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High-entropy diboride has been arousing considerable interest in recent years. However, the low toughness and damage tolerance limit its applications as ultra-high-temperature structural materials. Here we report that a unique SiB6 additive has been first incorporated as boron and silicon sources to fabricate a high-entropy boride (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2–SiC composite though one-step boro/carbothermal reduction reactive sintering. A synergetic effect of high-entropy sluggish diffusion and SiC secondary phase retarded the grain growth of the (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2–51SiC composites. The small grain size was beneficial to shorten the diffusion path for mass transport, thereby enhancing the relative density to ~99.3%. These results in an increase of fracture toughness from ~5.2 in HEBS-1900 to ~7.7 MPa·m1/2 in HEBS-2000, which corresponded to a large improvement of 48%. The improvement was attributed to a mixed mode of intergranular and transgranular cracking for offering effective pinning in crack propagation, resulting from balanced grain boundary strength collectively affected by improved densification, solid solution strengthening, and incorporation of SiC secondary phase.

During the spark plasma sintering at 1900 °C, SiB 6 decomposes into cubic silicon carbide and boron carbide, owing to the reducing environment of the furnace. For the HfB 2 -SiB 6 ceramic improvement in hardness (24.5 ± 0.7 GPa) was attributed to the formation of the B 12 (C,Si,B) 3. Fracture toughness by indentation (6.8 ± 2.4 MPa·m1/2), single-edge notched bend specimens (4.6 ± 0.4 MPa·m1/2) and room-temperature strength (513 ± 21 MPa) of the HfB 2 –SiB 6 composite produced by spark plasma sintering was higher or on the same level as the HfB 2 –SiC ceramics. The high-temperature flexural strength tests suggested that the strength would decrease monotonically with an increase in temperature. At or below 1600 °C, only a linear stress-strain response was observed, and resulted into a mean strength of ~320 MPa. During the tests at 1800 °C, we observed a nonlinear deformation indicating ongoing plastic deformation which led to a strength decrease down to 230 ± 30 MPa. [ABSTRACT FROM AUTHOR]

Silica-based ceramic core is an extremely critical component in the manufacture of hollow blades during investment casting. However, the traditional preparation methods rely more on the molds, and the manufacturing costs are relatively high. In this study, silica-based ceramics with silicon hexaboride (SiB 6) addition were prepared via 3D stereolithography printing. And the effects of the SiB 6 content on mechanical properties of the obtained ceramic samples were explored. As the SiB 6 content increased to 2.0 wt%, the linear shrinkage gradually decreased, while the room temperature and high temperature flexural strength were enhanced at the SiB 6 content from 0 to 1.0 wt% and reduced as the SiB 6 content further rose. As the SiB 6 content increased to 1.0 wt%, the linear shrinkage was reduced to 1.86% resulting from the oxidation reaction of SiB 6. Furthermore, with 1.0 wt% SiB 6 addition, the flexural strength of the samples at room temperature was enhanced from 6.75 MPa to 14.63 MPa due to the sintering promotion of oxidation product B 2 O 3 , and the flexural strength at 1550 °C was improved from 7.68 MPa to 13.08 MPa because of the enhanced β-cristobalite content, which is suitable for high temperature casting of ceramic cores. Therefore, it demonstrates the capability of fabricating SiB 6 reinforced silica-based ceramic cores with high performance via stereolithography. [ABSTRACT FROM AUTHOR]

In this research, the effects of NanoSilicon hexaboride (NanoSiB 6 ) additive, as a sintering aid, on mechanical properties and the sinterability of Boron Carbide (B 4 C) have been investigated. In addition, the results of several equations used for determining the Indentation fracture resistance ( K IFR ) by Vickers indentation test have been evaluated. For this purpose, 0, 2, 4, 6 and 8 wt% NanoSiB 6 additive have been added to B 4 C in order to improve its sinterability at temperatures 2050, 2150 and 2250 °C. The findings indicate that by adding NanoSiB 6 , up to 4 wt%, to B 4 C, its properties such as relative density, Young's modulus, microhardness and K IFR improve as the sintering temperature rises; however, these properties diminish with the further increase of the mentioned sintering aid. Also, a comparison between K IFR values shows the closeness of the results obtained by different equations and the satisfactory accuracy of the equation for determining K IFR by crack area method compared to the results of other equations; with a difference of < 20% between the two results. [ABSTRACT FROM AUTHOR]

• We proposed a novel silicon hexaboride, Cmca -B 6 Si. • The proposed structure is dynamically and mechanically stable at ambient pressure. • The formation energy of Cmca -B 6 Si is lower than those of Pnnm -SiB 6 and SiB 4. • Cmca -B 6 Si may be obtained by removing Li from LiB 6 Si in a thermal "degassing" process. • Cmca -B 6 Si is metallic and has a novel two-dimensional metallicity. Using first-principles calculations, we propose a novel metallic silicon hexaboride, Cmca -B 6 Si. This structure is dynamically and mechanically stable at ambient pressure. Furthermore, the formation energy of Cmca -B 6 Si is lower than those of synthesized Pnnm -SiB 6 and SiB 4 , indicating that Cmca -B 6 Si is thermodynamically stable. The calculations of thermodynamic stability and structural inheritance indicate that Cmca -B 6 Si may be obtained by removing Li from the synthesized LiB 6 Si, that is, by imitating the preparation of Si 24 and T-Graphene C 4. The structure of Cmca -B 6 Si is a covalent three dimensional framework linked by B 12 icosahedrons and Si atoms. The analysis of band structure and density of states reveals that Cmca -B 6 Si is metallic because of the partially occupied 2 p and 3 p electrons of B and Si atoms, respectively. The electron orbitals reveal that Cmca -B 6 Si has a novel two-dimensional metallicity. This study gives a fundamental investigation on the structural, phonon, and electronic properties of Cmca -B 6 Si. [ABSTRACT FROM AUTHOR]