Two types of silicon carbide (SiC) substrates, CVD‐SiC and SA‐Tyrannohex™ (SA‐THX) are diffusion bonded using three different metallic interlayers: Ti (10 and 20 μm) coating on SiC deposited using physical vapor deposition (PVD), Ti foils (10 and 20 μm), and a Mo–B foil (25 μm). The microstructures of the resulting interfacial reaction zone are examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in samples prepared using focused ion beam (FIB) technique. The effect of interlayer material and thickness, and the direction of the SiC fibers in SA‐THX, on the interfacial microstructure and phase composition are studied. Sound diffusion bonds developed in most samples except a few that showed microcracking. In SA‐THX/PVD‐Ti samples, chemical reactions occurred more rapidly with thin (10 μm) interlayers and SA‐THX fibers bonded parallel to the interlayer. Microcracks developed in samples with large amounts of an intermediate phase and small amounts of stable phases. This microcracking occurs due to presumably large coefficient of thermal expansion (CTE) mismatch and CTE anisotropy. The SA‐THX samples with the Mo–B interlayer formed crack‐free bonds even in the presence of reaction phases with anisotropic CTEs. This was attributed to a lower CTE mismatch of chemical phases with the SA‐THX substrate in Mo‐B interlayer system than in samples with Ti interlayers.
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