Abstract
Brazed joints were made between silicon nitride and Ni-based and Fe-based super alloys. Room temperature shear (torsion) strengths ranged from 75-242 MPa for Si3N4-to-Incoloy 909 joints and from 30-127 MPa for the Si3N4-to-Inconel 718 joints. At 500°C the joint strength was 120 MPa while at 650°C and 950°C the joint strengths were less than 20 MPa. These low strengths at 650°C and 950°C were attributed to a reduction in the shrink-fit and to low braze strength at these high temperatures. Finite element analysis (FEA) and a probabilistic failure theory (CARES) were used to predict the joint strengths. The predicted joint strengths agreed well with measured joint strengths in torsional loading at 20°C. Torsion tests were also performed at 650°C. Aspects of the material systems, residual stresses, mechanical behavior, and strength predictions and presented. Two new braze alloys based on the Au-Ni-Cr-Fe system were used to overcome the poor high-temperature strength. Joints made with these brazes had good strength (85 MPa and 35 N-m) at 650°C.
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