiferroics, are characterized by the lack of inversion symmetry, a fact that may give rise to a Dzyaloshinskii-Moriya (DM) interaction. For sufficient strength, the DM interaction will favor spiral spin configurations of definite chirality. In order to harness such systems for applications, it is important to understand the conditions under which these spiral spin configurations form and how they can be controlled via an external field. Here, we present exact solutions of the 1D magnetization profiles in such systems for arbitrary material parameters in closed form. Determining the energy per unit length exactly, we are able to present the critical strength of the DM interaction, at which spiral solutions are energetically favorable. These magnetization profiles, in general, take the form of a domain wall or soliton lattice, with all solitons having the same chirality, whose sign is dictated by DM interaction. Conversely, given an energetically favorable spiral solution, we determine quantitatively how the magnetization profile changes as a function of the applied field.

Plasticity in the nanoscale Cu/Nb single-crystal multilayers as revealed by synchrotron Laue x-ray microdiffraction

Abstract

HARDNESS STUDY OF THE PULSE ELECTRODEPOSITED NANOSCALE MULTILAYERS OF CR-NI

Nanoscale multilayers of Cr - Ni coatings were deposited on low carbon steel by pulse electroplating and agitation modulation in chromium (III)-nickel (II) bath. The coatings were characterized by using SEM, EDS, XRD and microhardness techniques. For hardness measurement, the total thickness of the coatings was fixed at 5 μm and the thickness of the monolayers in the coatings varied from 20 to 100 nm. Single chromium and single nickel coatings with similar thickness of 5 μm were also produced by using DC electroplating. Microhardness testing was carried out on the surface of the coatings by using a range of loads from 25 to 1000 gf. A composite hardness model was used to estimate and compare the hardness of the single and multilayer coatings. It was shown that multilayer deposition significantly increases the hardness of the coating and the hardness increases further as the thickness of the nano- monolayers is reduced.

Surface modification of 316L stainless steel with magnetron sputtered TiN/VN nanoscale multilayers for bio implant applications

Nanoscale multilayered TiN/VN coatings were developed by reactive dc magnetron sputtering on 316L stainless steel substrates. The coatings showed a polycrystalline cubic structure with (111) preferential growth. XPS analysis indicated the presence of peaks corresponding to Ti2p, V2p, N1s, O1s, and C1s. Raman spectra exhibited the characteristic peaks in the acoustic range of 160-320 cm(-1) and in the optic range between 480 and 695 cm(-1). Columnar structure of the coatings was observed from TEM analysis. The number of adherent platelets on the surface of the TiN/VN multilayer, VN, TiN single layer coating exhibit fewer aggregation and pseudopodium than on substrates. The wear resistance of the multilayer coatings increases obviously as a result of their high hardness. Tafel plots in simulated bodily fluid showed lower corrosion rate for the TiN/VN nanoscale multilayer coatings compared to single layer and bare 316L SS substrate.

Strengthening mechanism of metallic nanoscale multilayer with negative enthalpy of mixing

Unlike sharp interfaces usually formed in multilayer thin films with positive enthalpy of mixing, intermixing at interface due to negative enthalpy of mixing can reduce the effectiveness of the interface as dislocation motion barrier and thus weaken the hardness/strength of the multilayer thin film. Through first-principles calculations, however, we find that the large negative enthalpy of mixing may lead to negative interface energy and thus the formation of intermetallic compounds in the interface. The resulting intermetallic compounds may have much higher shear moduli than those of the constituent metals, which can make positive contribution to the hardness enhancement of the nanoscale multilayer thin film.

Friction and Wear Properties of WC/C Nano-Scale Multilayer Coatings on Technical Surfaces

This article reports on the tribological performance of a WC/C multilayer coating with a periodicity WC/C = 15/5 nm deposited on bearing steel substrates. These substrates were initially polished to achieve different RMS roughness values ranging from 15 to 900 nm. Tribological tests were performed using a precision reciprocating microtribometer under ambient conditions with a steel ball as a counter-sample. The worn sample surfaces were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX). The results indicate that the combined effects of the real contact area, lubricating effect of the coating, as well as the formation of a transfer layer and its subsequent removal, all contribute to a dependence of surface roughness on the coefficient of friction (CoF).

Compressive flow behavior of Al–TiN multilayers at nanometer scale layer thickness

The mechanical behavior of nanoscale Al–TiN multilayered composites was studied using micropillar compression experiments, which were compared with previously performed nanoindentation experiments, and the microstructure was investigated using scanning and transmission electron microscopy. It was found that at extremely small layer thicknesses (<5 nm) the nanoscale multilayers acquire remarkable hardness (∼6 GPa), high flow strengths (∼4.5 GPa maximum) and high compressive deformability (5–7% plastic strain). These high strengths were accompanied by extraordinarily high strain-hardening rates in the Al layers, which were of the order of 16–35 GPa (∼E/4–E/2) in “regime II” of the stress–strain curve of the compression tests in the 2–4 nm layered films and 5–9 GPa in the Al-18 nm–TiN-2 nm layered films, where Al layers deform plastically and TiN layers are deformed elastically. The high strengths and high work-hardening rates are discussed and analyzed using the concepts of dislocation motion and interactions within the confined nanoscale Al layers.

Intermixing of metal oxide/Ag/metal oxide nanoscale multilayers prepared by rf reactive magnetron sputtering

This study mainly aims to evaluate the intermixing between layers during the deposition of dielectric/metal/dielectric nanoscale multilayer coatings. TiO2 and WO3 films as dielectric materials are developed using radio frequency reactive magnetron sputtering on unheated soda lime glass substrates. Ag films are grown using radio frequency argon sputtering. Auger electron spectroscopy and ultraviolet–visible–near infrared spectrometry are employed to estimate the degree of layer intermixing in TiO2/Ag, WO3/Ag, TiO2/Ag/TiO2 and WO3/Ag/WO3 films. The morphological properties are also investigated under a field emission scanning electron microscope and an atomic force microscope. The transmittance curves for bilayer films exhibited heat mirror effects significantly. However, multilayer films showed a little amount of heat mirror effects. Auger electron spectroscopy depth profiles confirmed a minor diffusion of Ag at the TiO2/Ag interface. However, significant interactions of WO3 with Ag were observed for the WO3/Ag film. Besides, severe intermixing and oxidation of Ag films were observed for the multilayer films.

Nanoalloyed Bi2Te3, Sb2Te3 and Bi2Te3/Sb2Te3 Multilayers

ABSTRACTIn this work, thin films of Bi2Te3 and Sb2Te3 were synthesized by the nanoalloying approach: Nanoscale layers of the elements Element nanoscale layers of Bi, Sb and Te are stoichiometrically deposited on a cold substrate using a MBE setup and subjected to an annealing process in which a solid state reaction yielding Bi2Te3 and Sb2Te3 takes place. Besides the two binary compounds, nanoscale multilayer (ML) stacks of 9 nm Bi2Te3/9 nm Sb2Te3 were created. The electrical transport properties of the binary compounds were determined in dependence of composition. Compound formation was directly observed in temperature-dependent in-situ XRD scans and was found to start at ∼100 °C. The stability of the Bi2Te3/Sb2Te3 ML nanostructure against temperature-driven interdiffusion during annealing was examined by SIMS and TEM for an annealing temperature of 150 and 250 °C, respectively. A comparative TEM study of the as grown and annealed state is presented.

Self-Organization of Nanoscale Multilayer Liquid Metal Films: Experiment and Theory

Surfaces made from composite nanostructured materials are potential multifunctional platforms for detection, sensing, and energy harvesting in biological and inorganic systems. However, robust and cost-effective synthesis routes are required to create the required arrays of nanostructures with tailorable size, morphology, and composition. Here we show that self-organization via spontaneous pattern formation in nanometer thick bilayer liquid films could lead to such nanostructure arrays. Experimentally, bilayers of immiscible metallic liquids show different self-organized patterning characteristics based on their order of arrangement on a substrate. Energy rate theory based on equating the rate of free energy change to viscous dissipation was used to explain this result. The different bilayer arrangements change the signs of intermolecular interactions, which changes the mode of coupled deformations and the patterning characteristics. Patterning length scale characteristics from nanosecond pulsed laser induced self-organization of Ag and Co liquids on SiO₂ substrate were in good agreement with theory.

Structure and Magnetic Properties of Gd/Ti Nanoscale Multilayers

Structure and magnetic properties of nanoscale [Gd/Ti]n multilayers prepared by rf-sputtering are studied. It is found that the decrease in the Gd layer thickness LGd leads to beginning of the structure transformation in Gd layers from the fine-crystalline to amorphous state when LGd becomes less than 2 nm. The Curie temperature TC decreases as a function of the Gd layer thickness in the same way as in early studied epitaxially grown Gd films, i.e. in case for which the finite-size effect plays most important role. A deviation of the TC(LGd) behaviour at very low LGd from the fit according to the finite-size law is probably caused by the island-like structure of the Gd layers.

Properties of TiAlCN/VCN Nanoscale Multilayer Coatings Deposited by Mixed High-Power Impulse Magnetron Sputtering (HiPIMS) and Unbalanced Magnetron Sputtering Processes—Impact of HiPIMS During Coating

Nanoscale multilayer TiAlCN/VCN coating has been deposited by pure unbalanced magnetron sputtering (UBM) and high-power impulse magnetron sputtering (HiPIMS)-UBM techniques. The V+ HiPIMS etching used in both processes has shown excellent adhesion (Lc > 50) of the coating to the substrate. The plasma compositional analysis of V+ HiPIMS etching has shown high metal-to-gas ion ratio with ionization states of V up to 5+. Moreover, during the coating of TiAlCN/VCN, the plasma analysis has confirmed the higher production rate of metal ions in the case of HiPIMS-UBM in contrast to pure UBM. This has resulted to a denser closed columnar microstructure of the coating during the HiPIMS-UBM technique than UBM. A thermogravimetric analysis has shown increased oxidation-resistance temperature for coatings deposited by HiPIMS-UBM (w780°C) with significantly lower mass gain. The scanning electron microscope and X-ray diffraction studies of the oxidized surface of the coating have revealed the formation of lubricant Magneli phase oxides of V2O5 and TiO2 at elevated temperature. The wear coefficient of the coating deposited by HiPIMS-UBM has shown two orders of magnitude lower value than that for the UBM-deposited coatings, which represents significant advantage for coatings deposited by UBM. This enhanced performance in oxidation-resistance dry sliding wear conditions can be attributed to the extremely dense structure of the HiPIMS coatings, which could be promising in elevated temperature applications.

In situ observation of rapid reactions in nanoscale Ni–Al multilayer foils using synchrotron radiation

The observation of rapid reactions in nanoscale multilayers present challenges that require sophisticated analysis methods. We present high-resolution in situ x-ray diffraction analysis of reactions in nanoscale foils of Ni0.9V0.1–Al using the Mythen II solid-state microstrip detector system at the Material Science beamline of the Swiss Light Source Synchrotron at Paul Scherrer Institute in Villigen, Switzerland. The results reveal the temperature evolution corresponding to the rapid formation of NiAl intermetallic phase, vanadium segregation and formation of stresses during cooling, determined at high temporal (0.125 ms) and angular (0.004°) resolution over a full angular range of 120°.

Theoretical explanation of Ag/Cu and Cu/Ni nanoscale multilayers softening

Relationship between metallic multilayers hardness and monolayer thickness has been investigated and explained for electroplated Ag/Cu and Cu/Ni multilayers using a modified Thomas–Fermi–Dirac electron theory. Experiments reveal that the peak hardness of Ag/Cu multilayers occurs at the monolayer thickness of about 25 nm, while the peak hardness of Cu/Ni multilayers occurs at about 50 nm. Critical monolayer thickness corresponding to the peak hardness is approximated by the grain size limit of stable dislocations in Ag crystals for the Ag/Cu multilayers and in Cu crystals for Cu/Ni multilayers. Grains size limits are calculated based on a modified Thomas–Fermi–Dirac electron theory. Developed relationship between the critical monolayer thickness and the grains size limit helps understand nanoscale metallic multilayers softening.

Residual stress characterization of Al/SiC nanoscale multilayers using X-ray synchrotron radiation

Nanolayered composites are used in a variety of applications such as wear resistant coatings, thermal barrier coatings, optical and magnetic thin films, and biological coatings. Residual stresses produced in these materials during processing play an important role in controlling their microstructure and properties. In this paper, we have studied the residual stresses in model metal-ceramic Al/SiC nanoscale multilayers produced by physical vapor deposition (magnetron sputtering). X-ray synchrotron radiation was used to measure stresses in the multilayers using the sin 2 Ψ technique. The stresses were evaluated as a function of layer thicknesses of Al and SiC and also as a function of the number of layers. The stress state of Al in the multilayer was largely compressive, compared to single layer Al stresses. This is attributed to a peening mechanism due to bombardment of the Al layers by SiC and Ar neutrals during deposition. The stress evolution was numerically modeled by a simplified peening process to qualitatively explain the Al thickness-dependent residual stresses.

Mechanism for shear banding in nanolayered composites

Recent studies have shown that two-phase nanocomposite materials with semicoherent interfaces exhibit enhanced strength, deformability, and radiation damage resistance. The remarkable behavior exhibited by these materials has been attributed to the atomistic structure of the bimetal interface that results in interfaces with low shear strength and hence, strong barriers for slip transmission due to dislocation core spreading along the weak interfaces. In this work, the low interfacial shear strength of Cu/Nb nanoscale multilayers dictates a new mechanism for shear banding and strain softening during micropillar compression. Our findings, supported by molecular dynamics simulations, provide insight on the design of nanocomposites with tailored interface structures and geometry to obtain a combination of high strength and deformability. High strength is derived from the ability of the interfaces to trap dislocations through relative ease of interfacial shear, while deformability can be maximized by controlling the effects of loading geometry on shear band formation.