Reaction layers and mechanisms for a Ti-activated braze on sapphire

A study was conducted to understand the wetting phenomena observed in brazing of a Ti-containing active filler metal on sapphire substrates. The goal of the study was to understand the interfacial reactions that permit wetting of commercial Ag-Cu-Ti active filler metal to pure alumina, despite the lower thermodynamic stability of TiO2 relative to Al2O3. Based upon transmission electron microscope, electron microprobe, and Auger analyses, it is proposed that two coupled reactions and diffusion of reactants take place. The oxides TiO, Ti2O, and Cu3Ti3O were observed at the braze/ceramic interface. It is suggested that the complex oxide Cu3Ti3O grows at its interface with TiO, and the oxide TiO is produced by reaction of Ti and sapphire and is subsequently consumed at its interface with Cu3Ti3O. It is also suggested that Ti2O forms from Ti and TiO while cooling from the brazing cycle.