LAUSR

Abstract A systematic investigation of the brazing of Al2O3 to Kovar® (Fe–29Ni–17Co wt.%) using the active braze alloy (ABA) Ag–35.25Cu–1.75Ti wt.% has been undertaken to study the chemical reactions at the interfaces of the joints. The extent to which silica-based secondary phases in the Al2O3 participate in the reactions at the ABA/Al2O3 interface has been clarified. Another aspect of this work has been to determine the influence of various brazing parameters, such as the peak temperature, Tp, and time at Tp, τ, on the resultant microstructure. As a consequence, the microstructural evolution of the joints as a function of Tp and τ is discussed in some detail. 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. The presence of Kovar® next to the ABA does not change the intrinsic chemical reactions occurring at the ABA/Al2O3 interface. However, the extent of these reactions is limited if the purity of the Al2O3 is high, and so it is necessary to have some silica-rich secondary phase in the Al2O3 to facilitate the formation of a Ti3Cu3O layer on the Al2O3. Breakdown of the Ti3Cu3O layer, together with fracture of the Fe2Ti layer and separation of this layer from the Kovar®, has been avoided by brazing at temperatures close to the liquidus temperature of the ABA for short periods of time, e.g., for Tp between 820 and 830 °C and τ between 2 and 8 min.