Active metal brazing of Al₂O₃ to Kovar® (Fe–29Ni–17Co wt.%) using Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%)

Abstract

The application of an active braze alloy (ABA) known as Copper ABA ® (Cu-3.0Si-2.3Ti-2.0Al wt.%) to join Al 2 O 3 to Kovar ® (Fe-29Ni-17Co wt.%) has been investigated. This ABA was selected to increase the operating temperature of the joint beyond the capabilities of typically used ABAs such as Ag-Cu-Ti-based alloys. Silica present as a secondary phase in the Al 2 O 3 at a level of ~5 wt.% enabled the ceramic component to bond to the ABA chemically by forming a layer of Si 3 Ti 5 at the ABA/Al 2 O 3 interface. Appropriate brazing conditions to preserve a near-continuous Si 3 Ti 5 layer on the Al 2 O 3 and a continuous Fe 3 Si layer on the Kovar ® were found to be a brazing time of ≤15 min at 1025 °C or ≤2 min at 1050 °C. These conditions produced joints that did not break on handling and could be prepared easily for microscopy. Brazing for longer periods of time, up to 45 min, at these temperatures broke down the Si 3 Ti 5 layer on the Al 2 O 3 , while brazing at ≥1075 °C for 2-45 min broke down the Fe 3 Si layer on the Kovar ® significantly. Further complications of brazing at ≥1075 °C included leakage of the ABA out of the joint and the formation of a new brittle silicide, Ni 16 Si 7 Ti 6 , at the ABA/Al 2 O 3 interface. This investigation demonstrates that it is not straightforward to join Al 2 O 3 to Kovar ® using Copper ABA ® , partly because the ranges of suitable values for the brazing temperature and time are quite limited. Other approaches to increase the operating temperature of the joint are discussed.

Effects of Cu interlayers on the microstructure and mechanical properties of Al2O3/AgCuTi/Kovar brazed joints

“…Moreover, its coefficient of thermal expansion (CTE) is 5.5‐7.5 × 10 −6 K −1 , which is generally much lower than that of metal. Recently, Al 2 O 3 ceramic has typically been joined with metals by active brazing because of its simplicity and cost‐effectiveness . AgCuTi is usually used as a filler metal for brazing ceramics or composites with metals .…”

Recent Advances in Brazing Fillers for Joining of Dissimilar Materials

“…As reported in several studies, the formation of interfaces and IMCs is of significant interest in brazing because these compounds and their geometries are crucial for the process. Other reports on interface studies involve ceramic systems in active metal brazing, such as SiC [29][30][31], diamond [32][33][34], WC [35,36], Al 2 O 3 [37,38], and ZrO 2 [6,39]. It can be inferred from these investigations that studies related to the brazing joint interface in dissimilar systems include improvements in wetting and minimization of interfacial compounds by suitable filler metals on a case-by-case basis.…”