Preparation and TEM characterization of interfaces in co-sintered metal-ceramic composites

Tape casting with subsequent cosintering can be used for manufacturing metal -ceramic composites with a high efficiency for a wide range of products, e.g. for fuel cell technology or for surgical instruments. The mechanical, chemical, and long-term properties of the products are mainly determined by the interface of the components, which requires a complete understanding of its microstructure in the nanoscale. Thus the established characterization methods (optical and scanning electron microscopy) must be supplemented with imaging and analytical transmission electron microscopy (TEM). The key enabler for a successful TEM investigation is a sitespecific preparation method, which is very challenging for a combination of different classes of materials. A well established preparation method is Focused Ion Beam (FIB) technique. The in-situ lift-out technique allows to choose the region of interest with a submicrometer precision. Due to the porosity of a sintered material a stabilization by infiltration with an epoxy material and subsequent curing was necessary. The final TEM-sample covers several micrometers on both sites of the interface. For the imaging characterization of complex materials the Scanning TEM technique (STEM) leads to the best results. This way of operation enables the combination of imaging of lattice defects and chemical information, e.g. from Energy-dispersive X-ray spectroscopy (EDX) or Electron Energy Loss Spectroscopy (EELS). The EEL-spectroscopy of the near edge structure, especially of the oxygen edge enables to understand the chemical bindings of the involved elements. Structural information can be achieved by electron diffraction. A major result are differences in the microstructure of the ceramic in the vicinity of the interface. The interface is decorated by precipitations of steel alloying elements. The density of precipitations in the steel decreases rapidly in a region close to the interface. The knowledge about the phase configuration at the phase boundary is a precondition for the understanding its corrosion behaviour and the key to a long term stable design.