Journal of Student Research 2014

Characterization of Vacuum Brazed Advance Ceramic & Composite Joints

St. Gobain Si 3 N 4 (NT-154) containing 4 wt% Y 2 O 3 as a sintering aid was used for silicon nitride joints and three ZrB 2 -based ceramics, called ultra-high-temperature ceramics (UHTC) were used to create joints. The three ZrB 2 -based ceramics, named ZSS, ZSC and ZS, have the following compositions: (i) ZSS (ZrB 2 + 20 v/o SiC p + 35 v/o SiC f ), contains SCS-9a fiber and particulate SiC, (ii) ZSC (ZrB 2 + 14 v/o SiC p + 30 v/o C p ), contains particulate SiC and carbon, and (iii) ZS (ZrB 2 + 20 v/o SiCp), contains particulate SiC. These ceramics were fabricated by hot-press diffusion bonding. The ZSC and ZS achieved full-densification but the hot-pressed ZSS had ~30% residual porosity (determined from weight and volume measurements) and micro-cracking due presumably to residual stresses resulting from the mismatch of coefficient of thermal expansion (CTE) between the SCS-9a fibers (CTE~4.3×10 -6 /K) and the matrix. The metallic substrates (Table 1) used in bonding were obtained either from Inco Specialty Metals or from Good Fellow. The interlayer metals included foils of W (20 µm), Ta (50 µm), and Mo (20 µm) and were used to create multilayer joints with Si 3 N 4 . The CTE and yield strength of the selected substrate and interlayer materials are given in Table 3.

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