Joining of partially sintered alumina to alumina, titanium, Hastealloy and C–SiC composite using Ag–Cu brazes

The joining behavior of polycrystalline alumina sintered at different temperatures (1473–1773 K) and times (0.5–4 h) to itself and to Ti, Hastealloy and a CVI C–SiC composite using two Ag–Cu active metal brazes (Cusil–ABA and Ticusil) containing Ti has been evaluated. Partial densification and the resulting high porosity content of Al2O3 substrates sintered at low temperatures led to braze infiltration of Al2O3 pores in 20 min contact but no penetration occurred in dense Al2O3 substrates sintered at high temperatures. The scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) examination of the joints showed that chemical reactions between Al2O3 and braze constituents formed a Ti-rich reaction layer at braze/Al2O3 interface regardless of the sintering conditions and degree of densification of the Al2O3. Some redistribution of the substrate and braze constituents across the joint interfaces was observed due to dissolution and interdiffusion. The Knoop microhardness of sintered alumina was consistent with the degree of Al2O3 densification achieved, and microhardness profiles across the joint regions displayed a minimum (∼70–160 KHN) in the braze region and a maximum (1900–2012 KHN) in the alumina region. The polished CVI C–SiC composites led to better quality Al2O3/C–SiC composite joints and greater segregation of Ti at the composite/braze interface than unpolished composite specimens.