Diffusion kinetics and phase formation in Ag/Al and Ru/Al multilayer thin films

Thin films demonstrate peculiar mass-transport and chemical reactions, which is a manifestation of their unique microstructure properties compared to bulk. High interfacial densities found in multilayer thin films induce various effects on the kinetics and thermodynamics which require further attention. Multilayers deposited by DC magnetron sputtering undergo calorimetric and microstructural analyses. The microstructure is extensively studied on the atomic scale to provide quantitative information on concentration gradients, grain boundary segregations and reaction mechanisms. Two multilayer thin films systems of miscible elements are chosen for the investigation, namely, Ag/Al and Ru/Al, analyzed in as-deposited and annealed conditions, respectively. The interdiffusion in the Ag/Al films is studied under different deposition conditions and period thicknesses. The magnitude of interdiffusion is found to be inversely proportional to the period thickness and reduced using low sputtering power. Layer thicknesses in Ag/Al below 6 nm have revealed the influence of the interfacial gradient energy on enhancing interdiffusion. Metastable phases were detected in Ag/Al reactions which form as transient or product phases. Asymmetric interdiffusion is detected in Ru/Al multilayers, related to non-equilibrium segregations during deposition.Dünnschichten zeigen Besonderheiten im Stofftransport und bei chemischen Reaktionen, was ein Ausdruck ihrer einzigartigen Gefügeeigenschaften im Vergleich zu Bulk-Materialien ist. Die hohe Grenzflächendichte in Multischichten hat verschiedene Auswirkungen auf Thermodynamik und Kinetik, was besondere Aufmerksamkeit verdient. Mittels DC-Magnetronsputtern abgeschiedene Multischichten werden kalorimetrisch und mikrostrukturell analysiert. Eingehende Gefügeuntersuchungen auf der atomaren Skala liefern quantitative Informationen über Konzentrationsgradienten, Korngrenzensegregation und Reaktionsmechanismen. Zwei verschiedene Multilagensysteme miteinander mischbarer Elemente, Ag/Al und Ru/Al, werden für die Untersuchungen ausgewählt und sowohl nach der Abscheidung, als auch nach Wärmebehandlung analysiert. Die Interdiffusion in Ag/Al-Schichten wird als Funktion der Abscheidebedingungen und Bilayerdicke untersucht, wobei sich ihr Ausmaß als umgekehrt proportional zur Bilayerdicke erweist und bei geringerer Sputterleistung weniger ausgeprägt ist. Einzellagendicken von weniger als 6 nm in Ag/Al zeigen den Einfluss der Grenzflächengradientenenergie auf die Interdiffusion. Die intermetallische ?-Phase nukleiert durch lokale Übersättigung in wärmebehandelten Ag/Al-Schichten. Metastabile Phasen wurden sowohl als Zwischen-, als auch als Endprodukte von Ag/Al-Reaktionen gefunden. Asymmetrische Interdiffusion als Folge von Ungleichgewichtssegregationen wurde in Ru/Al-Multilagen nachgewiesen

On the process of co-deformation and phase dissolution in a hard-soft immiscible CuCo alloy system during high-pressure torsion deformation

In this study, dual phase Cusingle bondCo composites with a total immiscibility in the solid state and a very different initial phase strength are deformed by severe plastic deformation. Nanocrystalline supersaturated solid solutions are reached in all Cusingle bondCo composites independent of the initial composition. The deformation and mechanical mixing process is studied thoroughly by combining scanning electron microscopy, transmission electron microscopy, three-dimensional atom probe tomography and nanoindentation. The indentation hardness of the Cu and Co phase and its evolution as a function of the applied strain is linked to deformation and mechanical mixing process to gain a better understanding how the phase strength mismatch of the Cu and Co phase effects the amount of co-deformation and deformation-induced mixing. Our results show that co-deformation is not a necessary requirement to achieve mechanical mixing

Structural evolution and strain induced mixing in Cu-Co composites studied by transmission electron microscopy and atom probe tomography

A Cu-Co composite material is chosen as a model system to study structural evolution and phase formations during severe plastic deformation. The evolving microstructures as a function of the applied strain were characterized at the micro-, nano-, and atomic scale-levels by combining scanning electron microscopy and transmission electron microscopy including energy-filtered transmission electron microscopy and electron energy-loss spectroscopy. The amount of intermixing between the two phases at different strains was examined at the atomic scale using atom probe tomography as complimentary method. It is shown that Co particles are dissolved in the Cu matrix during severe plastic deformation to a remarkable extent and their size, number, and volume fraction were quantitatively determined during the deformation process. From the results, it can be concluded that supersaturated solid solutions up to 26 at.% Co in a fcc Cu-26 at.% Co alloy are obtained during deformation. However, the distribution of Co was found to be inhomogeneous even at the highest degree of investigated strain

On the process of co-deformation and phase dissolution in a hard-soft immiscible Cu Co alloy system during high-pressure torsion deformation

In this study, dual phase CuCo composites with a total immiscibility in the solid state and a very different initial phase strength are deformed by severe plastic deformation. Nanocrystalline supersaturated solid solutions are reached in all CuCo composites independent of the initial composition. The deformation and mechanical mixing process is studied thoroughly by combining scanning electron microscopy, transmission electron microscopy, three-dimensional atom probe tomography and nanoindentation. The indentation hardness of the Cu and Co phase and its evolution as a function of the applied strain is linked to deformation and mechanical mixing process to gain a better understanding how the phase strength mismatch of the Cu and Co phase effects the amount of co-deformation and deformation-induced mixing. Our results show that co-deformation is not a necessary requirement to achieve mechanical mixing

Growth and thermal stability of TiN/ZrAlN: Effect of internal interfaces

Wear resistant hard films comprised of cubic transition metal nitride (c-TMN) and metastable c-AlN with coherent interfaces have a confined operating envelope governed by the limited thermal stability of metastable phases. However, equilibrium phases (c-TMN and wurtzite(w)-AlN) forming semicoherent interfaces during film growth offer higher thermal stability. We demonstrate this concept for a model multilayer system with TiN and ZrAlN layers where the latter is a nanocomposite of ZrN- and AlN- rich domains. The interfaces between the domains are tuned by changing the AlN crystal structure by varying the multilayer architecture and growth temperature. The interface energy minimization at higher growth temperature leads to formation of semicoherent interfaces between w-AlN and c-TMN during growth of 15 nm thin layers. Ab initio calculations predict higher thermodynamic stability of semicoherent interfaces between c-TMN and w-AlN than isostructural coherent interfaces between c-TMN and c-AlN. The combination of a stable interface structure and confinement of w-AlN to nm-sized domains by its low solubility in c-TMN in a multilayer, results in films with a stable hardness of 34 GPa even after annealing at 1150 °C.Peer ReviewedPostprint (author's final draft

Atom probe tomography investigation of 3D nanoscale compositional variations in CVD TiAlN nanolamella coatings

The cubic (Ti1−xAlx)Ny (TiAlN) phase with a nanolamella structure, synthesized via low pressure chemical vapour deposition (LPCVD), has been widely used in wear-resistant coatings during the latest years. The nanolamella structured TiAlN coatings contain periodic and epitaxially grown Ti-rich [denoted as Ti(Al)N] and Al-rich [denoted as Al(Ti)N] lamellae. However, the chemical compositions of these nano-structures have not been fully revealed. In this study, the microstructure of the nanolamella TiAlN coating was studied using scanning and transmission electron microscopy (SEM and TEM), and the chemical content was investigated using atom probe tomography (APT) that provides three-dimensional composition data with good accuracy and a spatial resolution down to the nanometer scale. It was found that over- and under-stoichiometries of N exist for the Ti(Al)N and the Al(Ti)N lamellae, respectively. According to the previous simulation results, such over- and under-stoichiometries are due to metal (Al and Ti) and N vacancies, respectively. The Al(Ti)N lamellae have a chemical formula of (Ti0.12Al0.88)N0.90, and have 10% N vacancies. The Ti(Al)N lamellae have a chemical formula of (Ti0.70Al0.30)0.97N, and have 3% metal (Al and Ti) vacancies. In addition to the nanolamella structure, compositional variations on a scale of a few nm were found in both types of lamellae. In the Ti-richest volumes, the composition corresponds to (Ti0.72Al0.28)0.88N so a maximum of 12% of metal vacancies exists. In the Al-richest volumes, the composition corresponds to (Ti0.07Al0.93)N0.64 so a maximum of 36% N vacancies exists. In addition, a small amount of Cl (around 0.1\ua0at.%) was found in the coating, which could originate from the incomplete dissociation of chloride precursors during the CVD surface reaction

Dynamic Impurity Redistributions in Kesterite Absorbers

Cu2ZnSn(S,Se)4 is a promising nontoxic earth-abundant solar cell absorber. To optimize the thin films for solar cell device performance, postdeposition treatments at temperatures below the crystallization temperature are normally performed, which alter the surface and bulk properties. The polycrystalline thin films contain relatively high concentrations of impurities, such as sodium, oxygen and hydrogen. During the treatments, these impurities migrate and likely agglomerate at lattice defects or interfaces. Herein, impurity redistribution after air annealing for temperatures up to 200 \ub0C and short heavy water treatments are studied. In addition, nonuniformities of the sodium distribution on a nanometer and micrometer scale are characterized by atom probe tomography and secondary ion mass spectrometry, respectively. Sodium and oxygen correlate to a greater extent after heat treatments, supporting strong binding between the two impurities. Redistributions of these impurities occur even at room temperature over longer time periods. Heavy water treatments confirm out-diffusion of sodium with more incorporation of oxygen and hydrogen. It is observed that the increased hydrogen content does not originate from the heavy water. The existence of an “ice-like” layer on top of the Cu2ZnSnS4 layer is proposed

Characterization of as-deposited cold sprayed Cr-coating on Optimized ZIRLO™ claddings

As-produced Cr-coated Optimized ZIRLO™\ua0cladding material\ua0fabricated with the cold-spray (CS)\ua0deposition process\ua0is studied. Cross-sectional\ua0electron microscopy, nano-hardness profiling,\ua0transmission electron microscopy, transmission Kikuchi diffraction, and\ua0atom probe tomography\ua0(APT) were performed to investigate the nature of the CS Cr-coating/Optimized ZIRLO™ interface, the microstructure of the coating, and the effects of the deposition on the Zr-substrate microstructure. The former surface of the Zr-substrate was found to have a highly deformed nano-crystalline microstructure, the formation of which was attributed to dynamic recrystallization occurring during\ua0coating deposition. This microstructural change, evaluated with\ua0electron backscattered\ua0diffraction and nano-hardness profiling, appeared to be confined to a depth of a few microns. Through APT analysis, a 10–20\ua0nm thick intermixed bonding region was observed at the interface between coating and substrate. The chemical composition of this region suggests that this layer originated from a highly localized shearing and heating of a thin volume of the outermost former surface of the substrate. The study of the intermixed bonding region\u27s crystalline structure was performed with\ua0high resolution transmission electron microscopy\ua0and revealed a distorted hexagonal close-packed structure

Microstructural influence of the thermal behavior of arc deposited TiAlN coatings with high aluminum content

The influence of the microstructure on the thermal behavior of cathodic arc deposited TiAlN coatings was studied as a function of isothermal annealing. Two compositionally similar but structurally different coatings were compared, a Ti0\ub734Al0\ub766N0.96 coating with a fine-grain structure consisting of a mixture of cubic (c) and hexagonal (h) phases, and a Ti0\ub740Al0\ub760N0.94 coating with a coarse-grain structure of cubic phase. By in situ wide-angle synchrotron x-ray scattering, spinodal decomposition was confirmed in both coatings. The increased amount of internal interfaces lowered the decomposition temperature by 50 \ub0C for the dual-phase coating. During the subsequent isothermal anneal at 1000 \ub0C, a transformation from c-AlN to h-AlN took place in both coatings. After 50 min of isothermal annealing, atom probe tomography detected small amounts of Al (∼2 at.%) in the c-TiN rich domains and small amounts of Ti (∼1 at.%) in the h-AlN rich domains of the coarse-grained single-phase Ti0\ub740Al0\ub760N0.94 coating. Similarly, at the same conditions, the fine-grained dual-phase Ti0\ub734Al0\ub766N0.96 coating exhibits a higher Al content (∼5 at.%) in the c-TiN rich domains and higher Ti content (∼15 at.%) in the h-AlN rich domains. The study shows that the thermal stability of TiAlN is affected by the microstructure and that it can be used to tune the reaction pathway of decomposition favorably

Effect of Ce addition on microstructure, thermal and mechanical properties of Al-Si alloys

In this study, commercial Al–12Si and Al-12Si-xCe (x = 0.5, 1, 2, 4, 8 and 12 wt% Ce) alloys were synthesized and the effect of cerium (Ce) content on the microstructure and on thermal and mechanical properties of the alloys was systematically investigated. The coefficient of thermal expansion decreased from 25.9 7 10−6 K−1 to 23.3 7 10−6 K−1 (50–493 \ub0C) with increasing amount of Ce in the alloys. XRD analyses revealed that α-Al, Si, and CuZn5 were present in all of the alloys. The addition of Ce resulted in the formation of Al9FeSi3, Al0.85CeSi1.15 and AlCeSi2. The chemical composition of the alloy and the existing phases was investigated with energy dispersive spectroscopy in a scanning electron microscope at micrometer scale and with atom probe tomography at nanometer scale in three dimensions. Ce was found to be exist within eutectic Si and Ce-rich intermetallic phases. The addition of 2 wt% Ce into the Al-12Si alloy improved the ultimate tensile strength of the alloy by 25–30%. Further Ce addition (4–12 wt%) resulted in a dramatic drop in the strength of the alloy. The low ductility of the Al-12Si alloy was remarkably improved for the alloys containing Ce up to 2 wt%