INTERFACIAL CHARACTERIZATION OF DIFFUSION-BONDED MONOLITHIC AND FIBER-BONDED SILICON CARBIDE CERAMICS

摘要

Diffusion bonding was used to join silicon carbide (SiC) to SiC substrates using 10 μm thick titanium interlayers. Two types of substrate materials were used: chemical vapor deposited (CVD)-SiC and SA-Tyrannohex~(TM) (SA-THX); the latter has microstructures consisting of SiC fibers and a carbon layer. Microstructures of the phases formed during diffusion bonding were investigated with transmission electron microscopy (TEM) and selected-area diffraction (SAD) analysis. Attention was paid throughout to the direction of the SiC fibers in SA-THX with respect to the Ti interlayer. For example, the width of bonding region was 9 or 15 μm depending on whether the SiC fibers were parallel or perpendicular to the Ti interlayer, respectively. From the SAD pattern analysis, Ti_3SiC_2, Ti_5Si_3C_x, and TiSi_2 were identified in all samples. TiC and unknown phases appeared when a monolithic CVD-SiC substrate was used with a SA-THX substrate with SiC fibers perpendicular to the Ti interlayer. A high concentration of Ti_3SiC_2 and a lower concentration of Ti_5Si_3C_x were formed when SA-THX with SiC fibers parallel to the Ti interlayer was processed. In contrast, a lower concentration of Ti_3SiC_2 phases and a higher concentration of Ti_5Si_3C_x were formed with fibers perpendicular to the Ti interlayer. These differences are caused by the presence of the hexagonal carbon layer on the SiC fiber surface that acts like a grain boundary at the Ti interlayer. It appears that this layer retards the migration of Si and C atoms into the Ti interlayer during diffusion bonding.