Transmission Electron Microscopy of Interfaces in Diffusion-Bonded Silicon Carbide Ceramics

Abstract

Diffusion bonding was used to join silicon carbide (SiC) to SiC substrates using three kinds of interlayers: physical-vapor-deposited (PVD) Ti coatings (10 and 20 μm) on the substrate, Ti foils (10 and 20 μm), and a Mo–B foil (25 μm). Two types of substrates were used: chemical-vapor-deposited SiC and SiC fiber bonded ceramic (SA-TyrannohexTM), the latter having a microstructure consisting of SiC fibers and a carbon layer. The microstructures of the phases formed during diffusion bonding were investigated using transmission electron microscopy (TEM) and selected-area diffraction analysis. TEM samples were prepared using a focused ion beam, which allowed samples to be taken from the reacted area. The effect of the interlayer material and the direction of the SiC fibers in the substrate with respect to the interlayer was evaluated. Scanning electron microscopy and TEM revealed good diffusion bonds in all samples; however, some samples exhibited small amounts of microcracking. The diffusion bonded CVD SiC sample using the 10-μm-thick PVD-Ti interlayer formed more of the stable phase and less of the intermediate phases than the sample using the Ti foil. This behavior was caused by the presence of columnar Ti grains in the interlayer, which may have enhanced the migration of Si and C atoms in the interlayer. In the SA-Tyrannohex samples using the Ti-foil interlayer, the chemical reaction proceeded more rapidly when the fibers were parallel to the interlayer than when they were perpendicular. This behavior was likely caused by the hexagonal carbon layer always facing the Ti interlayer in the sample with perpendicular fibers; this peculiar microstructure reduced the mobility of Si and C migrating into the interlayer. The SA-Tyrannohex sample using the Mo–B foil as the interlayer had excellent diffusion bonds with no microcracks or voids. In this system, Mo5Si3C, Mo2C, and Mo5Si3 formed. While phases have anisotropic coefficient of thermal expansion (CTE), the CTE mismatch between those phases and the substrate was apparently smaller than the mismatch in the samples using Ti interlayers.