1995

Heikinheimo, Liisa

The combination of the materials alumina and titanium was selected for the purpose of studying the effect of the metal-ceramic interface structure on the mechanical properties of the joints. The behaviour and structure of Al2O3 - Ti interfaces are important both for direct diffusion bonding and reactive brazing. The good reactivity of titanium with oxide ceramics is well known and there are several industrial applications in which this material couple can be readily used. The results show that various types of interfaces can be produced in Al2O3 to metal systems where the activity of Ti is nearly one. In a liquid state, using a Ag-Ti alloy or Ti-Ag-Ti multilayers, the phase next to the alumina is a-Ti(O). In the solid state, alumina is in contact with TiAlx intermetallics, either TiAl in infinite and finite couples or Ti3Al in equilibrated couples. The addition of Nb to Ti, 10 and 20 at. % leads to the formation of TiAlx(Nb) at the alumina interface. The kinetics of the reaction layer growth and furthermore the structural changes in the interface can be calculated by the experimentally derived equations for the infinite and finite diffusion couples. The shear mode testing which was selected has proved to be applicable for optimizing the bonding process and material selection. The fracture mechanics testing was introduced to measure the effect of the interface structure on the properties of the joints. The interface fracture energies for diffusion-bonded Al2O3 - Ti joints were calculated on the basis of Griffith's energy balance. From the various energy terms determined, values for both interface types, TiAl-alumina and Ti3Al-alumina, could be determined. The interface fracture energy of brazed joints is found to be almost a half less than the one for finite diffusion-bonded joints. The measured energy, as calculated directly from Kc, depends on the metal foil thickness and the bonding time. Taking various energy terms into account, the interface fracture energy is obtained. This is, unlike the measured energy in the testing, a constant that is independent of the metal/brazing foil thickness or the structural kinetic parameter dx:de.