Improved mechanical properties of SiC/SiC brazed joints via added Al in Si-Mg composite fillers

SiC components were joined under vacuum at 1100 degrees C for 10 min via Si-Mg composite fillers with added Al (XAl) ranging over 0-10 at.%. The joining mechanism involves Mg addition to Si, which lowers the melting point of Si. The transformation of the molten liquid phase into a solid Si layer is induced by Mg evaporation. In SiC/SiC joining, however, Mg evaporation was significantly suppressed when XAl = 0 at.%, with a large amount of Mg remaining in the joint, forming a brittle bonding layer of Si, Mg2Si, and MgO. Notably, Mg evaporation was enhanced by the addition of Al to the filler, resulting in the formation of a dense Si layer. The joint tensile strength increased with increasing XAl in the range of 0-6 at.% and then decreased with further increases in XAl. The changes in strength were correlated with microstructural changes in the bonding layer. From the microstructure and the phase diagrams, it was concluded that the added Al increased the thermodynamic degrees of freedom, which hindered the rate of Mg-evaporation-induced Si solidification from the liquid during joining. Because the amount of liquid surrounded and trapped by the solid Si decreased, more Mg in the liquid could diffuse to the surface.