A thermodynamic approach to the brazing of silicon carbide

Through the application of solution thermodynamic principles, brazing alloys based on the Ni/Cr ratio of AWS BNi-5 (Ni-18Cr-19Si (atom%) Ni/Cr = 3.46) were designed for brazing SiC to itself and Nb.For the SiC-Nb brazing study, the thermodynamic modelling procedure determined that a series of Ni-Cr-Si-Nb alloys were suitable for forming bonds. Brazing experiments were performed using two of the "optimum" brazing alloy compositions (i.e. Ni-11.6Cr-29Si-19Nb and Ni-11.2Cr-20Si-30Nb (atom%)) and two alloys of lower Si and Nb contents in order to assess the predictions of the thermodynamic model through evaluation of the joint microstructures. All of the joints exhibited porous reaction zones, with the thickness of the reaction zone decreasing as the Si and Nb content of the brazing alloys approached the computed optima. However, due to cracking and solidification porosity in the brazing alloy, these joints were found to be unsuitable for engineering applications.It was concluded that the solution thermodynamic method developed in this thesis was very useful for selecting Ni-based (Ni/Cr = 3.46) brazing alloy compositions suitable for forming SiC-SiC-Nb bonds.The optimum alloy composition for SiC-SiC joints was computed to be Ni-14.5Cr-35Si (atom%). Brazing experiments were conducted to assess the effect of changing the Si content of the brazing alloy away from the calculated optimum on the joint microstructures and strengths. For alloys containing less than 37 atom% Si excessive reaction was observed, resulting in the formation of a porous reaction zone at the brazing alloy/SiC interface and poor joint strengths, the thickness of the porous zone decreasing and the joint strength increasing as the brazing alloy composition approached 37 atom% Si. The most favourable joint microstructures and highest joint strengths (...