Design of lattice-matched AuNi(111)/α-Al2O3(0001) interfaces and their investigation by X-ray diffraction and electron microscopy
Metal-ceramic interfaces play an important role in many applications such as catalysts, power electronics etc . Due to different thermal expansion coefficients of metals and ceramics and, in general, a high lattice mismatch applications suffer from mechanical stresses making the metal-ceramic interfaces the weak point in the device. The usage of a binary alloy system on the metal side offers the possibility to tailor the mismatch, leading to a less strained interface with improved mechanical stability.
In the present study, this concept is elaborated for the epitaxial interface between AuxNi1-x and α-Al2O3. For this Au/Ni bilayers are e-beam evaporated on (0001) oriented α-Al2O3. Rapid thermal annealing of the bilayers at 935 °C for 120 s in reducing atmosphere is used to fabricate AuxNi1-x alloy nanoparticles (NPs), which are subsequently quenched to avoid phase separation. The lattice parameter of the NPs is controlled via the Au concentration. XRD out-of-plane measurements confirmed the formation of AuNi alloy NPs and show that the lattice parameter follows the expected values for AuNi bulk alloys. Furthermore, the dependence of the texturing on the atmosphere is shown, e.g. a predominant (111)-texturing for high H2-flow rates. The combination of XRD out-of-plane and in-plane measurements reveal a pronounced AuNi (111) [1-10]||α-Al2O3 (0001) [10-10] orientation relationship (OR1). (111) AuNi XRD pole-figures reveal the existence of two twin variants within OR1. In addition, two further twin variants can be found where the NPs are rotated by ±18 degrees around the <111> surface normal, changing the orientation relation to AuNi (111) [-321]||α-Al2O3 (0001) [10-10] (OR2). For NPs with a mismatch >1% a diffuse signal can be observed in the in-plane measurements which originates from buckling of the terminating layer at the interface (delocalized coherent interface) . The vanishing of the diffuse signal for NPs with 55 at% Au indicates a change in the interface structure from a delocalized coherent interface to a semi-coherent interface. These findings are complemented by TEM investigations of plan-view prepared lamellas showing the NP-substrate interface. EBSD measurements confirmed the presence of both twin variants of OR1 within the lamella. The TEM observations strengthen the picture of a semi-coherent interface obtained by in-plane XRD measurements due to the rise of hexagonal (Twin1) and triangular (Twin2) shaped misfit dislocation networks.
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