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Microstructure of the as-received materials. (ZS) ZrB 2 -20% vol. SiC. (ZSC) ZrB 2 -30% vol. C -14% vol. SiC. In the ZSC material, grain pullout and C removal during the metallographic preparation is evident.
Source publication
The high melting point of refractory metal diborides makes them promising materials for ultra high temperature applications. In this work, we study the compressive strength of two ZrB2-SiC and ZrB2-SiC-C composites. Samples have been studied in compression at room temperature, 1400°C and 1550°C, in atmospheric air. The degradation of the mechanical...
Contexts in source publication
Context 1
... conductive coating of either carbon or gold was applied to the specimens prior to observation. Figure 1 shows the as fabricated microstructure of the two compositions studied. Our observations match those reported in Ref. 7. The ZS composite appears to be fully dense, while the ZSC suffered from significant grain pullout during polishing. ...
Context 2
... ZS, ZrB 2 grains are equiaxed with reported grain size in the 6-12 πι range, while the SiC grains are elongated with sizes of approximately 1.5-3 ηι thick by 3-11 ιη wide/long 7 . In ZSC, the grain pullout during polishing made the estimation of grain size difficult, although it can be seen from Figure 1 that ZrB 2 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 anisotropie thermal expansion coefficient of ZrB 2 , which has been reported to be around 15 ηι 28 . ...
Context 3
... can also explain why at room temperature the composite containing carbon is stronger, as it has a smaller grain size due to the presence of grain-growth inhibiting carbon. Additionally, the weak bonding of C to ZrB 2 evidenced as grain pullout in Figure 1 can contribute to the relaxation of microcrack-inducing residual stresses developed upon cooling. At high temperatures however, carbon burn-out is responsible for the lower strength observed in ZSC when compared to ZS. ...
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Citations
In the last few decades, research on the processing and properties of ultra‐high‐temperature ceramics (UHTCs) has generated a substantial base of knowledge and left several unanswered questions. There is a large scatter in the literature data associated with the processing of UHTC borides prompting the non‐reproducibility and non‐uniformity in the microstructure and, thus, desired properties. Herein, the data on the oxidation behavior of UHTC borides ubiquitous in the entire literature are analyzed to understand the effect of composition, sintering parameters, densification, grain size, and oxidation conditions. A conjunction of graphical methods has been utilized to converge the scattered data and correlate the effect of variables and testing environment on the oxidation behavior. It was concluded that high densification (>95%), large grain size (∼8 μm), and 20–30 vol.% of Si‐containing additives (SiC, Ta5Si3, and TaSi2) could augment the oxidation resistance. The study elucidates that oxide scale thickness should be preferred over mass gain in future studies as a metric for measuring oxidation. The analysis presented here will allow the UHTC community to optimize diboride materials' design for hypersonic applications. The developed database on the oxidation performance of diborides will also transfer knowledge beyond the memory banks of the experts in the field. Furthermore, well‐structured databases such as the one developed herein could be employed in data‐driven approaches to optimize the design and manufacturing of ultra‐high‐temperature materials in an efficient scheme.
