Compressive strength degradation in ZrB2-based ultra-high temperature ceramic composites

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3. Results and discussion

3.1. Microstructure of as-received specimens

Fig. 1 shows the as-fabricated microstructures of three types of composites studied. The ZS composite appears to be fully dense, while the ZSC suffered from significant grain pullout during sample preparation. This is attributed to the weak bonding of C to the ZrB2 and SiC phases, which results in removal of the C phase during polishing. In ZS, ZrB2 grains (gray phase) are equiaxed with reported grain size in the 6–12 μm range, while the SiC grains (dark phase) are elongated with sizes of approximately 1.5–3 μm thick by 3–11 μm wide/long. [7] In ZSC, the grain pullout during polishing made the estimation of grain size difficult, although it can be seen from Fig. 1 that ZrB2 grain size is smaller in ZSC than in ZS. It should be noted that, at least for the ZS composite, the grain size is close to the critical grain size for microcracking due to the anisotropic thermal expansion coefficient of ZrB2, which has been reported to be around 15 μm.30

The fiber composite microstructure is shown in Fig. 1C and D. Fibers were distributed in layers, and regularly spaced matrix cracking is produced due to residual thermal stresses that appear because of the differences in thermal expansion coefficients of the fibers and the matrix. A detailed view is presented in Fig. 2, which also shows that significant amount of porosity is present in the matrix. Fig. 3 shows elemental maps of the ZSS composite obtained by EDS. It can be seen that the SCS-9a fibers contain a carbon core surrounded by SiC. From Fig. 1 and Fig. 3 it can be concluded that the fiber coating remained intact during the uniaxial pressing step of the fabrication route.

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