The fundamental chemical processes leading to the formation of braze joints between alumina of ≥95 wt.% Al2O3 and Kovar® (Fe–29Ni–17Co wt.%) using Ag–Cu–Ti-based active braze alloys (ABAs) have been clarified in this work. In addition, the effects on the resultant microstructure of altering the peak brazing temperature (Tp), time at Tp and the purity of the alumina with up to ~5 wt.% silica have been determined.

Two commercially available Ag–Cu–Ti-based ABAs containing 1.8 and 4.5 wt.% Ti commonly used for the brazing of alumina were used in this work. Initially, alumina/Ag–Cu–Ti/alumina joints were prepared to eliminate metal dissolution from the Kovar® into the ABA so that the chemical interaction between the ABA and the alumina was undisturbed and could be studied separately. Conditions simulating those of a commercial process result in a bi-layered structure at the ABA/Al2O3 interface that is mostly comprised of Ti3Cu3O, which is in contact with the ABA, alongside a thin γ-TiO layer on the Al2O3. Brazing experiments using single crystal Al2O3 revealed that the titanium in the ABA first reacts with the Al2O3 to form a transient Ti2O layer, which then enables the ABA to wet the Al2O3. The Ti2O layer breaks down quickly to support the growth of Ti3Cu3O particles that nucleate behind it and are in contact with the ABA. Particles of γ-TiO are shown to be the last interfacial phase to form in the joint by a reaction between Al2O3 and titanium which has diffused through the Ti3Cu3O.

The addition of Kovar® next to the ABA does not change the intrinsic chemical reactions occurring at the ABA/Al2O3 interface. However, the extent to which the Ti3Cu3O and γ-TiO layers grow is limited as a consequence of the reactions occurring at the ABA/Kovar® interface. Intergranular silica in the alumina is required to establish a chemical bond between the alumina and ABA using typical brazing conditions. The formation of a Fe2Ti layer on the Kovar® and its growth, along with adjacent Ni3Ti particles in the ABA, dominate the microstructural developments at the ABA/Kovar® interface. Joints with uniform microstructures containing continuous reaction layers indicative of good chemical bonding at each interface were only produced by brazing for short periods of time using a Tp very close to the ABA liquidus temperature. This was also the case for brazing with the high melting temperature ABA Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%).