Plasticity Considerations in Probabilistic Ceramic-to-Metal Joint Design

Introduction Ceramic materials are being developed for use in advanced heat engine applications. A key issue in their design and manufacture is joining the ceramic rotor to a metal shaft to transmit power. Design concepts for ceramic-to-metal joints were described in an earlier paper . The goals of this work were to develop new methods for the design and analysis of ceramic-to-metal joints, to predict performance of the joint, and to construct and test ceramic-tometal joints that could support a 20.9 N-m (50 MPa) torque load at 650°C and 950°C with a braze area of 2.0 cm 2 . Some expectation of the strength of the ceramic joint was necessary so that different joint geometries and materials could be explored without the difficulty of making each different design. The difficulty with realizing a predictive design tool for these joints was the probabilistic nature of the ceramic material properties, and the interaction between the metal, ceramic, and braze materials. The heat engine applications of the joints considered here are primarily loaded with high-temperature torsional stresses. Therefore, the test joints were evaluated in torsion, torsional fatigue, and thermal fatigue tests. At the completion of the work, it was desired to not only have a prototype joint design manufactured, but also to confirm the analytical models derived for joint design by comparison with life-tests of the final joint prototypes

THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS PERFORMANCE TESTING AND STRENGTH PREDICTION OF CERAMIC-TO-METAL JOINTS

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 joints 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 are 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