|Ph.D Thesis||Department of Materials Science and Engineering|
|Supervisor:||Prof. Kaplan Wayne D.|
Intergranular amorphous films (IGF) with a thickness of ~1nm exist in many ceramic systems. The equilibrium thickness of the IGF is dominated by a balance between a London dispersion attractive force and a steric repulsive force. Recently, such films were observed at metal-ceramic interfaces (Cu-Al2O3). The formation of these films is usually a result of sintering additives or impurities. The films can have a profound effect on microstructural evolution and the final properties of the material, whether electrical, chemical, or mechanical.
In this work the nature of IGF’s at metal-ceramic interfaces was investigated via detailed microstructural characterization of model metal-Al2O3 nanocomposites, and wetting/dewetting experiments. Cu-Al2O3 and Ni-Al2O3 composites, with and without glass-forming additives, were produced and characterized. Cu/Ni-sapphire wetting experiments were performed with and without the presence of anorthite glass.
The composites doped with glass-forming additives contained amorphous pockets at the triple junctions, as well as IGF’s at the metal-Al2O3 interfaces. No amorphous phase was observed in the undoped composites. In the doped composites the metal particles were found at triple junctions, grain boundaries, and as occluded particles within the alumina grains. No occluded particles were found in the glass-free samples. High spatial resolution energy dispersive spectroscopy showed a difference in the chemical composition of the films at Ni-alumina and Cu-alumina interfaces. The results indicate a specific role of IGF’s in particle occlusion, resulting in significant changes to the nanocomposite microstructure. From the present investigations it is evident that the films are stable, since they were retained during the sintering and occlusion process. Calculation of Hamaker constants for the metal-ceramic interfaces resulted in a stronger attractive force for IGF’s at metal-alumina interfaces, compared to alumina grain boundaries, correlating to the film thickness measured in this work.