Published online by Cambridge University Press: 01 June 2006
The silver-copper oxide–based reactive air brazing technique was developed as a method of joining complex-shaped ceramic parts. To investigate the viability of this approach for high-temperature application, a series of air-brazed alumina joints were independently exposed to either oxidizing or reducing atmosphere at 800 °C for 100 h. Those samples that were thermally aged in air maintained good joint strength, similar to that of the original as-brazed samples. Microstructural analysis revealed no significant change in joint microstructure after long-term oxidation at elevated temperature, indicating excellent stability of the Ag–CuO-based filler metal in this environment. On the other hand, exposure of the air-brazed alumina joints to hydrogen under the same aging conditions resulted in a measurable decrease in joint strength. Scanning electron microscope analysis conducted on the fracture surfaces of the broken hydrogen-exposed specimens indicated that the source of joint failure was debonding along the interface between the filler metal and alumina substrate. This was due in large part to internal reduction of CuO precipitates within the filler metal to copper and accompanied by the simultaneous formation of porosity at these sites, both within the bulk of the joint as well as along the filler metal/substrate interfaces. Pore formation was noticeably present in filler metals prepared with a high concentration of copper oxide.
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