Effects of atmospheres on bonding characteristics of silver and alumina

Joints prepared using the silver–copper oxide based reactive air brazing (RAB) technique are known to experience a significant decrease in joint strength when exposed to a high-temperature reducing environment. To investigate the effects of ambient atmosphere on the bonding characteristics of ceramic joints brazed with Ag–CuO filler metals, alumina joints prepared using a series of Ag–CuO compositions were exposed to a reducing hydrogen atmosphere and re-oxidized in air at 800 °C. As previously reported, joints exposed only to hydrogen revealed significant reduction in flexural strength and exhibited interfacial de-bonding between the filler metal and the alumina substrate. In the case of the joints brazed with a filler metal containing a high copper content, 8 mol% CuO, the formation of interfacial porosity caused by the reduction of interfacial oxide phases led to an extremely weak interface, which was not recovered after subsequent reoxidation in air at 800 °C. However, no such microstructural change or formation of interfacial porosity was observed in joints brazed with filler metals containing no or low copper content and the substrate/filler metal interface remained intact even though interfacial strength was found to be relatively weak. Subsequent reoxidation of these joints resulted in the recovery of interfacial strength and flexural strength. The results clearly indicate that interfacial adhesion in this type of braze filler metal is significantly influenced by the oxidizing/reducing characteristics of the surrounding atmosphere during high-temperature exposure. XPS analysis conducted on the in situ fractured surfaces of as-brazed and hydrogen-treated samples prepared using a filler metal with 2 mol% CuO indicates that it is the concentration of oxygen in the silver matrix which is critical to the level of bond strength between silver and alumina. This was confirmed by exposing an equivalent set of joining specimens to an inert atmosphere at high temperature, which subsequently also displayed low flexural strengths and de-bonding along the silver/alumina interface.