|Ph.D Student||Meltzman Hila|
|Subject||Correlation of Structure and Energy at Ni-A12O3|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Wayne D. Kaplan|
|Full Thesis text|
The structure and properties of metal-ceramic interfaces have been the focus of many studies since the 1960’s, mostly due to the potential for numerous technological applications. The complexity of solid-solid interfaces arises from the wide variety of defects that can form, and the unique structural and compositional characteristics of each interface. The correlation between structural and thermodynamic interfacial properties is extremely important for the basic understanding of interface stability, however, to date, understanding of this correlation is far from being complete, mainly due to experimental limitations which include specimen equilibration and transmission electron microscopy specimen preparation.
Ni-Al2O3 interfaces are widely used in commercial turbine blades of jet engines. Given the importance of this system, the goal of the present work is to overcome most of the experimental limitations and to determine the structure and energy of Ni equilibrated in contact with sapphire. The effect of Hf segregation was also investigated, since Hf segregation was predicted to stabilize the interface.
Samples were based on thin Ni films which were dewetted in the solid-state to reach equilibrium. Characterization of the interface atomistic structure was mostly done by advanced techniques for quantification of transmission electron microscopy data.
For the first time the nickel equilibrium crystal shape was determined and correlated to the chemical composition of the surface. Fe contamination decreased the anisotropy dramatically, de-stabilizing high-index planes.
The solid-solid interface energy was measured and aberration corrected high-resolution transmission electron microscopy was used to identify the atomistic structure of the same interfaces. It was found that the interface between Ni and (0001) sapphire undergoes reconstruction, resulting in a unique interfacial unit cell and the creation of shorter and stronger Ni-O bonds. The interface energy was determined to be 2.16 J/m2. It is believed that the reconstructed interface accommodates the misfit which results from very different structures at the interface, and this is predicted to be a viable mechanism to reach a low energy interface between very dissimilar materials. To the best of the authors’ knowledge, this is the first report of reconstruction at metal-ceramic interfaces.
In contrast to the existing literature, Hf was not found to segregate to Ni-alumina interfaces. Furthermore, under the current P(O2) level the Hf oxidized and formed small HfO2 particles from which the Ni films dewetted. This is explained by the measured difference in interface energy between Ni-alumina versus Ni-HfO2 (2.7±0.4 J/m2).