Active metal brazing is a relatively simple and versatile technique to join a wide range of ceramics to themselves or to metals. Braze filler alloys based on the Ag–Cu–Ti system have been used extensively to bond Al₂O₃ to itself. Although there is extensive scientific literature available on the interfacial structures between Ag–Cu–Ti-based alloys and Al₂O₃, the reaction processes responsible for the evolution of the interfacial phases, and ultimately the formation of strong bonds, have not been fully understood. Using a range of electron microscopy-based techniques it has been shown that when the Ag–Cu–Ti-based alloy melts, Ti first reacts with the Al₂O₃, dissolving significant amounts of oxygen, to form a transient reaction layer that enables the braze to wet the Al₂O₃. This reaction layer breaks down within tens of seconds to support the growth of Ti₃Cu₃O particles that nucleate behind it and are in contact with the liquid braze. Particles of γ-TiO are shown to be the last interfacial phase to form in the joint. At extended bonding times, much longer than would be used in an actual brazing operation, γ-TiO becomes the only interfacial bonding phase. The detailed experimental observations as a function of bonding temperature in this work have been used to establish a definitive bonding mechanism for the joining of high purity Al₂O₃ with Ag–Cu alloys activated by small amounts of Ti.

Figure: a-f) Schematic mechanism for the evolution of a series of metastable interfacial phases at a Ag–Cu–Ti/sapphire interface during brazing, such as Ti₂O_1–x and Ti₃Cu₃O, g-h) and their disappearance so that γ-TiO is left as the most significant interfacial phase for overlong joining times, such as 10 h.