Reaction and bonding of Hf and Zr containing alloys to alumina and silica

We have investigated the wetting and reaction behavior of two less common transition element additives to braze alloys to learn more about their reaction and bonding mechanisms. Alloys of Ag with different amounts of Hf or Zr were reacted in a controlled atmosphere furnace with sapphire, alumina of 99.6 and 96% purity, and fused silica. We determined contact angles during heating and examined cross sections after cooling using electron analytical techniques. Different interfacial microstructures were obtained when the active metal was varied in an otherwise constant system. The Hf-containing alloys reacted with polycrystalline Al2O3 and had a dispersion of Hf-containing particles near the interface. If there were a reaction layer with the Al2O3 it was below the resolution of the analysis. By contrast, Zr reaction products were more evident and appeared to have diffused away from the interface into the alloy. The microstructural observations are interpreted using available thermodynamic data and known phase relations.