The issue of metal-ceramic interfaces is being explored through three interconnected topics. In the first, wetting of ceramics by metals (both liquid and solid) are explored. This is conducted using the sessile drop technique for liquids on solids, and Winterbottom analysis of “dewetted” and equilibrated films to determine the energy of solid-solid interfaces. The second topic related to metal-ceramic interfaces deals with processing of ceramic matrix composites and ceramic-metal and ceramic-ceramic joins, where data from wetting experiments is used as the basis for process design. Finally, in-depth electron microscopy of the interfaces formed from the wetting and joining processes is conducted. Major issues include the formation of adsorbate films at metal-ceramic interfaces. Emphasis is placed on analysis of the atomistic structure and chemistry of the same interfaces for which the interface energy is determined. A description of the major methodologies and systems under study is given below.
Sessile drop experiments are conducted in a unique UHV Wetting Furnace. This unique system is dedicated to in-situ investigations of sessile drops at very high temperatures (up to 1800°C) under very controlled gas partial pressures. The system is based on a UHV chamber and includes a water cooled tungsten heating system, an ion-sputtering system for cleaning the surfaces of samples in-situ, a residual gas analyzer for monitoring gas partial pressures, and mass-flow controllers and various vacuum and pressure gauges for controlling the total pressure and partial pressure within the system. Kaplan’s lab includes two additional secondary sessile drop systems for working with metals with high vapor pressures.
Data from the sessile drop experiments are analyzed using image analysis software developed by Kaplan’s group, for the automatic measurement of contact angles, and to extract liquid surface energies from the drop shape. The focus is to analyze the thermodynamic work of adhesion and interface energies of metal-ceramic interfaces, as a function of temperature, chemistry and gas partial pressure.
In addition to liquid-solid wetting, activities have been extended to solid-solid interface energies. These studies are based on Winterbottom analysis, using cross-section TEM of site-specific samples prepared using FIB. Again, the goal of these studies is to obtain a correlation of solid interface energy to processing temperature, gas partial pressure, chemistry, and crystallographic orientation. Systems under study currently include Ni, Pt and Au (both pure and doped) in contact with ZrO2, Si, SiO2, TiO2, SrTiO3, and of course Al2O3.
Composite and Joint Processing
Data from the wetting experiments are important in their own right, but are also used for designing processing schemes for metal-ceramic joints and composites, which is the second topic related to metal-ceramic interfaces which is studied in Kaplan’s group. Kaplan’s group has developed a unique method for producing nanocomposites.
The process is based on the infiltration of metallic salts into a fired (green) ceramic preform. During sintering, the gas phase in the sintering furnace is controlled to promote reduction of the salts, to form nano-sized metallic particles within the open pores of the ceramic body, which is then sintered to full density.
Current Research Projects
Ni-YSZ interfaces and in-situ precipitation studies of Ni in YSZ;
High temperature solubility limits of key dopants and impurities in ceramics;
Grain boundary migration as a function of dopant adsorption in Al2O3;
The influence of external fields on grain boundary mobility;
The correlation between atomistic structure/chemistry and interface thermodynamics.