Abstract text (incl. figure legends and references)

Metal/ceramic interfaces are of great scientific and technological importance, with applications in electroceramic devices, structural composites and catalysis [1]. Due to different thermal expansion coefficients of metals and ceramics and, in many cases, 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 Au_xNi_1-x and α-Al₂O₃. For this Au/Ni bilayers are e-beam evaporated on (0001) oriented α-Al₂O₃. Rapid thermal annealing of the bilayers above the miscibility gap at 890°C for 120s in reducing atmosphere is used to fabricate AuxNi1-x alloy nanoparticles (NPs), which are subsequently quenched into a metastable supersaturated solid solution. Here, the Au concentration (c_Au) is utilized to tune the {220} d-spacing of the alloyed NPs towards lattice matching with the {30-30} d-spacing of α-Al₂O₃. The influence of c_Au on the structure of the NP and the interface as well as the interplay of solute atoms and the substrate is monitored by complementary X-ray and electron techniques.

XRD out-of-plane scans (Fig.1A) confirmed the formation of AuNi alloy NPs. Furthermore, an influence of the lattice mismatch (e.g. at%Au) on the texturing of the NP can be seen. While all NPs show the typical <111>-OOP texture, interestingly this texture is partially lost for samples close to lattice matching (0.3%≥ δ≤ -1%). In XRD in-plane measurements (Fig. 1B), the convergence of the NP lattice parameter towards lattice matching as well as the evolution of the diffuse background can be traced. For 59at% Au the in-plane NP and substrate peak match and the diffuse background, which stems from disorder at the interface, vanishes, both revealing the (semi-)coherent nature of the interface in contrast to an incoherent or delocalized coherent [2] interface for larger lattice mismatch. Furthermore, EBSD ODFs revealed i) a pronounced AuNi (111)[1-10] || α-Al₂O₃ (0001)[10-10] orientation relationship (OR1) for NP with δ>0.3% and ii) a change in OR to AuNi (111)[1-10] || α-Al₂O₃ (0001)[11-20] (OR2) with δ<-1%.

To get a deeper understanding of the interface and the interplay of solute atoms and the substrate, cross-sectional EM-investigations on a <110> oriented lamella were carried out on a Ni-rich particle (Fig.2). The ADF image reveals a modulated contrast in the range of two interfacial layers, correlating perfectly with the disordered interface seen in IP XRD. Additionally HR HAADF imaging confirmed the semi-coherent nature of the interface with the appearance of stand-off misfit dislocations (DL). Via EDX we revealed the segregation of Au on the interface, which is driven by the minimization of strain energy by i) reducing the misfit and ii) shifting the DL to higher stand-off positions [2].

Overall, we demonstrate an elegant route to produce lattice matched alloy NPs on a ceramic substrate and provide insight into the structure and chemistry of the system by complementary scattering probes. In our contribution, the origin of the findings reported above will be discussed in light of existing literature and basic physical principles.

[1] Sinnott et al., 10.1016/j.mser.2003.09.001

[2] Herre et al., 10.1016/j.actamat.2021.117318