Metallic Multilayer Films Research Articles

Bone Fracture Healing under the Intervention of a Stretchable Ultrasound Array.

Ultrasound treatment has been recognized as an effective and noninvasive approach to promote fracture healing. However, traditional rigid ultrasound probe is bulky, requiring cumbersome manual operations and inducing unfavorable side effects when functioning, which precludes the wide application of ultrasound in bone fracture healing. Here, we report a stretchable ultrasound array for bone fracture healing, which features high-performance 1-3 piezoelectric composites as transducers, stretchable multilayered serpentine metal films in a bridge-island pattern as electrical interconnects, soft elastomeric membranes as encapsulations, and polydimethylsiloxane (PDMS) with low curing agent ratio as adhesive layers. The resulting ultrasound array offers the benefits of large stretchability for easy skin integration and effective affecting region for simple skin alignment with good electromechanical performance. Experimental investigations of the stretchable ultrasound array on the delayed union model in femoral shafts of rats show that the callus growth is more active in the second week of treatment and the fracture site is completely osseous healed in the sixth week of treatment. Various bone quality indicators (e.g., bone modulus, bone mineral density, bone tissue/total tissue volume, and trabecular bone thickness) could be enhanced with the intervention of a stretchable ultrasound array. Histological and immunohistochemical examinations indicate that ultrasound promotes osteoblast differentiation, bone formation, and remodeling by promoting the expression of osteopontin (OPN) and runt-related transcription factor 2 (RUNX2). This work provides an effective tool for bone fracture healing in a simple and convenient manner and creates engineering opportunities for applying ultrasound in medical applications.

Exploring nanoscale metallic multilayer Ta/Cu films: Structure and some insights on deformation and strengthening mechanisms

Nanoscale metallic multilayer (NMM) films are systems offering insight into the role of interfaces in metal plasticity, deformation, and strengthening mechanisms. Magnetron sputtering was used to fabricate the Ta/Cu NMM films with a periodicity (equal Ta and Cu layer thickness) from 6 to 80 nm, and with the structure exhibiting immiscible tetragonal β-Ta and face-centred cubic Cu phases. Transmission electron microscopy and X-ray diffraction analyses revealed that, irrespective of the period, all films manifested a polycrystalline structure. The growth direction of both Cu and Ta layers was found to be along 〈001〉 β-Ta || 〈111〉 Cu directions, with the crystallite size constrained by the layer thickness. The studies showed that the Ta/Cu NMMs exhibited compressive residual macro-stress and flow strength, and enhanced elastic recovery at the periodicity of ≤12 nm. Activation volume V⁎ value of 11 to 20 b3 as determined from the indentation creep test under a constant load, may indicate a mixed deformation mechanism. This mechanism likely involves the emission of dislocations from the incoherent Ta/Cu interfaces, as well as the formation of screw dislocations within Cu grains. The high-load indentation test, TEM studies, and the rCLS model collectively demonstrate that all NMM films predominantly undergo plastic deformation. This plastic deformation primarily occurs within the soft Cu layer, while the propagation of dislocations across the incoherent interface is largely excluded.

Exotic Topological Magnetic States in Thin Co/Pd Ferromagnetic Films

AbstractThin ferromagnet/heavy metal multilayer films are considered as prospective media for a magnetic recording and Co/Pd films are a good example of such materials. In this work, the magnetic properties and micromagnetic structure of Co/Pd multilayer films are studied with different bilayer thicknesses ([Co(0.3 × t nm)/Pd(0.5 × t nm)]10), but with the same ratio Co versus Pd. Transmission electron microscope and X‐ray diffraction studies allow authors to suppose that the investigated films are highly mixed alloys. Magnetic force microscopy and Lorentz transmission electron microscopy showed the presence of various micromagnetic features in the films. Along with skyrmions that are well‐known magnetic topological artifacts some new features are revealed, which are interpreted as 360° domain walls, skyrmioniums and the combination of the above two. It is found that the type and density of micromagnetic features strongly depend on the bilayer thickness parameter (t). The effect is associated with the peculiarities of interfacial magnetic interactions in the samples with highly mixed interfaces. The tooling coefficient represents a useful tool of the electron beam evaporation technique enabling wide manipulation of micromagnetic particles, in particular, skyrmioniums that are currently considered a prospective media for current driven magnetic recording.

Interfacial random roughness effects on the magnetoresistance of Co–Cu metallic superlattices

The purpose of this study is to better understand the interfacial random geometric roughness contribution regarding the magnetoresistance (MR) effect in structure consisting “Ferromagnetic (FM) metal/Non-magnetic (NM) metal” metallic multilayer (ML) film by treating the interfaces in two different approaches. The calculation of the film in-plane conductivity is realised for two configurations: the FM layers aligned (i) parallel and (ii) antiparallel to each other. The results reveal in the first approach that the MR behaviour is strongly influenced by interfaces state by varying their chemical composition and also their magnitude of its geometric roughness. The scattering effects across non-rough interfaces on the magnetoresistive signal amplitude are observed in the second approach. The results show that a [Substrate//Co(1.1 nm)/Cu(1.1 nm)//Co(1.1 nm)//Capping layer] ML with smooth interfaces exhibit a giant magnetoresistive ratio of 97.5 % for the largest asymmetric spin-dependent scattering.

Light-induced ultrafast spin transport in multilayer metallic films originates from sp - d spin exchange coupling

Ultrafast interaction between the femtosecond laser pulse and the magnetic metal provides an efficient way to manipulate the magnetic states of matter. Numerous experimental advancements have been made on multilayer metallic films in the last two decades. However, the underlying physics remains unclear. Here, relying on an efficient ab initio spin dynamics simulation algorithm, we revealed the physics that can unify the progress in different experiments. We found that light-induced ultrafast spin transport in multilayer metallic films originates from the sp - d spin-exchange interaction, which can induce an ultrafast, large, and pure spin current from ferromagnetic metal to nonmagnetic metal without charge carrier transport. The resulting trends of spin demagnetization and spin flow are consistent with most experiments. It can explain a variety of ultrafast light-spin manipulation experiments with different systems and different pump-probe technologies, covering a wide range of work in this field.

Excellent impact resistance of multilayer metallic glass films subjected to micro-ballistic impact by overcoming dynamic size effects

Size effects are key issues that hinder the enhancement of impact resistance of films with increasing thickness. In this paper, we consider Ni2Ta amorphous metallic alloy as a prototype thin film and demonstrate that the impact resistance of metallic glass (MG) nanofilms with surface oxidation subjected to micro-ballistic impact can be increased significantly by lamination of thin monolayers, overcoming significantly the size effects in the impact resistance of MG nanofilms. Shear band formation and delamination are the dominant energy dissipation mechanisms for multilayered films under impact. Our molecular dynamics (MD) simulations confirmed that the interfaces between thin layers as modified by surface oxidation play an important role in the impact resistance of the multilayered films. Surface oxidation of multilayered films increases significantly the impact resistance due to oxidation-induced curly structure and the increase of the interfacial strength, which contributes greatly to the energy dissipation during impact. However, excessive oxidation initiates defects near the surfaces of the monolayers to therefore reduce greatly impact resistance of the multilayered films. Our work suggests an effective pathway for fabricating high-performance multilayered MG films with extraordinary impact resistance by overcoming the size effects through the lamination of monolayers.

Self-Propagating Exothermic Function in Metallic Multilayered Films and Its Reactive Bonding Applications

Self-Propagating Exothermic Function in Metallic Multilayered Films and Its Reactive Bonding Applications

Dissolution of Mg(OH)2 by swift heavy ion irradiation in CoFe2O4/MgO/ZnFe2O4 multilayer thin films

In-situ system for metallic multilayer thin films used for magnetic tunnel junction (MTJ) required a multi-target system to avoid the formation of impurities which make these devices very costly. In this work, the study was performed to resolve this issue by using a single target system for multilayer formation and impurity phases were dissolved by swift heavy ion (SHI) irradiation. A pristine MgO/Si(100) and CoFe2O4/MgO/ZnFe2O4/Si(100) (CFMZF) multilayer thin film were prepared via radio frequency (RF) sputtering technique. These structures were irradiated by 75 MeV oxygen-ion (O-ion) fluence (5x1011, 1x1012, and 5x1012 ions/cm2) and 200 MeV silver-ion (Ag-ion) fluence (1x1012 ions/cm2) for the investigation of structural changes. The high-resolution X-ray diffraction (HRXRD) reveals that MgO and CFMZF thin films have impurity peaks, dissolved via O-ion and Ag-ion irradiation. Further HRXRD analysis confirmed that Ag-ion irradiation dissolves more peaks leaving one small intensity peak of (400) corresponding to MgO. The multilayer stack of CFMZF irradiated via Ag-ion shows that Mg(OH)2 phase were completely dissolved and appears only (311) less intense peak corresponding to the ferrite structure. The findings are also corroborated by cross-sectional field emission scanning electron microscopy of CFMZF multilayer thin film showing damaged layers with Ag-ion irradiation. Therefore, the SHI irradiation technique can be used to dissolve the impurity peaks and improve the interface of the multilayer stacks.

Interface-Driven Strain in Heavy Ion-Irradiated Zr/Nb Nanoscale Metallic Multilayers: Validation of Distortion Modeling via Local Strain Mapping.

Nanolayered metallic alloys are promising materials for nuclear applications thanks to their resistance to radiation damage. Here, we investigate the effect of ion (C, Si, and Cu) irradiation at room temperature with different fluences into sputtered Zr/Nb metallic multilayer films with periods 27 nm (thin) and 96 nm (thick). After irradiation, while a high strain in the entire thin nanoscale metallic multilayer (NMM) is observed, a quite small strain in the entire thick NMM is established. This difference is further analyzed by a semianalytical model, and the reasons behind it are revealed, which are also validated by local strain mapping. Both methods show that within a thick layer, two opposite distortions occur, making the overall strain small, whereas in a thin layer, all the atomic planes are affected by the interface and are subjected to only a single type of distortion (Nb─tension and Zr─compression). In both thin and thick NMMs, with increasing damage, the strain around the interface increases, resulting in a release of the elastic energy at the interface (decrease in the lattice mismatch), and the radiation-induced transition of the Zr/Nb interfaces from incoherent to partially coherent occurs. Density functional theory simulations decipher that the inequality of point defect diffusion flux from the inner to the interface-affected region is responsible for the presence of opposite distortions within a layer. Technologically, based on this work, we estimated that Zr/Nb55 with thicknesses around Zr = 24 nm and Nb = 31 nm is the most promising multilayer system with the high radiation damage resistance and minimum swelling for nuclear applications.

Ultrathin Gold Microelectrode Array using Polyelectrolyte Multilayers for Flexible and Transparent Electro‐Optical Neural Interfaces

AbstractElectro‐optical neural interface technologies provide great potential and versatility in neuroscience research. High temporal resolution of electrical neural recording and high spatial resolution of optical neural interfacing such as calcium imaging or optogenetics complimentarily benefit the way information is accessed from neuronal networks. To develop a hybrid neural interface platform, it is necessary to build transparent, soft, flexible microelectrode arrays (MEAs) capable of measuring electrical signals without light‐induced artifacts. In this work, flexible and transparent ultrathin (<10 nm) gold MEAs are developed using a biocompatible polyelectrolyte multilayer (PEM) metallic film nucleation‐inducing seed layer. With the polymer seed layer, the thermally evaporated ultrathin gold film shows good conductivity while providing high optical transmittance and excellent mechanical flexibility. In addition, strong electrostatic interaction via the PEM alters the electrode‐electrolyte interfaces, thereby reducing the electrode impedance and baseline noise level. With a simple modification of the fabrication process of the MEA using biocompatible materials, both excellent transmittance, and electrochemical interface characteristics are achieved, which is promising for efficient electro‐optical neural interfaces.

Towards hexagonal C2v systems with anisotropic Dzyaloshinkii-Moriya interaction: Characterization of epitaxial Co(101¯0)/Pt(110) multilayer films

Ferromagnet/heavy metal (FM/HM) multilayer thin films with ${C}_{2v}$ symmetry have the potential to host antiskyrmions and other chiral spin textures via an anisotropic Dzyaloshinkii-Moriya interaction (DMI). Here, we present a candidate material system that also has a strong uniaxial magnetocrystalline anisotropy aligned in the plane of the film. This system is based on a new Co/Pt epitaxial relationship, which is the central focus of this work: hexagonal closed-packed $\mathrm{Co}(1\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}\overline{1}\phantom{\rule{0.16em}{0ex}}0)[0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}1]$ $\ensuremath{\parallel}$ face-centered cubic $\mathrm{Pt}(1\phantom{\rule{0.16em}{0ex}}1\phantom{\rule{0.16em}{0ex}}0)[0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}1]$. We characterized the crystal structure and magnetic properties of our films using x-ray diffraction techniques and magnetometry, respectively, including $q$-scans to determine stacking fault densities and their correlation with the measured magnetocrystalline anisotropy constant and thickness of Co. In future ultrathin multilayer films, we expect this epitaxial relationship to further enable an anisotropic DMI while supporting interfacial perpendicular magnetic anisotropy. The anticipated confluence of these properties, along with the tunability of multilayer films, make this material system a promising testbed for unveiling new spin configurations in FM/HM films.

DEPOSITION OF Al/SnAl OHMIC CONTACT ON SILICON FROM A LIQUID PHASE

Background. Single- and multi-layer metal films are widely utilized in modern electronics and optoelectronics as ohmic contacts. As a rule, the films are deposited by thermal evaporation, ion sputtering and chemical vapour deposition. However the methods of deposition from a liquid phase are the most simple and cost-effective. Thus the ohmic contact deposition by these methods is still an actual problem. Objective. The purpose of the paper is to study the possibility of deposition of multi-layer ohmic metal films over a semiconductor wafer surface from a liquid phase, particularly by scanning liquid phase epitaxy technique. Methods. In this work we considered the influence of a long-term temperature gradient at the interface metallic solution-melt – semiconductor wafer on the possibility of deposition of multi-layer ohmic metal films on the semiconductor wafer surface during segmental contact between the solution-melt and the wafer. For this purpose we carried out the simulation of heat transport process, wafer wetting process as well as the process of wafer cleansing off the solution-melt taking into account capillary phenomena in the mask openings using the method of scanning liquid phase epitaxy. For experimental confirmation of adequacy of the model proposed we carried out the deposition of Al/SnAl layer on silicon wafer in the above mentioned conditions. Results. We have deposited the contact layer Al/SnAl on the surface of silicon wafer from Al-Sn solution-melt by scanning liquid phase epitaxy technique using supplementary heater for the wafer and mask installed in the apparatus. The contact layer is made as three identical pads located at different distance one from each other. By the analysis of current-voltage characteristic we determined that the metallic film contact with the semiconductor is a non-rectifying, i.e. ohmic contact. The specific contact resistance was determined by the Transmission Line Method using linear configuration of the contact pads (LTLM). Its value was 7.2∙10-4 Ohm·cm2. Conclusions. The principal possibility of obtaining of multi-layer ohmic contacts to the semiconductor by scanning liquid phase epitaxy technique in conditions of segmental contact between the solution-melt and the wafer as well as long-term gradient at the contact interface was shown. The conditions were realized by using extra heating of the wafer back side and the high-temperature mask through which the solution-melt contacted the wafer.

Improvement of flow strength and scratch resistance of Ti/Cu nanocrystalline metal multilayer thin films by tailoring layer thickness and modulation ratio

In this work, nanocrystalline Ti/Cu multilayer thin films were fabricated using direct current magnetron sputtering with different layer thicknesses, namely 250, 100, 50, 25 and 5 nm. Additionally, a group of the film specimens were prepared with various modulation ratios where each bilayer was set to 100 nm. The size-dependent deformation behavior of the films was studied using the nano-indentation and nano-scratching techniques. The results showed that the flow strength of the films tended to increase as the layer thickness decreased. Moreover, the strength and scratch resistance of the films can be further enhanced by tailoring the thicknesses of two adjacent layers, while keeping the thickness of each bilayer unchanged. Finally, the size-dependent strengthening mechanism was discussed by recourse to microscopic examination.

Molding Wetting by Laser-Induced Nanostructures

The influence of material characteristics—i.e., type or surface texture—to wetting properties is nowadays increased by the implementation of ultrafast lasers for nanostructuring. In this account, we exposed multilayer thin metal film samples of different materials to a femtosecond laser beam at a 1030 nm wavelength. The interaction generated high-quality laser-induced periodic surface structures (LIPSS) of spatial periods between 740 and 790 nm and with maximal average corrugation height below 100 nm. The contact angle (CA) values of the water droplets on the surface were estimated and the values between unmodified and modified samples were compared. Even though the laser interaction changed both the surface morphology and the chemical composition, the wetting properties were predominantly influenced by the small change in morphology causing the increase in the contact angle of ~80%, which could not be explained classically. The influence of both surface corrugation and chemical composition to the wetting properties has been thoroughly investigated, discussed and explained. The presented results clearly confirm that femtosecond patterning can be used to mold wetting properties.

Inducing LIPSS on multilayer thin metal films by femtosecond laser beam of different orientations

The occurrence of laser-induced periodic surface structures (LIPSS) has been known for a while. Multilayer thin films, like Al/Ti, are suitable for LIPSS formation and attractive for applications—due to their wearing behavior and corrosion resistance; LIPSS generation may improve their properties as well. LIPSS properties depend not only on the material but also on the beam characteristics, like wavelength, polarization and scanning directions, etc. After exposing with NIR femtosecond pulses from Coherent Mira 900 laser system in several beam exposures, we have analyzed the samples of thin metal film systems with Tescan Mira3 SEM and NTegra AFM. The formation of LIPSS is most probably due to the generation of surface plasmon polariton, through the periodic distribution of energy in the interaction zone which lead to thermal processes in layers and interfaces. Two types of LIPSS were generated, which differ in shape, orientation and in ablation pronounced or not. For consecutive interactions in the same direction, LIPSS maintained its orientation, while for orthogonal passes LIPSS with mutually orthogonal orientation were generated. LIPSS period fluctuated between 320 and 380 nm and structures with pronounced ablation have significantly smaller width. Probable mechanism is that for greater accumulated energy pronounced ablation takes place giving LIPSS in the form of trenches or grooves, while for less accumulated energy the buildup of the material—probably due to pronounced oxidation—lead to LIPSS in the form of hills or ridges.

Modeling of Catalytic Centers Formation Processes during Annealing of Multilayer Nanosized Metal Films for Carbon Nanotubes Growth.

The paper presents a theoretical model of the catalytic centers formation processes during annealing of multilayer nanosized metal films for carbon nanotubes growth. The approach to the description of the model is based on the mass transfer processes under the influence of mechanical thermoelastic stresses, which arise due to the difference in the thermal expansion coefficients of the substrate materials and nanosized metal layers. The thermal stress gradient resulting from annealing creates a drop in the chemical potential over the thickness of the film structure. This leads to the initiation of diffusion mass transfer between the inner and outer surfaces of the films. As a result, the outer surface begins to corrugate and fragment, creating separate islands, which serve as the basis for the catalytic centers formation. Experimental research on the formation of catalytic centers in the structure of Ni/Cr/Si was carried out. It is demonstrated that the proposed model allows to predict the geometric dimensions of the catalytic centers before growing carbon nanotubes. The results can be used to create micro- and nanoelectronics devices based on carbon nanotube arrays.

Rapid synthesis of metallic and alloy micro/nanoparticles by laser ablation towards water

A versatile method named laser ablation towards water (LATW) was developed to rapidly and simply synthesize metallic and alloy micro/nano particles, even high-entropy-alloy nanoparticles (NPs). A nanosecond pulsed laser beam passed through a transparent glass substrate, ablated its coated metallic film in the air or contacting with water, and synthesized a colloidal solution contained the micro/nano particles. In this study, Cu microparticles (MPs, Daverage = 1.8 μm), Cu MPs with Cu2O NPs, Cu2O NPs and Au NPs (Daverage = 9.5 nm) were successfully synthesized by the laser ablation of 10 μm Cu film in the air, 10 μm Cu film in the water, 200 nm Cu film in the water, 200 nm Au film in the water, respectively. The content of bimetallic NPs can be adjusted by the relative thickness ratio of bilayer film using laser ablation. Moreover, the ternary (Au-Ag-Cu), quaternary (Au-Ag-Cu-Pt), and quinary (Au-Ag-Cu-Pt-Ni) alloy NPs were also fabricated by the laser ablation of multilayer metallic film in the water. Therefore, LATW is a flexible technique for the high-effective synthesis of MPs, NPs, and various alloy NPs.

Discrete dislocation dynamics algorithms and their application in modeling of plastic behaviors of crystalline materials

The plastic behaviors of crystalline materials are induced by the evolution of a series of dislocations. Discrete dislocation dynamics (DDD) can be used to model plastic deformation by taking into account the nucleation, annihilation, motion of dislocations, and formation of jogs, junctions, networks, and other dislocation structures directly. As a result, it can be used to investigate the physical correlation between material microstructures, dislocation structures, and plastic responses. It can also model the intrinsic size effect of plastic behaviors at the micron/submicron scales. Since its modeling scale is much larger than that of the microscopic molecular dynamics but is smaller than that of the macroscopic finite element method (FEM), DDD plays a connecting link role in multiscale modeling. In the present manuscript, three categories of DDD schemes, i.e., the DDD-FEM superposition algorithm, the DDD-FEM direct coupling algorithm, and DDD-extended FEM (XFEM) coupling algorithm, are further developed and optimized. In the DDD-FEM superposition algorithm, the consideration of dislocation climb is specifically included to take into account the high-temperature effect. In the DDD-FEM direct coupling algorithm, the so-called virtual dislocations are introduced to distribute the Eigen plastic strain by dislocation evolution to the FEM module more appropriately, and a special scheme is developed to calculate the Peach-Koehler force on the dislocation efficiently. To model the problems with strong or weak interfaces (such as cracks, voids, particles, and grain boundaries), the scheme coupling the DDD and XFEM, which is not sensitive to the finite element mesh, is further developed carefully. Each of these three DDD schemes has its own special characteristics, and so they are suitable for solving different plastic problems. Based on these three kinds of DDD algorithms, the plastic deformation mechanisms of nickel-based superalloy, the damage and fracture behaviors of crystalline materials, and the size effect and microstructure dependency of heterogeneous crystalline materials have been investigated systematically. For the nickel-based superalloy, the dislocation mechanisms for the hardening responses, the abnormal yielding behavior with increasing temperature, the loading rate effect, the lattice misfit effect, and the crystalline orientation effect and the dislocation climb effect have been studied in detail. Based on these dislocation mechanisms and the special two-phase material microstructures, a crystal plasticity constitutive law informed by dislocation and microstructure mechanisms suitable for single crystal nickel-based superalloy was here developed. Further, the microvoid growth mechanism, the crack tip-microvoid interaction, the crack tip ratchetting responses, the crack shielding effect by dislocation, and the effect of crack-tip dislocation emission on crack growth have also been investigated systematically through DDD simulations. In addition, the plastic behavior, its size effect, and intrinsic dislocation mechanisms for materials with heterogeneous structures, such as particle-enhanced metal matrix composites, multilayered metallic films, and polycrystalline materials, have also been studied carefully using DDD models, especially at elevated temperature. This type of DDD research has facilitated better understanding of the intrinsic mechanisms of hardening response, cyclic plasticity, fracture, damage evolution, size effect, and microstructure dependency. DDD can be further used to study the plastic behaviors of crystals under the extreme environmental conditions, such as high temperature, high pressure, high loading rate (shocking), chemical erosion environment, and high neutron irradiation. DDD has recently become a powerful model of the behavior of metallic materials at microscale in a more physical manner than existing plasticity models.

High-resolution transmission electron microscopy investigation of diffusion in metallic glass multilayer films

Lack of plasticity is one of the main disadvantages of metallic glasses. One of the solutions to this problem can be composite materials. Diffusion bonding is promising for composite fabrication. In the present work the diffusion process in glassy multilayer films was investigated. A combination of advanced transmission electron microscopy (TEM) methods and precision sputtering techniques allows visualization and study of diffusion in amorphous metallic layers with high resolution. Multilayered films were obtained by radio frequency sputter deposition of Zr-Cu and Zr-Pd. The multilayers were annealed under a high vacuum (10−5 Pa) for 1 and 5 h at 400 °C, that is, well below the crystallization temperatures but very close to the glass-transition temperatures of both types of the glassy layer. The structural evolution in the deposited films was investigated by high-resolution transmission electron microscopy. It was observed that, despite the big differences in the atomic mass and size, Pd and Cu have similar diffusion coefficients. Surprisingly, 1 h of annealing results in formation of metastable copper nanocrystals in the Zr-Cu layers which, however, disappear after 5 h of annealing. This effect may be connected with nanovoid formation under a complex stress state evolving upon annealing, and is related to the exceptionally slow relaxation of the glassy layers sealed with a Ta overlayer.

Investigation into nanoscratching mechanical performance of metallic glass multilayers with improved nano-tribological properties

Friction and wear are the two most important properties that determine the long-term performances of metallic glasses as much as strength and ductility. With this in mind, a multilayered strategy was proposed to improve the nano-scratching properties of metallic glass thin films and the well-characterized Zr-based metallic glasses were chosen to be the representative model materials in current work. It was found that the metallic glass could exhibit improved wear resistance and reduced friction coefficient if it is made in the form of multilayers. The complexity in the temporal scale of the lateral force signal is investigated, elucidating the inhomogeneous (and homogeneous) shear-banding processes during the nanoscratching process. The statistical and dynamic analyses of the stick-slip behaviors showed a transition from a chaotic to a self-organized critical state in the multilayers, indicating that the hetero-interfaces promote extensive shear band-interface interactions and induce the scratch hardening effect, both of which are responsible to the excellent tribological properties. The current work not only highlights the significant roles of hetero-interfaces in improving the nano-tribological properties but also make the multilayered metallic glass thin films a promising candidate towards structural and functional applications for metallic glasses.