THERMODYNAMIC BRAZING ALLOY DESIGN FOR JOINING SILICON-CARBIDE

Using solution thermodynamic theory, a Ni-Cr-Si alloy, based on the Ni/Cr ratio of AWS BNi-5 (Ni-18Cr-19Si (atom%)), was designed for brazing SiC ceramics. The optimum thermodynamic composition was computed to be Ni-14.3Cr-36Si (atom%). Brazing experiments were conducted to assess the effect of changing the Si content away from this composition on the joint microstructures. For alloys containing less than 36 atom% Si, an excessively vigorous joining reaction occurred, resulting in the formation of a porous reaction zone at the brazing alloy/SiC interface, the amount of porosity decreasing as the brazing alloy composition approached the thermodynamically predicted optimum. It was found that the most likely mechanism for the formation of the porous zone was CO evolution during the SiC decomposition reaction. The best microstructures were attained for 40 atom% Si alloy joints, closely agreeing with the thermodynamic model, whereas higher Si content alloys exhibited localized debonding of the brazing alloy from the SiC.