Small scale model systems for in situ TEM studies: multilayered and alloyed nanowires

Abstract text (incl. figure legends and references)

To understand the behavior of complex alloys and composites under mechanical and thermal loading small scale model systems are needed. Only such model systems provide the opportunity to analyze mechanisms of plastic deformation, interdiffusion and phase transformation down to the atomic scale. Tailored nanowires are ideal in this role as they can be directly studied with in situ TEM to elucidate the effect of compositional complexity, multilayered coatings and local ordering.

Multilayered coatings prevent the underlying core material from degradation and enhance the mechanical and wear-resistant behavior. Such a coating consists for example of alternating nm-sized layers of metals and metal-oxides. The coating life-time crucially depends on crack initialization and propagation at the interfaces. To further optimize the performance and to prevent early failure like layer delamination, the underlying failure mechanisms on the atomic scale have to be studied. Small scale model systems of such coatings allows characterizing application-limiting factors inside the Transmission Electron Microscope (TEM) with a view to optimize the mechanical stability of the bulk counterpart.

In this work, we use a combinatorial process based on advanced Physical Vapor Deposition (PVD) and Atomic Layer Deposition (ALD) to create complex one-dimensional nanowires (NW). These wires are ideal model-systems that enable studying the performance of different coatings. In order to analyze the effect of multilayers on the mechanical behaviour, we compare conventional coatings on metals (Al2O3-Cu) (Figure 1a) with nanolaminates consisting of alternating layers of Al2O3-ZnO (Figure 1b). Defect nucleation at the metal-metal oxide interface as well as crack propagation are studied via in situ mechanical testing inside the TEM. Figure 1c shows a cracked multi-shell NW after mechanical loading.

Figure 1: Core-shell NWs as small scale model system for bulk-coatings. Exemplary STEM images. a) Al2O3-Cu core-shell NW. The Cu NW has been subsequently coated with a conformal ALD layer. b) Nanolaminate on Cu NW. The Energy-dispersive X-ray spectroscopy (EDX) map shows the layered Al2O3-ZnO structure. c) Nanolaminate after mechanical loading.

Next to metal-metal oxide structures, intermetallic (Cu-Au) nanostructures are used to study the effect of compositional complexity on the material properties as well as diffusion processes at interfaces. By using co-sputtering at elevated temperatures alloyed NWs (Cu3Au/Au3Cu) are directly achieved. Figure 2 a shows an exemplary intermetallic Cu3Au NW with corresponding EDX mapping. The line scan across the NW is shown in figure 2c. The High-Resolution Scanning Transmission Electron Microscopy (HR-STEM) image in figure 2c emphasizes the ordering and figure 2d shows the corresponding diffraction pattern of the NW. In comparison with solid-solutions (of the same composition), such intermetallic model systems are used to characterize the effect of atomic ordering on the mechanical behavior, which is of great interest for the development of complex alloys. Beside the direct creation of the ordered phase via co-sputtering, intermetallic NWs can also be produced by heating core-shell NWs which consist of a copper core covered by a gold layer. These bimetallic nanostructures are ideal systems to study diffusion phenomena and ordering in situ in TEM. Figure 2e shows an EDX mapping of an initial Cu-Au NW before and after heating and figure 2f the corresponding line scans for different time intervals (initial, 50s to 2327s). Based on the Wagner equation, we calculated the interdiffusion coefficients of Cu (8.75· 10-18 m2/s) and Au (3.27·10-18 m2/s). To achieve the intermetallic phase and to observe the ordering, the NWs are annealed at 300°C (see diffraction pattern in figure 2g).

Figure 2: Intermetallic NWs as small scale model system for complex alloys. a) Exemplary Cu3Au NW directly obtained via co-sputtering. STEM image with EDX mapping. The corresponding line profile is shown in figure 2b. c) HR-STEM image with Fast Fourier Transform (FFT) and d) diffraction pattern of the NW. Cu-Au core-shell NWs as small scale model system to analyze diffusion processes. e) EDX mapping of Cu-Au NW before and after heating at 400°C. g) Diffraction pattern before and after annealing at 300°C. Reversible formation of an ordered intermetallic phase.