Microstructural and mechanical characterization of actively brazed alumina tensile specimens

Alumina (94 and 99.8% grade compositions) was brazed directly to itself with gold-based active brazing alloys (ABAs) containing nominally 1,2 or 3 wt-% vanadium. The effects of brazing conditions on the joint properties were investigated. Wetting behavior, interfacial reactions, microstructure, hermeticity and tensile strength were determined. Wetting was generally fair to good for the ABA and base-material combinations. Microanalysis identified a discontinuous reaction product at the alumina-braze interface as an Al-V-O spinel-type phase. Tensile strength of the 94% alumina specimens was generally not sensitive to the vanadium concentration and consistent within an 85-105 MPa range. The tensile data for the 99.8% alumina specimens, however, had significantly greater variability, with values ranging from 25 to 95 MPa. The highest value was obtained for the samples brazed with the filler metal containing 3 wt-% vanadium. Fractures occurred either along the braze-alumina interface for the 99.8% alumina specimens or through the alumina piece with the 94% alumina samples.