Flexural Strength and Shear Strength of Silicon Carbide to Silicon Carbide Joints Fabricated by a Molybdenum Diffusion Bonding Technique

Abstract

The capability to form robust SiC‐to‐SiC joints is needed to enable the fabrication of complex SiC‐based structures. In this work, molybdenum foils are used to develop a diffusion bond with SiC during a vacuum heat treatment at 1500°C for 10 h. The bond consists of a central region of Mo2C with Mo5Si3+carbon and/or Mo5Si3C phases at the SiC interface. Flexure strength determined using a standard 4‐point flexural test method (ASTM C1161) over the temperature range from room temperature to 1100°C showed that the molybdenum foil joined SiC failed at a somewhat lower value (263–50 MPa) than similarly tested monolithic chemically vapor deposited (CVD) SiC without any bond (443–197 MPa). Differences in elastic properties and coefficient of thermal expansion between SiC and the phases produced in the molybdenum foil joint region likely result in the formation of slightly larger flaws in the SiC near the joint, which are fracture initiation sites that result in lower flexural strength values for molybdenum‐joined SiC. Shear strength results obtained using a double‐notched specimen are also compared for the joined and monolithic specimens, which provides fracture strength data that can be used for comparisons. The shear strength values for the molybdenum‐joined SiC (264–61 MPa) are found to be within the data scatter of the monolithic material (397–31 MPa). These results indicate that a molybdenum foil diffusion bonding technique can be used to produce joints that are as strong as continuous bars of monolithic SiC in shear‐type loading, but they have lower flexural strength values than monolithic SiC. However, the flexure strength values for molybdenum foil‐joined SiC are within the range of values or higher than values reported in literature for the joining of SiC using reaction forming, reaction‐bonding, polymer precursors, and niobium diffusion bonding.